摘要:

FEBRUARY, 1968 Voi. 93, No. I103 THE ANALYST Separation of Silicon and Phosphorus with Ammonium Molybdate and their Successive Determination BY A. M. G. MACDONALD AND F. H. VAN DER VOORT (Departwent of Chemistry, The University, Birmingham 16) A method for the successive determinations of silicon and phosphorus is described. Phosphate is separated cleanly from silicate as ammonium molybdophosphate; phosphate in the precipitate and silicate in the filtrate are determined by conventional quinoline procedures. The method is suitable for the separation and determination of 0.2 to 2 mg of each of the ions. The separation method can be applied to organic materials, after decomposition with peroxide. A study of organic bases for the precipitation of molybdo- silicate showed that none is preferable to quinoline.IN a recent paper,f a study of various organic bases for the quantitative precipitation of the molybdophosphate ion was described; a method based on 2-methylpyridine was found to be less sensitive to interferences from silicate, arsenate and citrate than the well known quinoline method, although most of the favourable characteristics of the quinoline method were retained. The investigation described below was undertaken in an attempt to find an organic base that would prove analogously less sensitive to interfering effects than quinoline for the determination of silicate. This aim was not accomplished but a satisfactory method was developed for the separation and successive determination of phosphate and silicate, which readily proved applicable to the analysis of organic materials.EXAMINATION OF ORGANIC BASES FOR PRECIPITATION OF MOLYBDOSXLICATE QUALITATIVE STUDY- Sensitivity tests were made in 1 N, 1.5 N and 2 N hydrochloric acid medium for both known and possible precipitants for molybdosilicate; for these tests, precipitation was made at 70" to 80" C by the addition of ammonium molybdate solution and the organic base solution. The sensitivities quoted in Table I are the least amounts of silicon (in a final volume of 6 ml) that could be detected when compared with a blank solution prepared simultaneously; at acidities below 1 N, precipitation of the molybdosilicate could scarceIy be distinguished from that of the base molybdate itself. Better sensitivities for some of the reagents could probably be achieved with a change in the conditions, for example, King and Watson2 recommend a sulphuric acid medium for pyramidon, but the hydrochloric acid medium used seemed satisfactory for comparison purposes.As expected, the sensitivities deteriorated rapidly as the acidity of the medium increased. Hexamine,s pyridine? 1-naphthylamine, piperidine, 2,4-dimethylpiperidine, cyclohexylamine, dibutylamine, 2-aminopyridine, benzotriazole, 2,2'-pyridoin and methyl and ethyl nicotinate proved of no value for molybdosilicate precipitation. Where their behaviour was unknown, the bases listed in Table I were tested for their sensitivity to the molybdophosphate ion. All showed greater sensitivity to molybdophosphate than to molybdosilicate, in the same way as quinoline and 8-hydroxyquinoline. QUANTITATIVE STUDY- The quantitative precipitation of molybdosilicate with the bases listed was examined, the procedure used being analogous to the gravimetric quinoline method of Miller and Chalmers.6 Generally, the precipitates were formed in about 1 N hydrochloric acid medium at 70" to 75' C, cooled in ice - water, washed with ice-cold water, and dried at 120" to 140" C for 1 hour; they were then weighed without ignition, their hygroscopicity being generally 66; 0 SAC and the authos.66 MACDONALD AND VAN DER VOORT: SEPARATION OF [APzdySt, VOl.93 negligible with a reasonable amount of care. The methods were tested for about 1.5 and 3.5mg of silicon as silicate; the results for the former amount are listed in Table I. All results were calculated on the basis of the formation of 4 (base).12Mo03.Si02.2H,0.TABLE I QUALITATIVE AND QUANTITATIVE RESULTS ON PRECIPITATION OF SILICATE Sensitivity as mg of silicon per 6ml* of Base 8-Hydroxyquinoline . . 2,4Dimethylquinoline 2-Methylquinoline . PMethylquinoline . . 2,4-Dimethylpyridine PAminopyridine . . 2-Benzoyipyridine . . Tributylamine . . N,N-Diethylaniline . . 4-Picolylamine . . 2-Aminoquinoline . . 3-Aminoquinoline . . 8-Aminoquinoline - . Quinoline . . .. Pyramidon .. .. Reference .. 7,s . . 5, 6, 12 .. 5 .. 5 .. 6 . * 2, 9, 10 * . - - .. - .. - . . - .. - .. - .. I .. - .. r ~~ 1 1.5 N 2N hydrochloric hydrochloric acid acid 0.0 1 0.01 0.02 0-05 0.05 0.50 0.02 0.05 0.02 0.05 0.50 0.50 0.1 0-50 0.05 0.05 0.02 0.10 0.05 0.50 0.02 0.10 0.06 0.10 0.02 0.02 0.02 0.05 0.02 0.10 Silicon present, mg 1.464 1.470 1-470 1.464 1.635 1.470 1.452 1.470 1.470 1.635 1.635 1.580 1.665 1-665 - Silicon foundt , mg 1.442 1.464 1.467 1.450 1.636 1-366 1.202 1.438 1-457 1.610 1.323 1.578 1.651 1.658 - Error t - 0.02 1 - 0.006 - 0.003 -0.014 + 0.001 -0.104 - 0.250 - 0.032 -0.013 - 0.025 -0.312 - 0.002 - 0.014 - 0.007 - * Qualitative test (see text).Average of two results. $ Blanks generally lay in the range 5 to 10 pg of silicon and have been deducted. The behaviour of these bases as quantitative precipitants for molybdosilicate ions falls into three groups. I. Quinoline,6 2,4-dimethylq~inoline,~ 4-rnethylq~inoline~ and 8-aminoquinoline all gave satisfactory results ; the 2-aminoquinoline precipitate tended to creep and was less satisfactory.11. 8-Hydro~yquinoline,~ s 8 2-methylq~inoline,~ 3-aminoquinoline and tributylamine gave slightly low results. These would have been improved by calculation on the basis of a monohydrated complex, but none of the previous evidence on base molybdosilicates seems to favour this formula; a proper thermogravimetric study would be of value. The low results obtained with 2-benzoylpyridine and N,N-diethylaniline can be explained by the slight solubility of the precipitates and a tendency to slight decomposition on drying. 111. 2,4-Dimethylpyridine, 4-aminopyridine and 4-picolylamine gave poor results, because of the solubility of the precipitates. A formula containing only three molecules of base has been favoured for pyramidon rnolybdosili~ate,~~~~~~~~ but none of the three above bases yielded sufficiently reproducible results for an analogous formula to seem justified. Heslop and Pearsonll have shown that 2,4-dimethylpyridine has a much better sensitivity for molybdophosphate than for molybdoarsenate, similarly to 2-methylpyridinel ; the molybdosilicates of the two bases appear to be more soluble than the molybdoarsenates.None of the bases examined had any marked advantage over quinoline itself for pre- cipitation of molybdosilioate. For the compounds in group 11, modification of experimental conditions would probably have improved the results but none of these bases showed any favourable characteristics in comparison with quinoline, hence this did not seem worthwhile. Accordingly, other procedures were sought for the separation of silicate and phosphate. As the method was ultimately intended for samples containing silicon in the range 1 to lOrng, the titximetric6 and gravimetric5 methods of completing the determination of silicate were compared ; the gravimetric method has been preferred by most later Under the conditions described in the Experimental it was found that silicon in the range 0.2 to 36mg could be determined gravimetrically with excellent accuracy and precision, whereas for the titrimetric method, the precision was less, an empirical factor was essential (as mentioned by Wilson6) and the range was only 0.2 to 1 nig, above which the end-points were masked by the formation of molybdenum blue, even when potassium chloratel5 was added.February, 19681 SILICON AND PHOSPHORUS WITH AMMONIUM MOLYBDATE 67 However, for the determination of phosphate, the titrimetric procedure of Wilson16 pr7 was preferred to the gravimetric method.18 No trouble arises from molybdenum blue in the phosphate titration whereas, for the gravimetric method, the molybdophosphate is more finely divided and requires more care in filtering on sintered crucibles than quinoline molybdosilicate.SEPARATION AND DETERMINATION OF PHOSPHATE AND SILICATE- Ion exchange and solvent extraction have often been used for the separation of phosphate and silicate. Ion exchange was not considered in the present work because of its general tediousness. Recently, extraction with isobutyl acetate at pH 1 has proved useful for the separation of phosphate from silicate,lg and methods for the selective formation of molybdophosphate and molybdosilicate in mixtures have been described.20 In both methods colorimetric procedures were used, which were considered inappropriate for the present purpose, i.e.,occasional organic analysis.Another colorimetric procedure for the simultaneous determination of silicon and phosphorus, which has been applied to organic compounds after fusion with sodium peroxide, involves the molybdosilicate blue and molybdovanadophosphate procedures.21 Brabson, Duncan and Murphyl3 have used quinoline for the simultaneous precipitation of molybdophosphates and silicates, the phosphate being determined on a separate aliquot as quinoline molybdopho~phate.1~~~7 So far as is known, no methods are available for the consecutive determination of the two ions on a single solution based on titrimetric or gravimetric techniques involving the heteropolyanions. The work done in these laboratories on the precipitation of molybdophosphates and silicates with organic bases indicated the impossibility of obtaining clean separations of the two ions by such means; the masking of one ion by, for example, citric or oxalic acid to allow precipitation of the other is valid only for small amounts of the unwanted ion, for there is always a slight effect on the required ion also if much of the masking agent is added.Accord- ingly, attention was re-directed to the ammonium heteropolymolybdates. There is little definite information in the literature about the precipitation of ammonium inolybdophosphate in the presence of silicate.Mellor and Thompson22 quoted many early workers in support of the statement that silicate can be co-precipitated as molybdosilicate, and mentioned that it is usually separated by a preliminary dehydration, although it may be removed by washing the ammonium molybdophosphate with ice-cold water. Hillebrand and Lunde1123 also state that silicate retards the precipitation and may co-precipitate as ammonium molybdosilicate, but most modern textbooks simply mention the interference of silicate; several imply that the gravimetric procedure for phosphate is affected by the formation of hydrated silica rather than by molybdosilicate. In contrast, Feig12* recommends a spot test in which molybdosilicate is detected in the filtrate after the bulk of the phosphate has been precipitated as ammonium molybdophosphate. Although Feigl’s separation was, of course, only for qualitative purposes, it was considered worthwhile to examine its quanti- tative possibilities. If ammonium molybdophosphate could be precipitated in a pure form, it should be possible to determine molybdosilicate in the filtrate by the quinoline method. The ammonium molybdophosphate precipitate could be dissolved and determined analogously by a quinoline rnethod,l6s17 for no advantage would accrue from the use of 2-methylpyridine in the absence of silicate.Measuring the ammonium molybdophosphate as such was un- attractive, because the conditions for the classical micro-gravimetric procedure are critical, requiring an empirical factor, and the micro-titrimetric procedure was shown to be inaccurate because of poor end-points despite several variations (cf.Stockdale25 and WilsonlSJ7). The procedure for the precipitation of ammonium molybdophosphate is based on that of Thistlethwaite26 and on the recommendations of Archer, Heslop and Kirby.27 REAGENTS- Standard silicate solution, 0-5 m g of silicon per mLWeigh about 200 mg of pure precipi- tated silica (British Drug Houses Ltd.) into a weighed platinum crucible, ignite to constant weight and fuse with 3 g of analytical-reagent grade sodium carbonate for 1Q hours. After cooling, transfer the crucible and contents to a beaker and dissolve the melt in distilled water ; then dilute to 100ml. Standard Phosphate solution, 0-5 mg of phosphorus $er md-Prepare from analytical- reagent grade potassium dihydrogen phosphate.EXPERIMENTAL68 MACDONALD AND VAN DER VOORT: SEPARATION OF [Auzdyst, Vol. 93 Ammonium molybdate reagefit-Dissolve 35 g of analytical-reagent grade ammonium molybdate in 50 ml of water and add 50 ml of analytical-reagent grade ammonia (sp. gr. 0.88). Dilute 168 ml of nitric acid to 360 ml with water, cool, and add the molybdate solution. Leave for 24 hours, dilute to 900 ml with water and filter. Sodium moly bdate solzttiort-Dissolve 43.2 g of analytical-reagent grade molybdenum trioxide and 24.0 g of analytical-reagent grade sodium hydroxide in water, add 58-0 ml of 6 N hydrochloric acid, dilute to 500 ml with water and filter. QztGzoZine soZu,tkon-Prepare a 2 per cent.solution of quinoline in 0.26 M hydrochloric acid. Ammonizm nitrate wash solzction-Dissolve 16 g of analytical-reagent grade ammonium nitrate in 2 litres of water. Store all reagent solutions in polythene containers. Water from an all-glass distillation apparatus was used throughout. SEPARATION OF PHOSPHATE- To a 10-ml sample of the mixed phosphate - silicate solution, add 2 ml of 1 per cent. nitric acid and heat to 50" to 55' C on a water-bath. Heat the ammonium molybdate reagent to the same temperature and add 12 ml of it to the sample solution, while stirring. Digest at 50" to 55" C for 30 minutes with occasional stirring, and then cool for 30 minutes. Filter through a 1.5-cm layer of paper pulp supported on a Witt plate, transferring and washing the precipitate with 30ml of ammonium nitrate solution in four portions.Suck the paper pad dry and use the filtrate (volume 55 to 60 ml) for the determination of silicon. A suction "bell-jar" apparatus, similar to that described by Hillebrand and L~nde11,~~ was used. DETERMINATION OF PHOSPHORUS- Add 10.0 ml of 0-5 M sodium hydroxide solution to the original precipitation vessel and use this to dissolve the ammonium molybdophosphate off the pulp pad in small portions, collecting the filtrate in a 250-nil ground-glass stoppered conical flask. Wash the vessel, funnel and pad with five 4-ml portions of water and suck the filter dry. Neutralise the filtrate, the volume of which is about 30 ml, with 6 M hydrochloric acid in the presence of methyl red indicator, acidify with 8 ml of the acid and add 200 mg of citric acid followed by 6 ml of sodium molybdate reagent.Heat to boiling, and add 6 ml of quinoline solution. Re-heat to boiling and cool in ice - water to about 15" C. Filter on a pulp pad, washing the flask and filter with water until they are free from acid. Transfer the pad and precipitate back to the flask, add an excess of 0.1 N sodium hydroxide solution, shake to dissolve the precipitate and back-titrate the excess of alkali with 0.05 M hydro- chloric acid in the presence of phenolphthalein indicator. DETERMINATION OF SILICON- To the filtrate of the amnionium molybdophosphate separation, add 1.8 ml of 6 M hydro- chloric acid and 6 ml of sodium molybdate solution and allow to stand for 5 minutes. Heat in a water-bath to 65" to 70"C, but not higher, and add 6*2ml of 6~ hydrochloric acid and 6 ml of quinoline solution.Digest at 70" C for 10 minutes and then cool to about 15" C in ice - water. Filter on a weighed No. 4 sintered-glass crucible, transferring the precipitate and washing it with small portions of cold water (20ml in all). Dry the crucible and pre- cipitate for 1 hour at 120" to 140"C, cool in a desiccator over phosphorus pentoxide for 30 minutes and weigh immediately. In every determination, establish blank values by taking 0.5 mg of the relevant ion through the entire procedure. DECOMPOSITION OF ORGANIC MATERILU- Place some M.A.R. sodium peroxide in a weighed nickel "fluorine" bomb (C. W. Cook Ltd., Birmingham) and add the weighed (5 to 8 mg) organic sample, followed by more sodium peroxide to give 200mg in all.Seal the bomb with a copper gasket (not asbestos-filled) and heat in a muffle furnace €or 15 minutes at 500" C. Swirl the contents of the bomb after 10 minutes and again when removing the bomb from the furnace. Allow the bomb to cool and open it, rinsing any of the melt on the lid with water into a platinum dish. Rinse the outside of the bomb, wipe with a tissue and half-cover the bomb in the dish with water. Cover, heat gently to boiling to dissolve the melt and cool. PROCEDURESFebruary, 19681 SILICON AND PHOSPHORUS WITH AMMONIUM MOLYBDATE 69 Place the amount of 4 M nitric acid needed to neutralise the melt (1-80 ml for 200 mg of sodium peroxide) in a polythene beaker and rinse the watch-glass, bomb and the contents of the platinum dish into the beaker with water.Add 300ml of 1 M sodium hydroxide solution, allow to cool and finally adjust the pH to 7 with 2 M nitric acid (pH meter). Transfer the resulting 70 to 80 nil of solution to a glass beaker and use it for the separation and deter- mination of silicon and phosphorus as described above. RESULTS AND DISCUSSION DETERMINATION OF PHOSPHORUS- For the titrimetric determination of phosphate as quinoline molybdophosphate, it was shown that the presence of the additional molybdate, ammonium ion and the sodium chloride arising from the neutralisation had no effect on the accuracy of the final results. When phosphate was separated first as the ammonium complex, some low results were obtained when sintered-glass filters were used; there was no escape of the ammonium salt with pulp-pad filters.Because of the general doubt surrounding the stoicheiometry of ammonium molybdo- phosphate, it was felt necessary to check that all the phosphorus was present in the precipitate. This was done by precipitating the molybdate in the precipitate after dissolution, as lead molybdate ; ThistlethwaiteZB has shown that the molybdenum - phosphorus ratio in ammonium molybdophosphate is always constant whatever the total composition is. Accordingly, the excellent results obtained for molybdate in the above tests indicate that none of the original phosphate escaped precipitation. Ten determinations of standard phosphate solution by the two successive steps showed an average recovery (after blank deduction) of 9994 per cent.for 1 mg of phosphorus, with an average error of 0.6 per cent.; the slightly low recovery can certainly be ascribed to solubility losses. DETERMINATION OF SILICON- For the gravimetric determination of silicon as quinoline molybdosilicate, it was shown in the same way as for the phosphorus determination that the various ions present in the filtrate of the ammonium molybdophosphate precipitation did not affect the accuracy. Good results were obtained whether pure silicate solutions or those containing the additional ions were analysed. For example, six determinations of standard silicate solutions containing 1 mg of silicon with all the extraneous ions added showed an average recovery of 99.8 per cent. (average error 0.3 per cent.); when the same amount of silica was taken through the entire separation procedure, the average recovery was 99.65 per cent.and the average error was 0.6 per cent. for six results. Blank values, when determined by straightforward blank determinations, varied beyond the permissible limits, a common feature of heteropolymolybdate procedures, and were best determined by using a standard solution of the relevant ion. TABLE 11 CONSECUTIVE DETERMINATION OF PHOSPHATE AND SILICATE Phosphorus Phosphorus taken, found, mg mg 1.992 2.01 1 1.992 2.010 0.996 0.992 0.906 0.986 0.996 1.005 0.996 0.999 0.747 0.746 0.498 0.602 0,249 0.246 0.249 0.246 0.100 0.090 Error, mg + 0.019 + 0.018 - 0.004 - 0.010 + 0.009 + 0.003 -0.001 + 0.004 - 0.004 - 0.004 - 0.010 Silicon taken, mg 0.990 1.980 0.099 0.248 0.495 0-743 0.990 0.990 0.990 0.248 0.990 Silicon found, m g 0.979 1.961 0.101 0.243 0.497 0.736 0.981 0-987 0.981 0.237 0.978 Error, m g - 0.01 1 -0.019 + 0.002 - 0.006 + 0.002 - 0.008 - 0.009 - 0.003 - 0.009 -0~011 -0.012 The range over which the separation method could be used was about 0-2 to 2 mg of each of the ions concerned. Typical results are shown in Table 11.With amounts of silicon70 MACDONALD AND VAN DER VOORT: SEPARATION OF [Analyst, VOl. 93 larger than 2 mg there was a slight co-precipitation with the ammonium molybdophosphate, so that the method could not be used; at the 2-mg level, the relative error does not exceed 1 per cent. ANALYSIS OF ORGANIC COMPOUNDS- An attempt was made to use the oxygen-flask combustion for the decomposition of silicon compounds.Reverchon and Legrand2* have described a method in which a nickel flask is used, sodium peroxide is added to the sample and the sample holder is boiled for 45 minutes in the 1 M sodium hydroxide absorbent solution to recover all the silica from it. In the present work, thick-walled, 500 or 1000-ml polypropylene flasks were used and the products were absorbed in 0.1 M alkali or acid. Under no conditions could quantitative recoveries of silicon be achieved, despite the addition of sodium peroxide, sodium carbonate, sucrose, fully fluorinated or fully chlorinated compounds to the sample before the combustion. The various combustion aids were added both singly and in various mixtures; sodium peroxide was certainly the most effective additive, but the recoveries of silicon were still 5 to 10 per cent.low. Because of the plastic flasks, it was not possible to boil the absorbent solution. Christopher, Fennell and Webb21 have used fusion with sodium peroxide in a nickel bomb for the decomposition of semi-micro amounts of organic silicon and phosphorus compounds, and this method was successfully scaled down for 5 to 8-mg samples. Nitric acid was used to acidify the solution because chloride tends to retard the precipitation of molybdophosphate ; the amount of sodium nitrate arising from neutralisation of the bomb leachings had no effect on the separation and determination process. The volume of solution obtained from the bomb leaching and subsequent treatment was 70 to 80m1, and it was shown on standard silicate and phosphate mixtures that the increase in volume scarcely affected the accuracy of the final results.Results obtained for mixtures of silicon and phosphorus compounds are shown in Table 111. Only one compound containing both elements was available and the results are also shown in Table 111. It can be seen that all the results obtained are within the acceptable limits for elemental organic analysis. The method is probably less convenient for continual routine use than methods based on solvent extraction and spectrophotometry. It is, however, useful for occasional analysis, as only common reagents are required. We are grateful to Prof. R. Belcher for his interest in this work, and to Mr. R. W. Fennell for providing the special sample. TABLE I11 ANALYSIS OF ORGANIC COMPOUNDS Phosphorus, per cent.Silicon, per cent. Sample weight, mg 8.260 3.863 3.468 3.972 3.960 5.761 5.942 3.604 6.640 4.500 4.769 3.876 6.664 1 Theoreti- cal 11.81 Theoreti- cal 12-97 Compound Diphen ylsilanediol Diphenylsilanediol Diphenylsilanediol Hexaphenyldisiloxane Hexaphenyldisiloxane + triphenylphosphine + triphenylphosphine + triphenylphosphine + triphenylphosphine + triphenylphosphate C,*H,*O,P*Si, Found 11.96 Error +O.lS Found 12.91 Error - 0.06 11.81 11-73 - 0.08 12.97 13.06 + 0.09 11.81 11.93 +0.12 12.97 13-15 +O*l8 11.81 12.10 + 0-29 10.60 10.77 +0.27 9.62 9.36 -0.16 10.60 10.18 -0.32 8.04 7.28 + 0.26 +Om31 + 0.28 8.18 8.13 8-06 3-0.14 + 0.09 +O.Ol 7.63 7.59 7-66 REFERENCES 1. Macdonald, A. M. G., and Rivero, A.-M., Analytica Chim. Ada, 1967, 37, 625.2. King, E. J., and Watson, J. L., Mikrochamie, 1936, 20, 49. 3. Duval, C., “Inorganic Thermogravimetric Analysis, ” Second Edition, Elsevier Publishing Co., Amsterdam, London and New York, 1963, p. 239.February, 19681 SILICON AND PHOSPHORUS WITH AMMONIUM MOLYBDATE 71 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 10. 20. 21. 22. 23. 24. 25. 26. 27. 28. Babko, A. K., J. Appl. Chem., USSR, 1937, 10, 374. Miller, C. C., and Chalmers, R. A., Analyst, 1953, 78, 24. Wilson, H. N., Ibid., 1949, 74, 243. Voluinetz, M. Y., Zav. Lab., 1936, 5, 162. Su, Y.-S., Campbell, D. E., and Williams, J. P., Analytica Chinz. Ada, 1965, 32, 559. Hecht, F. , and Donau, J ., “Anorganische Mikrogewichtsanalyyse ,’, Springer-Verlag, Wien, 1940, Furuya, M., Bunseki Kagaku, 1962, 11, 1247. Heslop, R. B., and Pearson, E. F., Analytica Chim. Ada, 1967, 37, 516. Armand, M., and Berthoux, J., Ibid., 1953, 8 , 610. Brabson, J. A., Duncan, R. D., and Murphy, I. J., Analyt. Chem., 1963, 35, 1102. Christopher, A. J., and Fennell, T. R. F. W., Talanta, 1965, 12, 1003. Fernlund, U., and Zechner, S., 2. analyt. Chem., 1955, 146, 111. Wilson, H. N., AnaZyst, 1951, 76, 65. Fennell, T. R. F. W., and Webb, J. R., Talanta, 1959, 2, 105. Paul, J., Mikrochim. Acta, 1965, 836. Chalmers, R. A., and Sinclair, A. G., Analytica Chzm. Acta, 1966, 34, 412. Christopher, A. J., Fennell, T. R. F. W , and Webb, J. R., Talanta, 1964, 11, 1323. Mellor, J. W., and Thompson, H, V., “A Treatise on Quantitative Inorganic Analysis,” Second Edition, Charles Griffin & Co., London, 1938, p. 671. Hillebrand, W. F. and Lundell, G. E. F., Revised by Lundell, G. E. F., Bright, H. A., and Hoffman, J. I.,“Applied Inorganic Analysis,” Second Edition, John Wiley & Sons Inc., New York; Chapman 6 Hall Ltd., London, 1953, pp. 100 and 701. Feigl, F., “Spot Tests. Inorganic Applications,” Elsevier Publishing Co., Amsterdam, 1968, p. 336, Stockdale, D., Analyst, 1968, 83, 24. Thistlethwaite, W. F., Ibid., 1947, 72, 531. Archer, D. W., Heslop, R. B., and Kirby, R., Analytica Chim. Ach, 1964, 30, 450. Reverchon, R., and Legrand, Y., Chim. Analyt., 1966, 47, 194. p. 244. -, Ibid., 1954, 79, 535. Received September 20th, 1967

ISSN:0003-2654    

DOI:10.1039/AN9689300065

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

72 Analyst, February, 1968, Vol. 93, pfi. 72-78 Molecular-emission Spectroscopy in Cool Flames Part 11.8 The Behaviour of Phosphorus-containing Compounds BY R. M. DAGNALL, K. C. THOMPSON AND T. S. WEST (Chemistry Department, Imperial College, London, S. W.7) A sensitive and selective molecular-emission method for the determina- tion of phosphorus is described, in which the intense green emission obtained by nebulising orthophosphoric acid solution into a cool, nitrogen - hydrogen diffusion flame is measured a t 528 mu. The limit of detection in aqueous solution under the conditions of measurement IS 0.1 p.p.m. of phosphorus. Most cations produce a depressive matrix effect that can be readily overcome by a preliminary ion-exchange separation. Fifty-fold molar excesses of acetic, hydrobromic, hydrochloric, nitric, oxalic, sulphuric and tartaric acids do not interfere.IN the previous paper in this series we reported on the application of cool, nitrogen - hydrogen diffusion flames to the determination of sulphur.1 The procedure was based on the blue emission from the S, species that is formed in such flames. Like sulphur, phosphorus has its principal atomic resonance lines in the far ultraviolet region of the spectrum (1775, 1783 and 1788 A) and, because of the almost total absorption by air, flame gases and quartz optics in this region, it is not possible to apply atomic-absorption or atomic-fluorescence spectro- scopy in their usual modes. Consequently, we have examined the determination of phos- phorus by molecular-emission spectroscopy in cool diffusion flames.The more usual methods of determining phosphorus by emission-spectroscopic techniques involve indirect procedures, such as the depressive effect of phosphate on the emission of calcium and magnesium.2 However, only a somewhat limited range of phosphate concen- trations can be examined by using a given concentration of calcium. In addition, the method is not very sensitive; by using a 100 p.p.m. calcium solution the estimated limit of detection is about 30 p.p.m. of phosphorus. A direct method, with a limit of detection of 3 p.p.m., has been described by Brite,s who measured the emission from the continuum at 540 mp obtained with an oxy-hydrogen total- consumption burner and organic solvents. Davis and co-workers4 also used an oxy-hydrogen turbulent-flow burner to obtain a limit of detection in aqueous solution of 8 p.p.m. at a slit width of 2.0 mm.Wide slits are necessary to gather sufficient energy from the continuum. Unfortunately, the use of organic solvents leads to high background emission, which must be subtracted from the phosphorus emission. Another method,b which is also concerned with the determination of phosphorus in organic compounds, involves the measurement of the emission from the H-P-0 species. In this instance, organic vapours leaving a gas- chromatographic column, with nitrogen as carrier gas, were mixed with a volume of oxygen to give the same nitrogen-to-oxygen ratio as occurs in air. This mixture was then burned, with hydrogen as fuel gas, in a flame-photometer burner.The use of narrow band-pass interference filters and a photomultiplier resulted in a very sensitive and selective detector for sulphur and phosphorus compounds. The limit of detection at 526mp is 0.0063 p.p.m. of phosphorus. However, it is applicable only to organophosphorus compounds and, in this instance, only to those which can be handled directly by gas-phase chromatography. * For details of Part I of this series, see reference list, p. 78. 0 SAC and the author.DAGNALL, THOMPSON AND WEST 73 The nitrogen-hydrogen diffusion flame has been found by us to offer considerable advantages as an atom reservoir in both atomic-absorption and atomic-fluorescence spectro- scopy.6~7 This arises because its background radiation is less than that of pre-mixed flames, and because its low temperature results in very low excitation of even the alkali elements.It also exhibits negligible absorption of radiation in the far ultraviolet region of the spectrum, so that increased signals may be obtained in atomic-absorption and atomic-fluorescence spectroscopy for elements such as arsenic.' The flame also has high reducing properties and limited quenching action. In addition, because of its low temperature and the limited supply of oxygen to these flames, it permits and promotes the existence of species not normally observed in usual pre-mixed flames, e.g., S,.1 The emission from phosphorus-containing compounds is a further instance of this phenomenon. In view of the voluminous literature available concerning organophosphorus compounds, we have concentrated on inorganic applications in this study.THE NITROGEN - HYDROGEN DIFFUSION FLAME- The burner head used was the standard 1.8 x 76cm air-acetylene emission head supplied with the Unicam SPSOOA flame spectrometer. The burner head is perforated near one end by a 1-cm square pattern of 13 holes. However, any other suitable emission burner head could be used in its place. Nitrogen was used as the nebulising gas (at 15 p.s.i.), with the conventional SPSOOA nebulising system. Hydrogen was introduced, as usual, at the bottom of the burner, at a pressure somewhat above that necessary to prevent the flame from lifting off the burner. The actual pressure of hydrogen was found to be uncritical. The principal characteristics of this flame are that it is colourIess and cool (temperature ranges from 280" to 850" C, depending upon position in the flame),l and gives weak OH band emission (about forty times less than a pre-mixed air-hydrogen flame) and only weak sodium emission (emission from a 2 p.p.m.sodium solution at 689mp is about fifty times less than in an air - hydrogen flame .) The green phosphorus emission, which is at a maximum at 528 mp, is easily obtained by spraying a solution of orthophosphoric acid. As with sulphur, it was observed that the signal was drastically decreased by the addition of small amounts of air or nitrous oxide through a third jet in the base of the burner.* Temperature measurements show that this is due to the increase in flame temperature obtained by the addition of oxygen.The green colour was noted to be particularly prominent in the cool inner regions of the diffusion flame (about 350" C), and was most marked about 1 cm above the burner top (280" C). A nitrogen - hydrogen total-consumption burner gave negligible emission, probably because the turbulent nature of the flame causes considerable entrainment of air, and hence gives rise to a much hotter flame than that obtained with the simple diffusion flame. The separated air - hydrogen flame, previously designed to aid the breakdown of sulphates and so give a more uniform sulphur emission,l was also investigated. The experimental arrange- ment was exactly as used for the sulphur studies, Le., a pre-mixed air - hydrogen flame with a cooled borosilicate glass or quartz sheath, but it was found not to be particularly advantageous in this instance.The green colour persisted a long time after nebulisation had been terminated. This is thought to be caused by condensation of phosphoric acid or phos- phorus pentoxide on to the walls of the glass sheath and subsequent slow evaporation. The emission occurred mainly around the cool walls of the sheath, just above the burner (about 350" C), and was only slightly more intense (about 20 per cent.) than in the normal diffusion flame. IDENTITY OF NEBULISED SPECIES- The green emission was most marked in the nitrogen - hydrogen diffusion flame on nebu- lising orthophosphoric acid. Metallic phosphates gave only a weak signal from lack of dissociation in the relatively cool flame (see Interference studies).A spectral scan of the emission obtained while spraying a 1.2 x 1 0 - 2 ~ aqueous solution of orthophosphoric acid, with a slit width of 0.03 mm, is shown in Fig. 1. The air - hydrogen flame yielded an exactly similar spectrum, but was at least five times less intense, and there was much more of a continuum. Also, the comparatively large background emission of the air - hydrogen detracts from the sensitivity towards dilute orthophosphoric acid solutions. In aU instances emission corresponds only to the H-P-0 species.6 EXPERIMENTAL74 DAGNALL, THOMPSON AND WEST : MOLECULAR-EMISSION [Analyst, Vol. 93 8o t Wavelength, mp Fig. 1. Emission spectrum of H-P-0 species obtained by nebulising a 1.2 x 10-ZM solution of orthophosphoric acid into the nitrogen - hydrogen diffusion flame The intensity of the emission at 528 mp in the diffusion flame exhibits a linear response with orthophosphoric acid solutions over the range 0.2 to 500 p.p.m.of phosphorus. Larger amounts of phosphorus were not examined. The limit of detection (signal-to-noise ratio = 1) was 0.1 p.p.m. of phosphorus, with a slit width of 0-17 mm. The use of filters and a more sensitive photomultiplier (E.M.I. 9601 B was used in these studies) would give considerably lower limits. There is also little doubt that the use of a fast monochromator or a simple filter system to isolate the desired radiation around 528 mp would improve the sensitivity considerably. The emission at 528mp was not very dependent upon the hydrogen pressure or the height of measurement in the flame, because it extends uniformly over most of the inner flame regions. Measurements were optimised with the hydrogen pressure somewhat above that necessary to prevent flame lift-off, and with the top of the burner 3 cm below the bottom of the monochromator slit.INTERFERENCE STUDIES- Resfionse towards various phosphorus-containing compounds-Table I shows the emission signals obtained for 2 x M solutions of various phosphorus-containing compounds usually encountered. The readings were obtained by taking the peak (at 528 mp)-to-trough (at 515mp) height from a recorded spectrum. This was considered to be more accurate than an emission signal reading at 528 mp because it eliminates any errors caused by a slight sodium continuum at 528 mp. The peak-to-trough height was found to vary almost linearly with orthophosphoric acid concentration.TABLE I EMISSION INTENSITIES OF VARIOUS PHOSPHORUS COMPOUNDS IN THE NITROGEN - HYDROGEN DIFFUSION FLAME Compound, 2 x 1 0 - 3 ~ Orthophosphoric acid . . .. .. .. Sodium dihydrogen orthophosphate . . .. Disodium hydrogen orthophosphate . . .. Disodium hydrogen phosphite . . .. .. Sodium pyrophosphate . . .. .. .. Calcium hydrogen orthophosphate. . .. .. Emission reading, peak-to-trough height 48 16 6 5.6 6 12February, 19681 SPECTROSCOPY I N COOL FLAMES. PART I1 76 All readings were obtained with a slit width of 0-05mm and a gain of 3-10, The sodium-to-phosphorus ratio has a critical effect on the emission signal; as the ratio increases so the emission signal decreases. The addition of hydrochloric acid (concentrations up to M) to the solution of &sodium hydrogen orthophosphate, or an increased hydrogen flow to the diffusion flame, did not produce any increased response.TABLE I1 EFFECT OF SOME EXTRANEOUS IONS ON THE H-P-0 EMISSION IN THE DIFFUSION FLAME Interference Aluminium chloride . . .. Ammonium chloride . . .. Ammonium iron (I1 I) sulphate Ammonium iron(I1) sulphate . . Cadmium chloride . . .. Calcium chloride .. .. Cobalt(I1) chloride . . .. Copper(I1) chloride . . .. Lead nitrate . . .. .. Lithium chloride .. .. Sodium chloride .. .. Magnesium chloride . . .. Concentration, M (5-6 x 1 0 - 3 ~ with respect to H3P0,) .. 1 x 10-2 .. .. .. .. .. * . .. .. .. .. .. 1 x 10-8 1 x 10-8 5 x 10-8 1 x 10-2 1 x 10-8 5 x 10-2 1 x 10-2 3 x 10-2 3 x 10-2 1 x 10-2 5 x 10-2 1 x 10-8 3 x 10-2 1 x 10-2 1 x 10-8 2 x 10-3 Emission reading, peak-to-trough height 54 0 55 22 17 21 21 13 4 22 > 100 32 23 18 2 6 3 2 Efect of extraweom ions-Interference studies (Table 11) were carried out with a 5.6 x 1 0 - 3 ~ solution of orthophosphoric acid and optimised diffusion flarne conditions.Following the addition of the extraneous ion, all of the solutions were made 0 . 2 ~ with respect to hydrochloric acid. As before, all of the readings are peak (at 528 mp)-to-trough (at 515 mp) heights to eliminate any light leaks in the monochromator, etc. In this instance the slit width was 0.03 mm and the gain was 3-9. The only increased response is caused by copper and arises from the strong Cu-Cl and Cu-H band emission in the same region as the phosphorus emission.It would appear from Table I1 that the degree of the depressive effects is not related to the volatility or stability of the various metal phosphate species produced in the flame. However, there is some correlation between these effects and the ease with which the species may be reduced by hydrogen. This may be illustrated with reference to lead orthophosphate [Pb,(PO,),], which has a higher melting-point than either lithium orthophosphate (Li,PO,) or sodium pyrophosphate (Na,P,O,). Also, iron(II1) forms a more stable phosphate species than sodium or lithium but grves less interference. These effects could be explained if cool diffusion flames, unlike normal pre-mixed flames, are assumed to produce atoms from metal salt solutions principally by reduction, and not by thermal breakdown.In a normal pre-mixed flame no interference from compound form- ation is observed with sodium, because its salts are thermally unstable at temperatures of about 2000" C. Phosphate compounds of elements such as cadmium, cobalt, lead and iron are much more likely to be reduced by hydrogen at temperatures of about 400" C1 than aluminium, lithium, sodium and magnesium compounds. The depressive effect increases with increasing concentrations of sodium and lithium. This appears to be a mass-action effect, because increasing the metal chloride concentration must increase the free metal concentration in the flame and, as the chlorides would be expected to be thermally less stable than the corresponding phosphates, the dissociation of the metal phosphate would be depressed.The effect is not so marked with increasing concentrations of cadmium, lead and iron, suggesting that the breakdown process is not entirely thermal but a reduction process that is not very dependent on metal-ion concentration.76 DAGNALL, THOMPSON AND WEST : MOLECULAR-EMISSION [A fldyst, VOl. 93 The effect of varying concentrations of sodium, lithium and calcium on a solution 5 x M with respect to orthophosphoric acid and 0.2 M with respect to hydrochloric acid is shown in Fig. 2. It can be seen that the sodium and calcium curves show a pronounced change of slope at a metal concentration equivalent to that of the phosphorus, corresponding to the formation of sodium dihydrogen orthophosphate (NaH,PO,) and calcium hydrogen orthophosphate (CaHPO,).Lithium does not show such a change until the metal con- centration is twice that of phosphorus, Le., corresponding to the formation of dilithium hydrogen orthophosphate (Li,HPO,) . The curve then levels off until the metal-to-phosphorus ratio (3: 1) corresponds to the fonnation of lithium orthophosphate (Li,PO,). A gradual decrease is then observed, with further increase in lithium concentration. Thus it would appear that the lithium dihydrogen orthophosphate is not as stable in the diffusion flame as the dilithium hydrogen orthophosphate. The following reaction presumably occurs- BLiH,PO, -+ Li,HPO, + H,PO,. It is a well known fact that lithium salts differ in behaviour from the corresponding sodium and potassium salts.Metal-to-phosphorus ratio Fig, 2. Effect of sodium, lithium and calcium on the phosphorus emission : curve A, varying lithium concentrations; curve B, vary- ing sodium concentrations; curve C, varying calcium concentrations At low calcium-to-phosphorus ratios (below 06), the calcium graph is linear and when extrapolated gives a ratio of 1 : 1. As this ratio increases above 0.5, the curve becomes rounded and extrapolation tends to give a calcium-to-phosphorus ratio of 1-5:l. Thus it would appear that in solutions containing a 2-fold, or more, molar excess of orthophosphoric acid over calcium, calcium hydrogen orthophosphate (CaHPO,) is formed, but in solutions containing greater amounts of calcium, calcium orthophosphate [Ca,(PO,),] is formed. However, acetic, hydrobromic, hydrochloric, nitric, oxalic, sulphuric and tartaric acids, when present in a 50-fold molar excess, produced no variation in response equal to, or greater than, +5 per cent.February, 19681 SPECTROSCOPY I N COOL FLAMES.PART I1 77 The presence of a 100-fold molar excess of the above acids, except for sulphuric acid, produced about a 10 per cent. decrease in signal because of the increased viscosity and attendant depression of nebulisation. Strong solutions of sulphuric acid produced an increase of signal caused by the intense blue S, emission. The presence of 10 per cent. v/v of organic solvents miscible with water produced with all about an 85 per cent. decrease in signal. This was almost identical with the effects that they produced With the S2 emission.The reason is not known at present, but the cause is most probably the quenching action of CH, CHO radicals, etc. Temperature measurements proved that the decrease in signal is not caused by any increase of temperature. Removal of cationic interferertces-It was considered that cationic interferences could generally be overcome by removal on a cation-exchange resin, in its hydrogen form. According to Samuelson,Q phosphate can be quantitatively separated on such a resin from the following cations: Li+, Na+, K+, NH,+, Rb+, Cs+, Mg2+, Caz+, Sr2+, Ba2+, Znw, MnZ+, Co2+, Ni2+, Cd2+, AP+ and Fes+. Alternatively, it should also be possible to remove phosphate on an anion-exchange resin and then regenerate it as phosphoric acid. By using such a method it might be possible to obtain a concentration step.Some preliminary measurements have been carried out with the cation-exchange resin Zeo-Karb 225. It was found that aluminium, calcium, iron and magnesium could be removed quantita- tively simply by shaking a few grams of the washed Zeo-Karb resin, in the hydrogen form, with about 30ml of solution containing orthophosphoric acid plus a 10-fold molar excess of the interfering ion. The interference caused by sodium and potassium was reduced by about 95 per cent., but this is presumably because Zeo-Karb 225 is not an efficient resin for univalent cations and neither is the batch-method of exchange. There is no reason to assume that most other cationic interferences cannot be removed by such methods. The separated air - hydrogen flame was also investigated, but it did not show any advantage in minimising interferences.In fact, the results tended to be inferior because of the higher temperature and the increased emission signals from easily excited elements. Elements and compounds with emission bands and lines close to 628 mp gave large apparent increases in signal. PREPARATION OF CALIBRATION GRAPH APPARATUS- A Unicam SP9OOA flame-emission /at omic-absorption spec t ropho t ometer , fitted with standard air - acetylene (rectangular) burner head and E.M.I. 9601B photomultiplier, was used. Fuel gas-Hydrogen, from a cylinder. Diluent gas-Nitrogen, from a cylinder. REAGENTS- M-Prepare by diluting AnalaR “syrupy” orthophosphoric acid (90 per cent. w/w), standardising and subsequently adjusting the solution to exactly lo-* M.Orthophosphoric acid, 1 ml of M orthophosphoric acid = 310.2 pg of phosphorus. PROCEDURE- Calibration graph for 3.1 to 31 P.9.m. of phosphorus-Transfer, by pipette, 1 to 10-ml aliquots of 1 0 - s ~ orthophosphoric acid into a series of 100-ml calibrated flasks and dilute to volume with distilled water. Nebulise the solutions into the nitrogen - hydrogen diffusion flame under the following experimental conditions : nitrogen pressure, 15 p.s.i. ; hydrogen flow-rate, about 15 cm on ,the dibutyl phthalate filled manometer; top of burner head, 1 cm below the bottom of the monochromator slit; slit width, 0-06mm; gain, 3.10; and band- width, 3. Measure the emission in the usual way at 528 mp and plot signal against concen- tration of phosphorus. Other concentration ranges (between 0-3 and 310 p.p.m.) can be prepared by suitable dilution.78 DAGNALL, THOMPSON AND WEST Linear calibration graphs over the ranges 3-1 to 31 p.p.m. and 31 to 620 p.p.m. of phosphorus have also been prepared in a similar manner, but in the presence of 0.1 M con- centrations of hydrochloric acid. In these instances, slit widths of 0.06 mm and 0.02 mm, and gains of 3-10 and 3.5, respectively, were used. We are grateful to I.C.I. Ltd. for the award of a grant to one of us (T.S.W.) for the purchase of the flame spectrometer used in this study, and to the Science Research Council for the award of a Research Studentship to K.C.T. Thanks are also given to Mr. R. 0. Walker of this Department for assistance with some of the experimental work. REFERENCES 1. Dagnall, R. M., Thompson, K. C., and West, T. S., Analyst, 1967, 92, 506. 2. Dippel, W. A., Bricker, C . E., and Furman, N. H., Analyt. Chem., 1954, 26, 553. 3. Brite, D. W., Ibid., 1955, 27, 1815. 4. Davis, A., Dinan, F. J., Lobbett, E. J., Chazin, J. D., and Tufts, L. E., Ibid., 1964, 36, 1066. 5. Brody, S. S., and Chaney, J. E., J. Gas Chromat., 1966, 4, 42. 6. Dagnall, R. M., Thompson, K. C., and West, T. S., Talanta, 1967,14, 1467. 7.--- , Ibid., in the press. 8. Maclkon, R., Analyst, 1964, 89, 745. 9. Samuelson, O., “Ion Exchangers in Analytical Chemistry,” John Wiley & Sons Inc., New York: NOTE-Reference 1 is to Part I of this series. Received August 22nd, 1967 Almquist & Wiksell, Stockholm; Chapman & Hall Ltd., London, 1953, p. 24,

ISSN:0003-2654    

DOI:10.1039/AN9689300072

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Analyst, February, 1968, Vol. 93, p p . 79-82 79 The Suppression of Some Interferences in the Determination of Molybdenum by Atomic-absorption Spectroscopy in an Air -Acetylene Flame BY D. J. DAVID (Division of Plaizt Industry, C.S.I.R.O., Cnnbewa, A .C.T., Austvah) Earlier published work on the determination of molybdenum with an incandescent air - acetylene flame showed that aluminium could be used effectively to suppress the interferences of such elements as strontium, calcium, manganese and iron, but this has not been confirmed for manganese and iron by other workers. Indications are that this is caused by instru- mental characteristics. Ammonium chloride, subsequently shown to be effective in the suppression of manganese and iron interference in metallurgical analysis, was found to suppress the interferences of alkaline earth chlorides, but t o have no effect on a severe interference by calcium ion, sulphate and phosphate in combination.As aluminium ion was found to suppress this interference, a mixture of aluminium and ammonium chlorides is suggested for general application. DAVID,~ when using a laboratory-assembled instrument to determine molybdenum, showed that aluminium eliminated the interferences of strontium, calcium, manganese and iron. However, Mostyn and Cunningham,z by using a Perkin-Elmer model 303, found that the combined interference of iron, manganese, chromium and nickel was not completely suppressed by the addition of aluminium ion, but was by the addition of 2 per cent. of ammonium chloride. This has been confirmed here with a Perkin-Elmer instrument.As the gas supplies and designs of the sampling systems were identical, the different interference-suppressing effects of aluminium between the two instruments must be ascribed to differences in optics. The Perkin-Elmer model 303 has a beam convergent before, and divergent after, a focus in the flame, whereas the laboratory-constructed instrument1 has a 4 x 1-mni parallel beam. This work was intended to extend the study of ammonium chloride as an interference suppressor beyond the metallurgical field. Severe interference, unaffected by ammonium chloride, was encountered, and further investigation involved identification and suppression of it. Wide validity of results prompted the use of the commercial, rather than the labora- tory-assembled, instrument.EXPERIMENTAL The equipment used in this work was the Perkin-Elmer model 303 atomic-absorption spectrophotometer with the following conditions : burner, barrel type, 10-cm path length; air, pressure 30 lb per sq. inch and flow, 7.5 (10.3 litres per minute); acetylene, pressure, 8 lb per sq. inch and flow, 13-5 (5.9 litres per minute); slits, position 4 (1.0 mm); wavelength, 3132.6 A; burner height, 1.5 (1.8 cm from the burner top to the centre of the beam); solution uptake, 5.8 ml per minute; scale expansion, x 1 ; lamp, molybdenum hollow-cathode discharge tube supplied by Atomic Spectral Lamps Pty. Ltd., Melbourne; lamp current, 25mA. Settings with units unstated refer to the manufacturer’s arbitrary scales on the Perkin-Elmer model 303 instrument. The visible flame, resulting from the stated air and acetylene flow-rates, was about 8 inches in height, with about seven eighths of this height showing incandescence.As 2 per cent. of ammonium chloride was found to suppress the interference of alkaline earth chlorides almost completely, as seen in Table I, an attempt was made, by using am- monium ion as interference suppressor, to determine molybdenum in “molybdenised” super- phosphate. A 5-g sample of superphosphate was shaken intermittently for 1 hour with 3.1 ml of 12 N ammonia solution and 5 ml of water. The resulting slurry was filtered and the residue washed with water. The filtrate and washings were then acidified by adding 5 ml of 6 N hydrochloric acid and made up to volume with water in a 100-ml calibrated flask, giving a solution containing 2 per cent.of ammonium chloride or its equivalent in other ammonium salts. This solution gave an atomic-absorption reading amounting to about only 12 per cent. of that expected from the stated molybdenum content of the superphosphate, and 0 SAC and the author.80 DAVID SUPPRESSION OF SOME INTERFERENCES [AutabSt, VOl. 93 TABLE I THE EFFECT OF 2 PER CENT. OF AMMONIUM CHLORIDE IN THE INTERFERENCE OF ALKALINE EARTH CHLORIDES WITH THE ATOMIC ABSORPTION OF MOLYBDENUM AT 3132.6 A The results are means of duplicate atomic-absorption readings Solution Absorption, per cent. w i t h Without 2 per cent. of ammonium ammonium chloride chloride 20 p.p.m. of molybdenum . . .. .. .. .. .. . . 23.0 22.6 20 p.p.m.of molybdenum in the presence of 600 p.p.m. of strontium. . 21.4 20.7 20 p.p.m. of molybdenum in the presence of 600 p.pm. of barium . . 19.3 20 p.p.m. of molybdenum in the presence of 600 p.p.m. each of stron- tium, calcium and barium . . .. .. .. .. .. 6.2 31-4 2.1 5.8 8.6 20 p.p.m. of molybdenum in the presence of 600 p.p.m. of calcium . . measurement of a 2 per cent. ammonium chloride dilution showed that this depression was not lessened by increasing the proportion of ammonium chloride to extract solution. Confirmation of the assumption that this depression was caused by interference rather than poor extraction of molybdenum was obtained: by adding molybdenum to the extract solution, which increased the reading by only about 11 per cent. of that to be expected in an inter- ference-free situation ; from colorimetric analysis of the extract solution by using the dithiol method3; and by adding aluminium chloride to the extract solution, which enhanced the molybdenum reading about 7-fold.Data from the atomic-absorption tests are given in Table 11. The colorimetric result appears in Table 111. TABLE I1 MEANS OF DUPLICATE ATOMIC-ABSORPTION READINGS FOR MOLYBDENUM AT 3132.6 A ON AN ACIDIFIED AMMONIA SOLUTION EXTRACT OF “MOLYBDENISED” SUPERPHOSPHATE UNDER VARIOUS CONDITIONS COMPARED WITH THE INCREMENT TRATION IN 2 PER CENT. AMMONIUM CHLORIDE IN ABSORPTION PRODUCED BY A 20 P.P.M. INCREASE IN MOLYBDENUM CONCEN- Absorption, Solution per cent. Acidified ammonia solution extract of molybdenum superphosphate . . .. 3.1 2.2 9 ml of acidified extract + 1 ml of 200 p.p.m.molybdenum in 2 per cent. ammonium chloride . . .. .. .. .. .. . . .. 4.6 20 p.p.m. of molybdenum increment (10 to 30 p.p.m.) in 2 per cent. ammonium chloride . . .. .. .. .. .. .. .. .. 16.0 10 ml of acidified extract + 16 ml of 3333 p.p.m. of aluminium in 2 per ceni. ammonium chloride . . .. .. . . .. .. .. .. 16.5 10 ml of acidified extract + 16 ml of 2 per cent. ammonium chloride . . .. TABLE I11 RECOVERIES OF ADDED MOLYBDENUM BY ATOMIC-ABSORPTION ANALYSIS AND COMPARISON OF ATOMIC ABSORPTION WITH COLORIMETRIC RESULTS IN THE DETERMINATION OF MOLYB- DENUM IN ACIDIFIED AMMONIA SOLUTION EXTRACTS OF SUPERPHOSPHATE The results are derived from the means of duplicate atomic-absorption readings 16 ml of extract solution + 10 ml of additive to give Additives in standards Molyb- A r -i P denum, Recovery Estimate Am- Added Am- p.p.m., in of added monium Alu- Phos- molvb- monium Alu- Phos- extract 4- molvb- chloride, per cent.2 2 2 2 2 9 Y minium, p.p.m. 1000 1000 1000 1000 1000 1000 phorus, denum, chloride, p.p.m. p.p.m. per cent. - 2 - 20 2 2 600 600 20 s 2000 - 2 2000 20 2 - - minium, p.p.m. 1000 1000 1000 1000 1000 1000 phorus, additions den;m, p.p.m. solution per cent. - 17.0 - 39-0 110.0 2000 16.4 2000 34.1 94.1 200@ 16.2 2000 36.3 103.5 DithioP colorimetric result of molyb- denum in solid, p.p.m. 567 613 640 52 1February, 19681 I N THE DETERMINATION O F MOLYBDENUM 81 Arc-emission analysis showed, in addition to molybdenum, the presence of phosphorus, calcium, magnesium and silicon, with traces of copper, aluminium, vanadium and sodium. Sulphur and zinc would also have been present.In 2 per cent. ammonium chloride, these elements, individually, had little effect, but calcium with sulphate or phosphate, or both, severely depressed molybdenum absorption ; aluminium lessened the depression (see Table IV). Concentrations of the interfering ions different from those given in Table IV produced less interference. TABLE IV THE EFFECTS OF ALUMINIUM AND 2 PER CENT. OF AMMONIUM CHLORIDE ON THE INTERFERENCE OF CALCIUM ION, SULPHATE AND PHOSPHATE WITH THE ATOMIC ABSORPTION OF MOLYBDENUM AT 3132.6 The results are means of duplicate atomic-absorption readings Solution composition* r Molybdenum, p.p.m. 20 20 20 20 90 20 20 20 Calcium, Sulphur, Phosphorus, p.p.m.p.p.m. p.p.m. - - - - - 600 - 400 - - - 400 600 400 - 600 - 400 600 400 400 600 400 400 Absorption, per cent. 1 -7 Without In 2 per .cent. Aluminium, ammonium ammonium p.p.rn. chloride chloride - 23-3 22.6 - 5-6 21.4 - 21-9 21.3 - 24-1 23.0 - 2.3 18.0 - 4.9 10.4 - 2.6 3.6 1000 24.0 22.6 * Compounds used : molybdenum trioxide, calcium chloride, sulphuric acid, phosphoric acid and aluminium chloride. 30 U 2c L aJ n S Y .- P 0, 2 IC I I I I I I I 1 500 loo0 1500 2000 2m 3000 3500 Aluminium in solution, p.p.m. Fig. 1. The effect of aluminium ions, at two concen- trations of phosphate, in the suppression of the combined interference of calcium, phosphate and sulphate on the absorption of solutions containing 20 p.p.m. of molyb- denum. Measurements were carried out a t 3132.6 A.Curve A: 20 p.p.m. of molybdenum, 600 p.p.m. of calcium, 400 p.p.m. of sulphur, 400 p.p.m. of phosphorus and 2 per cent. of ammonium chloride. Curve B: 20 p.p.m. of molybdenum, 600 p.p.m. of calcium, 400 p.p.m. of sulphur, 2000 p.p.m. of phosphorus and 2 per cent. of ammonium chloride. Curve C: 20 p.p.m. of molybdenum and 2 per cent. of ammonium chloride82 DAVID SUPPRESSION OF INTERFERENCES- Although ammonium chloride was shown to have little effect on the interferences under study, consideration of its usefulness in the suppression of other interferences made its presence in test solutions desirable. The 2 per cent. concentration found most effective by Mostyn and Cunningham2 was used. Two series of solutions were prepared, in each of which the aluminium ion was varied over a wide range at constant concentrations of molybdenum, phosphate, sulphate and calcium ions.Two phosphate concentrations were used to examine its enhancing effect. Atomic-absorption results derived from these solutions are displayed in Fig. 1. Conclusions to be derived from Fig. 1 are that 600 p.p.m. or more of aluminium in solution effectively eliminates the combined interference of calcium ion, sulphate and phosphate ; at 1000 p.p.m. of added aluminium, 800 p.p.m. of aluminium in solution originating in the sample can be tolerated. STUDY OF SUPERPHOSPHATE EXTRACT- Acidified ammonia solution extract, prepared as outlined earlier, was used in the tests to be described. Three recovery tests for added molybdenum, the details of which are shown in Table 111, were carried out.Having found an enhancement of 10 per cent. in the first test, the subse- quent two tests were designed to test the assumption that phosphate was the cause of the enhancement. Colorimetric analysis of the extract solution by the molybdovanadophos- phate method4 indicated the presence of 30 p.p.m. of phosphorus. Variation of phosphate at constant molybdenum (20 p.p.m.), ammonium chloride (2 per cent.) and aluminium (lo00 p.p.m.) showed maximum enhancement at between 2000 and 3000 p.p.m. of phosphorus. DISCUSSION As this work has shown that interference can be overcome, it is suggested, for the following reasons, that ammonia solution, rather than the more conventional acid,5 extraction of superphosphate would be preferable : molybdenum trioxide is more soluble in alkaline than in acid solutions, ammonia solution dissolves less of other components and experience here indicates that an acid extract is less stable than an ammonia solution extract.Agreement between the extent of enhancement shown by the third recovery test and comparison of the colorimetric with the atomic-absorption result when 1000 p.p.m. of aluminium, 2000 p.p.m. of phosphorus and 2 per cent. of ammonium chloride were present in both sample and standard solutions (Table 111) indicates that the effect is caused by experimental error. Phosphate interacting with other components is probably the cause, but as it is small it can be ignored for most purposes. The effectiveness of ammonium chloride2 in some instances, and of aluminium ion in others, suggests that a mixture of the two might be suitable for general application.A test showed that, in the absence of nitrate, 1000 p.p.m. of aluminium did not impair the effective- ness of 2 per cent. of ammonium chloride in the suppression of iron and manganese inter- ference in the determination of molybdenum in a simulated stainless-steel solution. Nitrate could easily be removed as a routine measure during the preparation of stainless-steel solutions for analysis. Aluminate may exclude molybdate from solid solution in a relatively involatile calcium sulphate or calcium phosphate solid phase in the sample particle in the flame. Reduction of particle size in the flame arising from the low decomposition temperature of ammonium chloride may be the active principle where it is effective, and alkali salts’ may act similarly in their limited range of effectiveness. REFERENCES The mechanisms of suppression of these interferences are obscure. 1. David, D. J., Analyst, 1961, 86, 730. 2. Mostyn, R. A., and Cunningham, A. F., Analyt. Chem., 1966, 38, 121. 3. Sandell, E. B., “Colorimetric Determination of Trace Metals,” Second Edition, Interscience Publishers Inc., New York and London, 1960, p. 459. 4. Snell, F. D., and Snell, C . T., “Colorimetric Methods of Analysis,” Volume 2, Third Edition D. Van Nostrand Co. Ltd., London and New York, 1961, p. 672. 6. Horwitz, W., Editor, OfficiaI Methods of Analysis of the Association of Official Agricultural Chemists,” Eighth Edition, The Association of Official Agricultural Chemists, Washington, D.C., 1965. Received April 7th, 1967

ISSN:0003-2654    

DOI:10.1039/AN9689300079

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Analyst, February, 1968, Vol. 93, $9. 83-92 83 A Solvent-extraction Absorptiometric Method for Determining Nickel in Boiler Feed-water BY A. L. WILSON (Central Electricity Research Laboratories, Cleeve Road, Leathevkead, Surrey) A method has been developed for determining nickel in boiler feed-water. The nickel complex with furil a-dioxime is extracted into chloroform, and is then determined absorptiometrically. The criterion of detection was about 0-3p.g of nickel per litre, and the standard deviation of results varied from about 0.15 to 0-3 pg of nickel per litre in the range of concentrations 0 to 50 pg of nickel per litre; within this range, the calibration graph was linear. No appreciable interference u 7 a s caused by other impurities likely to be present in feed-water, and a batch of ten samples could be analysed in about 24 hours.IMPURITIES entering the feed-water of modern high pressure boilers by corrosion in the condenser and feed-system may initiate corrosion failures in boiler tubes. Such failures result in costly outages, and, accordingly, one of the aims of chemical control in power stations is to control the concentrations of corrosion products in feed-water. Nickel constitutes an important proportion of certain alloys that are used in feed systems. Therefore, a method for determining nickel in boiler feed-water was required for use in power stations of the Central Electricity Generating Board. Because of the minute concentrations of nickel normally present (0 to 10 pg per litre), the method had ta be capable of giving results with a standard deviation no greater than 1 pg of nickel per litre. It was decided that a solvent-extraction absorptiometric method, in which the reagent furil a-dioxime was used, should be suitable.I have previously reported1 the results of a detailed investigation of the effects of several factors involved in the formation and solvent extraction of the nickel complex with this oxime. The work referred to showed the technique to be suitable for a precise and sensitive method. The purpose of this paper is to present full details of the method that has been developed and the tests made of its performance. EXPERIMENTAL REAGENTS, APPARATUS AND TECHNIQUE- All of the experimental details were exactly as previously reported,l and measurements of optical density were always made at 435mp.Distilled water that had been de-ionised with a laboratory-scale mixed-bed column was used for all tests, except where otherwise stated. BASIS OF THE METHOD- The previous results1 had shown suitable conditions for the formation and solvent extraction of the nickel complex. However, the analysis of feed-water requires certain precautions that affect the details of the analytical method. For example, earlier work* on the determination of traces of copper in feed-water had shown the necessity for collecting samples into sufficient hydrochloric acid to give a final acidity of 0.1 N. This acidification was required to prevent losses of copper. As a safeguard, the same acidification technique was adopted when determining nickel. Further, at this stage of the investigation, there was the possibility that some specific treatment might be required to dissolve particulate forms of nickel present in samples; such treatment might have produced samples of varying acidity, Therefore, it was decided to control the pH and ammonia concentration (both parameters are important1) of the solution before the solvent-extraction stage.This was carried out by first adjusting to a pH of about 8-5 with sodium hydroxide solution and dilute hydrochloric acid, and then adding a fixed optimum amount of ammonia solution. The presence of about 0.1 N sodium chloride in the final solution (from the neutralisation of the hydrochloric acid added initially) should aid the extraction of the nickel complex.184 WILSON A SOLVENT-EXTRACTION ABSORPTIOMETRIC METHOD [Analyst, VOl 93 As relatively large amounts of iron would sometimes be present in samples, tartrate was added to the sample before neutralisation to prevent precipitation of the iron.Finally, to prevent interference from i r ~ n ( I I ) , ~ a small amount of hydrogen peroxide was added to the acidified sample to cause oxidation to iron(II1). Hydrogen peroxide, rather than the potassium dichromate used by Taylor,8 was used because the dichromate reduced the optical density due to a given concentration of nickel. The technique finally devised was then as given under Method, and the tests of the effects of several experimental factors not considered previously are reported as follows. EFFECT OF VARIATIONS IN THE AMOUNTS OF REAGENTS- The effects of variations in the amounts of sodium chloride (from the neutralisation of hydrochloric acid), tartrate, furil cc-dioxime and ammonia have already been reported.l Addition of ten times the amount of the phenolphthalein solution specified in the method caused no significant effect, i e ., less than 0.4 pg of nickel per litre. Addition of 0.1 or 0.3 ml of the hydrogen peroxide solution did not significantly affect the results obtained from blank determinations or solutions containing 20 pg of nickel per litre. Finally, during adjustment of the pH of the sample, before the solvent extraction, the addition of 5 drops in excess of the dilute hydrochloric acid (1 + 99) caused no significant effect on the results for blank determinations and for solutions containing 20 pg of nickel or 100 pg of copper per litre.Thus, none of the amounts of the reagents appeared to be critically important. EFFECT OF TEMPERATURE- Solutions containing either 0 or 20pg of nickel per litre were analysed, as described under Method, except that the temperatures of the solutions during the solvent extraction were adjusted to three different values. The temperatures of the chloroform extracts during measurement of their optical densities were within 0.6' C of ambient temperature. Duplicate determinations were made for each condition tested, and the mean results given in Table I show no significant effect of temperature between 21" and 29" C. The efficiency of extraction of the nickel was apparently reduced slightly at the lower temperatures of 12-5" to 14.5' C.TABLE I EFFECT OF TEMPERATURE ON THE EXTRACTION OF NICKEL Mean optical density Mean temperature of 7- sample during extraction, "C Blank hrckel, 20 pg per htre* 12.5 to 14.6 0.024 0.342 21 0.036 0.361 29 0.034 0.352 * These results have been corrected for the blank. The effect of temperature on the optical density of the nickel furil a-dioximate in chloro- form was determined approximately by either warming or cooling a chloroform extract, and then measuring its optical density as the temperature approached ambient. The results indicated a linear relationship between optical density and temperature in the range 18" to 30" C, the optical density at 20' C changing by about 0.3 per cent. per "C. DETERMINATION OF THE CONCENTRATION OF NICKEL IN THE WATER USED FOR BLANK DETER- A technique that seemed suitable for this determination was to analyse 200 and 400-ml portions of the water in exactly the same way, and to equate the difference between the two optical densities to the optical density due to nickel in 200 ml of the water.Tests showed, however, that appreciably greater amounts of chloroform dissolved in the 400-ml than the 200-ml samples. Thus, for a given amount of nickel, the optical density was greater for 400 than 200-ml samples. This effect was overcome by measuring the volume of the extracts before measuring their optical densities. An allowance for the effect of this difference in volumes can then be made as described under Method. For this technique to give accurate results, it is also necessary that the optical density of furil a-dioxime in the chloroform is not affected by the volume of the aqueous phase.Tests in which 200 and 400-ml portions of water were analysed, after addition of 5 and 10 ml MINATIONS-February, 19681 FOR DETERMINING NICKEL IN BOILER FEED-WATER a5 of the furil oc-dioxime reagent solution, showed that the optical density due to the oxime was 0-003 for both 200 and 4Wml samples. Finally, it is also essential that small concentrations of nickel are quantitatively extracted from 400-ml samples. To check this, duplicate 400-ml portions of water, to which either 0 or 0*80pg of nickel was added, were analysed. The mean recovery of the added nickel was 0.84 0.04pg (95 per cent. confidence limits). It was concluded, therefore, that the technique described under Method was satisfactory for determining the nickel content of the water used for blank determinations. DISSOLUTION OF FORMS OF NICKEL INSOLUBLE IN FEED-WATER- Tests were made to determine whether samples were likely to contain insoluble forms of nickel that do not react directly with furil a-dioxime.For this purpose, samples of con- densate and feed-water from one power station were each analysed by two methods. In method A, 200-ml portions of the acidified samples were analysed, as described under Method, except that the initial boiling of the sample was omitted. In method B, the initial boiling was continued until the volume of the sample had been reduced to about 6 ml. This residual volume was diluted with water (containing less than 0-1 pg of nickel per litre) to a final volume of 200 ml, and this solution was then analysed as before.Four portions of each sample were analysed by each method, and blank determinations by both methods were carried out. The results are given in Table 11 and indicate that these particular samples contained in- appreciable amounts of insoluble forms of nickel. Both of these samples had been taken during steady operation of the power station, and, by analogy with iron and copper, it was thought that greater nickel concentrations might be encountered during periods while the turbine and feed-system were being brought into operation. Such greater concentrations might be associated with insoluble forms, and other tests were made to check this possibility. The tests were kindly made by the Scientific Services Department of the South Eastern Region of the Central Electricity Generating Board.Two samples of feed-water were collected during two different start-up operations at the same power station as for the previous samples. The experimental designs were exactly as for the previous tests; methods A and B were used, but the method proposed in this paper (k, samples just brought to the boil and then cooled before analysis) was also used. For method A, samples were analysed within 1 hour of being collected. The results of these tests are also given in Table 11. TABLE I1 EFFECT OF PRE-TREATMENT OF SAMPLES ON APPARENT NICKEL CONTENT Apparent nickel content,* pg per litre, as determined by- - \ Type of sample method A method B proposed method Condensate during steady operation .. . . 1.3 1.6 - Feed-water during steady operation (1) . . 1.6 1.4 - Feed-water during start-up (2) . . 7.6 13.3 - Feed-water during start-up (3) . . 8-8 11.1 10.8 * 96 per cent, confidence limits for these results are about f0-3 pg of nickel per litre. The results indicate that the samples taken during start-up contained small amounts of insoluble forms of nickel that were dissolved by method B. The results obtained by the proposed method were not significantly different from those obtained by method B on the one sample that was tested. Other tests by the South Eastern Region Scientific Services Department showed that the apparent nickel content given by method A increased slowly as the time between sampling and analysis increased.For example, results of 10-0 and 12-45 pg of nickel per litre were obtained after 1 and 3 days, respectively, for feed-water (2). Greatest interest attached to the precise and accurate determination of nickel during steady operation of power stations. Under these conditions, subsequent work has shown that the nickel content of condensates and feed-waters from modern stations is almost invariably less than 6 pg per litre, and is often less than 2 pg per litre. On this basis, it appears that the proposed method should provide a sufficiently rigorous pre-treatment of the sample. The results also showed that the evaporation treatment of method B could be used, if required for any special purposes, without any important loss of precision.86 WILSON : A SOLVENT-EXTRACTION ABSORPTIOMETRIC METHOD [Analyst, Vol.93 METHOD REAGENTS- Except where otherwise stated, analytical-reagent grade chemicals should be used whenever possible. Water-Use water with a small nickel content (preferably less than 0.5pg per litre) for preparing reagents and for blank determinations. Water containing less than 0.2 pg of nickel per litre has been consistently obtained by passing distilled water (from a Manesty still) through a laboratory-scale, mixed-bed, de-ionisation column. Determine the nickel content of the water used for blank determination as described under Procedure. Ethanol-Industrial methylated spirit, 74" O.P. Hydrochloric acid, about 5 r;-Dilute 500 ml of hydrochloric acid (sp.gr. 1.1s) to 1 litre with water.Sodium hydroxide solution, 2.5 N-For all of the results quoted in this paper this reagent was prepared by diluting concentrated volumetric sodium hydroxide solutions (British Drug Houses Ltd) . Preparation of the reagent from sodium hydroxide pellets (analytical-reagent grade) tended to give slightly greater optical densities for blank determinations. Store this reagent in a sealed polythene bottle. Hydrogen peroxide solution (1 + 9)-Dilute 10 ml of hydrogen peroxide (100 vol) to 100ml with water. Prepare this solution freshly each week. Phenolphthalein solution, 0.5 per cent. w/v. Sodiunz potassium tartrate solution, 10 per cen,t. w/v-Dissolve 25 g of sodium potassium tartrate tetrahydrate in water, and dilute with water to 250ml. Prepare this solution freshly each week.Hydrochloric acid (1 + 99). Furil a-dioxime solution, 0.15 per cent. w/v-Dissolve 0.75 g of furil a-dioxime in industrial methylated spirit, and dilute to 500 ml with industrial methylated spirit. Store in a sealed, glass bottle, and keep the bottle in the dark when not in use. This solution has been found to be adequately stable for at least 8 months. Tests showed no significant (95 per cent. confidence limits) difference in the optical densities due to a given amount of nickel when furil a-dioxime from two different suppliers was used. Ammonia solution, 2 ih-Dilute ammonia solution (sp.gr. 0.88) with water so that the normality of the final solution is 2.0 0-1 N. Store the solution in a sealed, glass bottle; its concentration has been found to change by less than 1 per cent. after 2 weeks.Chloroform. Stapzdard nickel solution A-Dissolve 1-000 g of pure nickel metal (rod, foil or wire) by warming with 20 ml of dilute nitric acid (1 + 3). When dissolved, add 10 ml of hydrochloric acid (sp.gr. 1.18) and 100ml of water, cool, and dilute with water to 1 litre in a calibrated flask. 1 ml of solution = 1000 pg of nickel. Determine the normality of the diluted acid. This solution was found to remain stable for at least 6 months. Standard nickel solution B-Dilute 20 ml of standard nickel solution A with 10 ml of hydrochloric acid (sp.gr. 1.18) and water to 1 litre in a calibrated flask. This solution was found to remain stable for at least 6 months. 1 ml of solution = 20 pg of nickel. Standard nickel solution C-Dilute 20ml of standard nickel solution B with water to 1 litre in a calibrated flask.Prepare this solution freshly each day as required. APPARATUS- 1 ml of solution = 0.4 pg of nickel. Care is required to minimise contamination of apparatus when not in use. Pyrex-glass conicaZJEasks, 500-wzZ capacity-Soak the flasks overnight in a cleaning solution of chromic acid, and then wash well with water. Add 250ml of dilute hydrochloric acid (1 + 1) to each flask, and heat the flasks until only 5 to 10 ml of acid remain. Wash the flasks well with water, and store with 50-ml beakers inverted over the necks of the flasks. Pyrex-glass beakers, 50-ml ca#acity-Clean the beakers in the same way as the 500-ml flasks, but with 25 instead of 250ml of the dilute hydrochloric acid.Pyrex-glass separating fzmnels, 500-ml capacity-Soak the funnels overnight in a cleaning solution of chromic acid, and then wash well with water. Before using the funnels for analysis,February, 19681 FOR DETERMINING NICKEL IN BOILER FEED-WATER 87 carry out a blank determination in each funnel, discarding the contents of the funnels after the solvent-extraction stage. Finally, wash each funnel well with water of low nickel content. Do not grease the taps of the separating funnels. Pyrex-glass, stoppered graduated cylinders, 25-ml capacity-Soak the cylinders and their stoppers overnight in a cleaning solution of chromic acid, and then wash well with water. Dry in an oven. Polythene bottles-These bottles were found to be suitable for collecting samples, and any convenient size may be used.Soak the bottles in dilute hydrochloric acid (1 + 1) for 2 to 3 days, and then wash well with water. PROCEDURE- Sample collection-Place sufficient hydrochloric acid (about 5 N) into a polythene bottle to ensure that the final acid concentration, when the sample has been collected, will be 0.1 N (k 0.005 N). Samples acidified in this way were found to be stable for at least several weeks. Make allowance for the acid contained in the sample when measuring the volume required for analysis. AnaZ_vsis of samples-Transfer a volume of acidified sample, equivalent to 200 ml of feed- water, into a 500-ml conical flask. Cover the neck of the flask with an inverted SO-ml beaker, and heat the flask until the contents just begin to boil.Cool the flask, and transfer its contents into a 500-ml separating funnel. Wash the flask with two 5-ml portions of water, and add these washings to the separating funnel. The temperature of the sample should now be between 16' and 30" C. Add to the sample 4 drops of the dilute hydrogen peroxide solution, 2 drops of the phenolphthalein solution and 5 ml of the sodium potassium tartrate solution. While swirling the separating funnel, add the 2.5 N sodium hydroxide solution until the phenolphthalein just turns pink. Then, with swirling, add dilute hydrochloric acid (1 + 99), dropwise, until the pink colour is just discharged. Add 5.0 ml of the furil a-dioxime solution, followed im- mediately by 25 ml of 2 N ammonia solution, and swirl the funnel for a few seconds. Add 15.0 ml of chloroform froin a burette, shake the funnel gently for a few seconds and carefully release the excess of pressure in the funnel.Shake the funnel vigorously (about 200 shakes per minute) for 1 minute, and allow the two phases to separate for at least 5 minutes. Do not alfow direct sunlight to fall on the chloroform extract. Run a small portion of the chloroform phase through a 9-cm Whatman No. 541 filter- paper, discarding the filtrate. Pass the remaining chloroform through the same filter-paper into a clean, dry 4-cm cuvette. Discard the filter-paper. Measure the optical density of the chloroform extract at 435 mp against a 4-cm reference cuvette containing pure chloro- form; the measurement should be made within 2 hours after the extraction.Let the measured optical density be As. After measurement, discard the contents of the sample cuvette, rinse it well with pure chloroform, and allow it to dry before filtering the next extract into the cuvet te. BZnnk determinations-A blank determination should be made with each batch of sample determinations. For this, place 200 ml of water (low and known nickel content, see below) in a 500-ml conical flask, and add sufficient hydrochloric acid (about 5 N) to ensure that the final acid concentration is 0.1 N (k 0.005 N). Analyse this solution exactly as described above. Determination of nickel in the water used for blank determinations-Place 200 ml of the water used for the blank determination in a 500-ml separating funnel and 400ml of the same water in another funnel.Add to each funnel the same volume of about 5 N hydrochloric acid as that added for the blank determination. Treat both funnels exactly as described under Analysis of samples, beginning with the addition of hydrogen peroxide. On completing the solvent extraction, allow at least 30 minutes for the two phases to separate. Pass as much as possible of the two chloroform extracts through 9-cm Whatman No. 541 filter-papers (previously moistened with a few drops of chloroform, so that no excess drops of chloroform remain in the filter-paper or filter funnel) into 25-ml graduated cylinders. Allow all of the chloroform extracts to drain through the filter-papers, insert the stoppers of the cylinders, and measure the volumes of the two extracts. Let the measured volumes for the 200 and 400-ml samples of water be VT and Vp, respectively.Measure the optical densities of the extracts as described above. Let the measured optical densities for the 200 and 400-ml samples of water be AT and AF, respectively. Let the measured optical density be A g .88 for the blank determination is given by the expression- WILSON : A SOLVENT-EXTRACTION ABSORPTIOMETRIC METHOD [A .nabst, Vol. 93 CaZczcZation ofreszclts-The optical density, Ac, due to nickel in the 200 ml of water used The optical density, .AR, due to nickel in the sample is given by the expression- and the concentration of nickel in the sample can then be determined from AK and the calibration graph. PREPARATION OF CALIBRATIOX GRAPH- Into a series of separating funnels, transfer 200, 198, 195, 190, 185, 180 and 175ml of water of low nickel content, and then add 0.00, 2-00, 5-00, 10.00, 15-00, 20*00 and 25.00 ml, respectively, of standard nickel solution C.Add sufficient hydrochloric acid (about 5 N) to each funnel to ensure that the final acid concentration is 0.1 N (2 0-005 N). Treat these solutions exactly as described under Analysis of samples, beginning with the addition of hydrogen peroxide. Repeat this series of determinations until the calibration graph is defined with the desired precision. Subtract the mean optical density for the solution with no added nickel from the mean optical densities of each of the other solutions, and plot the corrected optical densities against the concentration of nickel added to the sample.The calibration graph was linear to at least 50 pg of nickel per litre in samples when 4-cm cuvettes were used, and to at least 230 pg of nickel per litre for l-cm cuvettes. When measurements were made in 4-cm cuvettes with the Spekker absorptiometer and Ilford No. 601 filters, the calibration graph was linear to 30pg of nickel per litre, but the departure from linearity at 50pg of nickel per litre was only 8 per cent, Optical densities measured with No. 601 filters were about 80 per cent. of those obtained by measuring at 435mp. SENSITIVITY AND PRECISION- To determine the precision of the basic solvent-extraction absorptiometric technique, the initial boiling of the sample was omitted. For these tests, on each of 10 days, duplicate analyses were made at concentrations of about 0, 2, 10, 50 and 230 pg of nickel per litre, as described under Preparation of calibration graph; 4-cm cuvettes were used for the first four solutions, and l-cm cuvettes for the last. These analyses were made during a period of 4 weeks, Analytical results are obtained by subtracting the blank value from that of the sample; the precision of these corrected results was, therefore, calculated by allowing for the variability of both the blanks and the samples.For all four levels of nickel there were no statistically significant (95 per cent. confidence limits) “between-days” variations; all of the results for each level of nickel were, therefore, used to calculate the standard deviation of the deter- minations. The “within-days” standard deviation of the blank value was also calculated.These standard deviations are shown in Table 111. The amount of nickel extracted was calculated from the average volume of an extract (13.2 ml) and the molar extinction coefficient1 (1.82 x lo$). TABLE I11 SENSITIVITY AND PRECISION OF NICKEL DETERMINATIONS A R = A S - A B + A C , RESULTS Concentration of nickel added, pg per litre 0.00 1.87 9-32 46.70 233.9 Mean optical density*- corrected for blank value nickel per litre per 10 pg of - --t 0.0340 0.186, 0.176, 0.188, 0.876, 0.187, 1.098 0.046, Amount of nickel extracted, per cent. 98.9 99.7 99.4 99.6 - Standard deviation, pg of nickel per litre 0.1 4 0.13 0.32 2.1 0.11: Degrees of freedom 9 19 19 19 19 * 4-cm cuvettes were used for all of the solutions, except that containing 233.9 pg of nickel t The mean optical density of the blank solutions was 0.014,.$ This is the “within-days” standard deviation only. per litre, for which l-cm cuvettes were used.February, 19681 FOR DETERMINING NICKEL IN BOILER FEED-WATER 89 The precision of anallysing actual samples was determined by a similar experiment with standard solutions containing 0 and 50 pg of nickel per litre and samples of condensate and feed-water from power stations. For these tests, the method given under Analysis of samples was followed exactly, and one batch of analyses was made on each of 5 days. The results were analysed, as before, and are summarised in Table IV. TABLE IV PRECISION OF ANALYSING POWER Mean concentration found, Sample pg of nickel per litre Blank .. .... 49.8 Condensate .. .. 0.0 Feed-water .. .. 10.6 - Nickel, 60.1 pg per litre . . STATION WATERS Standard deviation, Degrees of pg of nickel per litre freedom 0-14* 6 0-28 0 0.20 9 0.22 9 * This is the “within-days” standard deviation only. BIAS- AutaZysis of a feed-water-Twelve portions of a feed-water were analysed, but the equivalent of 1Opg of nickel per litre was added to six of these portions before analysis. Substance Copper(I1) . . .. Zinc(11) .. .. Calcium(I1) .. Iron(II1) .. .. Manganese(I1) . . Cobalt(I1) . . . . Magnesium(I1) . . Chromium(II1) . . Aluminium(II1) . . Molybdenum(V1) . . Vanadium(V) . . Vanadium(1V) . . Titanium(1V) . . Tungsten(V1) . . Silicate . . .. Orthophosphate . . Nitrate . . .. Fluoride . . .. Fulvic acidt . . Detergentst . .Cyclohexylamine . . Tin(I1) . . .. Hydrazine . . .. Blorpholine .. Octadecylamine . . EFFECT Concentration of substance, pg per litre 1000 100 1000 1000 100 100 10,000 1000 100 1000 100 1000 1000 1000 100 10,000 10,000 1000 2500 0000 10,000 1000 100,000 10,000 100,000 10,000 10,000 1000 TABLE V OF OTHER SUBSTANCES Nickel recovered, pg per litre* 0.0 Gg per litre added 2.5 0.3 0.0 2.4 0.0 -0.1 0.1 0.3 0.0 - 0.1 - 0-2 0.0 0-2 0.1 0.1 - 0.2 0.2 - 0.1 0.6 -0.1 - 0.2 0.0 0.2 0.4 - 0.1 - 0.2 - 0.1 9.3 pg per litre 93.4 pg pe; litre added added 11.3 95.4 9-6 94.0 9.2 92.5 10.2 93.8 9.5 95-4 9-3 93.1 9.7 Not tested 9-5 8.4 9-1 6.3 9.0 9.3 9.1 9.3 9.6 86.9 90.5 92-8 63.7 94.9 94.2 Not tested Not tested Not tested 9.4 93.7 9.6 Not tested 9.7 94-4 9.4 9.1 9.3 9.0 9.9 9.0 9.3 9.1 Not tested 93.4 Not tested 92.3 Not tested 91.3 Not tested 92.1 * The ranges of recoveries expected, assuming no interference from the other substances, were (95 per cent.confidence limits)- 0.0 f 0-4 (when 0.0 pg of nickel per litre was added) ; 9.3 f 0.4 (when 9.3 pg of nickel per litre was added) ; 93-4 f 2.4 (when 93.4 pg of nickel per litre was added). t Prepared as described previously.4 1 Omo, Daz, Surf, Dreft, Blue Tide and Quix (1000 pg of each per litre) were used.90 WILSON : A SOLVENT-EXTRACTION ABSORPTIOMETRIC METHOD [ArtdySt, VOl. 93 The difference between the two sets of results was equivalent to 10.1, pg of nickel per litre, with 95 per cent. confidence limits of 3-0.2. Thus, on this particular sample, the recovery of the added nickel was satisfactory. Tests made by other laboratories within the C.E.G.B.have also shown satisfactory recoveries for many different feed-waters. Eflect of other substances-The effects of some other substances were investigated at each of three concentrations of nickel, Le., 0 and about 10 and 90 pg per litre. The analyses were made singly by the proposed method, omitting the initial boiling of the sample; l-cm cuvettes were used for the solutions containing 90 pg of nickel per litre. Solutions containing nickel alone were analysed with each batch of analyses, and the results are shown in Table V. ROBUSTNESS OF THE METHOD- To test the sensitivity of the method to deviations from the recommended conditions, solutions containing known amounts of nickel were analysed by the proposed procedure, except that the amounts of reagents added were varied in the direction thought most likely to cause incomplete extraction of the nickel.The amounts used were: 8 drops of the hydrogen peroxide solution, 4 drops of the phenolphthalein solution, 10 ml of the tartrate solution, 10 drops in excess of the dilute hydrochloric acid (1 + 99), 2.5 ml of the furil a-dioxime solution and 30 ml of the 2 N ammonia solution. All of the solutions were shaken for I minute during the solvent-extraction stage, but both the normal shaking rate (about 240 shakes per minute) and a slower rate (about 120 shakes per minute) were used. One analysis was made for each condition tested, and the results are given in Table VI. TABLE VI EFFECT OF DEVIATIONS FROM THE RECOMMENDED PROCEDURE Shaking rate, Concentration of nickel Concentration of nickel* shakes per minute added, pg per litre found, pg per litre -240 9.3 9.3 93.4 93.0 -120 9.3 9.0 93.4 90.8 * The optical densities of the blank determinations at the two different shaking rates were 0-006 and 0-007.CALIBRATION GRAPH- The results in Table I11 indicate that the calibration graph was linear in the range 0 to 230 pg per litre. The equation of the graph, for 4-cm cuvettes, was: C = 53.5 A where C is the concentration of nickel in pg per litre and A is the optical density due to nickel in the sample. Although 50 pg per litre is regarded as the normal upper limit of the method when 4cm cuvettes are used, the results show that the range of the method can be simply extended to at least 230 pg per litre by using l-cm cuvettes.The results in Table I11 also show that one solvent extraction was sufficient to give essentially complete extraction of the nickel from the sample. PRECISION- The standard deviations of optical density measurements were determined separately by repeated measurements of portions of chloroform solution containing the nickel furil a-dioximate at various concentrations. From these determinations the standard deviations expected for analytical results from measurement errors alone were calculated. These standard deviations ranged from 0.08 to 0.19 pg per litre at concentrations of 2 and 50 pg per litre, respectively. Comparison of these calculated values with those found experimentally (see Tables I11 and IV) shows that the measurement errors were not the sole source of random error.Thus, errors arising during the chemical treatment of the sample were also of im- portance; calculations show that these errors were of about the same magnitude as the measurement errors. The precision of the solvent-extraction technique (Table 111) was satisfactory for my purpose. The results in Table IV show that the precision was not adversely affected by the initial boiling of the sample and that the precision for samples of condensate and feed- water was also satisfactory. DISCUSSION OF THE METHODFebruary, 19681 FOR DETERMINING NICKEL IN BOILER FEED-WATER 91 Since this method was developed, independent determinations of the standard deviations of results have been made in many laboratories of the Central Electricity Generating Board.In one series of tests in five other laboratories values were obtained ranging from 0.11 to 0.59 pg per litre for the standard deviation of results from standard solutions containing 50 pg of nickel per litre. Thus, there appears to be no difficulty in obtaining precise results with this method, as would be expected from the results given under Robustness of the method. The sensitivity of the method can be improved, if desired, by extracting the nickel from larger volumes of the sample. The effect of such a modification has not been examined in detail, but results (see Determination of the concentration of nickel in the water used for blank determinations) indicate that a useful gain in precision might be achieved.CRITERION OF DETECTION- If the criterion of detection5 (95 per cent. confidence) is taken as 2-33 times the “within- batch” standard deviation of blank determinations, it is equivalent to about 0.3 pg of nickel per litre. Again, it should be possible to decrease this criterion of detection by extracting larger volumes of the sample. BIAS- The results given under Bias and from other laboratories of the Central Electricity Generating Board show no signs that the efficiency of extraction of nickel is adversely affected by other impurities present in condensates and feed-waters. The results in Table V show that several of the substances tested can significantly affect analytical results, although none of the effects was important in feed-water analysis, because the necessary concentrations of the interfering substances were greater than would normally occur.The effect produced by the combinations of nitrate, fluoride, fulvic acids and deter- gents is probably caused by partial extraction of fulvic acids, which absorb appreciably at 435 mp. Iron(II1) did not significantly affect results for solutions containing up to 10 pg of nickel per litre. The negative effect of iron at the level of 93 pg of nickel per litre seems likely to be caused by competition between iron and nickel for the furil a-dioxime; even this effect became non-significant for 1OOpg of iron per litre. The effect of chromium is interesting as 1000 pg per litre did not significantly affect the blank, but depressed the result for both 9.3 and 93 pg of nickel per litre by about 30 per cent.As this effect became non- significant at a chromium concentration greater than that expected in feed-water, no further investigation of the nature of the effect was made. The most important interfering substance is copper. The results in Table V show that 10o0 pg of copper per litre was equivalent to about 2 pg of nickel per litre, and that this effect was approximately directly proportional to the concentration of copper. As samples usually contain only a few micrograms of copper per litre, the effect of copper is normally un- important. However, tests of the method in other laboratories of the Central Electricity Generating Board showed that the effect of copper varied a little with different batches of the furil cc-dioxime solid reagent.The greatest effect found was that IOOpg of copper per litre was equivalent to 1.1 pg of nickel per litre. This effect is still unimportant for normal feed-water analysis, but it could cause unexpected errors in solutions containing abnormally large concentrations of copper. For the most precise work with solutions with high ratios of copper to nickel, it seems desirable to check the magnitude of the interference caused by copper for each batch of solid furil a-dioxime. The results in Table I1 indicate that there is little difficulty in converting insoluble forms of nickel (present in the original sample) to forms that react with furil a-dioxime. For many samples, the initial boiling of the sample is likely to be unnecessary, but it is included in the method as a simple precautionary measure. Nevertheless, the detailed tests have been made on relatively few actual samples, and the possibility should be borne in mind that certain samples may contain forms of nickel requiring a more vigorous pre-treatment to dissolve them. It is normal practice in the Central Electricity Generating Board to set the acidified samples aside for 1 day before analysis, to allow partial dissolution of insoluble forms of nickel, before applying the proposed method. This procedure is a useful precaution when analytical results are not required urgently but is unlikely to have any important beneficial effect.92 WILSON SPEED OF ANALYSIS- The method has been shown to be simple to carry out, and a batch of 10 samples may be analysed in about 24 hours. This time would be reduced to about 1Q hours if the initial boiling of the sample were omitted. This paper is published by permission of the Central Electricity Generating Board. I thank Mr. D. Leighton for his co-operation in independently investigating the need for pre- treatment of samples before solvent extraction. I also thank many colleagues in the Central Electricity Generating Board who took part in independent tests of the method, and some of whose results have been quoted in this paper. REFERENCES 1. Wilson, A. L., in Shallis, P. W., Editor, “Proceedings of the SAC Conference, Nottingham, 1965,” 2. - , Analyst, 1962, 87, 884. 3. Taylor, C. G., Ibid., 1956, 81, 369. 4. Wilson, A. L., J . Appl. Chem., 1959, 9, 501. 6. Roos, J. B., Analyst, 1962, 87, 832. W. Heffer & Sons Ltd., Cambridge, 1965, p. 361. Received June 29th, 1967

ISSN:0003-2654    

DOI:10.1039/AN9689300083

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Artalyst, February, 1968, Vol. 93, $9. 93-96 93 A Gravimetric Method for the Determination of Mixed Oxides (Niobium and Tantalum Pentoxides) in Niobium - Tantalum Minerals BY J. B. POLLOCK (Geological Survey and Mines Department, P.O. Box 9, Entebbe, Uganda) A method is described in which the sample is dissolved by repeated evaporation with hydrofluoric acid, and the final solution then considerably diluted and filtered. The insoluble residue is reserved for determining tin, uranium and thorium, if required, and the filtrate made up to a fixed volume. A suitable aliquot is evaporated to fumes with sulphuric acid, diluted and the combined oxides precipitated with tannin. A second aliquot may be used, after evaporation to fumes with sulphuric acid, for determining niobium pentoxide by Pickup’s method.HYDROFLUORIC acid was first proposed as a solvent for niobium and tantalum in the analysis of samarskite by Smith1 nearly 90 years ago. His procedure was reviewed in 1888 by Hille- brand,a some of whose observations were incorrect in that he declared iron and manganese to be insoluble in hydrofluoric acid, and again, in 1924, by Wells.3 More recently, a mixture of hydrofluoric and sulphuric acids has been applied to the analysis of niobium-bearing loparites by Cherniltov and Uspenskaya? and Chernikov and Goryu~hina.~ EXPERIMENTAL The method described here can be used in the analysis of columbite - tantalite, microlite and samarskite, whether alone or in mixed concentrates with wolfram and .cassiterite, and is a useful adjunct to the determination of uranium and thorium.Although not original, it offers the following advantages over methods that begin with the more customary potassium (or sodium) hydrogen sulphate fusion. (i) Silica need not be filtered off as it is volatilised by the hydrofluoric acid in the sub- sequent stages of the analysis.8 (ii) Cassiterite is not attacked but remains with the insoluble residue. This is important in the analysis of mixed columbite - cassiterite concentrates. The tin reported by Wells to be present in his hydrofluoric acid solution was derived from the samarskite itself. (iii) Thorium, bismuth (from bismuthotantalite), calcium (from microlite) and rare earths are all, likewise, conveniently separated. Wells’s analysis for his samarskite sample indicated that while the quadrivalent uranium was insoluble in hydrofluoric acid, the sexavalent uranium passed into solution, but experience with radioactive niobium - tantalum minerals in Uganda has shown, from whatever cause, that any uranium present remains entirely in the residue.INTERFERING ELEMENTS TITANIUM INTERFERENCE- According to Schoeller’s work,’ niobium and tantalum can be separated quantitatively from titanium by precipitation with tannin in dilute sulphuric acid (4 + 96), subject to a slight negative error. By inference, therefore, titanium will not interfere in the determination of niobium and tantalum pentoxides by the present method, but, because of its importance, the point was checked experimentally. It was found from the first that precipitation in 3 per cent.sulphuric acid (3 + 97) gave a slightly better recovery of niobium and tantalum pent- oxides than in the 4 per cent. acid recommended by Schoeller, and the former concentration was, therefore, used in the following investigation. Weighed mixtures of niobium and tantalum pentoxides (about 0-2 g for each experiment to simulate the maximum weight to be precipitated when assaying a mineral concentrate) were fused with 3-6 g of sodium hydrogen sulphate in 20-ml Vitreosil crucibles for 2 hours. 0 SAC and the author.94 [Autalyst, vol. 93 When cold, each melt was ejected by gently dropping the crucible, mouth downwards, on to an iron plate. The crucible and lid were placed in a 260-ml beaker containing 40 in1 of water and 15 ml of sulphuric acid (1 + 1).After warming the beaker for 20 minutes, they were removed, washed and well scrubbed with a rubber-tipped rod, and the melt was placed in the solution. At this stage any titanium required was added by transferring into the solution, with a pipette, the necessary volume of a standard solution of 0.25g of titanium dioxide plus 25 ml of sulphuric acid (1 + 1) per 100 ml, the sulphuric acid used for dissolving the nielt being adjusted beforehand to keep the total acid concentration constant. The solution was then evaporated to fumes under an infrared heat lamp. This gave a clear solution with the high niobium mixtures, but to dissolve the high tantalum mixtures com- pletely it was necessary to digest them on the hot-plate for up to 3 hours after the fuming stage had been reached.When cold, the solution was diluted, transferred into an 800-ml beaker, diluted to 250 ml, with addition of paper pulp, and boiled. Four grams of tannin dissolved in 40 nil of hot water were added and the procedure completed, as described below. The results obtained (Table I) show that there is a slight negative error but, contrary to Schoeller’s statement, it is greater when the amount of tantalum exceeds that of niobium. Secondly, part of the titanium is co-precipitated with the mixed oxides, about one fifth or less, varying according to the ratio of niobium to tantalum, and about one tenth in the two experiments in which precipitation was carried out in 4 per cent. sulphuric acid. In the fourth column of Table I, 0.0125 g of titanium dioxide corresponds to 5 per cent.in a sample, 0.0250 g to 10 per cent. and 0.0500 g to 20 per cent. The usual titanium content of a columbite - tantalite or microlite concentrate produced in Uganda does not exceed 1 per cent., and is generally less; 5 per cent. might be found in dirty low grade material, but for practical purposes, titanium interference can be disregarded. TABLE I POLLOCK: A GRAVIMETRIC METHOD FOR THE DETERMINATION RECOVERY OF NIOBIU?tl p l z l S TANTALUM AND TITANIUM 7 Niobium pentoxide 0-1734 0.0290 0.1753 0.0297 A 0.1744 0.1751 0.0266 0.1752 0.0260 0-174G 0-0249 B 0.1751 C 0-0259 Taken, g Tantalum pentoxide 0.0250 0.1760 0.0250 0.1760 0.0265 0-0254 0-1757 0.0267 0.1752 0.0216 0.1736 0.0266 0.1752 Total 0.1984 0.2050 0.2003 0.2057 0.2009 0.2005 0-2023 0.2019 0.20 12 0.1992 0.1985 0.201 7 0.201 1 -7 Titanium dioxide Nil Nil 0.0250 0.0250 Nil 0.0125 0.0125 0.0125 0.0 125 0.0125 0,0125 0.0500 0.0250 Found, g & Niobium and tantalum Titanium pentoxide dioxide 0.1970 Nil 0.2010 Nil 0.1965 0.0054 0.2012 0.0036 0-1978 Nil 0.1945 0.0021 0.1965 0.0015 0.1986 0.0035 0.1965 0.0023 0-1967 0.0031 0.1940 0.0022 0.1953 0-0058 0.1943 0-0022 Recovery, per cent.+---7 Apparent True N.A. 99.29 N.A. 98.05 100.79 98.1 1 99.56 98-12 N.A. 98.47 98.23 07-02 97-87 97.13 100*10 98.40 99.81 97-66 100-31 98.74 98-84 97.73 99-71 96.83 97-71 96.62 N.A. Not applicable. Experiment A included iron equivalent t o 0.05 g of iron(I1) oxide. Experiments B and C were carried out in 4 per cent. v/v sulphuric acid and are shown for comparison. MANGANESE INTERFERENCE- Manganese tends to be precipitated by fuming with sulphuric acid and remains insoluble after dilution.In one mineral deposit examined by the author, the columbite occurred closely associated with wad, and the manganese content of samples ranged up to 40 per cent. of manganese(I1) oxide. These samples were assayed by one of the author’s colleagues (Mr. Is. C. Patel) by the method described here, who found that the addition of 1 ml of ethanol to the hydrofluoric acid solution entirely prevented manganese precipitation, either on fuming with sulphuric acid or on subsequent dilution, and was equally effective in preventing manganese interference when niobium was determined by Pickup’s method8February, 19681 OF MIXED OXIDES (NIOBIUM AXD TANTALUM PENTOXIDES) 95 TUNGSTEN INTERFERENCE- The only significant interference is from wolfram and other tungsten minerals, as they are partly attacked by hydrofluoric acid, and the tungsten dissolved will be carried down in the tannin precipitate.However, it is usually necessary to determine tungsten trioxide (WO,) as well as niobium and tantalum pentoxides in a mixed concentrate, and the sample should first be assayed for tungsten by the aqua regia - ammonium tungstate method.' The niobium - tantalum minerals present will remain in the insoluble residue left after dissolving the tungsten trioxide in ammonia solution, and can be determined as described here. Alterna- tively, a method of dealing with tungsten interference is given below. METHOD PROCEDURE- Weigh 0.5000 g of sample into a 3-inch diameter platinum dish, add 10 ml of concentrated hydrofluoric acid solution and evaporate to dryness on a water-bath or carefully regulated sand-bath.Repeat the evaporation twice, with 15ml and then 20ml of acid, stirring occasionally with a polythene strip held in platinum-tipped tongs. Remove the dish from the water-bath, add 5 ml of hydrofluoric acid, stir and cover with a polythene circle. Leave for 2 or 3 hours, or overnight, as convenient. Remove and wash down the cover and stirrer, dilute the solution considerably and filter it through a 12.5-cm Whatman No. 541 filter-paper, in a polythene funnel, into a polythene beaker. Wash the platinum dish and filter-paper each three times with water. Reserve the residue. Rinse a 500-ml calibrated flask with dilute hydrofluoric acid (1 + 99), place into it 100 to 200 ml of water and pour in the sample solution, washing the polythene beaker three times.Make the volume up to the mark, shake the flask well and measure out 250 ml of the solution in a 250-ml flask that has been first rinsed out with a few millilitres of sample solution. It should be noted that hydrofluoric acid, at this dilution, has no effect on brief contact with glass, except to clean it by stripping off gelatinous silica. Keep the unused sample solution in a polythene bottle. Transfer the contents of the 250-nil flask (equivalent to 0.2500 g of sample) to an 800-ml glass beaker, washing the flask out three times, add 15 ml of sulphuric acid (1 + 1) and evaporate to fumes under an infrared heat lamp.The beaker will be etched, for which reason the same vessels should be kept aside for future analyses to avoid spoiling new glassware, but any silica abstracted is driven off by the fuming. Dilute to 250 ml, add some paper pulp, boil, add 4 g of tannin dissolved in 40 ml of hot water, and boil for 10 minutes. Turn the hot-plate switch to "low," leave for 2 hours and then allow to stand overnight. At this concentration of sulphuric acid, 3 per cent. v/v, tannin gives a clear-cut separation of niobium and tantalum from iron and manganese and an acceptable separation from titanium. Filter through a 15-cm Whatman No. 541 filter-paper, wash the beaker and precipitate three times each with dilute sulpliuric acid (2-5 + 97.5) and place the precipitate in a weighed Vitreosil crucible, which in turn should be put in a cold electric furnace. Switch on the furnace, ignite at 800" C until all carbon is burned off, allow the crucible to cool in a desiccator and re-weigh.If the precipitate is to be ignited over a burner it must be carefully dried at 105" C before ignition, to prevent loss by explosive boiling of the voluminous wet mass in the crucible. Multiply the weight of precipitate by 4 x 100 to obtain the percentage of niobium and tantalum pentoxides. CORRECTION FOR TUNGSTEN INTERFERENCE- If wolfram present in the original sample has not been separated by a preliminary aqua regia - ammonium tungstate treatment the tungsten derived from it will be precipitated as a yellow powder when the hydrofluoric acid solution is evaporated to fumes with sulphuric acid.(NOTE-A white precipitate is iron(II1) sulphate, which can be ignored, but should be re-dissolved on dilution before adding tannin.) In this event, transfer the concentrated sulphuric acid solution into a 25-ml calibrated flask, washing the beaker three times with more concentrated acid. Make up to the mark with concentrated sulphuric acid, cooling if necessary, and allow all of the precipitate to settle. By using a pipette fitted with a pipette filler, measure 15 ml of clear solution into a 1-litre beaker, dilute to 500 ml and precipitate with 4 g of tannin, as directed above.96 POLLOCK According to Pickup, the solubility of tungsten trioxide in concentrated sulphuric acid is about 8.4 mg per 10ml of acid.Therefore, deduct (1.5 x 04084g), i.e., 0.0126g from the weight of the final precipitate to allow for tungsten retained in the sulphuric acid solution. DETERMINATION OF NIOBIUM Returning to the unused part of the hydrofluoric acid solution, measure, with a 100-ml pipette fitted with a pipette filler, 100 ml into a 250-ml beaker, add 50 ml of sulphuric acid (1 + 1) and evaporate to fumes under an infrared heat lamp. The procedure for determining niobium pentoxide is then as described by Pickup, and tantalum pentoxide may be calculated by difference. TREATMENT OF INSOLUBLE RESIDUE DETERMINATION OF TIN- When tin is to be determined in a mixed mineral concentrate, the insoluble residue left after the hydrofluoric acid treatment should be wholly transfened into the filter-paper, which is gently heated in a 50-ml nickel crucible to burn off carbon.Tin may then be determined by the Pearce - Low iodine titration m e t h ~ d , ~ by using 0.002 N iodine for small amounts. DETERMXNATION OF URANIUM AND THORIUM- If uranium and thorium are to be determined in a radioactive niobium - tantalum mineral, the hydrofluoric acid insoluble residue should be washed back off the filter into the platinum dish, where it can be treated for the determination of uranium and thorium by any suitable method,lO for example, by dissolving it in nitric acid as a preliminary to column chromatography. COMPARISON OF RESULTS Several samples of columbite - tantalite that had been assayed by the proposed method were re-assayed by using the standard procedures described by Schoeller and Powell. The comparative figures are given in Table I1 and show closer agreement than would be expected from the experimental results in Table I, probably because recovery is not complete by any method.TABLE I1 COMPARISON OF RESULTS Assay by proposed method, Assay by standard method, Sample niobium and tantalum pentoxides, niobium and taotalum pentoxides, No. per cent. per cent. 1 76.8 77.7 2 74.9 74.9 3 72.1 72.8 4 76.9 77.8 5 73.6 74.5 6 73.1 73-0 7 64.1 64.5 8 60-2 00.4 Samples Nos. 7 and 8 contained 1 per cent. of titanium dioxide. whose 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. The author thanks the Commissioner of Geological Survey and Mines, Entebbe, with permission this paper is published. REFERENCES Smith, J . L.. Amer. J . Sci., 1877, 13, 359. Hillebrand, W. F., Colo. Scient. SOC. Proc., 1888, 3, 38. Wells, R. C., J . Amer. Chem. Soc., 1928, 50, 1017. Chernikhov, Yu. A., and Uspenskaya, T. A,, Nauchno-Tek. Otch. Tyemnye, 1936, No. 83, Ciredment, Chernikov, Yu. A., and Goryushina, U. G., Zav. Lab., 1945, 11, 876. Langmyhr, F. J., and Graff, P. R., Alzatytica Chim. Ada, 1969, 21 (4), 334. Schoeller, W. R., and Powell, A. R., “The Analysis of Minerals and Ores of the Rare Elements,” Third Edition, Charles Griffin & Company Limited, London, 1966. Pickup, R., Colon. Geol. Miner. Resouv., 1966, 5, 174. Weinig, A. J., and Schoder, W. P., “Technical Methods of Ore Analysis for Chemists and Colleges” based upon the text by Albert H. Low, Eleventh Edition, John WiIey 62 Sons Inc., New York, 1939. “The Determination of Uranium and Thorium, Handbook of Chemical Methods for their Deter- mination in Minerals and Ores,” National Chemical Laboratory, H.M. Stationery Office, London, 1963. Received November 28th. 1966 Moscow.

ISSN:0003-2654    

DOI:10.1039/AN9689300093

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Analyst, February, 1968, Vol. 93, pp. 97-100 97 Determination of Sulphate by Automatic Colorimetric Analysis BY MORRIS E. GALES, JUN., WILLIAM H. KAYLOR AND JAMES E. LONGBOTTOM (US. Department of the Interior, Federal Water Pollution Control Administration, Division of Pollution Surveillance, 1014 Broadway, Cincinnati, Ohio 45202, U.S.A .) An automatic colorimetric method for determining sulphate in natural water in the range of 5 to 400 mg per litre is presented. The method is based on the precipitation of barium sulphate and the release of the coloured acid chloranilate ion. A manifold has been developed for the determination of sulphate on the Technicon AutoAnalyzer. The proposed method will run 15 samples per hour. SULPHATE, which is widely distributed in nature, is present in natural water in concentrations ranging from near zero to several thousand milligrams per litre, and may also be discharged in numerous industrial wastes, such as those from tanneries, pulp mills, textile mills and other plants in which sulphuric acid is used.In the past, analytical procedures used for determining sulphate in water and waste discharges were cumbersome and tedious, e.g., the time-consuming standard gravimetric method,l and the titrimetric method, which is not very accurate or precise. This investigation was undertaken to develop a method for determining sulphate in natural water in the range of 5 to 400 mg per litre on the Technicon AutoAnalyzer. Several methods were tried : Technicon's turbidimetric method,2 the methylthymol blue method? and the barium chloranilate method, as described by Bertolocini and Barney4 and Agterdenbos and Martini~s.~ The turbidimetric procedure was abandoned because the barium sulphate coated the inside of the mixing coils and the flow cell, and the thymol blue method proved too sensitive for the range desired.The chloranilate method gave the best results. In the chloranilate procedure, when solid barium chloranilate is added to a solution containing sulphate, barium sulphate is precipitated, releasing the highly coloured acid chloranilate ion. The colour intensity in the resulting chloranilic acid is proportional to the amount of sulphate present. EXPERIMENTAL REAGENTS- and dilute to 1 litre with distilled water. 6.4 ml of acetic acid and dilute to 1 litre with distilled water. hydrated disodium salt of EDTA in distilled water and dilute to 1 litre.(Na,SO,) in distilled water and dilute to 1 litre. APPARATUS- Barium chloranilate-Add 3 g of barium chloranilate (BaC,C1,0,) to 333 ml of ethanol Acetate buffer, p H 4.63-Dissolve 13.6g of sodium acetate in distilled water. Add Sodium hydroxide - EDTA solzction-Dissolve 65 g of sodium hydroxide and 6 g of Ion-exchange resin-Dowex 50W-X8, ionic form, H+, 20 to 50 mesh. Stock sulphate solution, 1000 mg per litre-Dissolve 1.479 g of dried sodium sulphate Standard Technicon AutoAnalyzer equipment and accessories. Heating bath with double delay coil, at 45" C. Double delay coil. Single delay coil. Continuous jilter. 0 SAC and the authors.98 Lm GALES, KAYLOR AND LONGBOTTOM DETERMINATION OF [AIzabSf, VOl.93 ion exchange - I &Barium chlorani late ,065 ,045 - - 4.63 Buffer Continuous filter ,025 1. I00 - EDTA-NaOH Sample Recorder Colorimeter 520mj.1 Sm = Small mixing coil D40 = 40-foot Delay coil Lm = Large mixing coil 080 = 80-foot Delay coil Fig. 1. Manifold for determination of sulphate on the AutoAnalyzer: sampling time, 2-0 minutes; 1 wash tube MANIFOLD PROCEDURE- Alternating samples and wash tubes were pumped at the same rate of 5.0 ml per minute for 2 minutes per tube and passed through an ion-exchange resin to remove cations, because calcium, aluminium and iron will precipitate the chloranilate. The ion-exchange column consisted of 12 inches of $-inch 0.d. sleeving, packed with Dowex 50W-X8. 80 409 Fig. 2. Recording of the sulphate standard, mg per litre )portioning I per litre Fig.3. Precision of the barium chloranilate procedure (raw river water)February, 19681 SULPIIATE BY AUTOMATIC COLORIMETRIC ANALYSIS 99 The barium chloranilate is added to water and ethanol to form a slurry, and the mixture placed on a magnetic stirrer. Ethanol is added to decrease the solubility of barium sulphate. Air is pumped into the reagent line to keep the reagent moving. After emerging from the ion-exchange column, the sample is mixed with the buffer and the solid barium chloranilate and passed into a 45" C heating bath to increase the reaction rate. After a delay time of 15 minutes, by passing the sample through a series of delay and mixing coils, the barium sulphate and the unused barium chioranilate are removed by iiltra- tion. The sample, before passing into the flow cell, is mixed with sodium hydroxide - EDTA reagent to remove any barium sulphate that may come through the filter. The colour is measured at 520mp in a 15-mm flow cell.The entire flow diagram for this procedure is shown in Fig. 1 and the calibrztion curve obtained with it in Fig. 2. TABLE I RECOVERY OF SULPHRTE ADDED TO ARiciiNSAS RIVER WATER Total, Recovered, Recovered, Amount added mg per litre mg per litre per cent. - - i ! 31 10 mg per litre . . { I] 90 60 mg per litre . . { ;; q 94 100 mg per litre . . { E} 101 None . . .. .. [ 31 40 47 7 8 47 J 133 102 133 10.2 METHOD EVALUATON- To check the recovery of known amounts of sulphate, 10, 50 and 100mg per litre of sulphate were added to a sample of Arkansas river water.Ninety per cent. of the 10-mg per litre, 94 per cent. of the 50-mg per litre and 101 per cent. of the 100-mg per litre sample were recovered. These results are shown in Table I. Two samples containing different concentrations of sulphate were run through the system to determine the precision of the automatic procedure. The results, depicted in Fig. 3, show this method to be reproducible. Additional results confirming the reproducibility are given in Table 11. TABLE I1 PRECISION OF THE AUTOMATIC SULPHATE PROCEDURE Number of Coefficient of river water Range, Mean, St an dard variation, samples mg per litre rng per litre deviation per cent. 6 50 to 52 50.8 f 0-3 0.6 9 92 to 97 94.0 & 1.0 1.1 10 168 to 174 170.0 f 3.0 1.8 The reliability of this procedure was checked by comparing the results obtained by the AutoAnalyzer with the standard gravimetric methodl and a turbidimetric methodG on a group of representative river water samples.These comparisons, shown in Tables I11 and IV, indicate that this method is of comparable accuracy.100 GALES, IUE’LOR AND LONGBOTTOM TABLE 111 COMPARISON OF THE GRAVIMETRIC METHOD WITH THE BARIUM CHLORANILATE METHOD ON RIVER-WATER SAMPLES Gravimetric method , mg per litre 324 303 187 283 252 Barium cliloranilate method, mg per litre 340 318 192 276 246 Gravimetric I AutoAnalyzer, per cent. 95 95 97 102 10s TABLE IV COMPARISON OF THE TURBIDIMETRIC METHOD WITH THE BARIUM CHLORANILATE METHOD ON RIVER-WATER SAMPLES Concentration, 0 to 50 mg per litre Concentration, 60 to 400 mg per litre Turbidimetric Barium chloranilate Turbidimetric Barium chloranilate method method method method 8 6 4 13 16 15 13 13 10 45 35 23 Average deviation .. 10 6 6 16 15 15 20 17 10 42 30 22 2.2 mg per litre 91 215 235 239 240 80 85 105 225 115 320 - 82 196 220 219 236 72 80 103 231 110 320 - 8.9 mg per litre CONCLUSION The method can be used to determine sulphate in the range of 5 to 400 mg per litre without any sample dilution. The accuracy and precision are equal to, or better than, those methods used for comparative purposes in this study, and those reported for the standard turbidimetric meth0d.l REFERENCES 1. Orland, H. P., Editor, “Standard Methods for the Examination of Water and Wastewater,” Twelfth Edition, American Public Health Association Inc., New Yorlr, 1965. 2. “Technicon AutoAnalyzer Methodology-Sulfate VIb,” Technicon Controls Inc., Chauncey, New York. 3. Lazrus, A. L., Hill, K. C., and Lodge, J. P., “A New Colorimetric Microdetermination of Sulfate Ion,” Technicon Symposia 1965, Automation in Analytical Chemistry, Mediad Inc., New York, 1966, p. 291. Bertolocini, R. J., and Barney, J. E., Analyt. Chern., 1957, 29, 283. Agterdenbos and Martinius, Talanla, 1964, 11, 875. Water & Sewage Analysis Procedures, Catalogue No. 9, Hach Chem. Co., Ames, Iowa, 1966, p. 42. Received May 2nd, 1967 4. 5. 6.

ISSN:0003-2654    

DOI:10.1039/AN9689300097

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

A~zalyst, February, 1968, Vol. 93, #J. 101-106 101 Automated Neutron-activation Analysis of Biological Material with High Radiation Levels BY K. SAMSAHL* ( A B Atomenergi, Nykoping, Swedeii) A method has been deveIoped for the chemical separation and subsequent y-spectrometric analysis of the alkali metals, the alkaline earths, the rare earths, chromium, hafnium, lanthanum, manganese, phosphorus, scandium and silver in neutron-activated biological material. The separation steps, which are fully automatic, are based on a com- bination of ion exchange and partition chromatography and require 40 minutes for their completion. MANY procedures suitable for chemical separations of radionuclides by anion exchange from hydrochloric acid solutions have appeared in the literature. However, little information is available about simple and rapid sub-grouping of trace elements that are non-adsorbable by this method.The aim of the present work was to develop a method for routine separations of these elements in neutron-activated biological material. Because of the high level of sodium-24 radiation and sometimes also the long-lived alkaline earth activities, manual chemical processing may require the use of rigorous safety precautions or excessively long decay periods.lJ To overcome these difficulties and to hasten and simplify the chemistry involved, an almost fully automated, remote-control separation method is needed. This implies the exclusion of many conventional steps in analytical chemistry, e.g., extractions, precipitations, titrations and evaporations, as these are all more or less unsuited for auto- mation.A better solution to the present problem seems to be the application of ion exchange or related techniques. However, the high ionic strength of the sample solution (strong hydrochloric acid) severely limits the possibility of choosing many of the common separation procedures. Methods that do not often suffer from this disadvantage include separations from halogen acid solutions with anion-exchange resins, from nearly neutral or alkaline solutions with chelating resins and inorganic ion exchangers, and from a wide variety of acidic solutions with part it ion-chromat ographic technique. Since the pioneering work in partition chromatography with reversed-phases, carried out on different mixtures of inorganic constituents by Siekierski and Kotlinskaya,3 Siekierski and Fidelis4 and Gw6Zdi and Siekierski,6 the method has found an ever-increasing number of applications in radiochemical analysis.s The ability of chelating ion exchangers to separate traces of different cations from strong electrolyte solutions has been studied by Sides and Kenner.7 The ion-exchange properties of several inorganic materials have been studied by Kraus, Phillips, Carlson and Johnson, who developed extremely simple procedures for separations within the groups of the alkali metals and the alkaline earths.8 In the present method seven groups of chemical constituents are separated in the following sequence: hafnium and scandium are adsorbed from strongly acidic solution on a column of kieselguhr impregnated with di(2-ethylhexyl) orthophosphate (HDEHP) (Fig.1, column C) ; lanthanum and the rare earths are then adsorbed in an analogous way from weak acid (column D); traces of silver are separated as the bromide complex with anion-exchange resin (column E); a column of Chelex-100 chelating resin adsorbs chromium and manganese from weakly acidic solution (column F), whereas the alkaline earths are separated in a similar way from strong alkali (column G) ; and the series is ended with a column of zirconium ortho- phosphate, which selectively adsorbs the heavier alkalis, potassium, rubidium and caesium, from a weakly acid solution (column H). The remaining strong activities of sodium-24 and phosphorus-32 are passed to the effluent of the column series and fonn the seventh group of elements separated.* Present address : 8000 Munchen-45, Riemerschmidstrasse 6/111, Germany. 0 SAC and the author.102 SAMSAHL : AUTOMATED XEUTRON-ACTIVATION AXALYSIS [Ana&St, VOl. 93 The separations, which are based on selective sorption, are carried out simultaneously with a peristaltic pump, and are in direct relationship with the anion-exchange separation step from 8 N hydrochloric acid solution described e1sewhere.l The desired composition of the various influent solutions is obtained by introducing, between the columns, appropriate amounts of strong solutions of sodium acetate buffer, sodium hydroxide, sodium bromide, nitric and orthophosphoric acids. EXPERIMENTAL APPARATUS- A peristaltic pump (type Desaga, Heidelberg) was used for the separation and the working principles are shown in Fig.1. The pump is equipped with twelve Tygon plastic tubes with inner diameters of 1-6mm. Teflon tubes with 1-mm inner diameter are used to connect the columns in the series B, C, D, E, F, G and H. The various influent solutions are thoroughly mixed by pumping them through the mixing coils, J. The coils, which are made of Teflon tubing, each consist of eight turns with a diameter of 15mm. As shown in Fig. 1, the effluent solution from column B is led back to the suction side of the pump before entering the remaining columns in the series. This is done to decrease the flow-rate resistance in the system and to make the proportioning control easier. I 1 1 A = Peristaltic pump B = 5 x 200-mm Column of Dowex-2 (Cl-, 200 C = 5 x SO-mm Column of HDEHP-treated D = 7 x 50-mm Column of HDEHP-treated E = 7 x 50-mm Column of Dowex-2 (Cl-, 200 F and G = 16 x 50-mm Columns of Chelex-I00 H = 22 x 90-mm Column of Bio-Rad ZP-l (100 to 400 mesh) kieselguhr kieselguhr to 400 mesh) (Na+, 100 to 200 mesh) to 200 mesh) J = Mixing coils K = Point at which ION nitric acid - M sodium dihydrogen phosphate ( I + I ) is intro- duced L = Point at which 8N sodium hydroxide is intro- d uced M = Point at which 4M sodium acetate- ION sodium hydroxide - 5N sodium bromide (15 + 3 + 2) is introduced N = Point at which 8N hydrochloric acid -con- centrated sulphuric acid sample i s intro- duced 0 = Test-tube Fig.1. Scheme of the separation system The columns B, C, D, E , F, G and H, of borosilicate glass, are fitted with Teflon stoppers with a neoprene ring.The stoppers are kept securely in position with Teflon screw-caps. The distance between the lower end of a stopper and the top of the column is kept within 3 to 5 mm to minimise the amount of washing solution needed for a separation. The inner diameters of columns B and C are 5 mm, of D and E, 7 mm, of F and G, 16 mm, and that of H, 22 mm. The columns have glass filter discs of porosity G-1 or G-2 at the bottom. Details not shown in Fig. 1 are a lead-brick shielded calibrated flask used to receive the effluent from column H and bottles containing the stock solutions K, L, M and N.‘February, 19681 OF BIOLOGICAL MATERIAL WITH HIGH RADIATION LEVELS 103 PREPARATIONS AND CALIBRATION- Column B is filled by suction to a height of 200 mm with wet Dowex-2 x 10, 200 to 400- mesh, resin in the chloride form, and column E to a height of 50 mm with the same resin form.As indicated in Fig. 1, column B may also be sub-divided into three smaller, coupled columns, the first being 100mm high and the remaining two each 50mm high. This arrangement is made to allow for a further sub-grouping into five different groups of the many trace elements adsorbed on this column.1 Except for the thorough removal of colloidal particles, the commercially available resin in the chloride form may be used without pre-treatment. The resin is stored in distilled water. Columns C and D are both filled to a height of 50 mm with wet kieselguhr pre-treated with dk(2-ethylhexyl) orthophosphate (HDEHP) and are washed with a little water on a suction flask until the effluent ceases to foam and finally drained.The preparation of the column material was carried out as follows. Celite 545 (Johns-Manville) was mixed with water several times, decanting after each mixing, to remove colloidal particles. After drying, the kieselguhr was siliconised with dimethyldichlorosilane vapour and again dried; 15 g of the material were then stirred into a solution of 1.5 g of HDEHP in 50 ml of di-isopropyl ether. The bulk of the solvent was volatilised by allowing it to stand at room temperature, the last traces being removed under reduced pressure, The impregnated kieselguhr was stored in distilled water. Columns F and G are prepared with wet Chelex-100, 100 to 200 mesh, in the sodium form (Bio-Rad Laboratories).The resin is stored in distilled water after removing colloidal particles. To avoid shrinkage during operation the columns are washed with composite solutions, in each instance corresponding to the influent in practical runs, and are then drained by suction. The final height of both columns of resin mass should be 50 mm. The last column in the series, H, is filled to a height of 90 mm with dry ZP-1, 100 to 200 mesh (Bio-Rad Laboratories), which is washed with 4~ sodium acetate until the pH of the effluent is between 4 and 5. The column is finally drained. Columns B, C, D, E, F, G and H are then coupled in series and connected to the peristaltic pump, as shown in Fig. 1. They are then pre-equilibrated and the whole system calibrated by running the pump for some minutes.At the beginning of the running-in period the influent solutions to the last three columns, F, G and H, are by-passed to waste. This is done to decrease the time needed for the calibration and to obviate the risk of destroying the zirconium orthophosphate exchanger. At point N, 8 N hydrochloric acid is fed into the system through two Tygon tubes from a washing solution reservoir. The solution passes through column B and then back to the suction side of the pump via a small test-tube, 0, which continuously stores 0-5 to 1 ml. The solution is now pumped unchanged, and at the same speed, through column C. The effluent is automatically diluted to three times its volume before entering columns D and E. The dilution is effected by introducing through four different Tygon tubes at point M a solution, a stock of which is prepared by mixing 600 ml of 4 M sodium acetate, 120 ml of 10 N sodium hydroxide and 80 ml of 5 N sodium bromide.The influent to column F is obtained by injection of 8 N sodium hydroxide through one tube at point L. The resulting solution, which by-passes column F, is controlled for acidity with pH paper strips and, at the beginning of the running-in period, will show a strongly alkaline reaction. However, after a few minutes, when the air has been forced out of the system and the strong hydrochloric acid has reached full action at the end of the series, the pH will become stabilised at a value of 5 to 5-5. The pH of the influent to the foregoing columns, D and E, is about 36, at which pH the acetate system still has sufficient buffer capacity for the present separations.Column F is now attached to the series by turning the three-way stopcock ahead of the column. The effluent is mixed with a further portion of 8 N sodium hydroxide, introduced at point L. When it is strongly alkaline, the solution is passed through column G and finally brought back again to the buffering interval. This is effected by pumping in through two tubes at point K a solution consisting of 10 N nitric acid - M sodium dihydrogen orthophosphate (1 + 1). After the pH of the resulting solution has become stabilised at between 4 and 4.5 the last column, H, is also switched into the series. Its effluent should have an acidity falling within the same pH range, and the flow-rate is controlled at about 5 ml per minute.This corresponds to a flow-rate of about 1 ml per minute through B, the first column in the series.1 The pump is now stopped and the apparatus is ready for the separation.104 SAMSAHL : AUTOMATED NEUTRON-ACTIVATION ANALYSIS [ A nalyst, Vol. 93 The preparation of the columns, as well as the calibration step, is advantageously carried out before completing the irradiation of the samples, especially if the determination of short-lived nuclides is needed. PROCEDURE- Irradiation and #re-se$aration-The biological material, ranging in amount from about 15 mg for hard tissue, to 200 mg dry weight for soft tissue, is sealed in a quartz tube and irradiated for 2 days with a thermal neutron flux of about 2 x lofs neutrons per cm2 per second.The irradiation may be followed by a decay period of 1 to 3 days, according to the level of sodium-24 activity present. The sample may then be re-irradiated for 10 to 30 minutes to make possible also the determination of short-lived nuclides. Shortly after the second irradiation, it is transferred into a distillation apparatus, together with 60 pl of a 48 per cent. hydrobromic acid carrier solution containing 1 pg each of Ba2+, Ca2+, C9+, Cs+, Hf4+, K+, La3+, Mn2+, Na+, PO?--, rare earths, Rb+, Sc3+ and Sr2+; 1 pg of Ag+ is added from a weak sulphuric acid solution. For simultaneous determinations of elements, separated as chloride complexes on column B, an additional sulphuric acid carrier solution containing 1 pg each of Au3+, Cd2+, Co2+, Cu2+, FeS+, Ga3+, Ins+, Mos+,W6+ and Zn2+ is also added to the flask.l The organic matter is then destroyed with a mixture of concentrated sulphuric acid and 50 per cent.hydrogen peroxide, and radionuclides of the elements arsenic, bromine, chlorine, ger- manium, mercury, iodine, osmium, rhenium, rubidium, antimony, selenium and tin are distilled from the solution. Further details concerning the irradiation and the distillation procedures are given el~ewhere.~ tg After the distillation, concentrated sulphuric acid solution remaining in the flask is boiled down to 0.5 ml and diluted with 10 ml of 8 N hydrochloric acid. Any calcium sulphate precipitate is dissolved by gentle heating. After cooling, the solution is introduced at point N (Fig.1) and allowed to flow through the column series at the same rate, and with the same dilutions between the columns, as described above. On its way to the first column, B, the sample passes a small glass filter disc of porosity G 4 . Immediately following the sample, and separated from it by an air bubble, a wash solution, consisting of 15 ml of 8 N hydro- chloric acid, is also passed through the system in the same way. Column B is now removed from the system. As a result of preparing the column in three different parts, adsorbed trace elements may be easily sub-divided into five gr0ups.l A second wash solution, consisting of 15 ml of 8 N hydrochloric acid, is now passed through the remaining series of columns C, D, E, F, G and H in the same way The pump is then stopped, and each column attached to a suction flask for a further rapid rinsing.Columns C, D and E are washed with 10 to 20 ml of water, and columns F, G and H with the same amount of solutions, the composition of which corresponds in each instance to the influent. After a final draining, the various column materials are transferred into polythene tubes, homogenised and counted. A multi-channel y-spectrometer attached to a 3 x 3-inch well-type sodium iodide crystal is suitable for the measurements. RESULTS AND DISCUSSION RECOVERY AND REPRODUCIBILITY STUDIES- The results obtained in recovery and reproducibility experiments with seventeen different radioactive trace elements are given in Table I. The strongly irradiated samples shown in the first column of Table I were separately dissolved in concentrated sulphuric acid, or in hydrochloric acid plus hydrogen peroxide , and diluted. Various mixtures containing suitable activities of 6 to 8 of the trace elements and carriers, as described above, were then treated with mixtures of hot sulphuric acid and 30 per cent.hydrogen per~xide.~ Finally, each solution was heated to strong fumes of sulphur trioxide and boiled down to 0.5 ml, which was used for the separations. These were carried out with a peristaltic pump, as described above. The activities of the separated groups were compared with reference samples. The values given are the means of three experiments, and the standard error in no instance exceeded 3 per cent. DESTRUCTION OF ORGANIC MATERIAL AND TOTAL PROCESSING TIME- The mixture of concentrated sulphuric acid and 50 per cent.hydrogen peroxide acts not only as a simple and rapid wet-ashiiig agent for biological material, but will also quantitativelyFebruary, 19681 Irradiated material OF BIOLOGICAL MATERIAL WITH HIGH RADIATION LEVELS TABLE I RECOVERY VALUES FOR THE RADIOISOTOPES SEPARATED Mean value of yield, per cent. .A r Isotope Column measured B &NO, . . Silver-110 - RaC1, . . . . Barium-131 - CaC1, . . . . Calcium47 - Ce,(SO,), . . Cerium-141 - CrC1, . . . . Chromium-61 - CSCl . . . . Caesium-134 - Hf02 ., . . Hafnium-181 (-9) KC1 . . . . Potassium-42 - La(NO,), . . Lanthanum-140 - LU,O,. . . . Lutecium-177 - XlnSO, . . Manganese-56 (-2) NaCl . . . . Sodium-24 - NH,H,PO, . . Phosphorus-32 - RbCl .. . . Rubidium-36 - sc,o, . . . . Scandium-46 - Sm2% . . Samarium-153 - SrC1, . . . . Strontium-87 - Column Column C D - 91 - 100 (-2) 98 100 - - 100 105 convert the trace elements into appropriate oxidation states, which are ideally suited for subsequent chromatographic group separations. As it is a powerful oxidant in an acidic medium, hydrogen peroxide will oxidise many of the elements to their normally highest valencies, although a few of them, e.g., molybdenum and tungsten, are dissolved as peroxy- compounds. On the other hand, the biologically important elements, manganese and chrom- ium, are reduced from higher valency states to the bivalent and tervalent state, respectively, thus securing quantitative separation in one group of the scheme. The rare earths, including cerium, will be present in the tervalent state in the final hydrochloric acid sample solution, thereby making possible their separation, as a single group, from all other elements.The risk of precipitating typical trace elements, such as silver, tungsten and uranium, during the chemical processing may be regarded as non-existent, because of their low concentration in biological material, and the minute amount of carriers used. The destruction of organic matter can be carried out at a distance behind a lead shield in less than 10 minutes. If coupled to a distillation step for the simultaneous determination of volatile elements, as mentioned above, the time required to obtain the sample solution will increase to 30 to 40 minutes. Accordingly, the total processing time will vary from about 50 to 80 minutes, depending on the number of elements sought.Further, one non- scientific operator can process three samples simultaneously with three machines in 14 hours, provided the distillation step is omitted. In routine analysis of such samples no cleaning of the apparatus is needed, the preparation for a fresh separation consisting in mounting a new set of columns from stock, followed by a running-in period lasting for a few minutes. Chemical group separations similar to those described have also been carried out manually on human tissues. Under these conditions one skilled chemist could process only one sample at a time in 1 Q to 3 days.2JO APPLICABILITY OF THE METHOD- The use of the present method is restricted by the amounts of alkaline earths and silicon present in the samp1es.l The chemical separations were not extended to more than seven groups.This is probably sufficient for most routine applications, especially if advantage is taken of the different decay rates of the nuclides, and the measurementsare combined with a spectrum stripping technique. However, in the event of more detailed studies being required complementary separations will be needed. The determination of hafnium-181 in the presence of the high energy scandium-46 in the first group will certainly, in many instances, be difficult or impossible. However, a chemical separation has so far not been tried, because of, inter alia, the exceedingly long counting times needed for reliable hafnium determinations (at least with medical sampleslO).For lanthanum and the rare earths separated in the second group of the scheme, more detailed studies will probably always be impossible without a further chemical sub-grouping.106 SAMSAHL For example, in typical medical samples only the determination of cerium, lanthanum and samarium is possible.1° However, by splitting this large group of elements into two or more sub-groups, with selective elution steps,ll a much better resolution may be possible. As regards chromium and manganese in group IVY further chemical processing is rarely needed. An important exception occurs with blood samples, because of the formation of large amounts of manganese44 with fast neutrons. A simple chemical separation seems possible by selectively eluting manganese with weak hydrochloric acid.and the determination will probably not be affected by radionuclides of barium and strontium. Also, determinations of strontium based on strontium-87un will hardly cause difficulties. However, barium determinations, with either barium-131 or barium-139, may sometimes be impossible. Purification of barium activities after the elution of the alkaline earth group with weak acid is possible with ion exchange on zirconium orthophosphate.12 Individual chemical separations of the heavier alkali metals in group VI will hardly be necessary, provided a well-type detector is used in combination with spectrum stripping after the decay of potassium-42. The problem can be solved in a similar way in the analysis of phosphorus and sodium in group VII.The phosphorus-32 bremsstrahlung remaining after the decay of sodium-24 may, at least theoretically, be contaminated by the long-lived sodium-22. The spectrilm of this nuclide is then stripped away, the 1.02 MeV sum-peak being utilised in this connection. In contradistinction to an extensive chemical sub-division, the adding together of different groups may also be shown to be advantageous. For example, the relationship between silver-110 and chromium-51 is, in many kinds of neutron-bombarded biological material, sufficiently favourable to permit simultaneous measurement. Sometimes also the scandium-46 activity in group I may be added, thus saving long, expensive counting times. THE PROPORTIONIKG PUMP SYSTEM- Practical experience shows that the peristaltic pump used for the experiments will work in a reliable way, provided that careful calibrations are carried out.However, this type of apparatus seems to be rather sensitive to differences in flow resistance of the columns used for the separation, which may lead to slightly inexact volumes of liquid being supplied to the series. The same effect may also be caused by differences in the swelling of the plastic tubes transferring the strongly acid or alkaline solutions. In the present work, these difficulties were largely overcome in various ways, e.g., by frequently changing the plastic tubes of the pump, by using a large amount of acetate buffer for the separation, and by applying com- paratively coarse-sized and broad resin beds towards the end of the column series.The present separations may probably be improved by using a machine with a piston drive.1 With this kind of drive the apparatus will work accurately, being almost completely independent of flow-resistance problems and the concentrations of acid and alkali used. However, adjustments of a more minor nature in the composition at present used of the solutions injected into the system may be necessary. Because of the high degree of reliability obtained, it may also be possible to extend the number of columns in the series, e.g., to split the rare-earth group into sub-groups by selective sorption steps on small columns of HDEHP- treated kieselguhr.ll I am grateful to Dr. Erik Haeffner, Head of the Chemistry Department, for his interest in this work. Calcium-47, or sometimes calcium-49, can be used for calcium analysis in group V, I also thank Sigrid Hackbarth for skilful technical assistance. 1 . 2. 3. 4. 5. 6. 5. r 9. 10. 11. 12. REFERENCES Samsahl, K., Nukleonik, 1966, 8, 252. Samsahl, K., Brune, D., and “ester, P. O., Int. J . AppZ. Radial. Isotopes, 1965, 16, 273. Siekierski, S., and Kotlinskaya, B., Atomw i!??zerg., 1959, 7, 160. Siekierski, S., and Fidelis, J., J . Chromat., 1960, 4, 60. Gw6tdi, R., and Siekierski, S., Nztkleonzka, 1960, 5, 671. Hedrick, C. E., and Fritz, J. S., U.S. Atomic Energy Commissioii Report, IS-950, 1964. Sides, J. L., and Kenner, C. T., Analyt. Chem., 1966, 38, 707. Kraus, K. A., Phillips, H. O., Carlson, T. A., and Johnson, J . S., Int. Co~f. Peaceful Uses Atomic Samsahl, K., AnaZyt. Chenz., 1967, 39, 1480. Wester, P. O., Scafad. J . Clin. Lab. Invest., 1965, 17, 357. Sochacka, R. J., and Siekierski, S., J . Chromat., 1964, 16, 376. Maeck, W. J., Kussy, 31. E., and Rein, J. E., Analyt. Chem., 1963, 35, 2086. Enevgy, Geneva, 1958, 28, 3. Received March 28th, 1967

ISSN:0003-2654    

DOI:10.1039/AN9689300101

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Analyst, February, 1968 ,Vol. 93, $p. 107-110 107 A Method for the Determination of Vitamin A, a- and p- Carotene in Margarine, including the Results of a Collaborative Test BY C. D. USHER, D. J. FAVELL (Unilever Research Laboratory, Colworth House, Shavnbrook, Beds.) AND H. LAVERY ( Unilever Research Laboratory, The Frythe, Welwym, Herts.) The statutory method for the determination of total vitamin A in margarine has been modified to enable vitamin A, u- and p-carotene to be determined separately. A collaborative test by six laboratories has shown that the proposed method is more accurate than the statutory method, particularly for margarine coloured with synthetic /&carotene, and there is no loss of precision. THE colour of most margarines produced in Great Britain is caused mainly by their synthetic /3-carotene content.This has not always been so. Before about 1960 very little synthetic /I-carotene was available and red palm oil was used as the source of colour. The latter is a mixture of a-, /3- and inert carotenoids; a-carotene has only about one half the activity of /3-carotene as a pro-vitamin A. The vitamin A content of margarine and the method for its determination1 are governed by Act of Parliament. The total is made up from the actual vitamin A and the calculated vitamin A contribution of the /3-carotene. Because the present regulations were drafted when palm oil was widely used, a conversion factor of 358 (based on the average a- and 8- carotene contents) was quoted. Today, when synthetic ,%carotene, with a true conversion factor of 667, is used, analysts must still use the statutory conversion factor.It has, of course, been difficult to separate u- and /3-carotene from each other, and from vitamin A, and to recover all of them quantitatively. This paper gives details of a method of separation. It also provides evidence to show that the method is more accurate than, and as reliable as, the present statutory method. In addition, the method makes it relatively easy to determine whether red palm oil or p-carotene has been used in the manufacturing process. EXPERIMENTAL PRINCIPLE- The unsaponifiable matter extracted from a portion of margarine, after refluxing with ethanolic potassium hydroxide, is chromatographed on a column of neutral alumina. The yellow carotene eluate, which precedes vitamin A off the column, is chromatographed again on a column of magnesium oxide to separate the a- and /3-carotene.The colourless vitamin A eluate is passed through a column of alkaline alumina. The concentration of a- and p-carotene and vitamin A in each eluate is measured with a spectrophotometer. APPARATUS- The apparatus has been fully described in a previous paper.2 REAGENTS- The reagents are identical with those listed in a previous paper,2 with the following additions. Potassiwn hydroxide solution, 60 per cent. wlv, aqueous. Additional developing solvent-Light petrolcum containing 50 per cent. of diethyl ether. Cyclohexane-Spectroscopic grade. Magutesia-Heat magnesium oxide (heavy) at 100" C for 2 hours. Cool in a desiccator 0 SAC and Unilever Ltd.and set aside for 3 or 4 days in an air-tight bottle before use.108 PROCEDURE- USHER, FAVELL AND LAVERY: METHOD FOR THE DETERMINATION [Analyst, Vol. 93 The whole process to take place as quickly as possible in subdued light. Saponijcation and extraction-Weigh 10.0 g of margarine into a 250-ml flat-bottomed flask. Add 20 mg of quinol, 60 ml of ethanol, 10 ml of potassium hydroxide solution and finally 10 ml of light petroleum. Boil under reflux for 30 minutes, protecting the flask from light. (Use flasks covered with a shield of aluminium foil.) Cool, and add 80 ml of distilled water. Transfer the solution quantitatively into a 500-ml separating funnel and rinse out the flask with a further 80 ml of distilled water. Add the rinsings to the separating funnel.Extract the unsaponified material with 100ml and three 50-ml portions of diethyl ether. Combine the ether extracts and wash with four 50 ml-portions of distilled water; carry out the first washing by swirling and the following three by gentle shaking. Evaporate the unsaponifiable extract to dryness on a water-bath at 50" C, with a stream of inert gas. The last stages of the evaporation require full attention, because the residue in the flask must not be allowed to remain dry longer than is absolutely necessary. Immediately after all of the diethyl ether has been removed, add 2 ml of absolute ethanol and again evaporate to dryness in a current of inert gas; if the residue appears wet, repeat the addition of absolute ethanol and evaporation to dryness.Immediately dissolve the residue in 5ml of light petroleum and again evaporate to dryness in a current of inert gas. Repeat the dissolution in light petroleum and evaporation to dryness twice more. Finally, dissolve the residue in 2 to 3 ml of light petroleum for chromatography. Chromatographic separatioiz of vitami% A-The chromatographic separation and deter- mination of vitamin A has been fully described elsewhere.2 The fraction containing carotenes is chromatographed again on magnesia to separate a- and /3-carotene. Chromatographic separation of cc- and /3-carotene-Evaporate the fraction containing carotene to dryness on a water-bath at 50" C, with a stream of inert gas. Dissolve the extract in 2 ml of light petroleum before chromatography. Place a pledget of cotton-wool in the lower tip of the upper chromatographic tube.Pour in light petroleum to a level half-way up the centre section, and add 3 g of magnesia. Add the cc- and /3-carotene extract and rinse the flask with 2 ml of light petroleum. Develop the chroniatogram, under pressure if necessary, with light petroleum containing diethyl ether. The precise conditions required to produce the separation must be determined by experiment, and vary with different batches of magnesia. The optimum mixture lies within the range 4 to 12 per cent. of diethyl ether in light petroleum and gives a complete separation of a- and p-carotene, in sharp bands. If difficulty is experienced with the flow-rate, and with adequate separation of a- and /3-carotene, re-dry the magnesia immediately before use (24 hours).Collect the lower, pale yellow-coloured zone of a-carotene. After the elution of a-carotene, collect the deeper orange-coloured zone of p-carotene. Use 50 per cent. of diethyl ether in light petroleum to speed up the elution of p-carotene. Evaporate the two zones separately to dryness on a water-bath at 50" C, with a stream of inert gas. Dissolve the a- and /!-carotene in cyclohexane and dilute to 10 and 25m1, respectively, with cyclohexane. Measure the optical densities of these solutions in 1-cm glass cells against cyclohexane, at 2-mp intervals, from 440 to 456mp, repeating those at the peak for confirmation. CALCULATIONS- optical density x volume of carotene solution . 10 x 100 EiZ = a-Carotene potency = Et& x 334. /3-Carotene potency = E:,%, x 667.Total carotene potency (i.u. per g) = (Et:'; value of /3-carotene solution + $ Ei:& value, of a-carotene solution) x 667. Total carotene potency x 0.8 = vitamin A equivalent (i.u. per g). This vitamin A equivalent must be added to the vitamin A figure, already determined, to give the total vitamin A potency of the margarine. COLLABORATIVE TEST Two margarines, in sufficient amount for the whole test, were prepared in a conventional margarine plant.TABLE I COMPARISON OF THE RECOVERIES OF TOTAL VITAMIN A FROM TWO MARGARINES BY THE STATUTORY AND PRESENT METHODS ON THREE OCCASIONS Total vitamin A (all laboratories) 7 A , Method of analysis Statutory { { Present 1 week - Mean Mean Sample Actual, recovery, value, Standard number i.u.per g per cent. i.u. per g deviation 1 30.4 81 24.7 1-07 2 32-0 86 27.4 1.69 1 30.4 91 27.6 1.04 2 32.0 88 28.1 1.81 7 Mean recovery, per cent. 77 83 88 88 7 weeks Mean value, Standard i.n. per g deviation 23.5 1.63 26-6 0.76 __h______\ 26.7 1.94 28.0 0.80 7 Mean recovery, per cent. 75 84 85 88 14 weeks --h-----? Mean value, Standard i.u. per g deviation 22.9 0.85 264 0.94 25.9 0.83 28.1 1 -08 TABLE I1 COMPARISON OF THE SEPARATE RECOVERIES OF VITAMIN A AND CAROTENE FROM TWO MARGARINES BY THE STATUTORY AND PRESENT METHODS ON THREE OCCASIONS Vitamin A and “carotene” (all laboratories) 1 week 7 weeks 14 weeks P - w - 7 c A I Mean Mean Mean Mean Mean Mean Method of Sample Actual, recovery, value, Standard recovery, value, Standard recovery, value, Standard analysis number i.u.per g per cent. i.u. per g deviation per cent. i.u. per g deviation per cent. i.u. per g deviation Statutory { Present { 1 24.0 89 21.4 1.32 86 20.6 1.72 84 20.2 0.86 30.0 86 26.9 1.46 84 26.2 0.69 86 26.6 0.99 1 24.0 91 21.8 0.95 90 21.6 1.77 88 21.0 0-61 30.0 88 26.6 1.76 89 26-6 0.89 89 26-7 1.03 Vitamin A added 1 6.4 63 3.4 0.25 47 3.0 0.16 42 2.7 0.10 2.0 70 1-4 0.17 70 1.4 0.08 66 1.3 0.12 1 6.4 89 5.7 0.37 78 6.0 0.32 75 4.8 0-37 2.0 80 1.6 0.29 70 1.4 0.13 70 1.4 0.2 1 Statutory { Present { or red palm oil as vitamin ,4110 USHER, FAVELL AND LAVERY Sample 1 contained 24.0 i.u. per g of vitamin A and synthetic /3-carotene equivalent Sample 2 contained 30.0 i.u. per g of vitamin A and red palm oil equivalent to 2.0 i.u. Samples were despatched from a central depot in time for each of the six participating laboratories to analyse the margarines after 1, '7 and 14 weeks at 15" C.At each of the three stages, single determinations were made on each margarine on each of 3 consecutive days by the two methods of analysis (statutory method and the method described above). Appropriate precautions were taken to avoid sampling errors, and every sample was analysed under code. to 6.4 i.u. per g of vitamin A. per g of vitamin A. RESULTS AND DISCUSSION The results of the collaborative test are shown in Tables I and 11. The recovery of added vitamin A is 90 per cent. by each method, with no difference in the precision. The recovery of pro-vitamin A from margarine containing red palm oil is 70 per cent. by each method. Recovery from margarine containing synthetic IS-carotene is 80 per cent. by the present method and less than 50 per cent. by the statutory method. During the collaborative test some participants had difficulty with the flow-rate through magnesia, and with adequate separation of 01- and /3-carotene. To overcome this problem a minor modification has been included in the method, These difficulties do not appear to have reduced significantly the precision of recovery of pro-vitamin A, compared with the recovery of vitamin A added as such (see Table 11). The authors acknowledge the help and advice given by Mr. G. Walley during the preparation of the method, and by Mr. K. T. Boyd and Mr. A. Rook on the design of the experiment and the analysis of results. They also thank the Margarine and Shortening Manufacturers Association for its help in making possible the collaboration of several of its member companies, and the Directors of Unilever Limited for permission to publish this paper. REFERENCES 1. 2. The Food Standards (Margarine) Order, 1964, S.I. 19541613. Analytical Methods Committee, Analyst, 1964, 89, 7. Received January 16th, 1967

ISSN:0003-2654    

DOI:10.1039/AN9689300107

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

ANaZyst, February, 1968, Vol. 93, j!@. 111-115 111 The Extraction and Determination of Disodium Octaborate in Sitka Spruce BY A. I. WILLIAMS (Forest Products Research Laboratory, Princes Risborough, Aylesbury. Bucks.) A method is described for the rapid determination of the boric acid equivalent of disodium octaborate tetrahydrate in Sitka spruce. The boron- containing compounds are leached from thin sections of wood with sodium hydroxide solution and determined by the spectrophotometric measurement of the red complex formed between boric acid and curcumin. The procedure is particularly useful for the study of the distribution of boron-containing preservatives in wood, THE current interest in the “Timbor” process for preserving timber against attack by wood- destroying fungi and insects has resulted in the need for a rapid, simple and accurate method for the determination of the boric acid equivalent of “Timbor” in treated wood.“Timbor” is the trade name of a water-borne preservative containing a crystallised mixture of boric acid and sodium borate that corresponds approximately to the formula Na2BS0,,.4H2O, disodium octaborate tetrahydrate. It is usual to express the disodium octaborate content as the boric acid equivalent. The “Timbor” process consists in immersing, or spraying, freshly converted green timber in disodium octaborate solution, then closely stacking and covering to prevent drymg, so that diffusion of the salt into the wet wood can take place over a period of time. To ensure that the treatment has been sufficient to meet the specification of the British Wood Preserving Association it is necessary to analyse the treated timber to determine the loading, distribution and penetration of preservative achieved during diffusion.There are available spray techniq~esl1~,~,~16,~ for the determination of loading and penetration, but they are often not accurate enough for research purposes involving diffusion and distribution experiments and weathering tests. Also there are methods based on ashing and leaching followed by titration,297 but although they have the required accuracy they are not practicable when large batches of specimens need to be examined, and cannot be used to determine the distribution of preservative over small areas. The problem posed requires for its solution, first, a simple quantitative procedure for separating the disodium octaborate from the matrix, and secondly, an accurate and precise method for determining 0.01 per cent. upwards of the boric acid equivalent.It was found that disodium octaborate tetrahydrate can be rapidly leached from thin sections of wood. Sitka spruce, Picea sitchertsis, a relatively impermeable species, was used for the experimental work. The low permeability of the wood did not cause any apparent difficulties during the leaching out of disodium octaborate when sections of the order of 200 to 300p in thickness were used. Determination of boron by the spectrophotometric measurement of the rosocyanins complex, formed between boric acid and curcumin, is a well known procedure and has been described by Hayes and Met~alfe.~ Therefore it was decided, by using these techniques, to evolve a method for the determination of the boric acid equivalent.EXPERIMENTAL To examine techniques for determining the boric acid equivalent of disodium octaborate in timber it was necessary to prepare standard samples containing a known amount of preservative. This was achieved by impregnating wood with solutions of known concen- trations of disodium octaborate (boric acid equivalent, 0.014 to 1-370 per cent. w/w) by the full cell process1* and freeze-drying.ll 0 SAC; Crown Copyright Reserved.112 WILLIAMS : EXTR.4CTION AND DETERMINATION [Analyst, VOl. 93 PREPARATION OF STANDARD SAMPLES- Weighed quarter-sawn blocks of Sitka spruce of known moisture content, with over-all dimensions of 3 x 2 x 1 cm, cross-section 3 x 1 cm, radial face 3 x 2 cm and tangential face 2 x 1 cm, were submerged in disodium octaborate solutions of known concentrations and subjected to a vacuum of 71 cm of mercury for 3 hours to evacuate the air from the wood cells.The vacuum was released, and a pressure of 7 kg per cm2 was applied to the solution containing the specimens for half an hour. The pressure was released, the specimens removed from the solution, their surfaces were superficially dried on filter-paper to remove excess of solution, and weighed. The wood absorbed about three times its own weight of solution. To prevent re-distribution of disodium octaborate the specimens were freeze-dried to a moisture content of about 7 per cent. From the observed figures the percentage of disodium octaborate, expressed as the boric acid equivalent based on oven-dry wood, was found by calculation to range from 0.042 to 4-16 per cent.The structure of the wood in the small blocks used for the experiment varies. In “Timborised” timber, concentration gradients of deposited disodium octaborate can occur across the annual rings; more will be found in the spring or early wood, as the void space is greater than in summer or late wood. Therefore, for development work on the procedure, it was decided to use thin cross-sections that are more representative of the preservative in the bulk of the wood. Also, for leaching purposes, the cross-section is far more permeable than the tangential or radial sections. Cross-sections up to 300 p thick, and weighing up to 60 mg, were cut on a microtome.Attempts were made to leach the disodium octaborate out of thin cross-sections of standard samples with cold curcumin - acetic acid reagent solution, but varying recoveries, usually less than 60 per cent. of the calculated preservative content, were observed. Warming did not improve the recovery. Thin sections were leached with 0.5 ml of M sodium hydroxide solution, which greatly improved the recovery, but to obtain complete removal of the di- sodium octaborate from the wood it was found necessary to warm the sodium hydroxide solution at 50” C for 10 minutes. After leaching, curcumin - acetic acid reagent solution was added, followed by a mixture of sulphuric acid and acetic acid to convert the disodium octaborate to boric acid and to dehydrate the solution, so that the reaction between boric acid and curcumin could take place. All the operations were carried out in soda-glass boiling tubes.The complex-forming reaction was complete within 15 minutes; the mixture was then diluted with a solution of acetone and water to a known volume and mixed. The thin sections of wood were left in the solution, which was decanted for spectrophotometric measurement. SPECTROPHOTOMETRIC MEASUREMENT- Measurements of optical densities of the test solutions were made at wavelength 666 m p by using a Unicam SP600 spectrophotometer with 1-cm cells. The spectrophotometer cali- bration graph was constructed in the range 0 to 72 pg of boric acid, and a straight-line relation- ship was obtained.RESULTS The procedure described was applied to thin cross-sections of the standard samples and the results are given in Table I. Three thin cross-sections, up to 300p thick, were taken from each specimen, one from the outside, the second from 0.6 cm in, and the third from the middle of the specimen. The results showed some small variation through the specimen, but the average of the three determinations was in agreement with the calculated over-all boric acid equivalent content. The standard deviation, based on nine determinations, was +O.OlS per cent. at the 0.4 per cent. boric acid level. These results show that the proposed procedure is quantitative and possesses the required speed and precision. As a result of the success of the procedure, thin radial sections up to 200 p thick were examined.The radial sections taken for analysis were narrow strips, so that a representative portion of both spring and summer wood was included in the sample. Three adjacent samples were taken from the surface, and three 06cm from the surface of the standard sample. The results were similar to those obtained for cross-sections. The average of each set of determinations was in agreement with the calculated boric acid equivalent content of the standard samples.February, 19681 OF DISODIUM OCTABORATE IN SITKA SPRUCE TABLE I RESULTS OBTAINED BY USING THIN CROSS-SECTIONS Calculated boric acid equivalent, per cent. found, per cent. Average Boric acid equivalent 4.16 4.08 4.06 3.80 4.30 2-14 2.03 1-29 1-20 0.71 0.76 0-41 0.38 0.19 0.1 8 0.085 0.081 0.044 0.040 2.1 I 2.27 2-16 1.28 1.23 1.24 0.73 0.72 0.73 0.40 0.40 0.40 0.1 8 0.1 6 0.18 0.084 0.076 0.081 0.042 0.041 0.042 113 Tangential sections, up to 200 p thick, of either spring or summer wood were analysed.As expected, the spring wood contained much more disodium octaborate than the summer wood. Therefore, unless a representative sample of both spring and summer wood is taken for analysis, inaccurate results will be obtained with thin tangential sections. It is difficult to sample tangential sections to include a representative amount of spring and summer wood because the gradual change of cell wall thickness cannot be accurately judged during sampling. The difficulty does not arise with cross-sections or radial sections where it is a simple operation to cut thin sections across several annual rings.TABLE I1 The results are given in Table 11. RESULTS OBTAINED BY USING THIS TANGENTIAL SECTIONS Standard Calculated boric acid Boric acid equivalent sample No. equivalent, per cent. found, per cent. Average 126 Spring wood 0.78 Summer wood 0.7s 130 Spring wood 0.35 Summer wood 0.38 0.9 1 1.01 1.10 0.6 1 0.64 0.67 0.4 1 0.4 1 0.4 1 0.25 0.26 0.24 To demonstrate the usefulness of the proposed method the distribution of the boric acid equivalent of disodium octaborate through a 2-inch (51 mm) thick piece of “Timberised” wood, which had been on a weathering test for 18 weeks during which the rainfall was 11+ inches (292 mm), was investigated. In the outer Q inch (3.2 mm) of opposite surfaces of the test piece, five determinations of the boric acid equivalent were made on radial sections at intervals of & inch (0.8 mm).Further determinations were made to complete the distribution pattern through the test piece. The curve obtained by plotting the boric acid equivalent against thickness is shown in Fig. 1. The eighteen determinations, including sample cutting, made to form the distribution curve were completed within 2 hours.114 WILLIAMS : EXTRACTION AND DETERMINATION [Autdyst, VOl. 93 It can be seen from Fig. 1 that it is possible to evaluate rapidly the distribution of boron- This is not possible by previously available containing preservatives over small areas. methods of analysis. I 1 1 I I I I I 0 a t d " t l t ' d Exposed side Inches Underside Fig.1. Distribution of disodium octaborate, expressed as the boric acid equivalent, in a weathered specimen METHOD APPARATUS- A Unicam SP600 spectrophotonaeter. Soda-glass boiling tubes, size 150 x 25 mm. Graduated fEasks, 100-ml capacity. Use high purity reagents when possible. Curcumin solution-Dissolve 0.12 g of curcumin in 100 ml of warm glacial acetic acid, SuZ~FLuric acid - acetic acid solution-Add 50 ml of sulphuric acid, slowly with cooling, Acetone - water solution-Dilute 250 ml of acetone to 500 ml with water. Sodium hydroxide solution, M-Dissolve 4 g of sodium hydroxide pellets in 50 ml of REAGENTS- cool, and store in a polythene bottle. to 50 ml of glacial acetic acid. Store in a stoppered soda-glass bottle. water and dilute to 100ml. Store in a polythene bottle. PROCEDURE- Weigh the sample and place in a soda-glass boiling tube, add 0.5 ml of M sodium hydroxide solution, then warm on a water-bath at 50" C for 10 minutes. Remove the boiling tube from the water-bath and add, from a burette, 3 ml of curcumin solution rinsing the boiling tube wall down with the reagent.Allow to stand for 5 minutes then add, from a burette, 3 ml of sulphuric acid - acetic acid solution, mix and allow to stand for 16 minutes. PourFebruary, 19681 OF DISODIUM OCTABORATE IN SITKA SPRUCE 115 the reaction mixture into a 100-ml graduated flask containing 50ml of acetone- water solution and swirl to mix. Wash the reaction mixture remaining in the boiling tube into the graduated flask with acetone - water solution. Dilute the solution to the mark with the acetone - water solution and mix.Measure the optical density of the red rosocyanin complex against a reagent blank, prepared in a similar way, in l-cm cells at 555 mp on a Unicam SP600 spectrophotometer. To obtain the boric acid content in the test solution compare the spectrophotometer reading with a calibration graph. CALIBRATION PREPARATION OF STANDARD SOLUTION A- Dissolve 2.3140 g of freshly recrystallised sodium borate, Na,B,O,.lOH,O, in water. Transfer the solution to a 250-ml graduated flask, dilute to the mark with water and mix. Immediately store in a polythene bottle. 1-0 ml of standard solution A = 6000 pg of boric acid. PREPARATION OF STANDARD SOLUTION B- the mark with 1.1 M sodium hydroxide solution and mix. bottle. PREPARATION OF STANDARD SOLUTION C- to the mark with M sodium hydroxide solution and mix.bottle. Transfer by pipette, 10 ml of standard solution A into a 100-ml graduated flask, dilute to Immediately store in a polythene 1-0 ml of standard solution B = 600 pg of boric acid. Transfer by pipette, 10 ml of standard solution B into a 100-ml graduated flask, dilute Immediately store in a polythene 1.0 ml of standard solution C = 60 pg of boric acid. Transfer aliquots of 0.1, 0.2, 0-3, 0.4 and 0.5ml of standard solution C to soda-glass boiling tubes and add 0.4, 0.3, 0.2, 0.1 and O-Ornl, respectively, of M sodium hydroxide solution. Transfer aliquots of 0.1 ml of standard solution B and 0.1 ml of standard solution B plus 0-2 ml of standard solution C to boiling tubes and add 0.4 and 0.2 ml, respectively, of M sodium hydroxide solution. The aliquots taken contain 6, 12, 18, 24, 30, 60 and 72 pg of boric acid in 0-5 ml of M sodium hydroxide solution. Continue as described under Pro- cedure. Plot optical densities against micrograms of boric acid to obtain the calibration graph. This paper is published by permission of the Ministry of Technology. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. REFERENCES McMullen, 31. J., Analyst, 1953, 78, 442. Wilson, W. J., Analytica Chinz. Acta, 1958, 19, 516. Cockcroft, R., Holzforschung, 1960, 14, 117. Cummins, N. H. O., “Proceedings of the 6th Annual General Meeting of the New Zealand Wood British Wood Preserving Association, Provisional Standard 105, 12. British Wood Preserving Association, Provisional Standard 105, 14. Spicer, G. S., and Strickland, J. D., J. Chem. Soc., 1952, 4644. Hayes, M. R., and Metcalfe, J., Analyst, 1962, 87, 1041. CartFight, K. St. G., and Findlay, W. P. K., “Decay of Timber and its Prevention,’’ Second Edition, H.M. Stationery Office, London, 1958, p. 286. Smith, D. N. R., and Cockcroft, R., Nature, 1961, 189, 163. Received August 15fh, 1967 -, Ibid., 1959, 21, 2. Preservers’ Association” 1965, p. 39.

ISSN:0003-2654    

DOI:10.1039/AN9689300111

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

116 Analyst, February, 1968, Vol. 93, $p. 116-117 A Critical Evaluation of the A.O.A.C. Method for the Determination of Milk Protein in Milk Chocolate when Applied to Crumb Chocolate BY R. J. MOT2 (Mars Limited, Confectionery Dicision, Dundee Road, Slough, Bucks.) Comparatively small differences in manufacturing temperatures of chocolate crumb may lead to unreiiable results when using the A.O.A.C. method for the determination of milk protein in crumb chocolate. A COLLABORATIVE study1 was recently carried out by the members of the Technical Com- mittee of the Office International du Cacao et du Chocolat on the determination of milk protein in milk chocolate, by using the A.O.A.C. methodJe in which the amount of protein precipitated from an oxalate solution with tannic acid is measured.The results obtained by the committee cast doubts on the suitability of that method for milk chocolate made by the “Crumb Process,” which is used for producing most of the chocolate in the U.K. and, to an increasing extent, on the European Continent. This crumb process differs from that used by American manufacturers in that it subjects the wet “crumb paste” to elevated temperatures, usually under reduced pressure. EXPERIMENTAL Crumb batches were prepared, one set, A, by heating portions for different lengths of time at 95°C and the other set, B, by heating for the same length of time at different temperatures. TABLE I RECOVERY OF NITROGEN FROM EXPERIMENTAL CRUMB A, HEATED FOR THE INDICATED TIME AT 95” c Heating time, hours 0 1 2 4 6 Protein-nitrogen by A.O.A.C.method, per cent. 1-25 1-26 1-25 1.20 1.11 Total nitrogen, per cent. . . .. . . 1.71 1.71 1-71 1.70 1-72 TABLE I1 RECOVERY OF NITROGEN FROM EXPERIMENTAL CRUMB B, HEATED FOR 2 HOURS, AT THE INDICATED TEMPERATURES Not Approximate heating temperatures heated 104OC 122OC 13OOC 14OOC Protein-nitrogen by A.O.A.C. method, per cent. 1.23 1.20 0.43 0.16 0.12 Total nitrogen, per cent. . . . . . . 1.89 1-66 1-66 1.65 1.55 Total fat, per cent. . . . . . . . . 15.8 15-7 16.9 15.9 16.9 Lactose, per cent. . . . . . . . . 11.9 11.4 11.1 11.0 10-8 Recovery of fat and lactose The samples were analysed for total nitrogen and protein-nitrogen before, and after, drying, according to the A.O.A.C. method. The results are listed in Tables I and 11. 0 SAC; Mars Ltd., Copyright Reserved.February, 19681 MOT2 117 DISCUSSION As seen in Fig.1, a slight but significant loss of protein-nitrogen resulted from increasing the heating time while maintaining the temperature at about 95”C, whereas increasing the dry temperature led to the loss of a considerable proportion of protein-nitrogen. Because the loss of total nitrogen under the various heating conditions was comparatively small, it is concluded that heating reduced the solubility of protein in sodium oxalate solution. As the conditions of crumb making vary between chocolate manufacturers, the results obtained by the A.O.A.C. method for milk protein in their products will also vary. Crumb drying temperature, “C Fig. 1. Relationship between crumb-drying temperature and resulting losses of protein- nitrogen (A.O.A.C.- nitrogen), lactose and total nitrogen based on the composition of the same crumb dried in wucuo at room temperature over phosphorus pentoxide : A, protein-nitrogen; B, lactose; and C, total nitrogen The use of the A.O.A.C. method would therefore appear to be unsatisfactory for charac- terising accurately the milk content of a large proportion of chocolate in the absence of detailed information on the manufacturing process used. However, losses of lactose are probably not more than 5 per cent. under the normal range of conditions of crumb making, and the lactose content of a crumb chocolate may form, together with other results, a more reliable basis of determination of milk content. REFERENCES 1. 2. Int. Choc. Rev., to be published. Horwitz, W., Editor, “Official Methods of Analysis of the Association of Official Agricultural Chemists,” Tenth Edition, Association of Agricultural Chemists, Washington, D.C., 1966, p. 187. Received August 18th, 1967

ISSN:0003-2654    

DOI:10.1039/AN9689300116

出版商:RSC    

年代:1968

数据来源: RSC