ISSN:0003-2654    

DOI:10.1039/AN97499FX001

出版商:RSC    

年代:1974

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97499BX003

出版商:RSC    

年代:1974

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97499BP007

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

12 ArtaZyst, January, 1974, Vol. 99, $$. 12-18 A Colorimetric Method for the Determination of Phenacetin and Paracetamok Part 111." Studies of the Indophenol Reaction: an Alternative Manual Procedure for the Determination of Phenacetin and Paracetamol and its Application to the Determination of Other Pharmaceuticals, including Sulphonamides, Procaine and Related Compounds BY D. R. DAVIS, A. G. FOGG AND D. THORBURN BURNS AND J. S. WRAGG (Chemistry Department, University of Technology, Loughborough, Leicestershire, LE11 3T U ) ( A nalytical Research, Quality Control, The Boots Company Ltd., Pennyfoot Street, Nottingham) ,4n alternative manual procedure is described for the colorimetric deter- mination of phenacetin and paracetamol as indophenol dyes. The procedure differs from that described in Part I1 in that phenacetin and paracetamol are hydrolysed first to p-phenetidine and p-aminophenol, respectively, before reac- tion with hypochlorite to form p-quinonechlorimide, which then undergoes a reaction with phenol.By using solid p-quinonechlorimide as starting material, the molar absorptivity of the indophenol dye formed, a t its wavelength of maximum absorption (625 nm), has been shown to be 2.85 x lo4 1 mol-1 cm-1. With the introduction of the hydrolysis step, complete reaction of phenacetin and paracetamol is attained, which is expedient in any colori- metric procedure. Furthermore, the pH a t which oxidation with hypo- chlorite is effected is not so critical as in the procedure described in Part 11. The main disadvantage of the procedure compared with that in Part I1 is the increased analysis time.The potentialities of the indophenol reaction as a method of determining other pharmaceutical compounds have been investigated. The identification of the products of reaction of the hydrolysed compounds with hypochlorite has not been attempted but the apparent molar absorptivities ( 1041mol-lcm-l) a t 625 nm of the indophenol dye formed in each instance are as follows: p-aminobenzoic acid, procaine and benzocaine, 1.65 ; aniline and acetanilide, 2.11 ; sulphanilic acid, 1.01 ; sulphaguanidine, 1-07 ; sulphathiazole, 1.00; sulphanilamide, 0.49; folic acid, 0.76; o-aminobenzoic acid, 0.47; and ametho- caine, 0.29. AUTOMATIC^ and manual2 colorimetric procedures for the determination of paracetamol and phenacetin based on the indophenol reaction were described in Parts I and I1 of this series.The compound to be determined is made to react with acidified hypochlorite solution to form a quinonechlorimide, excess of hypochlorite is reduced with arsenic(II1) and the quinone- chlorimide is made to react with phenol to form an indophenol dye. The apparent molar absorptivities at the wavelength of maximum absorption (625 nm) of the indophenol dyes formed from phenacetin and paracetamol were different, being 1.79 x lo4 and 2.26 x 104 1 mol-1 cm-1, respectively.2 This difference could be due either to the formation of different indophenol dyes or to the incomplete reaction of one or both drugs. The present study was made in order to try to distinguish between these alternatives, and to extend the method to the determination of other pharmaceutical compounds. EXPERIMENTAL Absorbance measurements were made with a Uvispek 700 spectrophotometer in 1-cm Wavelength calibration was carried out with a didymium glass filter and the absorbance cells.scale was checked by means of standard neutral filters. * For details of Part I1 of this series, see reference list, p. 18. @ SAC and the authors.DAVIS, FOGG, THORBURN BURNS AND WRAGG 13 PREPARATION OF QUINONECHLORIMIDE INTERMEDIATES- p-Quinonechlorimide was prepared by the method of Gibbs.2 $-Aminophenol (5 g) was dissolved in 10 ml of concentrated hydrochloric acid P l ~ s 50 ml of water and 50 g of crushed ice was added. The cold mixture was then added slowly to 100 ml of sodium hypo- chlorite solution containing 12 to 14 per cent.of available chlorine and which also contained crushed ice. The addition of excess of $-aminophenol, indicated by the appearance of a permanent blue or dark brown colour, was avoided. 9-Quinonechlorimide separated as a yellow solid, which was washed on a filter with cold distilled water until free from the odour of chlorine. After being partially dried in air, the crystals were completely dried at 40 "C in an oven. Recrystallisation from light petroleum (boiling range 40 to 60 "C) gave yellow needles in 95 per cent. yield (m.p. 84 to 85 "C). Paracetamol and phenacetin (3 g) were refluxed separately with 100 ml of concentrated hydrochloric acid for 40 minutes. On adding the cooled solution to 150 ml of sodium hypo- chlorite solution containing crushed ice, a yellow crystalline product was obtained in each instance. The products were identified as 9-quinonechlorimide from their melting-points and infrared spectra.P-Quinonechlorimide is the expected product from paracetamol, as the latter compound is hydrolysed to @-aminophenol. 9-Phenetidine, formed by hydrolysis of phenacetin, clearly loses its ethoxy group on oxidation with hypochlorite. FORMATION OF INDOPHENOL DYE FROM 9-QUINONECHLORIMIDE- The formation of indophenol dye from 9-quinonechlorimide under the conditions of the colorimetric determination was studied. $-Quinonechlorimide (100 mg) was dissolved in 25 ml of ethanol and the resulting solution was diluted to 100 ml with water in a calibrated flask. A dilute standard solution was then prepared by a fifty-fold dilution of this solution with water.A 10-ml amount of the dilute standard solution plus 2 ml of 6 per cent. m/V phenol solution were placed in a calibrated flask and diluted to 50 ml with borate buffer (pH 10.0). The solution developed a green tinge owing to the formation of a second reaction product (Amax. = 394 nm) in addition to the indophenol dye (AmaL = 625nm). After the colour had been allowed to develop for 1 hour, the apparent molar absorptivity based on the absorbance at 625 nm was 1.62 x lo4 1 mol-l cm-1 and that at 394 nm was 1-05 x lo4 1 mol-l cm-l. When the solution was heated in a water-bath, the rate of colour development increased and the height of the peak at 625 nm was increased with a concomitant decrease in that of the peak at 394 nm (see Table I).The results of a study of the effect of the pH of the solution on colour development at room temperature are given in Table 11. Over the pH range 9.2 to 9.6, high apparent molar absorptivities were attained; a higher proportion of the indophenol colour compared with that of the second reaction product was formed at the lower pH values within this range. Absorbance measurements were made on this solution. The reaction was very slow. TABLE I fi-QUINONECHLORIMIDE (UNCATALYSED REACTION) EFFECT OF TEMPERATURE ON THE FORMATION OF INDOPHENOL DYE FROM Apparent molar absorptivity after 30 minutes/l mol-1 cm-l x 10-4 Temperature of A -l heating/'C 394 nm 625 nm 50 0.66 2.04 100 0.43 2-25 Several catalysts have been suggested for indophenol reaction^.^-^ We found mangan- ese(I1) to be effective, but the peak at 394 nm persisted and a cloudiness was formed owing to precipitation of manganese(I1) hydroxide.Ethylenediaminetetraacetic acid and acetone were both found to be ineffective under the experimental conditions used in this work. The rate of colour development in the procedure described in Part 112 was rapid compared with the results obtained so far in this work. The only reagent missing from the solutions used in this work was arsenic(II1) , which had been used in the earlier work to reduce excess of hypochlorite in the recommended procedure; arsenic(II1) had been omitted so far in this14 DAVIS et al. : A COLORIMETRIC METHOD FOR THE TABLE I1 [Analyst, Vol.99 EFFECT OF PH ON THE FORMATION OF INDOPHENOL DYE FROM 9-QUINONECHLORIMIDE AT ROOM TEMPERATURE (UNCATALYSED REACTION) Apparent molar absorptivity/l mol-1 cm-l x 10-4 r h > After 2 hours After 24 hours After 3 days --- PH 394nm 625 nm 394 nm 625 nm 394 nm 625 nm 11-74 0-74 0-44 0.67 0.22 0.56 0.05 9-62 1.00 1-69 1.02 1.70 - - 9-55 0.90 1-77 0.90 1.94 - - 9.44 0-78 1.95 0.79 1.94 - - 9.24 0.37 1.55 0.56 2.08 - - 9-58 0.00 0.08 0.18 0.57 0.22 0.95 work as no hypochlorite had been added. However, on studying the effect of arsenic(II1) on the rate of formation of indophenol dye, it was found to be an extremely efficient catalyst and that no peak was obtained at 394 nm when it was used. The effect of pH on the formation of indophenol dye at room temperature when 2 ml of 4 per cent.sodium arsenite solution was added to the solution described earlier is shown in Table 111. In the pH range 9.8 to 10.4, a constant apparent molar absorptivity a t 625 nm of 2.85 x lo4 1 mol-l cm-l was obtained after 30 minutes. This value was the highest apparent molar absorptivity obtained, and, for the purposes of this paper, has been assumed to be the true value of the molar absorptivity a t this wavelength of the indophenol dye formed. The coefficient of variation calculated from results of absorbance measurements on ten solutions at pH 10 was less than 1 per cent. TABLE I11 EFFECT OF PH ON THE FORMATION OF INDOPHENOL DYE FROM $-QUINONECHLORIMIDE AT ROOM TEMPERATURE [ARSENIC(III)-CATALYSED REACTION] Apparent molar absorptivity a t 625 nm/l mol-l cm-1 x 10-4 A r > PH After 7 minutes After 20 minutes After 30 minutes 8.8 0.64 1.21 1.49 9.16 1.45 2.44 2-67 9-50 1.74 2.52 2-68 9-83 2.30 2-82 2-85 9.96 2-52 2.83 2.85 10.46 2.64 2.83 2-85 REACTION OF PARACETAMOL AND PHENACETIN WITH ACIDIC HYPOCHLORITE- 9-Quinonechlorimide is the product of the reaction of 9-aminophenol and P-phenetidine with hypochlorite.This reaction does not prove conclusively, however, that $-quinone- chlorimide is the product of the reaction of paracetamol and phenacetin with acidified hypo- chlorite solution, as the possibility of introducing a chlorine atom into the benzene ring cannot be ruled out. The following ultraviolet study of the reaction with acidified hypochlorite was carried out in order to obtain further information on this aspect.The wavelength of maximum absorption of p-quinonechlorimide in aqueous solution a t pH 3.4 in the ultraviolet region is 287 nm and its molar absorptivity at this wavelength was found to be 2.13 x lo4 1 mol-1 cm-l. Solutions of paracetamol and phenacetin treated with acidified hypochlorite solution also developed a peak at 287 nm. At this wavelength, the molar absorptivity of both paracetamol and phenacetin is only 0.12 x l o 4 1 mol-1 cm-l. It was possible, therefore, to determine by ultraviolet spectrophoto- metry the amount of 9-quinonechlorimide formed (assuming this to be the product) during the reaction of hypochlorite with paracetamol and with phenacetin, with a negligible error due to absorption by the unreacted paracetamol and phenacetin (see Table IV).The procedure for the determination of paracetamol and phenacetin described in Part I1 was followed until just before the addition of arsenic(II1) to reduce excess of hypochlorite. These solutions were then made up to volume and absorbance measurements were made at 287 nm. The results obtained are given in Table IV, and are compared with the resultsJanuary, 19741 DETERMINATION OF PHENACETIN AND PARACETAMOL. PART 111 15 obtained using the full indophenol reaction as described in Part 11. The extents of reaction as determined by the absorption of p-quinonechlorimide at 287 nm and of the indophenol dye at 625 nm are in very good agreement and, although not conclusive, these results strongly suggest that the low apparent molar absorptivities obtained by using the procedure described in Part I1 are due to the incomplete hydrolysis of paracetamol and phenacetin, and that $-quinonechlorimide is the only product of the reaction of these compounds with hypochlorite.TABLE IV EXTENT OF REACTION OF HYPOCHLORITE WITH PARACETAMOL AND PHENACETIN Formation of p-quinonechlorimide A I 3 Formation of Apparent molar Extent of reaction, indophenol dye absorptivity a t per cent. 287 nm/l mol-l r-A-, Extent of reaction, - per cent. Compound cm-l x lo-* A* Bt p-Quinonechlorimide 2-13:: Paracetamol 1.71 Phenacetin 1.47 - 100 80 79 a2 69 67 65 - * Uncorrected for absorption by unreacted paracetamol or phenacetin. Corrected for absorption by unreacted paracetamol or phenacetin. True molar absorptivity. FORMATION OF INDOPHENOL DYE FROM $-AMINOPHENOL, AND, AFTER HYDROLYSIS, FROM PARACETAMOL AND PHENACETIN- The full procedure described in Part 11, vix., treatment with hypochlorite, removal of excess of hypochlorite with arsenic(II1) and reaction with phenol at pH 10, was carried out on $-aminophenol.As no hydrolysis reaction took place in this instance, it was expected that complete formation of the indophenol dye would be achieved and, indeed the apparent molar absorptivity at 625 nm was found to be 2.72 x lo4 1 mol-l cm-l. This value is about 3 per cent. lower than that obtained when starting from 9-quinonechlorimide but the coefficient of variation was again less than 1 per cent. Studies were next made of the time required for the complete hydrolysis of paracetamol and phenacetin by using concentrated hydrochloric acid.The hydrolysis procedure and subsequent formation and measurement of indophenol were carried out as described in the recommended procedure below. The results are shown in Table V; a hydrolysis time of 20 to 25 minutes was sufficient to obtain a molar absorptivity of 2.76 x lo4 1 mol-1 cm-1, i.e., complete reaction. TABLE V EFFECT OF TIME OF HYDROLYSIS OF PARACETAMOL AND PHENACETIN WITH CONCENTRATED HYDROCHLORIC ACID ON FORMATION OF INDOPHENOL DYE Compound Paracetamol . . Phenacetin Reflux time/ minutes 5 10 12 15 20 3 5 10 12 15 20 Apparent molar absorptivity a t 625 nm/l mol-l cm-1 2-51 2.69 2.78 2.76 2.76 2.10 2-25 2-61 2.69 2.75 2.76 x 10-4 In the procedure described in Part 11, the pH of the hypochlorite solution is highly critical (see Fig.4 in Part 11), which is to be expected if hydrolysis has to precede the reaction of paracetamol and phenacetin with hypochlorite. When the paracetamol and phenacetin16 DAVIS et al.: A COLORIMETRIC METHOD FOR THE [Analyst, Vol. 99 are completely hydrolysed before the reaction with hypochlorite, reaction with hypochlorite is complete provided that the pH of the hypochlorite solution is less than 6. This result is shown in Fig. 1, which also shows a titration curve for the acidification of a commercial sodium hypochlorite solution. 06 L i 6.0 2 3 4 5 6 7 8 9 10 11 pH of sodium hypochlorite - hydrochloric acid mixture Fig. 1. Curve A (left-hand axis) : effect of pH of hypochlorite solution on the completeness of the reaction of hypochlorite with hydrolysed paracetamol and phenacetin, as measured by the absorbance of the final indophenol dye solution.Curve B (right-hand axis) : typical curve for the titration of alkaline hypochlorite solution with 0.1 M hydrochloric acid Reaction times of 5 minutes with the hypochlorite and 10 minutes with arsenic(II1) In the present work, a reaction time of 1 minute was found were recommended in Part 11. to be sufficient in both instances. REAGENTS- Sodium hypochlorite solution containing 0.4 per cent. m/V of available chlorine-Prepare by dilution of a commercial sample containing about 16 per cent. m/V of available chlorine. Borate bzc$er solution (PH about 9-9)-Dissolve 20 g of boric acid, 24 g of potassium chloride and 11 g of sodium hydroxide in 2 litres of water.Hydrochloric acid, concentrated and 0.1 M. Sodium arsenite solution, 4 per cent. m/V. Phenol solution, 6 per cent. m/V. PROCEDURE- Pre-determine the amount of 0.1 M hydrochloric acid required to adjust the pH of 5 ml of the sodium hypochlorite solution to about pH 3.5 (the pH of the solution containing hypo- chlorite and hydrolysed sample must be less than 6). Transfer an accurately weighed amount of sample, containing up to 500 mg of paraceta- mol or 550 mg of phenacetin, into a flat-bottomed 50-ml conical flask fitted with a reflux condenser, add 10 ml of concentrated hydrochloric acid and reflux the mixture for 20 to 25 minutes on a hot-plate. Cool the solution, add 25 ml of ethanol, transfer the mixture into a 100-ml calibrated flask and dilute to volume with water.Transfer 25 ml of this solution by pipette into a 250-ml calibrated flask and dilute to volume with water. Transfer 10 ml of this solution by pipette into a second 250-ml calibrated flask and dilute to volume with water. To a 50-ml calibrated flask, add 5 ml of sodium hypochlorite solution and a pre-determined amount of 0.1 M hydrochloric acid so that the pH lies within the range 3 to 5. Add 10 ml of the diluted sample solution from a pipette held just above the surface of the hypochlorite solution, then mix and allow the solution to stand for 1 minute. Add 2 ml of sodium arsenite solution, METHODJanuary, 19741 DETERMINATION OF PHENACETIN AND PARACETAMOL. PART 111 17 mix and allow the solution to stand for 1 minute. Add 2 ml of phenol solution, dilute the solution to 50ml with borate buffer solution, then mix and allow the solution to stand for 30 minutes.Rectilinear calibration graphs are obtained when pure samples of paracetamol and phena- cetin are used. The coefficients of variation (ten determinations) obtained when this pro- cedure was applied to 300 mg of pure paracetamol and to 300 mg of pure phenacetin were 0.5 and 0.7 per cent., respectively. The molar absorptivity in both instances was 2-78 x lo4 1 mol-1 cm-l. The procedure was tested for the determination of paracetamol in an aspirin - paraceta- mol tablet mix previously analysed in this 1aborato1-y.~ The paracetamol content determined by the present method was found to be 42.5 per cent. m/m with a coefficient of variation of 0.5 per cent.(six determinations), which agrees well with the previous results (e.g., 43.5 per cent. m/m by a direct ultraviolet absorption procedure). APPLICATION OF THE INDOPHENOL METHOD TO OTHER COMPOUNDS Measure the absorbance of the solution at 625 nm against water in l-cm cells. Many other compounds are known to react with hypochlorite and to give indophenol dyes, and a brief study was made of some of these reactions. The procedures used were ana- logous to that described above, and the results obtained are shown in Table VI. Procaine and benzocaine, which are hydrolysed to $-aminobenzoic acid before reaction with hypo- chlorite, naturally give the same indophenol dye as $-aminobenzoic acid. Similarly, the reaction is equally sensitive for both acetanilide and aniline. TABLE VI APPARENT MOLAR ABSORPTIVITIES AT 725 nm OF INDOPHENOL DYES FORMED FROM OTHER PHARMACEUTICAL COMPOUNDS Hydrolysis time/ Compound minutes p-Aminobenzoic acid 0 10 Procaine .. .. 15" Benzocaine 10* Aniline .. .. Acetanilide . . .. Sulphanilic acid Sulphanilamide . . Sulphaguanidine Sulphathiazole Sulphadiazine Sulphadimidine Folic acid o-Aminobenzoic acid Amethocainet .. 0 25* 0 25 0 10 0 30 0 30 0 30 0 30 60 0 0 10" Apparent molar absorptivity/ 1.73 1.65 1.65 1.65 2.11 2.1 1 1-03 1.01 0.52 0.48 1.07 1.05 0.93 1.00 0.37 0.94 -0.2 1 0-75 0.76 0.47 0.12 0.29 1 niol-l cm-l x * Apparent molar absorptivity effectively constant at and above these hydrolysis times. t A yellow colloid that was formed on adding hypochlorite was redissolved by adding ethanol. DISCUSSION In the alternative manual colorimetric procedure for the determination of paracetamol and phenacetin described in this paper, complete formation of the indophenol dye appears to be effected, and the procedure is precise and reliable.It is not certain whether the indo- phenol dye formed is the parent indophenol or one of its derivatives, e.g., a chloro derivative. Corbett* gives the wavelength of maximum absorbance of the parent indophenol anion as 637 nm with a molar absorptivity of 3.16 x lo4 1 mol-l cm-l, which differs considerably18 DAVIS, FOGG, THORBURN BURNS AND WRAGG from the results obtained in the present work (625 nm, 2-85 x lo4 1 mol-l cm-l). On the other hand, Corbett gives the molar absorptivity of the 2-chloroindophenol anion as 2-82 x lo4 1 mol-l cm-l at the wavelength of maximum absorption (630 nm).When paracetamol and phenacetin are made to react with acidified hypochlorite solution according to the manual colorimetric procedure described in Part 11, a peak appears in the ultraviolet absorption spectrum of the resulting solutions at the same wavelength as that for +-quinonechlorimide (287 nm) . When these solutions undergo a further reaction with phenol, the ultraviolet absorption spectrum of the solution formed has a peak at the same wavelength (625nm) as that of the indophenol dye formed in the procedure described in this paper. The apparent molar absorptivities at both 287nm and 625 nm are consistent with an 82 per cent. reaction of paracetamol and a 65 per cent. reaction of phenacetin. This result is not conclusive proof, however, that the same indophenol dye is formed in this reaction, and that the lower apparent molar absorptivities obtained are caused by incomplete reaction.The same result would be obtained by the complete formation of other substituted quinone- chlorimides and indophenols with molar absorptivities in the same ratio as p-quinonechlori- mide and the indophenol dye formed here, and with the same peak wavelengths. Although this seems improbable, nevertheless, Corbett8 has shown that the spectra of substituted 9- quinonemonoimines and indophenols do not differ greatly from those of the parent compounds. Despite the greater uncertainty concerning the reactions that take place, the manual pro- cedure described in Part I1 has the major advantage over the present method that the reaction with acidified hypochlorite solution takes place in the cold and no previous refluxing with acid is necessary in order to hydrolyse the paracetamol and phenacetin. This advantage makes the procedure more convenient for routine use and it is especially suitable for use in an automated procedure, such as that described in Part I. Information on sensitivities is given with regard to the determination of other pharma- ceutical compounds, including procaine and sulphonamides, by using procedures similar to that described for the determination of paracetamol and phenacetin. Preliminary results with these reactions have been highly reproducible. The reactions could form the basis of manual procedures as indicated here, or of automated procedures similar to that described in Part I. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. NOTE-References 1 and 2 are to Parts I and I1 of this series, respectively. Murfin, J. W., Analyst, 1972, 97, 663. Murfin, J. W., and Wragg, J. S., Ibid., 1972, 97, 670. Gibbs, H. D., J . Bid. Chem., 1927, 72, 649. Russell, J. A., Ibid., 1944, 156, 457. Crowther, A. B., and Large, R. S., Analyst, 1956, 81, 64. Tetlow, J. A., and Wilson, A. L., Ibid., 1964, 89, 453. Fogg, A. G., Sausins, P. J., and Smithson. J. R., Analytica Chim. Acta, 1970, 49, 342. Corbett, J. F., J . Chem. SOC., B, 1970, 1502. Received May 31st, 1973 Accepted August 14th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900012

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Analyst, January, 1974, Vol. 99, $9. 19-25 19 Methods for Identification and Assay of Virginiamycin in Animal Feeds BY B. BOON AND R. DEWART (Recherche et Industrie Thhafleutiques, B-1320 Genval, Belgium) Methods for identification and assay of virginiamycin in feeds are pro- posed. The specific identity test is based on paper chromatography followed by bioautography with Corynebacterium xerosis (NCTC 9755). The assay is carried out by a paper disc agar-diffusion method with the same organism as the test organism. Some specific requirements for the assay of virginia- mycin are discussed, with particular reference to the marked synergism between its components, the M and S factors. VIRGINIAMYCIN*, an antibiotic substance currently used in many countries as an additive for animal feedingstuffs, is a mixture of two main active compounds, the M and S factors.Their structure and nomenclature, as well as those of some impurities of related structure, have recently been reviewed.1 A colorimetric assay method has been described for the M component2 whereas spectrofluorimetric techniques were applied for the S component deter- minati~n.~ The marked synergism between these factors has already been ~bserved.~,~ It is a property common to all antibiotics of the streptogramin group6 and the knowledge of this synergism is of fundamental interest to the analyst who has to define their biological activity and the correct conditions for their microbiological assay. The experimental part of this paper gives the synergistic graphs of M and S factors observed in vitro with some organisms of analytical interest.The methods for identification and assay of virginiamycin in feeds are described and their effectiveness is discussed. SYNERGISTIC GRAPHS- Pure M and S factors were mixed in various proportions and assayed by using Sta@zylo- coccus aweus 6538 P, Sarcina lutea ATCC 9341 and Coyynebacterium xerosis NCTC 9755 for their apparent potency, with the virginiamycin standard as reference. The assay method consisted in a one-dose procedure, by a classical cylindrical plate agar- diffusion method with a separate standard graph, as described by Grove and Randall.’ M and S factors were separately weighed and dissolved in methanol in order to prepare two stock solutions, each at a concentration of 1.0 mg ml-l. Their mixtures in various proportions were diluted with distilled water to obtain similar inhibition zones for the reference concentra- tions of virginiamycin and the solutions of M and S factors.The inocula and plates were prepared according to the procedure described below in Assay procedure. Standard solutions were prepared from the stock solutions by use of distilled water as diluent, with reference concentrations of 2-0 virginiamycin units per millilitre (u ml-1) for Corynebacterium xerosis and Sarcina lutea and 10.0 units per millilitre for Staphylo- coccus aureus. The synergistic graphs were obtained by plotting the potency of each combination versus the M and S factor ratio (Fig. 1). A high degree of synergism was observed with each test organism over a wide range of combinations; the potency of each preparation results mainly from its relative content of M and S factors.The optimum proportions of these factors are approximately the same, and their activity identical, for the organisms tested in this study. However, in spite of a similar pattern, the synergistic graphs show slight differences that are of importance for the analyst. The preparations with a less than optimum S-factor content (left-hand part of the graph) are less active (by about 10 per cent.) against Staphylo- coccus aureus than against Sarcina lutea, whereas with Corynebacterium xerosis they give the same response. The situation is reversed with preparations that have an S content higher than the optimum ratio; in that event, Sarcina Zutea and Stafihylococcus aureus give the same response and the activity against Corynebacterium xerosis is slightly lower.EXPERIMENTAL @ SAC and the authors. * Formerly known by its proprietary name, Staphylomycin.20 BOON AND DEWART: METHODS FOR IDENTIFICATION AND ASSAY OF [Analyst, Vol. 99 2500 2000 - I F -? 1500t t. a + Q m 0) - .- $ 1000 .- a 4 500 11 I I 1 I 1 80 60 40 20 M factor, per cent. 0 20 40 60 80 100 S factor, per cent. Fig. 1. Synergy between M and S factors for virginiamycin: ~ , Sta@y- lococcus aureus 6538P; ----, Sarcina lutea ATCC 9341; and ------, Corynebacterium xerosis NCTC 9755 The potency of virginiamycin is defined by its activity against Staehylococcus aureus. One of the first industrial production batches, with the best attainable purity at that time, was selected to constitute the first virginiamycin standard with an activity arbitrarily fixed at 1000.0 units per milligram.Since then, the production procedure has been improved so that the virginiamycin has an activity far exceeding the 1000.0 units per milligram (u rng-l) of the first standard, owing partly to improved purity and partly to a more favourable M to S ratio. The virginiamycin composition is now very consistent, corresponding to slightly higher M to S ratios than the optimum. The synergistic graphs of Staphylococcus aurcus and Corynebacterium xerosis are superimposed a t this part of the graph, so that both organisms are equally suitable as test organisms for assaying virginiamycin. Corynebacterium xerosis will often be preferred for testing biological fluids, finished feeds or other materials with a low virginiamycin concentration, owing to its much higher sensitivity.IDENTITY TEST EQUIPMENT AND REAGENTS- Chromatographic paper-Whatman Chroma 1 or paper of a similar quality. Chloroform. Methanol. Isobutyl methyl ketone. The above four reagents should be of analytical-reagent grade: the chloroform is stabilisedJanuary, 19741 VIRGINIAMYCIN I N ANIMAL FEEDS 21 with 0.7 to 1.0 per cent. V/V of ethanol. Extraction solvent system-Citric acid (0.1 M) - acetone (1 + 1 V / V ) was used. Developing solvent system-This was chloroform - methanol (99 + 1 VIYf. Culture media-See the media composition for the method of assay. Stock inoculum of Corynebacterium xerosis NCTC 9755-See the procedure described for the assay.CHROMATOGRAPHY- Paper chromatography is carried out by the ascending solvent technique in the normal way. After drying the chromatogram in air, apply the classical bioautographic techniques on large agar plates according to Betina,s allowing diffusion for 20 minutes before removal of the paper. PREPARATION OF SAMPLES- Take amounts of feed samples and extractant according to the virginiamycin content 9 as recommended under Assay procedure. Stopper the flask and shake it vigorously for 1 hour, maintaining the pH between 3.5 and 6 with 1 N hydrochloric acid. Centrifuge the contents and concentrate the supernatant liquid under vacuum to an aqueous residue. Add 50 ml of 0.001 N sulphuric acid and 200 ml of isobutyl methyl ketone. Stopper the flask and shake it vigorously for 15 minutes.Decant the upper layer and dry it with anhydrous sodium sul- phate; evaporate it to dryness under vacuum. Dissolve the residue in 5.0 ml of chloroform and spot 2 to 10-pl aliquots of the extract on to the paper, the volume depending on the antibiotic content, in order to ensure a minimum content of 0.1 virginiamycin unit per spot. Compare, on the same chromatogram, the unknown sample with a control obtained by adding to the acetone - citric acid feed extract an amount of virginiamycin equivalent to the estimated feed extract content. Virginiamycin is detected as a single inhibition zone (R, = 0-7 to 0.8, depending on the amount of feed extract spotted) corresponding to the position of both M and S factors together. The method has consistently facilitated the detection of 1 virginiamycin unit per gram of feed.Moreover, the test is specific for virginiamycin, excluding all other antibiotics currently used as feed additives, because the tetracyclines, streptomycin, zinc bacitracin and flavomycin are not mobile in the solvent system used and tylosin migrates at about the same position as virginiamycin and spiramycin at intermediate R, values ; however, both substances remain unextracted under the conditions selected for the test. The specificity is not affectedby tylosin even at very unfavourable ratios such as 20 p.p.m. of tylosin to 1 unit of virginiamycin. ASSAY PROCEDURE REAGENTS AND APPARATUS- Petri dishes-These were 90 mm in diameter and 15 mm deep. Paper discs-Use Schleicher and Schull paper No 2668 or a similar quality, with a disc Micropipettes-Use 20-4 Drummond Microcaps or syringes of the same capacity.Extraction solvent mixture-This was citric acid (0.1 M) - acetone (1 + 1 V/V). Sodium hypochlovite solution-Use a fresh, 15 per cent. m/V commercial preparation. Sodium thiosulphate solution, 0.1 N. Culture media-A, medium for base layer and strain maintenance : antibiotic medium No. 1 (Difco 0263) 30.5 g, agar 5.0 g and distilled water to 1.0 litre. B, medium for the seed layer : medium A diluted with an equal volume of distilled water. C, medium for the prepara- tion of the inoculum: antibiotic medium No. 3 (Difco 0243) 175g and distilled water t o 1.0 litre. diameter of 7.0 mm. PREPARATION OF THE INOCULUM- Maintain the stock culture of Corynebacterium xerosis NCTC 9755 on agar slants of medium A.Incubate the slants for 18 to 24 hours at 37 "C, store at 4 "C and discard after 2 weeks. Collect the growth from a single slant with 10.0ml of sterile distilled water and inoculate 100 ml of medium C, contained in a 250-ml conical flask, with 1.0 ml of this cell suspension. Incubate for 18 hours in a shaking-bath at 35 "C. This cell suspension constitutes the stock inoculum; maintain it at 4 "C and discard after 5 days.22 BOON AND DEWART: METHODS FOR IDENTIFICATION AND ASSAY OF [Analyst, Vol. 99 PREPARATION OF THE STANDARD SOLUTIONS- The virginiamycin working standard is stored a t 4 "C in sealed containers; its activity, which is stated on the label, is expressed in (U.K.) units per milligram or in per cent.A biological titre of 100 per cent. corresponds to 1000.0 units per milligram. Prepare a stock solution in methanol at 1000*0 units per millilitre and store at 4 "C, discarding it after 4 weeks. On the day of assay, dilute appropriate aliquots of the stock solution with acetone to obtain standard solutions, using 2.0 units per millilitre as the reference concentration. Apply 20 pl of each standard solution to paper discs and dry them for 1 hour under vacuum. Keep the discs in sterile vials. The use of a blank feed sample is recommended for a feed containing less than 50 units per gram. In this instance, proceed as described under Preparation of the blank feed extract and apply 20 p1 of each compensated standard solution to paper discs as described above. PREPARATION OF THE ASSAY SOLUTION- Use amounts of feed sample and extractant, according to the virginiamycin content of the feed, as follows.Volume of extraction Virginiamycin content/ solvent mixture/ u g-1 Sample sizelg ml 100 10 200 50 20 200 20 50 200 10 100 250 6 100 250 Stopper and shake the flask vigorously for 1 hour, maintaining the pH between 3.5 and 6.0 with 1 N hydrochloric acid. Dilute an aliquot of the feed extract with acetone to an estimated potency of 2.0 units per millilitre, corresponding to the reference concentration of the standard graph. For a feed containing 5 units per gram or less, assayed with an inactivated blank feed extract, dilute the feed extract slightly with water to an estimated potency of 1.6 units per millilitre.For the milk replacers, it is recommended first to mix the powder with hot water (at 70 "C), according to the usual milk preparation procedure, before diluting the sample with 2 volumes of acetone and shaking the mixture vigorously for 30 minutes. PREPARATION OF THE BLANK FEED EXTRACT- If unfortified blank feed is available, extract and dilute it according to the procedure recommended for the preparation of the assay solution and use this blank feed extract as a diluent for the preparation of the standard solutions. As unfortified blank feeds are seldom available under commercial conditions, however, a procedure was developed for the prepara- tion of such compensated standard graphs by inactivating the virginiamycin content. SODIUM HYPOCHLORITE INACTIVATION PROCEDURE- Treat an aliquot of feed extract by adding 20 per cent.VlV sodium hypochlorite solution (10 per cent. V/V for a feed at 20 or more units per gram), adjusting to pH 6 with concentrated hydrochloric acid and gently refluxing for 30 minutes. Evaluate the excess of sodium hypo- chlorite, if any, by iodimetric titration on an aliquot, and neutralise with a calculated amount of 0.1 N sodium thiosulphate solution. Dilute the solution to a suitable extent with acetone and use this inactivated extract to prepare the standard solutions. ASSAY- four plates per standard solution and eight plates per assay. Apply 20 pl of the assay solution to paper discs. Arrange the paper discs on petri dishes according to the usual one-dose procedure.' Use RESULTS AND DISCUSSION The feedstuffs grade virginiamycin is marketed either as a powder or as very small granules, formulated at 50 per cent.virginiamycin activity with methylcellulose and calcium silicate. Any suitable extraction solvent system must take into account the very low water solubility of virginiamycin as well as the necessity to use enough water to release the antibioticJanuary, 19741 VIRGINIAMYCIN IN ANIMAL FEEDS 23 from the granules by rendering the binder soluble. The assay method for virginiamycin in feed proposed in this paper meets these essential requirements. STANDARD GRAPH- A separate standard graph must be prepared for each different feed formulation assayed. The necessity of using blank feed extracts for virginiamycin concentrations lower than 50 units per gram is demonstrated by the difference existing between compensated and uncom- pensated standard graphs.In Fig. 2 composite dose responses obtained from five commercial mixed feeds supple- mented with 20 virginiamycin units per gram are shown. The uncompensated standard graph A is compared with graph B, which is compensated with an unfortified feed extract. The observed linearity, as well as the parallelism between the graphs, is excellent over the range of concentrations used. 0.5 0 6 0 0.1 0.2 0.3 0.4 Log (virginiamycin concentration)/u mI Fig. 2. Composite standard graphs : mean response from five commercial mixed feeds. A, uncompensated standard graph; and B, standard graph compensated with an un- fortified feed extract (extraction procedure corresponding to a feed at 10 u g-l) The effectiveness of the inactivation procedure was demonstrated in a comparative trial, the results of which are shown in Table I.Five different commercial feedstuffs, supple- mented with 20 and 5 virginiamycin units per gram, were assayed with the use of compensated standard graphs prepared with a blank feed extract from an unfortified feed sample and with an inactivated feed extract. The recoveries are little or not affected by the inactivation step and the observed differences remain within acceptable limits. TABLE I COMPARATIVE ASSAYS WITH COMPENSATED STANDARD GRAPHS Average potency within 95 per cent. confidence limits*/units per gram Feedstuff Broiler finishing mash Complete sow . . Pig fattening . . Chick grower . . Broiler finishing mash Complete sow .. Pig fattening . . Pig starter .. Chick grower . . Pig starter . . .. Assay with .. 18.02 f 0-96 .. 18-64 f0-74 .. 18.46f 1.04 .. 18.48 f0.68 .. 19.62 f 0.38 .. 4.37 f 0.24 .. 4*51&0-25 .. 5.00f0-30 .. 4.27k0.15 .. 4*32&0*38 unfortified blank feed Assay with inactivated feed extract 14.82 f 0.62 18.54f0.90 16.16f0.94 15-72f0.76 21.36% 1.28 6.19 f 0.45 4-41 5.26 f 0.35 5.17 f 0.26 5.83&0*54 * Average of ten samples, on the same day.24 BOON AND DEWART: METHODS FOR IDENTIFICATION AND ASSAY OF [Analyst, VOl. 99 RECOVERY- The recovery of virginiamycin (granulated feed grade) from feeds is satisfactory even at low levels of incorporation, as is shown in Table I1 by the assay results for several pilot batches prepared under well controlled mixing conditions.TABLE I1 VIRGINIAMYCIN RECOVERIES FROM VARIOUS COMMERCIAL MIXED FEEDS Feedstuff Milk replacer . . Milk replacer . . Pig starter A . . Pig starter B . . Pig starter C . . Pig fattening meal Pig pre-starter . . Pig grower A . . Broiler finishing mash Complete sow feed Pig grower A . . Pig grower B . . Broiler finishing mash Theoretical virginiamycin Average recovery with concentration/ 95 per cent. .. 80 112.3& 5.9 * . 20 91.6& 7.1 .. 20 96.8% 9-3 .. 20 89*9& 10.5 .. 20 99.2f 6.1 .. 20 91*6& 7.1 .. 20 97.0& 8.0 .. 10 102*0& 11.3 .. 10 100*5+ 9-3 .. 5 106.2f 4.4 u g-1 confidence limits*, per cent. .. 20 95.75 11.1 a . 20 94.9% 9.0 . . 10 844& 5.2 Mean recovery 97.1 * Average of ten samples, on the same day; assayed with blank feed extract.REPRODUCIBILITY- The reproducibility of the method of assay was tested by repeatedly assaying over 5 days the same broiler feed fortified with 20 and 5 units per gram of granulated feed grade of virginiamycin. The standard deviation for samples includes variations related to both the reproducibility of the assay and the homogeneity of the virginiamycin distribution in the feed. The granulo- metric properties of a granular feed play a leading part in the quality of its distribution, even Results are collected in Table 111. TABLE I11 REPRODUCIBILITY OF THE VIRGINIAMYCIN ASSAY Feed at 5 u g-l- Average potency with 95 per cent. confidence 9”Y limits*/u g-l 5.32 & 0.59 2 4.19 f 0.35 3 4.80 & 0.84 4 4-70 f 0.58 5 4.55 f 0.52 Standard deviation between samples Standard deviation between samples Feed at 20 u g-l- within days .. . . 0.81 betweendays . . 0.88 Average potency with 95 per cent. confidence Day limits*/u 8-1 1 26.3 5 2-16 2 24.3 f 1.35 3 23.1 5 1.06 4 23.3 5 1.06 5 22.4 f 0.79 Standard deviation between samples Standard deviation between samples * Average of ten samples; assayed with blank feed extract. within days . . . . 1.79 betweendays . . 2.27January, 19741 VIRGINIAMYCIN I N ANIMAL FEEDS 25 under perfect mixing conditions. The degree of heterogeneity related to such granule characteristics as their particle-size distribution curve and the mean number of particles per mass unit, far exceeds the deviation related to the assay and would explain the rather high standard deviation values reported in the tables.This granulometric effect can be predicted by the calculation method developed by Bruggemann and Nie~ar.~ The standard deviation for the distribution of a granular feed additive depends on the mean number n of particles per sample and on a correcting factor K deduced from the particle-size distribution curve, according to the equation- 100 + K Standard deviation, per cent. = dn- Based on the average granulometry of virginiamycin, K = 174 and n is calculated from the mean number of particles per milligram of granular feed grade virginiamycin, which is equal to 250. Some experimentally determined standard deviations for the assay of a commercial broiler feed supplemented with different particle-size classes of virginiamycin (granular feed grade) are compared in Table IV with the corresponding calculated values. It is obvious that the experimental and calculated deviations are very close to each other and that the standard deviation for the microbiological assay represents only a small part of the observed value.TABLE IV REPRODUCIBILITY OF THE ASSAY AS RELATED TO THE GRANULOMETRY OF THE VIRGINIAMYCIN FEED GRADE INCORPORATED I N THE FEED Particle-size range of virginiamycin, granular 0.25 to 0.50 0.20 to 0.25 0.12 to 0.20 0.07 to 0.12 Commercial mixture- 0.07 to 0.5 feed grade/mm Experimentally deter- mined standard deviation, per cent. 10-5 4.5 3.25 1.46 13.0 Calculated standard deviation, per cent. 10.0 5.5 3.5 1.6 16.5 CONCLUSION A microbiological method of assay and a paper-chromatographic method of identification are proposed for virginiamycin in feed. They are equally adaptable to the granular feed grade as to the powder form, do not require any special equipment and can be easily adopted for routine purposes. The assay method has been satisfactorily used in several laboratories for many years; its response graph is linear and allows complete recovery with an acceptable accuracy. The sodium hypochlorite inactivation procedure does not appear significantly to affect feed extracts when used in compensating standard graphs. The identification test is very specific to virginiamycin, excluding all other antibiotics currently used as feed additives. The authors express their gratitude to the Institut pour 1’Encouragement de la Recherche Scientifique dans 1’Industrie et l’Agriculture, I.R.S.I.A., for the financial support given to the work on virginiamycin. REFERENCES 1 . 2. 3. 4. 5 . 6 . 7. 8. 9. Crooy, P., and De Neys, R., J . Antibiot., 1972, 25, 371. Fils, F., J . Pharm. Belg., 1972, 27, 549. Crooy, P., and De Neys, R., in preparation. Van Dijck, P., Vanderhaeghe, H. and De Somer, P., Antibiotics Chemother., 1957, 7 , 625. Van Dijck, P., Chemotherapy, 1969, 14, 322. Chabbert, Y. A., and Courvalin, P., Path. Biol., Paris, 1971, 19, 613. Grove, D. C., and Randall, A., “Assay Methods of Antibiotics,” Medical Encyclopedia Inc., New Betina, V., Chromat. Rev., 1965, 7 , 120. Bruggemann, J., and Niesar, H. K., Ind. Aliment. Animale, 1965, 162, 39. York, 1955, p. 12. Received June 14th, 1973 Accepted August 13th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900019

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

26 Analyst, January, 1974, Vol. 99, pp. 26-37 Mass-spectrographic Analysis of Geological Samples Using the Low-voltage Discharge Source BY P. F. S. JACKSON AND A. STRASHEIM (National Physical Research Laboratory, South African Council for Scientific and Industrial ResearcP, P.O. Box, 395, Pretoria, South Africa) Problems associated with electrode performance were encountered in the application of the low-voltage discharge source to accurate quantitative analysis of non-conducting geological samples. By using compound elec- trodes of the sample and conducting powder, preferential loss of the con- ducting material from the electrode system led to imprecision and to eventual failure of the electrical discharge. The effects of discharge current and electrode composition on this denuding factor are discussed.Results are given for the analysis of three international standards: G2, BCR-1 and W-1, and a comparison is made between the results obtained by mass spectrography and by other analytical techniques used in the routine analysis of geological material. The analytical range for most elements is more than two orders of magnitude. Results reported indicate that analytical values with an accuracy of better than 10 per cent. can be obtained by using mass spectrography. THE first commercial spark-source mass spectrograph became available in the late 1950s. This apparatus was fitted with a radiofrequency spark source, which has been almost univer- sally used in this technique. Other sources, the laser and the low-voltage discharge sources, found limited applications but results from these sources were overshadowed by the volume of data produced by the use of the radiofrequency spark.A detailed description of the two types of spark source has been given by Fran2en.l Brown and \Volstenholme,2 in 1963, published mass-spectrographic survey analyses on several non-conducting matrices, including geological materials. By using the radiofrequency spark source and compound electrodes prepared with 50 per cent. m/m graphite, they obtained analytical results for almost fifty elements in each sample, some elements being determined down to well below 1 p.p.m. The analyses were described as quantitative, with the assumption that all elements had equivalent sensitivity in the spectrograph, which was soon proved to be erroneous. In the following years many author^^-^ incorporated internal reference standards and made use in their interpretative procedure of relative sensitivity factors determined from a known reference material.The use of other analytical matrices, such as silver and gold in the analysis of titanium(1V) oxide,s and silver in the analysis of rare earths (H. H. Whittaker, private communication, 1965), was made necessary by the relatively high abundance of major-element carbides formed in the radiofrequency spark when graphite was used as matrix. These carbides caused deconvolution problems that arose from the superimposed spectral lines, and made the determination of low-level impurities at the relevant mass positions impossible. Precisions and accuracies given in the literature varied from values close to 5 to about 300 per cent.in certain instances. Some of this imprecision stemmed from problems associ- ated with homogeneity and the very small amount of sample consumed during the analysis. Nicholls, Graham, Williams and Wood7 showed that by repeated fusion of the sample with rapid quenching and grinding, coefficients of variation of better than 5 per cent. could be obtained on known heterogeneous samples. Other authors6s9 preferred electronic gating of the beam, thus increasing sample consumption per unit exposure. Nearly all of these results were obtained by using the radiofrequency spark source. The low-voltage discharge spark was only very occasionallyl0>l1 used as a source of ions. Franzen, Schuy, and Maurerll developed an interpretative technique based on a comprehen- sive study of the photoplate and its reaction to ion bombardment.In this work, and in work on mass-spectrographic precision, they discussed the use of broad line profiles with homogeneous blackening in contrast to the distorted Gaussian profile normally obtained. @ SAC and the authors.JACKSON AND STRASHEIM 27 They proposed that the grain statistics of broad homogeneously blackened lines were naturally better than those otherwise obtained, and thus permitted a more accurate measurement for the photometric conversion of the analogue line intensity into the required digital counterpart. They indicated that with standard steel samples they were able to obtain a relative standard deviation of better than 1 per cent.for the isotopes of molybdenum, and analytical coefficients of variation of between 4 and 8 per cent. for some of the minor components of the steels. No successful analyses of silicate rocks by low-voltage discharge source mass spectro- graphy appear to have been reported and this paper describes an attempt to use such a source for the analysis of geological samples. DEVELOPMENT OF THE METHOD EXPERIMENTAL- A Varian-Mat SMIBF spark-source mass spectrograph was used throughout this work. In the course of developing the method the commercial apparatus was modified, in that a Pfeiffer 260 1 s-l turbomolecular pump was added to the source region instead of the Edwards 300 1 s-1 oil-diffusion pump and cold-trap. The butterfly valve leading from the pump to the source was replaced by a VAT gate-valve (Balzers Ltd.), and the optical baffles were removed except for the protective gauze at the entrance to the source.Minor modifications were also made to the electronic system so as to enable stabilised currents of as low as 1 A to be used. The graphite used was supplied by Ringsdorff (RW-A quality) and the silver powder by Cominco. Electrodes were prepared by admixing the dry powders by hand with a pestle in an agate mortar in order to obtain the best result. Mixtures produced by commercially available shakers and amalgamators proved unsatisfactory as in certain instances separation of the two phases could be seen even after mixing for 1 hour. The powder mixture was then placed in a heavy-duty polythene mould, so as to avoid contamination, and compressed to about 9 kbar.The press was supplied by Research and Industrial Instruments, London (Type 0025). The ion path was adjusted to give a broad, homogeneously blackened line-profile with a line width of about 0.1 mm. The electrodes, mounted on their respective holders, were positioned accurately within the source of the spectrograph in order to reduce variations in electrode position between sets of electrodes to a minimum. These variations had previously been shown12 to affect spectral quality and also precision. The positioning of the electrodes was made by using a calibrated Perspex spacer, the thickness of which controlled the position of the electrodes relative to the primary slit. The horizontal and vertical position of the electrode gap was set from the calibrations on this spacer, which itself is aligned with the source-gun.The discharge, under the conditions used, continues for approximately 100 ps after the ignition spark. In order to select material emanating only from the low-voltage discharge and not from the ignition spark, the gate, synchronised to the ignition spark, was kept closed for 20 ,US after ignition, then opened for approximately 80 ps. These conditions were maintained for the whole of the programme. In this manner any possible differentiation within the ion cluster produced by each discharge is avoided. OPTIMUM OPERATIKG CONDITIOKS- were studied; those investigated, including ranges, are given in Table I. To obtain optimum conditions, all parameters affecting the operation of the instrument TABLE I PARAMETERS STUDIED IN ORDER TO OBTAIN OPTIMUM EXCITATION CONDITIONS Discharge current ..1 t o 5 A Repetition rate . . . . 1 to 100 Hz lnitiator spark energy . . 0.125 to 0.625 J Silver to sample ratio 10 : 1 t o 5 : 1 Graphite to sample ratio 10 : 1 to 1 : 1 . . The criteria used to judge the performance of the system were precision and lifetime of Two factors were found to influence the performance of the system, vix., the The higher current settings also increased the These fragments, the electrode. electrode composition and current settings. number of flying fragments observed in the vicinity of the electrodes.28 JACKSON AND STRASHEIM : MASS-SPECTROGRAPHIC ANALYSIS OF [Analyst, VOl. 99 when the machine was used with the original pumping system, tended to cause the accelerating voltage to become unstable and many voltage breakthroughs were observed. The pressure in the source was found to vary, depending on the pulse repetition rate used.At 100 Hz, pressure peaks were observed to exceed loF5 torr, at which level the accelerating voltage could no longer be maintained. With the turbomolecular pump no pressure peaks were observed, and the vacuum, when sparking at 100 Hz, did not deteriorate beyond 5 x lo-' torr. The precision itself was not affected by the pulse repetition rate nor by the initiating spark energy and these were set to maximum repetition rate, for maximum ion output, and an intermediate value for the initiating spark energy of 0.375 J, which gave an adequately low resistance in the spark gap for the discharge to ignite. The influence of high current on the electrode surface is illustrated in Fig. 1.An electrode mix was prepared consisting of 9 parts of graphite and 1 part of sample, from which electrodes were prepared and exposed to 2, 4 and 6 A low-voltage currents for 1000 s. The local melting of the sample is increased at 6 A and the depth and size of the craters is also increased. The influence of low graphite to sample ratios on the performance of the electrode can be seen in Fig. 2. Electrodes consisting of 9 parts of graphite and 1 part of sample and also of 1 part of graphite and 1 part of sample were exposed to a current of 4 A for 500 s. Scanning electron micrographs of the resultant surfaces are shown in Fig.2. The structures are totally different with evidence of local fusing occurring in the case of the 1 : 1 system. The best precision was found for samples with a high conducting matrix content that were excited at low current values, From the evidence cited above, it was decided to use the more dilute sample elec- trodes, i.e., with a graphite to sample ratio of 6: 1, with a low discharge current of about 2 A. Initially, the results with this matrix were poor with relatively large coefficients of variation, notably for the alkaline earth elements, On pre-heating the original sample at 600 "C prior to preparation of the electrode, these coefficients of variation improved considerably. Also noticeable was the relatively poor sensitivity of the alkaline earth elements in this matrix.(The sensitivity was determined as the inverse of the correction factor required to convert the observed concentration, measured in terms of an internal standard, into the expected value, as defined by other techniques.) This phenomenon is much less prominent in graphite, and Table I1 gives an indication of the difference in sensitivity in the two matrices for six elements, all being determined relative to iron (the internal standard). Scanning electron micrographs of the sample surfaces can be seen in Fig. 1. Silver was also used as conducting matrix. TABLE I1 RELATIVE CORRECTION FACTORS USED IN THE ANALYSIS OF SILICATE MATRICES FOR SILVER AND GRAPHITE Relative correction factors - Silver Graphite Iron . . .. 1.0 1-0 Vanadium ..0.8 0.82 Chromium .. 0-7 0.69 Manganese .. 0.59 0.57 Calcium . . 3.1 0.4 Strontium . . 9-2 0.8 Barium .. 36 2.3 The final choice of electrode composition and source parameters is summarised in Table 111. STANDARD REFERENCE SAMPLE- The factors listed in Table I1 were determined from results obtained by using the inter- national rock standard G-1. Samples of the finely ground rock were mixed with silver and graphite and correction factors, relative to the iron concentration, were calculated for as many elements as possible. The comparison of these factors between the two conducting matrices0 + w [To face p. 28Fig. 2. Comparison of electrode surfaces after exposure t o a current of 4 A for 500 s. Electrode composition (ratio of graphite t o sample) : ( a ) , 9 : 1; and ( b ) , 1 : 1January, 19741 SAMPLES USING THE LOW-VOLTAGE DISCHARGE SOURCE 29 is in no way affected by the value adopted for the absolute concentration in the standard, as the same standard, from the same bottle, was used throughout these experiments.TABLE I11 SUMMARY OF SATISFACTORY ELECTRODE COMPOSITION AND DISCHARGE SETTINGS Discharge current . . 1.1 t o 2.2 A Pulse repetition rate . . 100 Hz Ignition energy . . . . 0.375 J Silver to sample ratio . . 9 : 1 m/m Graphite to sample ratio 6 : 1 m/m For calibration purposes, however, an absolute value had to be adopted. In view of the wide discrepancies between determinations in the compilation of Fleischer,13 it was decided to use a preferred value rather than a simple mean. The data of Fleischer were taken, to which were added any recent data available to us on the standard G-1.Care was taken to ensure that no systematic bias was included by over-emphasising any particular method, but due significance was given to those methods which, owing to the recent attention paid to them, have been proved to be precise and reliable, e.g., isotope-dilution mass spectrography for the determination of rubidium and strontium, the results for which are commonly quoted to three and four significant figures. A simple average was then taken and outliers were rejected by the normal statistical process suggested by the A.S.T.M. (E178-68 p. 156-165). The resulting data were then re-averaged and the residual data inspected for bias of the method. In general, little bias was found, most of the results displaying a random distribution about the weighted mean.This value was then adopted as a starting value. Table fV contains some of the preferred values adopted in this work for the purposes of comparison. TABLE IV VALUES ADOPTED FOR THE U.S. GEOGRAPHICAL SURVEY STANDARD G-1 I N THE CALCULATION OF RELATIVE CORRECTION FACTORS M a j o r elements ( a s oxides), p e r cent.- CaO 1.37 K,O 5-5 TiO, 0-26 MnO 0.024 P,O, 0.09 MgO 0.38 Trace elements, p.p.m.- Ba 1150 Ho 0.4 Sr Be 2.4 La 103 Tb Ce 160 Li 22 Tm co 2.2 Lu 0-12 v Cr 15 Nb 20 Y c u 15 Nd 55 Yb Dy 2-8 Ni 1.5 Zn Er 1.3 Pr 16 Zr Gd 5.0 Sm 8.0 Eu 1.2 Rb 220 250 0.6 0.16 15 10 0.80 42 200 DISCUSSION OF OBSERVATIONS DURING DEVELOPMENT OF METHOD As the transfer of material in a low-voltage discharge system has been shown to be almost entirely unidirectiona1,l a problem arises with compound electrode systems, in which the rates of consumption of the two intermixed, but discrete, phases are found to be dissimilar.The surfaces of three electrodes after sparking under different conditions have been shown (Fig. 1). With low currents (2 A) the depth of the craters is relatively shallow and siliceous protrusions do not dominate the sample surface. Under the influence of a higher current, and also with less graphite present (Fig. 2) in the electrode mix, a coral-like structure begins to appear to be dominant on the electrode surface, which is non-conducting and causes the spark discharge to become erratic and finally to be extinguished. At the extinction point, deep craters and large siliceous protrusions almost cover the sample surface.The formation of these structures has been described and discussed more fully elsewhere.l*30 JACKSON AND STRASHEIM : MASS-SPECTROGRAPHIC ANALYSIS OF [Analyst, Vol. 99 The remarkably high relative correction factors required to convert the observed mass- spectrographic values, which were obtained for the alkaline earth elements in a sample - silver mixture into quantitative data, indicate a very poor ion yield for these elements under the conditions selected. The ion yield for these elements is, however, dramatically improved by a small addition (2 per cent.) of graphite to the electrode mix. This observation was also noted by Scott, Strasheim and Jacksonlj when using the relatively low temperature plasma of normal-mode laser radiation to excite geological material for subsequent mass-spectro- graphic analysis.They proposed that the presence of carbon creates a reducing environment, which assists in the fracture of strong metal to oxygen bonds, as is found with refractory oxide type material. The same theory has been adopted in this paper to explain the remark- able change in the ion yield of the alkaline earth elements when graphite instead of silver is used as the conducting matrix. The oxides of these elements are very stable thermally and highly electrically insulating. Under the cool plasma conditions selected, these oxides would tend to remain behind in the cathode. The reducing nature of the graphite effects the release of these elements by means of a thermochemical process and thus accounts for their large change in sensitivity.An extension of this effect may also explain the poor precision obtained for the alkaline earth elements, prior to calcination of the original sample, when a silver matrix is used. It is suggested that residual carbon present in the original sample, which acts as such a releasing agent, was the cause of the erratic ion yields observed in these instances. Calcination of the original sample prior to electrode preparation removed this carbon and thus eliminated the cause of the extremely poor reproducibility. PRECISION- In the SMIBF mass-spectrographic system, provision is made for a total of thirty exposure positions on each photoplate, thus enabling the precision with which any single exposure may be recorded to be assessed. Replicate exposures were taken at the same exposure value, and a statistical analysis was made of the variations in line transmission, measured by micro- photometer, of the replicates for several mass lines, each of which had a different transmission, It was found that, between transmission values of 85 and 8 per cent., the standard devia- tion, which is defined as the positive square root of the variance, was almost independent of the isotope or element, but was strongly dependent on the value of the exposure selected.The precision of a single exposure of between 0.1 and 10 pC was found to be very poor, even for gating conditions so stringent that only about 0.1 per cent. of the total possible beam was transmitted to the photoplate.Exposures of such small magnitude were therefore considered too imprecise and their use was avoided, which restricted the upper limit of the concentration that could be determined to approximately 2000 p.p.m.a. for any specific isotopic species. Exposures in the range 10 to 10000 pC were found to be more precise. The standard deviation recorded in these instances indicated a certain dependence on the transmission value itself, with the smaller deviations being recorded at lower transmission. The trans- mission scale of the Optica Milano microphotometer used in this study is calibrated from zero (no light passing through the microphotometer) to 1000 (corresponding to clear glass on the photoplate). At a mean line transmission value of 120 scale units (12 per cent.transmission), the typical standard deviation was between 3 and 4 units. At a mean transmission value of 720 scale units (72 per cent. transmission), the typical standard deviation had deteriorated to close to 15 units, and at transmission values closer to the background value (clear emulsion at about 920 scale units) this value had deteriorated still further to approximately 28 scale units. Above 10 000 pC (below about 3 p.p.m.a.), the precision depended on the performance of the electrodes which, under the conditions proposed, did not change discernibly. EXPERIMENTAL- Three standards, six lunar basalts and eleven rocks of unknown composition were analysed by using graphite as the conducting matrix. A range of exposures covering the analytical range desired was run and the photoplate developed in the normal fashion.The transmission values of the line, read by microphotometer, and the exposure for each of the lines were fed into an IBM 360 computer, with which data processing was handled according to the Franzen, Scliuy and Maurer equations.ll APPLICATION OF METHODJanuary, 19741 SAMPLES USING THE LOW-VOLTAGE DISCHARGE SOURCE 31 With these equations, and provided sufficient data points are available for regression analysis, values can be obtained for the unknown parameters Ts, the saturation transmission of the photoplate due to primary blackening, and V , the parameter defining the slope of the function, for each ion species. When insufficient data points are available for this treatment, an estimate of T , can be obtained from the equation- where a, b and c are constants determined for each plate, and is the mass of the ion for which Tt9)(=, is required.The value of V in these cases is determined approximately by interpolation or extrapolation. (For each individual element, the values of Ts and V obtained over the whole series were almost constant.) The individual ion intensities could therefore be established, and, by comparison with the ion intensity for the iron, the concentration of which was determined by other methods, a value was calculated for the concentration of individual elements. The response function was developed from the measured responses of the isotopes iron-54, -56 and -57. The con- centration value for iron was taken to be the mean of the values obtained by four separate iron determinations, which were achieved by four dissimilar methods, vix., chemical analysis,ls X-ray powder analysis,17 X-ray fusion18 and atomic-absorption spectrophotometry.~~~~~ The coefficient of variation between these methods determined over ten test samples was less than 5 per cent., which is in agreement with that found by Strasheim and Jackson,21 who reported that the determination of iron in geological material gives highly reproducible results.This result could therefore be relied upon and contributed little or no systematic error to our results, even when the geological character of the sample changed considerably. RESULTS AND DISCUSSION The results obtained on the twenty rock samples have been subdivided into groups, and those for all of the elements determined are given only for the standard samples and are tabu- lated in Tables V, VI and VII.For the other seventeen samples, certain elements have been TABLE V T(s)(,, = a M(n) + b M(n)+ + c Iron was selected as internal standard in all silicate analyses. ANALYSIS OF G2 COMPARED WITH THE RECOMMENDED VALUES AND THE RANGE OF VALUES OBTAINED BY OTHER METHODS Major elements (as oxides), per cent.- Recom- This mended Oxide work value CaO 2.0 1.94 - 4.51 0.55 0.50 K O TiO, MnO 0-032 0-034 P,O, 0.14 0.14 MgO 0.80 0.76 Trace elements, p.p.m.- Recom- This mended Element work value Ba 2100 1870 Be 2.1 2.6 Ce 160 150 c o 5.4 5.5 Cr 15 7.0 c u 11 11.7 DY 2.42 2.6 Er 0-93 1.3 E U 1.7 1.5 Gd 6.0 5-0 Ho 0.34 0.4 La 87 96 Li 50 34.8 I,U 0.12 0.1 1 Range 1.8-2.3 4.3-5.1 0.42-0.5 7 0.02-0.04 0.1 1-0.23 0.34-1-08 Range 1500-3000 1-5-3 140-180 2-2 1 5-29 <2-17 2-5 0.8-2.6 1.3-3.2 3-7‘0 <0*3-0.7 76-250 25-63 0.10-0.2 This Element work Nb 15-5 Nd 58.1 Ni 5.7 Pr 16.8 Rb 240 Sm 9.05 Sr 500 Tb 0.47 Tm 0.12 V 37 Y 15 Yb 0.93 Zn 80 Zr 300 Recom- mended value 13.5 60 5.1 19 168 479 (0.3 35.4 12.0 85 300 7.3 0.54 0.88 Range 8-20 42-67 2-1 4 19-20 1 08-5 1 3 7-1 1 235-680 O .P l * O 0.3-0-5 26-60 8-17 0.5-1,o 42-138 250-40032 JACKSON AND STRASHEIM : MASS-SPECTROGRAPHIC ANALYSIS OF [AnaZySk, VOl. 99 TABLE VI ANALYSIS OF BCR-1 COMPARED WITH THE RECOMMENDED VALUES AND THE RANGE OF VALUES OBTAINED BY OTHER METHODS Major elements (as oxides), p e r cent.- Recommended Oxide This work CaO 7.0 1.55 2.11 TiO, MnO 0.18 0.39 Trace elements, p.p.m.- K2O p20.5 MgO - This Element work Ba 576 Be 1.59 Ce 49.0 co 36 Cr 14.5 c u 22 5.7 3.0 DY Er Eu 1.8 Gd 6.0 €30 1.06 La 23.8 Li 17 Lu 0.54 value 6.92 1.70 2.20 0.18 0.36 3.46 Recom- mended value 676 1.7 53.9 38 17.6 18.4 6.3 3.59 1-94 6.6 1.2 26.0 12.8 0.55 Range 6.14-8.3 1.49-1.82 1.83-2.45 0.1 0-0.20 0.2 8-0.4 7 24-3-7 Range 480-1 230 1-3 40-53 29-60 8-45 7-33 5.7-6.6 3.1-3.7 1.8-2.4 5-8.5 1-1.3 22-36 10-19 0.4-0.6 Element Nb Nd Ni Pr Rb Sm Sr Tb Tm V Y Yb Zn Zr This work 11 28.7 20 6.3 50 6.0 330 0.84 0.43 340 27 140 200 2.9 Recom- mended value 13.5 29.0 15.8 7 46.5 6.6 1.0 0.6 37.1 330 399 3.36 120 190 Range 10-97 22-34 8-30 5-7 40-150 5.5-7-5 244-525 0.7-1.2 <0.4-0*7 120-700 20-52 2.3-5.0 94-278 144-275 selected to demonstrate the mass-spectrographic value compared, in most instances, with the average value obtained by X-ray fluorescence, atomic-absorption spectrophotometry and chemical analysis. These results are plotted on log - log graphs (Figs.3 to 9). TABLE VII ANALYSIS OF W-1 COMPARED WITH THE RECOMMENDED VALUES AND THE VALUES OBTAINED BY MASS SPECTROGRAPHY Major elements (as oxides), per cent.- Oxide This work value CaQ 11 10.96 0.65 0.64 1.15 1-07 TiO, MnO 0-17 0.17 0-14 0.14 - 6-62 Recommended K2O p205 MgO Trace elements, p.fi.m.- Recom- This mended Element work value Ba Be Ce Co Cr CU DY Er Eu Gd Ho La Li Lu 165 0.59 18.8 46 99 105 3.7 2.0 0.96 3-8 0.62 9.4 0.30 11 160 23 47 114 110 4 2.4 1.1 1 4 0-69 9.8 14.5 0.35 0.8 N i c h o 11 s et al.’s value 145 - 17-7 42 98 110 3-89 2.08 1.2 3.82 0.63 11.9 - 0.20 Taylor’s value Element 200 Nb - Nd 18 Ni Pr - Rb - Sm 2.6 Sr 1.8 Tb 0.95 Tm 3.0 V 0.78 Y 14 Yb Zn - Zr - - This work 6-5 13-4 70 2.7 20 3.2 195 0.44 0.29 260 20 85 110 2.1 Recom- mended value 9.5 15 76 21 190 3.4 3.6 0.65 0.30 264 25 86 105 2.1 1 qicholls et al.’s value 12.5 76 4.0 21 3.3 1 175 0.49 0.28 - 260 25 75 90 - Taylor’s value 11.0 2.5 - - 19 190 - 0.66 0.31 - 29 1.6 - 95January, 19741 SAMPLES USING THE LOW-VOLTAGE DISCHARGE SOURCE 33 1 10 100 1000 Average value, p.p.m.Fig. 3. Comparison of mass-spectrographic values with average values obtained by X-ray fluorescence, atomic-absorption spectroscopy and chemical analysis for rubidium : 0, test samples ; a, lunar samples; and x, U.S.Geological Survey standards As between 20 and 30 individual determinations (the transmission - exposures pairs) are made in order to create the mathematical response function, the precision within a single plate with which the ion intensity of any component can be measured bears no direct relation- ship to the precision of the individual exposure. Because of the variability of the standard deviation with transmission, and the non-linearity of the mathematical model, the evaluation of this plate precision is too complex and has been omitted. The precision, determined by replicate analysis of a single sample, indicates that for elements at concentration levels be- tween 1 per cent. and 1 p.p.m., a coefficient of variation of between 5 and 7 per cent. can be obtained for most elements, one of the limitations being the determination of those elements which are mono-isotopic, when the upper limit of detection is exceeded.The accuracy of the method is demonstrated in Figs. 3 to 9 and in Tables V to VII. The results obtained by I I 100 1000 10 000 Average value, p.p.rn. Fig. 4. Comparison as for Fig. 3, for phosphorus34 JACKSON AND STRASHEIM MASS-SPECTROGRAPHIC ANALYSIS OF [A'HdySt, VOl. 99 0.01 ~ 0.1 1 10 100 Average value, per cent. Fig. 5. Comparison as for Fig. 3, for calcium mass spectrography are compared with the average value by other methods. (This average is occasionally weighted in favour of the popular result by discarding the few obviously erroneous results from the collection.) Tables V and VI contain comparative22 results for the internationally recognised standards G2 and BCR-1. In Table VII are shown the results obtained by this method compared with the recommended and with those of Nicholls et a1.' and Taylor3 (results obtained with a mass spectrograph fitted with a radio- frequency source).In most instances and certainly at higher concentration levels the results follow a distinct 1 : 1 relationship when compared with the "average value". The differences become more apparent as the level of concentration decreases, as the other techniques with which comparison is made approach their individual detection limits. In 1969, Graham and Nicholl~,~~ discussing their results on W-1, commented upon their absolute error for the deter- minations of six of the rare-earth elements.In these instances the error appeared to be larger than expected and they suggested a possible systematic error in their assumed R values. The values presented in this paper, however, vary considerably from their originals and as this disparity was not discussed the more popularly quoted values' have been used for comparison. 0.0 1 0.1 1 10 Average value, per cent. Fig. 6. Comparison as for Fig 3,. for titaniumJanuary, 19741 SAMPLES USING THE LOW-VOLTAGE DISCHARGE SOURCE 35 The values for titanium (Fig. 6), manganese (Fig. S), calcium (Fig. 5) and phosphorus (Fig. 4) are very good, with the average error less than 5 per cent. except for phosphorus, with which a trend towards a systematic error is established at the lower level. Agreement is also seen between the results for these elements and the values obtained for the international standards with which the error in the determination of these elements is very small indeed.The comparisons of the results for chromium (Fig. 7 ) , copper (Fig. 9) and rubidium (Fig. 3) show larger differences, which are particularly noticeable below 20 p.p.m. As already stated, some of this error may be due to errors in the “accepted average result,” as these techniques are approaching their limits of detection (see degree of scatter indicated by bar in US. Geological Survey results). I I I 10 100 1000 10000 Average value, p.p.m. Fig. 7. Comparison as for Fig. 3, for chromium No strong inter-element effect has been noted for any of the elements determined (about 35) in each of the twenty rock samples.The same relative correction factors were used throughout, no matter which sample type was being analysed, and a significant third-partner effect should have been easily recognised. Owing to the uncertainty attached to the “accepted Average value, p.p.m. Fig. 8. Comparison as for Fig. 3, for manganese36 JACKSON AND STRASHEIM : MASS-SPECTROGRAPHIC ANALYSIS OF [A?i?a&St, VOl. 99 average,’’ no final conclusion can be drawn as to the over-all accuracy of the mass-spectro- graphic technique. It is believed that if the accuracy obtained at higher levels of concentra- tion could be extrapolated over the one or two orders of magnitude towards the very low results, a value of 10 per cent. may not be unrealistic. Although tests have demonstrated that no electrode deterioration is manifest after the exposure periods required for the lower levels of detection, it is felt that additional results by other techniques are required in order to give a significant comparison with mass-spectrographic values at the single part per million level and below.Clearly, however, the results demonstrate that the mass-spectrographic values will be very reliable “estimates” of these low levels. V I 1 1 10 100 1000 Average value, p.p.rn. CONCLUSION The results obtained from the use of the low-voltage discharge source in the mass- spectrographic study of geological materials indicate that, with care, accurate figures can be obtained for a large number of elements. The effect of carbon, as the conducting matrix, on the ion-yield of alkaline earth elements appears to be of a similar kind to that shown in laser source mass spectrography.The analytical precision that has been attained in this work confirms that, provided the effects of the various parameters are controlled, precise results can be achieved with this technique. The analytical accuracy of the mass-spectrographic system is once again demon- strated to be better than 10 per cent. in most instances, a figure which, for most geological work, is considered to be satisfactory. Fig. 9. Comparison as for Fig. 3, for copper 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. REFERENCES Franzen, J., “Analysis by Mass Spectrometry,” Chapter 2, Academic Press, New York and London, Brown, R., and Wolstenholme, W. A., Paper presented to the E l 4 Committee of the A.S.T.M. a t Taylor, S. R., Nature, Lond., 1965, 205, 34. ---, Geochim. Cosmochim. Acta, 1965, 29, 1243. Whitehead, J., and Jackson, P. F. S., Analyst, 1966, 91, 418. Whitehead, J., Jackson, P. F. S., Vossen, P. G. T., and Brown, R., Analyt. Chem., 1967, 39, 141. Nicholls, G. D., Graham, A. L., Williams, E., and Wood, M., Ibid., 1967, 39, 584. Jackson, P. F. S., “Proceedings of the Fifth Annual MS7 Mass Spectrograph Users Meeting,” Vossen, P. G. T., Analyt. Chem., 1968, 40, 3. Riddoch, A., “Proceedings of the Fifth Annual MS7 Mass Spectrograph Users Meeting,” April, 1965. Franzen, J., Schuy, K. D., and Maurer, K., 2. analyt. Chem., 1967, 225, 295. Strasheim, A., and Jackson, P. F. S., XVIth Colloquium Spectroscopicum Internationale, Heidel- 1972. the Eleventh Annual Meeting, May, 1963, San Francisco, U.S.A. April, 1965. berg, 1971.January, 19741 SAMPLES USING THE LOW-VOLTAGE DISCHARGE SOURCE 37 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. Fleischer, M., Geochim. Cosmochirn. Acta, 1969, 33, 65. Jackson P. F. S., and Strasheim, A., “South African Institute of Physics Conference, D19,” July Scott, R. H., Strasheim, A., and Jackson, P. F. S., Nature, Lond., 1971, 232, 5313. Strelow, F. W. E., Liebenberg, C. J., and von S. Toerien. F., Analytica Chim. Acta, 1969, 47, 251. Gricius, A. J., Proc. X V I I Colloquium Spect. Int., Florence, 1973, 02, 475. Wybenga, F. T., X-ray Spectrom., in the press. Butler, L. P. R., and Kokot, M. L., “Modern Methods of Geochemical Analysis,” Chapter 8, 1971, Butler, L. R. P., “Flame Emission and Atomic Absorption Spectrometry,” Marcel Dekker, New York Strasheim, A., and Jackson, P. F. S., “South African Institute of Physics Conference, Dl,” July Flannagan, F. J., Geochim. Cosmochim. Acta, 1973, 37, 1189. Graham, A. L., and Nicholls, G. D., Ibib., 1969, 33, 555. 1973, Pretoria. Plenum Press, New York. and London, in the press. 1973, Pretoria. Received APril 30th, 1973 Accepted August 2nd, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900026

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

38 Analyst, January, 1974, Vol. 99, p p . 38-42 The Thermometric Assay of Ascorbic Acid BY L. S. BARK AND J. K. GRIME* (Department of Chemistry and Applied Chemistry, University of Salford, Salford, Lancashire, M5 4WT) A method for the thermometric determination of ascorbic acid is described. The acid is oxidised by iodine monochloride in the presence of an excess of mercury(I1) ions. The effect of excipients has been studied. A comparison of the results obtained by using this method and the B.P. method shows that there is no significant difference in the accuracy of the two techniques. The main advantages of this method are those of time and the potential of the thermometric method for automation. THE titrimetric determination of milligram amounts of ascorbic acid in various materials has been reviewed.l The methods described are almost exclusively oxidimetric and involve such reagents as bromineJ2 N-bromosuccinimide,3 chloramine T4 and hexacyanoferrate( 111) ions5 ; however, the titrant most commonly used at present is 2,6-dichlorophenolindopheno16-g (Tillmans' reagent), the titration being carried out in media the acidic nature of which may vary from approximately pH 7 to relatively strongly acidic, depending on the material titrated and the presence of possible interfering substances.We have previously indicated1°-12 that many redox reactions have relatively high enthalpy changes and hence favour precision in thermometric titrimetry. Preliminary experiments indicated that although the oxidation of ascorbic acid with N-bromosuccinimide involves usable enthalpy changes, the use of highly concentrated titrant solutions, as demanded by the thermometric technique, was not possible for routine analysis as decomposition, resulting in loss of bromine and hence oxidative power, occurred on storing the concentrated solutions a t room temperature for periods of more than 8 hours.This disadvantage invalidated the use of an N-bromosuccinimide reagent. Aqueous solutions of potassium hexacyanoferrate( 111) also lose oxidative power on standing. In fact, all of the above mentioned reagents, with the exception of chloramine T, have a relatively short storage life in concentrated solutions; 2,6-dichlorophenolindophenol, in particular, must be standardised prior to each determination and it has been suggested that the reagent should not be used for 3 days after ~reparati0n.l~ There are several criteria that must be considered before a titrant can be used thermo- metrically, vix., stability, solubility, ease of standardisation, the kinetics of its reaction with analytes and a heat of reaction in excess of 4 kcal mol-l (16.64 kJ mol-l). It was found that iodine monochloride meets all of the above requirements. Titrimetric determinations of several organic substances based on substitution and addition reactions involving the use of this reagent have been previously reported14 and the possibility of using iodine mono- chloride as an oxidimetric reagent has also been examined.15J6 Iodine monochloride can be used as a titrant both in acidic and in weakly alkaline conditions.In acidic solutions the reagent can be reduced to iodide, if the redox potential of the titrated system is lower than + 0.4 V. The iodide ions thus formed can then be oxidised to iodine by further addition of iodine monochloride. Accordingly, two changes in potential can be observed in potentio- metric titrations and a similar effect would be expected in a thermometric titration. The calculated standard redox potential for the system I-/I+ is + 0.795 V. However, in the titration of strong reducing agents, iodine monochloride is reduced directly to elemental iodine, the reported standard potential for the reaction IC1,- + e- being + 1.06 V.1' -+ $1, + 2c1- The above mentioned reactions are used to determine the end-point by visual methods. The reagent is reduced to iodine, the first drop of reagent in excess liberates elemental iodine from the iodide present, and forms the well known blue colour with starch solution or colours a chloroform 1ayer.ls @ SAC and the authors.* Present Address : Department of Chemistry, Florida State University, Tallahassee, 32306, Florida, U.S.A.BARK AND GRIME 39 In the determination of certain substances, an intermediate separation of iodine occurs before the equivalence point so that the end-point is masked. In such cases mercury(I1) chloride can be added, the mercury(I1) ions complexing the iodide ions formed during the titration in accordance with the reaction- Hg2+ + 41- + Hg142- In the presence of an excess of mercury(I1) ions, red mercury(I1) iodide is eventually formed- Hg142- + Hg2+ -+ 2Hg12 The reaction proceeds more rapidly at the equivalence point, thus sharpening the visual end- point.As it was considered that the intermediate formation of elemental iodine, with the concomitant heat changes, would detract from the accuracy of the thermometric determination of ascorbic acid, the reaction has been investigated thermometrically both in the absence of mercury(I1) ions and in the presence of an excess of mercury(I1) ions. EXPERIMENTAL APPARATUS- and insulationll have been reported previously. 0.30 cm3 min-l (the actual rate being determined by gravimetry). magnetically. REAGENTS- Iodine monochloride standard solution, 0.3 M-The standard iodine monochloride solution is prepared by the oxidation of iodide with iodate in concentrated hydrochloric acid according to the equation- To prepare a 0.3 M solution, potassium iodide (50 g) is dissolved in water (100 cm3) and an aqueous solution of potassium iodate (62 g in 500 cm3 of solution) is added.Immediately, concentrated hydrochloric acid [200 cm3, shown to be free from iron(II1) and chlorine by the European Pharmacopoeia test] is added to the mixture. The solution becomes orange coloured and a brown precipitate is formed. The solution is slowly diluted to 1 litre with vigorous stirring until it becomes clear (gentle warming may be necessary to achieve a clear solution). A 0-5 to 0-0005 M solution is practically stable if sufficiently acidic (at least 2 M with respect to hydrochloric acid).la The solution is now standardised by the following method.An excess amount of potassium iodide solution is added to a known amount of iodine monochloride solution and the liberated iodine is titrated against standard sodium thiosulphate with starch as a visual indicator in the classical titration. Ascorbic acid-Because aqueous solutions of ascorbic acid are subject to considerable aerial oxidation, recrystallised solid ascorbic acid was used. In the tablet form, 50 mg (nominal) ascorbic acid tablets were used. Mercury(I1) chloride-A solution of mercury(I1) chloride (0.1 M) was made up in 2 M hydrochloric acid. The circuit for the basic electrical bridge systemlg and the details of the titration vessel The titrant was delivered at a rate of about The mixtures were stirred 21- + 103- + 6Hf = 31+ + 3H20 PROCEDURE In order to avoid any interferences from the heat of dilution of the acidic solvent, the sample is dissolved in a similar solution (10 cm3 of 2 M hydrochloric acid).Similarly, when the reaction occurs in the presence of an excess of mercury(I1) chloride, the sample is dissolved in a solution of mercury(I1) chloride, 0.1 M in 2 M hydrochloric acid. The mixture is next stirred in order to allow the system to reach thermal equilibrium (indicated by an electrical balance of the bridge and a constant trace on the recorder chart). The solution is then titrated with iodine monochloride solution (0-3 M in 2 M hydrochloric acid). The results obtained are shown in Table I. Several typical tablet excipients, i.e., magnesium stearate, lactose and starch, were added to the sample solutions and the effects on the accuracy and precision of the determination were noted (Table 11).40 BARK AND GRIME THE THERMOMETRIC [Analyst, VOl.99 TABLE I THERMOMETRIC DETERMINATION OF PURE ASCORBIC ACID Amount taken/mg Amount found/mg Error, per cent. 9.4 15-7 16-8 20-9 21.8 25.1 26.7 36.1 50-9 55.5 65.0 9.5 15.9 16.7 21.2 21.9 24.9 26.5 36.3 50.9 55.4 65.1 + 1.1 + 1-3 - 0.6 + 0.6 + 0-5 - 0.8 - 0.7 + 0.6 0.0 - 0.2 +0.1 Additional data: chart speed, 120 mm min-l; recorder sensitivity, 0.2 mV cm-l; titrant flow-rate, 0.3 cm3 min-l. For the determination of ascorbic acid in the dosage form, the compacted tablets are ground to a fine powder and then treated as solid ascorbic acid without separation of the matrix or excipient materials. It is necessary to stir the tablet mixture in the appropriate solvent for at least 5 minutes to ensure complete dissolution of the active ingredient.TABLE I1 EFFECT OF EXCIPIENTS ON THERMOMETRIC DETERMINATION OF Excipient and Amount of ascorbic Amount of ascorbic amount added/mg acid takenlmg acid found/mg Lactose Magnesium s tearate Starch In another series of powdered. An aliquot of dissolved and determined 30 36.7 60 30.5 30 29.4 60 37.9 30 30.4 60 30.8 36-3 30.2 29.0 37.5 30.0 30.7 ASCORBIC ACID Error, per cent. - 1.0 - 1.0 - 1.4 - 1.0 - 1.5 - 0.3 determinations, several ascorbic acid tablets were weighed and the powder equivalent to 50 mg of the active ingredient was next in the manner described above. The ascorbic acid content of a further aliquot of the powder was then determined by using the titrirnetric method recom- mended in the British Pharmacopoeia13 and the results are compared in Table 111.TABLE I11 COMPARISON OF THE THERMOMETRIC ASSAY AND BRITISH PHARMACOPOEIA ASSAY OF ASCORBIC Nominal amount of ascorbic acid present 50 mg ACID TABLETS Thermometric assay with iodine monochloride* Amount found/mg Average resultlmg 49.6 50.0, 49.1, 49.5, 49-6, 49.9 Maximum deviationlmg $0.4, -0.5 B.P. titrimetric method13t r \ 49.7, 49.1, 49-5, 49.2, 49.7 A Amount found/mg Average resultlmg 49.5 Maximum deviationlmg $0.2, -0.4 * Time taken for titration: approximately 1 minute. t Time taken for titration: approximately 3 to 5 minutes. The method used involves a titration of ascorbic acid against cerium(1V) ions with 1,lO-phenanthroline - iron(I1) sulphate as the visual end-point indicator.January, 19741 ASSAY OF ASCORBIC ACID DISCUSSION EFFECT OF THE PRESENCE OF MERCURY@) IONS- 41 Fig.l ( a ) represents a typical enthalpogram obtained in the absence of mercury(I1) chloride, while Fig. l ( b ) represents a typical enthalpogram obtained in the presence of an excess of mercury(I1) chloride. As can be seen from Fig. 1 (a) the titration in the absence of mercury(I1) chloride shows three definite stages. The portion BC of the graph represents the reaction between ascorbic acid and I+ to form I-, the portion CD represents the reactions between I+ and I- to form I, and the formation of the I,- ion (I, + I-) and the portion DE represents the evolution of iodine vapour and the dilution of the IC1 solution. The two volumes V , and V , are approxi- mately equal, the amount of the IC1 used in the second portion (CD) being governed by the amount of I- produced in the portion BC.In the presence of an excess of mercury(I1) ions [Fig. l ( b ) ] the intermediate formation of elemental iodine is arrested by the complexation of the liberated iodide ions. Conse- quently, the enthalpogram records only the heat of reaction of ascorbic acid with iodine monochloride (BC), with a sharply defined equivalence point at C. The latter method does not involve any extrapolative procedures and is therefore recommended. The thermometric determinations of ascorbic acid reported in Tables I, I1 and I11 are conducted in the presence of an excess of mercury(I1) chl6ride. I D ! t I- d Volume of titrant added ---L.Fig. 1. Thermometric titration of ascorb& acid with the use of iodine monochloride: (a) in the absence of mercury(I1) ions; and (b) in the presence of an excess of mercury(I1) ions. AB, temperature - time blank; BC, reaction I+/I-; CD, reaction I-/12; and DE, the evolution of iodine vapour EFFECT OF EXCIPIENTS- The maximum error reported for the determination of ascorbic acid in the presence of excipients (see Table 11) is -1.5 per cent. This compares with a maximum error of + 1.3 per cent. in the determination of ascorbic acid alone. The only limitation imposed is the maximum amount of insoluble material that can be effectively stirred without affecting the efficient transfer of heat through the mixture. Up to 100 per cent.excess of excipient can be tolerated without significantly affecting the accuracy of the determination. COMPARISON WITH B.P. METHOD- The results show that there is no significant difference in accuracy between the two tech- niques. The variations in any one procedure are those expected from the sampling of a mixture such as that found in commercially available tablets containing ascorbic acid. How- ever, the main advantages are those of time and the potential of the thermometric method for automat ion.42 BARK AND GRIME One of us (J.K.G.) thanks the Science Research Council for the provision of a grant during 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. this period of research. REFERENCES Ashworth, M. R. F., “Titrimetric Organic Analysis,” Part, I Interscience Publishers, New York, Tomicek, O., and Valcha, J., Colln Czech.Chem. Commun., 1951, 16, 113. Barakat, M. Z., Abd, El-Wahab, M. F., and El-Sadr, M. M., Analyt. Chem., 1955, 27, 536. Erdey, L., and Bodor, A., 2. analyt. Chem., 195213, 137, 420. van Pinxteren, J . A. C., and Verloop, E., Pharm. Weekbl. Ned., 1958, 93, 203. Tillmans, J., Hirsch, P., and Hirsch, W., 2. Lebensmittelunters. u. -Forsch., 1932, 63, 1. Kirkpatrick H. F. W., J . SOC. Chem. Ind., 1941, 60, 298. Lindner, K., 2. Lebensmittelunters. u -Forsch., 1955, 102, 37. Sitaramaiah, G., J. Indian Chem. Soc., 1957, 34, 147. Bark, L. S., and Grime, J. K., Analytica Chim. Acta, 1973, 64, 276. Bark, L. S., and Bark, S. M., “Thermometric Titrimetry,” Pergamon Press, Oxford, 1969, p. 52. “The British Pharmacopoeia 1973,” H.M. Stationery Office, London, 1973, pp. 36 and A55. Gengrinovich, A. I., Farmatsiya, 1946, 9 (6), 5 ; 1947, 10 (2), 23. Cihalik, J., and Vavrejnova, D., Chemicke’ Listy, 1955, 49, 693. Cihalik, J., Ibid., 1955, 49, 1167. Latimer, W. M., “Oxidation Potentials,” Prentice Hall, New York, 1952, p. 65. Berka, A., Vulterin, J., and Zyka, J., “Newer Redox Titrants,” Pergamon Press, Oxford, 1965, Bark, L. S., and Bate, P., Analyst, 1971, 96, 881. 1964, p. 463. , , Analyst, 1973, 98, 452. -~ p. 57. Received July 16th, 1973 Accepted August 16th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900038

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

AnaZyst, January, 1974, Vol. 99, $9. 4349 43 An Improved Procedure for the Complexometric Titration of Aluminium BY J. KRAGTEN (Natuurkundig Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65, Amsterdam, The Netherlands) Aluminium ions in solution show a marked tendency to hydrolyse with the formation of soluble polynuclear hydroxo complexes and precipitates of the trihydroxide, both of which are substantially unreactive. As the stability constant of the aluminium - EDTA complex is rather low, relatively high pH values are necessary to ensure complete formation of the complex; these pH values approach the critical range for hydroxide precipitation. Under these circumstances direct macro-scale titrations are of uncertain accuracy and direct micro-scale titrations become practically impossible to carry out.These problems can be overcome by the use of back-titration procedures coupled with carefully controlled pH adjustment in order to avoid locally exceeding the critical value for hydroxide precipitation. With macro-scale titrations the pH can be adjusted satisfactorily in a stepwise manner, but for micro-scale titrations the pH must be increased homogeneously. This type of increase can be achieved by using hexamine, which slowly releases ammonia in boiling aqueous solutions. Based on these observations, a pro- cedure is presented that provides for the complexometric determination of aluminium at concentrations down to 0.5 p.p.m., with an accuracy of 1 per cent. or better. THE behaviour of aluminium ions in solution renders special precautions necessary in the complexometric determination of this element. The more important factors to be taken into account are : firstly, that aluminium(II1) solutions show a marked tendency to hydrolyse; secondly, that the solubility of aluminium trihydroxide is very low; thirdly, that the precipi- tated aluminium trihydroxide and also the soluble polynuclear hydroxo complexes are rather unreactive; fourthly, that aluminium ions react relatively slowly with EDTA; and finally, that the stability constant of the aluminium - EDTA complex is rather low for a tervalent element .Many inve~tigatorsl-~ have described procedures for the titration of milligram amounts of aluminium in concentrations of about M. In most papers it is concluded that indirect titrations are superior to direct titrations; an accuracy of 1 per cent.can be attained when EDTA is added in excess at pH 3-0, the solution is boiled for several minutes, and finally the back-titration is carried out at pH 5 to 6. However, this procedure involving a stepwise change in pH is not suitable for the determination of aluminium(II1) at lower concentrations (lo-* M). Flaschka, ter Haar and Bazen7 were the only authors to report a micro-scale titration of aluminium; errors of about 10 pg in 100 to 900 pg were found. Much better results, however, can be obtained. In this paper it will be shown that even micromolar amounts of aluminium can be titrated in the p.p.m. range with a precision of 1 per cent., provided that a special neutralisation procedure is followed.THE FORMATION OF HYDROXIDES- The formation of soluble polynuclear hydroxo complexes and the formation of a pre- cipitate both depend on the pH as well as on the concentration of aluminium. At low concentrations only mononuclear complexes will be formed. Maximum concentrations exist at which polycomplexation and precipitation commences ; such a maximum concentration is a function of the pH. The equations for these maximum concentrations are derivedbelow. The values found in the literat~re83~ for the stability constants (*/I) of A1(OH)i (i = 1, 2, 3, 4) are log *PI = -4.3, log *pz = -9.3, log *ps = -15 and log */I4 = -22.0 (*pi = ‘M(OH)il x [HIi). In the absence of polycomplexes and a precipitate, the distri- bution of the various mononuclear complexes calculated with these values is as represented [MI @ SAC and the author.44 KRAGTEN: AN IMPROVED PROCEDURE FOR THE [Analyst, VOl.99 in Fig. 1. The corresponding side reaction coefficient,1° CCA~(OH), is given in Fig. 2. Polycomplexes are regarded as being absent when the concentration of each species is less than 1 per cent. of the total concentration of aluminium. This can be expressed mathe- PH Fig. 1. Distribution of the various mono- nuclear aluminium hydroxo complexes versus pH in the absence of other complexing agents. Curves: A, AP+; B, A10H2+; C , Al(OH),+; D, Al(OH),; and E, Al(OH),- (= H,AlO,-) PH Fig. 2. Log CCA~IOH) versus pH in the absence of poIynuclear complexes. This curve is the so-called mononuclear wall. Two groups of polynuclear complexes can be distinguished, according to the literature.8~~ For the 2:2 complex log p2:2 = 7.5.The composition of the second group is dubious; it seems significant, however, that the values for the Complexes 32: 13, 34: 13 and 17: 7 all lead to log [M'] values that coincide, within the limits of error. The corresponding curves are presented in Fig. 3; curve a relates to the 2: 2 complex and curve b to the second group. The formation of an aluminium hydroxide precipitate depends on the solubility con- stant, KsO: Combining equations (5) and (2) g' ives- Kso = [MI [OH]" or *Kso = [M]/[H]" . . .. * (5) log [M'Jmax. = log *Kso - PH + log ~ M ~ O H ) . . - * (6) For dilute solutions log *Kso = 9.5.899 In Fig. 3 curve c corresponds to this value. Wanninen and Ringb~m,~ and Schwarzenbach and Flaschkall concluded from less reliable results obtained by Brossetl2 that binuclear complexation determines the upper limitJanuary, 19741 COMPLEXOMETRIC TITRATION OF ALUMINIUM 45 of the pH at all concentrations of aluminium.This does not agree with the results in Fig. 3, from which it can be seen that polynuclear complexes do not exist in dilute solutions (less than W 3 M ) . l o / PH Fig. 3. Limits of the concentration as a function of pH. Curve a represents the limit for the formation of the (2 : 2) complex, curve b the limit for the formation of the second group of polynuclear hydroxo complexes and curve c the limit for precipitation of Al(OH),. The region in which the formation of complexes or precipitation of Al(OH), occurs lies under the curves.Curves d,, d, and d, represent total concentrations of A1 of and mol l-l, respectively, in equilibrium with a 25 per cent. excess of EDTA. The broken lines indicate the supposed pathways during the change in pH When a sample is dissolved for analysis a strongly acidic solution is generally produced. In order to establish the correct experimental conditions the pH is commonly increased by adding chemicals such as solid acetate, solid hexamine, ammonia solution , sodium hydroxide solution or a solution of hexamine. In the vicinity of the added chemical locally high pH values occur. It follows that the region under curve c (Fig. 3) is passed through by a part of the solution, with the implication that even if the final pH lies outside the region, at least some of the aluminium has been transformed into a form that is unreactive with respect to EDTA.Deviations will therefore occur and it is clear that a special neutralisation procedure will be necessary. THE COMPLEXOMETRIC TITRATION OF ALUMINIUM- considered. conclusions differ only in detail. In the following only the photometric, complexometric titration of aluminium will be The Analogous considerations can be made for other end-point techniques. For a direct titration the following titration conditions must be satisfied13- .. .. .. * . (7) log (C, x KMrI, ) > 1 .. .. .. (8) GdKhl’L’ > 3.5 log { c z } where C M is the total concentration of metal M and CI that of indicator I. KM’V and K ~ I T are the corresponding conditional constants. For aluminium this becomes log K M q t > 4.5 + PCA .. .. .. * * (9)46 KRAGTEN: AN IMPROVED PROCEDURE FOR THE [Analyst, VOl. 99 If we deal with a macro-scale titration in which CAI w M, equation (9) is satisfied for pH > 3.5, according to Fig. 4. As only negligible amounts of indicator are used in macro-scale titrations, the starting point P of the titration at pH 3.5 will lie rather close to the lines a, b and c in Fig. 3. Theoretically, it must be possible to reach P without pene- trating the area under c, but the prevention of a pH overshoot is hardly possible in practice. For this reason direct macro-scale titrations are inferior to back-titrations. For a micro-scale titration in which, for example, CAI w loF4, an equivalent amount of indicator is generally used, with the result that the concentration of aluminium not bound to the indicator is lower.Depending on the value of CIKMI, this concentration may be several powers of 10 less, but at least 10 times less, at the pH of the titration (equation 9). The corresponding point, Q, for which equation (10) is satisfied, will thus lie in, or just above, the precipitation region near pH 6. For a suitable titration Q should be reached, starting from point S, without passing line c. At S, generally no metal - indicator complex has yet formed and, as the hydroxide formation is much faster than the indicator - complex formation, it will be obvious that Q can only be reached by passing through the region under c. This phenomenon renders an accurate direct micro-scale titration impossible.4 6 8 10 PH Fig. 4. of A1 veysus pH. on ordinate: A, K A ~ Y ~ ; B, K A I ~ Y ~ ; and C, Q A ~ ~ O H ) Variation of the conditional constant Logarithms of functions plotted For back-titrations, in which the excess of EDTA is titrated with a metal M (for example, .. . . (10) lead), the following titration conditions must be satisfied14- log { (1 - yu) x C I K M t I t for M, and log (CAI X K~1’yt) - log .. . . (11) for aluminium. From equations (10) and ( l l ) , and the value of yAI, which can be taken as being 0-8 (EDTA is usually added in an excess of about 25 per cent.), it can be seen that the following condition must be satisfied- If CAl w 10-2, condition (12) is satisfied for a pH greater than 3.5; with a micro-scale titration log KA1,Y should exceed 9.1, which is satisfied for pH rn 5.5.These experimental conditions must be satisfied during the back-titration. From the considerations of the direct titration log K A y y , > 5.1 + PCU .. . . .. . . (12)January, 19741 COMPLEXOMETRIC TITRATION OF ALUMINIUM 47 it is obvious that these pH values cannot be reached in the absence of EDTA without passing through the precipitate region; EDTA should therefore be added before adjustment of the pH. It can be proved that, if aluminium remains in equilibrium with EDTA in excess while the pH is varied, the concentration of the aluminium not bound to EDTA (fi[Al(OH),]), as a function of the pH, is given by curves d,, d, and d, in Fig. 3. At low and high pH values K will not be large enough for complexation. In these instances aluminium remains free as AP+ and H,AlO,-, respectively.The horizontal parts of the curve represent total aluminium concentrations of and M, respectively. From Kd,y# = [AlY]/[Al'] x [Y'], it follows that when a 25 per cent. excess of EDTA is present p[Al'] = -l~gt[Al(OH)~] = log (t x KA1ty.) .. . . (13) This equation determines the shape of the middle section of curve d. The crucial point now is that when the pH is changed and p[Al'] follows curve d, the region of hydroxide precipitation is not passed. However, A1 and EDTA react slowly, even at 100 "C, with the result that special precautions have to be taken in order to ensure that equilibrium is approximately achieved during the pH change. In concentrated aluminium solutions at pH less than 1.5, no hydroxides that are inert with respect to EDTA will occur after heating; thus, all species that are present will be dissociated.If such solutions, which are generally obtained by simply dissolving the sample, are diluted to make a stock solution the pH will usually lie between 1.5 and 2. When aliquots are withdrawn and made up to the volume of the titration cell, the pH in the titration medium will be about 2.5. M), the concentration of aluminium not bound to EDTA will decrease to 10-4.5~ after boiling. When the pH is increased to 5.5 for the back-titration procedure, only negligible amounts of aluminium are lost as hydroxides precipitate. This small loss explains the success of the two-step procedure1-' mentioned previously. With a micro-scale titration this procedure is unsuccessful, as a solution of 1 0 - 4 ~ is insufficiently complexed at pH 2.5.Repeating the procedure at pH 4 did not give correct results; obviously the region under curve c was penetrated. Continuous change in pH by making use of the slow dissociation of hexamine into formaldehyde and ammonia at tempera- tures above 90 "C was found to be very suitable for our purpose. This sytem leads to a slow, uniform increase in the pH from 2 to about 6; the latter pH lies in the buffer region of the hexamine (some of the hexamine remains undissociated). If the pH of the solution to be titrated is too low (less than 1) more hexamine will be needed. In that event the dissociation velocity increases so much that the critical pH range of 2 to 6 may be passed too quickly.In some instances systematic deviations then occur, rendering the result of an individual titration less reliable. It is advisable to evaporate off the excess of acid after dissolution of the sample in order to obtain starting solutions of only moderate acidity. EXPERIMENTAL APPARATUS- The photometric titrations were performed in a Zeiss PMQ I1 spectrophotometer with a titration assembly for 10 and 20-ml cells (2-cm path length). The titrant solutions were added with Metrohm microburettes, Type E457. The tips of the 0.5-ml assemblies were bent upwards in order to prevent gravitational losses when they are immersed in the fluid. The water was purified by sub-boiling demineralised water in quartz distillation appara- tus15 (Quartz and Silice, Paris). REAGENTS- All solutions were prepared from analytical-reagent grade chemicals and sub-boiled water.15 Ethylenediaminetetraacetic acid, disodium salt, solution, 5 x 10-3 M-The EDTA solution was standardised against Specpure grade copper with purified l-(2-thiazolylazo)-2-resorcinol (TAR) as indicator.(TAR is commercially available from Fluka, A.G., Buchs, Switzerland; phenylazo-2-resorcinol and phenylazo-2-naphthol cannot be used because their KMI values are too large.13) It was found that the blanks were less than 0.1 per cent. and that the mean value was 4990 x When an excess of EDTA is added in a macro-scale titration M with a relative standard deviation of 0-15 per cent.48 KRAGTEN: AN IMPROVED PROCEDURE FOR THE [Analyst, Vol. 99 M cerium(I1I) nitrate at pH 3)-This solution was stan- dardised by photometric titration against EDTA with xylenol orange as indicator (5.01 x 10-3 MI. Hexamine-A 150-mg amount of hexamine was dissolved in about 20ml of water and brought to pH 2 with hydrochloric acid.A fresh solution must be prepared daily. Indicator-A solid mixture of xylenol orange - potassium nitrate (1 + 100). AEuminiuum soZutio-A 50-mg amount of aluminium (Johnson and Matthey, 99.9 per cent.) was dissolved in 5 ml of a 5 per cent. sodium hydroxide solution and the solution diluted to 20 ml. To this solution 2 ml of concentrated hydrochloric acid was added quickly. Rapid addition is necessary in order to prevent the formation of an unreactive hydroxide. The acidic solution is diluted to 500 ml and should have a pH between 1.3 and 1.7.Micro- molar amounts are taken from this solution, either directly, by means of a microburette (25Opl), or after 1 + 20 dilution, by withdrawing 5 ml in a pipette. PROCEDURE- A volume of solution containing the equivalent of about 1 pmol of aluminium is intro- duced into a 20-ml titration cell containing 5 to 10 ml of water. To this solution are added 300 to 400 pl of 5 x M EDTA, which corresponds to a 50 to 100 per cent. excess, and finally 2 ml of the acidified hexamine solution, which corresponds to a two-fold excess, over the amount required for neutralisation. The solution is heated for 5 minutes at almost 100 "C and after cooling the pH should lie between 4-5 and 5.5. If the pH is less than 4-5, the pro- cedure can be repeated with another 1 ml of hexamine solution; however, if the pH is greater than 5.5, too much hexamine has been added and errors may occur.In the latter instance the procedure should be repeated with a smaller amount of hexamine. Finally, the pH is adjusted to 5.5 & 0.1 with solid hexamine (10 to 30 mg) or dilute hydrochloric acid, and the excess of EDTA is back-titrated with lead or cerium(II1) solution with xylenol orange as indicator (in this work cerium(II1) solution was generally used). RESULTS AND DISCUSSION Some results are given for various amounts of aluminium in Tables I and 11. Cerium(I1I) solution (5 x The standard deviation can be taken as 0.005 pmol (w 0.14 pg) for a single titration. This value is of the same order as the approximate values 0.003 pmol and 0.004 pmol that follow from the correlation coefficients (Y) of series 111 and IV (Table 11) (r is an indication of the accuracy TABLE I PHOTOMETRIC BACK-TITRATION OF ALUMINIUM of fit).Series I Series I1 r 1 Taken/pmol Found/pmol 1.055 1.057 (28.5 P.g) 1-056 1-048 1.059 1.050 1.051 1.065 1.051 5, = 1.055 pmol ox = 0.006 r - \ Takenlpmol Found/pmol 0.408 0.400 0-409 0-404 0.402 0.405 (11.0 CLg) Z2 = 0.404 pmol U X = 0.004 In series I11 and IV the slopes deviate by only 0.3 per cent. from the theoretical values. Nevertheless, these deviations of 1 nmol per step suggest that the procedural errors ( b ) are significant. In both series b is slightly larger than the maximum amounts of impurities stated in the specifications of the reagent used. As far as is known, the procedure presented here is the only method in which aluminium can be determined with a precision of better than 1 per cent.in concentrations down to 10 pg in 20 ml (0.5 p.p.m.) ; it is more precise than atomic-absorption spectrophotometry. A drawback is that a separation usually has to precede the aluminium determination. Ion- exchange chromatography is very suitable for this purpose. Aluminium can be converted into the H,A103- ion in the presence of ammonia (pH 9-5 to 10) and can then be separatedJanuary, 19741 COMPLEXOMETRIC TITRATION OF ALUMINIUM TABLE I1 LINEAR INCREASE PROCEDURE IN THE PHOTOMETRIC BACK-TITRATION OF ALUMINIUM 49 Series I11 Taken Found +--7 & CLg pmol PQ pmol 9.8 0.363 9.9 0.367 10.0 0.370 19.6 0.726 19.9 0.736 19.8 0.733 29.4 1.089 29-6 1.097 29.6 1.097 39.2 1.452 39.4 1-460 39.4 1.461 I A -I Series IV 7- Taken Found & * Pg pmol pmol 9.8 0.363 9-9 0.367 9.8 0.365 19-6 0.726 19.7 0.731 19.7 0.729 29.4 1.089 29-7 1.100 29.6 1.096 39.2 1-452 39-3 1.458 39.3 1.468 Correlation coefficient (Y) = 0.999 995 Procedural error (b) = +0.006 pmol = 0.16 pg Slope = 0.364 pmol per step Correlation coefficient (r) = 0.999 98 Procedural error (b) = +0.003 pmol = 0.08 pg Slope = 0.364 pmol per step from copper and nickel on a cation-exchange column.In the author’s laboratory the alu- minium content was found with a precision of 1 per cent. in such cases. The whole procedure takes about 2 hours when used as a routine method. The author is grateful to Mrs. A. Briedk, Mr. J. Tan and Mr. A. Ph. Reynaert for their help in the practical performance of the determinations and to Professor Dr. G. den Boef for reading the manuscript. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. REFERENCES Milner, G. W. C., and Woodhead, J. L., Analyst, 1954, 79, 363. Flaschka, H., and Abdine, H., 2. analyt. Chem., 1956, 152, 77. Wanninen, E., and Ringbom, A., Analytica Chzm. Acta, 1955, 12, 308. Flaschka, H., and Abdine, H., Mikrochirn. Acta, 1955, 37. Gottschalk, G., 2. analyt. Chem., 1960, 172, 192. Pzibil, R., and VeselS;, V., Talanta, 1962, 9, 23. Flaschka, H., ter Haar, K., and Bazen, J., Mikrochirn. Acta, 1953, 345. Silldn, L. G., and Martell, A. E., “Stability Constants,” Special Publication No. 17, The Chemical Society, London, 1964. Society, London, 1972. London, 1963, p. 38. p. 184. -- 1 , “Stability Constants, Supplement No. 1,” Special Publication No. 25, The Chemical Ringbom, A., “Complexation in Analytical Chemistry,” Interscience Publishers, New York and Schwarzenbach, G., and Flaschka, H., “Complexometric Titrations,” Methuen, London, 1968, Brosset, C., Acta Chem. Scand., 1952, 6, 910. Kragten, J., Talanta, 1971, 18, 311. -, Ibid., in the press. Kiihner, E. C., Alvarez, R., Paulsen, P. J., and Murphy, T. J., Analyt. Chern., 1972, 44, 2050. Received April 4th. 1973 Accepted July loth, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900043

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

50 Analyst, January, 1974, Vol. 99, $9. 50-53 Gas- chromatographic Determination of Dilauryl pp’- Thiodipropionate and its Primary Oxidation Products BY J. SEDLAR, E. FONIOKOVA AND J. PAC (Research Institute of Macromolecular Chemistry, Tkalcovskd 2, Bmo, Czechoslovakia) A method is described for the gas-chromatographic determination of dilauryl pP’-thiodipropionate and its oxidation products, dilauryl sulphenyl- dipropionate and dilauryl sulphonyldipropionate. The sample is first hydrolysed in a 5 N methanolic solution of potassium hydroxide and the resulting lauryl alcohol is then determined gas chromatographically by using the internal marker technique. The method can be used to determine concentrations down to 10 p g ml-l, with a standard deviation of f3 per cent. DILAURYL PP’-thiodipropionate (DLTP, Negonox DLTP, Advastab PS 800), C12H2,0CO- CH2CH2-S-CH2CH2COOC12H25J is widely used as a component of synergistic compositions with phenolic antioxidants for the ~tabilisationl-~ of polypropylene, polyethylene, ethylene - vinyl acetate copolymers, acrylonitrile - butadiene - styrene resins and high-impact poly- styrene.The protective action of DLTP is believed2 to result from its ability to reduce hydroperoxides, via a non-radical process, to the corresponding alcohols and its sulphoxide and sulphone oxidation products. The isolation and determination of additives from polymer materials have been reviewed by several a ~ t h o r s . ~ - ~ The quantitative determination of DLTP in extracts is usually effected by determining the sulphur c ~ n t e n t , ~ , ~ although this is a time-consuming procedure.Gel-permeation chromatographys has been successfully applied to the separation and deter- mination of DLTP in multicomponent antioxidant mixtures, while infrared spectroscopy based on measurement of the extinction of carbonyl groups has been used for the direct quantitative determination of DLTP in undegraded polypropylene foils.lO A method in- volving the use of the esterification of dilauryl thiodipropionic acid by diazomethane has been reported as being suitable for the determination of DLTP in 1ard.ll Neureither and Bown2 used a high-speed polarographic technique for the quantitative determination of DLTP. Attempts to carry out a direct quantitative determination of this compound by using luminescence12 or gas-chr~matographicl~ methods have failed.No references to methods for the determination of the oxidation products of DLTP have been found. In the present work, we examined the conditions under which DLTP and its oxidation products are hydrolysed quantitatively to lauryl alcohol, thus making gas-chromatographic determination possible. The method described here has the advantages of being fairly rapid, accurate and simple. It has been applied successfully to the analysis of polypropylene samples containing 0.02 to 0.3 per cent. of DLTP and its oxidation products, the sample size being about 1.0 g of polymer. The last condition is hardly possible when the other methods are used. The additives, including DLTP, were extracted quantitatively from the sample under an atmosphere of nitrogen with a chloroform - ethanol - n-hexane (1 + 1 + 4) mixture by using a semimicro-extractor of the Soxhlet type.DLTP was separated from its oxidation products as well as from other additives by thin-layer chromatography on silica gel coated plates. The spots containing DLTP were extracted quantitatively by use of a chloroform - benzene (1 + 1) mixture and the extracts analysed by the method described under Procedure. METHOD REAGENTS- L aury 1 alcohol-Analyt ical-reagen t grade. Dilauryl PP’-thiodi$ro$ionate (DLTP)-This reagent was prepared by the esterification of thiodipropionic acid with lauryl alcohol in the presence of 9-toluenesulphonic acidl4; @ SAC and the authors.SEDLA~, FONIOKOVA AND PAC 51 sulphur assay, 6.23 per cent.(theoretical value, 6.23 per cent.); melting-point, 39.5 "C. For some analyses, commercial preparations were used. Dilaury sulphenyl-pp'-dipropionate (DLS0)-This was prepared by oxidation of DLTP with chromic acid in an acetic acid medium at 60 to 80 "C15 (melting-point, 74 "C). Dilauryl sulpJzonyl-~~'-dipropionate (DUO,)-This was also prepared from DLTP, by the method described for the oxidation of sulphides to sulphones16 with hydrogen peroxide solution at 90 "C (melting-point, 91 "C). n-Octadecane. Chloroform. Methanol. Potassium hydroxide. The last four reagents should be of analytical-reagent grade. Standard lauryl alcohol solution-Weigh accurately about 70 mg of lauryl alcohol into a Standard n-octadecane solution-Weigh accurately about 200 mg of n-octadecane into a Standard potassium hydroxide, 5 N solution in methanol-Dissolve 3 g of potassium hydrox- 100-ml calibrated flask and make the volume up to the mark with chloroform. 100-ml calibrated flask and make the volume up to the mark with chloroform.ide pellets in 2 ml of water and make the volume up to 10 ml with methanol. APPARATUS- A Perkin-Elmer, Model F 11, dual-column chromatograph equipped with flame-ionisation detectors and an isothermal column oven was used. The oven was operated isothermally at 165 "C and the injection port at 300 "C. Suitable column conditions were obtained with a 6-foot x $-inch glass column, packed with 1.5 per cent. of fluorosilicone oil FS-1265 on 80 to 100-mesh Chromosorb W AW-DMCS. Argon was used as the carrier gas at the flow-rate of 30 ml min-l.A 0-6-pl volume of the lauryl alcohol containing sample was introduced into the column by means of a 1-p1 Hamilton No. 7001 syringe. PROCEDURE- Place 5 ml of solution containing 0.1 to 3 mg of DLTP, DLSO or DLSO, in a test-tube (30 mm 0.d.) provided with a B29 ground-glass joint and evaporate off the solvent under a stream of nitrogen. Add 1 ml of freshly prepared methanolic potassium hydroxide solution, fit a reflux condenser or a cooling finger into the joint and immerse the bottom of the tube in a heating bath, the temperature of which is maintained at 80 "C. After heating for 30 minutes, transfer the contents of the tube quantitatively into a 25-ml cylindrical separating funnel (20 mm 0.d.) by using a total of 10 ml of water.Finally, rinse the tube with 2 ml of chloroform and add the rinsings to the water in the funnel. Shake the funnel well and when the layers have separated collect the bottom layer in a 10-ml calibrated flask. Repeat the extraction a further three times, using 1-5 ml of chloroform for each run. Add 1 ml of n-octadecane solution to the combined extracts in the flask and make the solution up to 10 ml with chloroform. Prepare the reference solution by placing 5 ml of standard lauryl alcohol solution in a 10-ml calibrated flask, adding 1 ml of n-octadecane solution and making the solution up to 10ml with chloroform. Run the chromatograms of both reference and sample solutions under the conditions described. Calculate the amount of lauryl alcohol formed by hydrolysis according to the equation where [LOH] is the concentration of lauryl alcohol in the sample in milligrams per 10 ml, a is the concentration of lauryl alcohol in the reference sample in milligrams per 10 ml, V, and V 3 are the peak heights due to lauryl alcohol in the reference and sample, respectively, V , and V4 are the peak heights due to n-octadecane in the reference and sample, respectively, and Q1 and Q, represent the sensitivities for the lauryl alcohol peak in the reference and sample run, respectively.RESULTS . . * * (1) == a V2V3Q2/VV,V4Q1 . . .. Under the column conditions described above the peaks due to the solvent, lauryl alcohol The retention data are summarised in and internal standard (n-octadecane) are well resolved. Table I.52 SEDLM et al.: GAS CHROMATOGRAPHY OF DILAURYL TABLE I RETENTION DATA [Analyst, Vol. 99 Relative retention Compound time Chloroform . . .. 0.10 Lauryl alcohol . . 0.68 n-Octadecane 1.00 (5.6 minutes) A series of hydrolyses at 80 "C in 5 N methanolic potassium hydroxide solution for 30 minutes was carried out within the concentration range of 0.15 to 3.00 mg of DLTP per 10 ml. The results are shown in Fig. 1, in which the amount of lauryl alcohol formed by hydrolysis is plotted against the amount of DLTP hydrolysed. 0 0.5 1 *o 1.5 2.0 2.5 DLTP hydrolysed/mg Fig. 1. Relationship between the amount of lauryl alcohol found and the amount of DLTP hydrolysed. Conditions of hydrolysis : temperature, 80 "C; reagent, 6 N potassium hydroxide solution in methanol; and time, 30 minutes The slope of the straight line, evaluated by the least-squares method, was found to be 0.7028.The mean deviation of the experimental points from the calculated values was & 2.5 per cent. over the whole concentration range. The theoretical value of the slope, assuming that the complete hydrolysis of DLTP gives two molecules of lauryl alcohol per molecule of DLTP decomposed, is 0.7238. Determination carried out under these conditions therefore gives results that are consistently low by a factor of 0.97. The conditions of hydrolysis described above were found to be the most suitable. At higher temperatures, the oxidation of lauryl alcohol occurs so that lower apparent amounts of lauryl alcohol are found. At lower temperatures, on the other hand, the hydrolysis becomes incomplete.Similar conditions hold for the hydrolysis of DLSO and DLSO,. The actual amount of the particular compound being determined in the sample can therefore be calculated by miiltiplying the observed value by a factor of 1.03, which accounts for the systematic error of the analysis. An example of the results obtained in the analysis of polypropylene sheets stabilised with pure DLTP is given in Table 11. TABLE I1 RECOVERY OF DLTP FROM POLYPROPYLENE SHEETS Concentration in polymer before Standard pressing/g kg-1 DLTP found in polymer/g kg-l deviation 3.00 2.67, 2.79, 2.79, 2.76, 2.94, 2.76 k0.09January, 19741 ,~P‘-THIODIPROPIONATE AND ITS PRIMARY OXIDATION PRODUCTS 53 The assay of lauryl alcohol in commercially produced DLTP frequently gives lower results, due mainly to the fact that these preparations contain, in addition to DLTP, other esters of thiodipropionic acid (e.g., cetyl, stearyl and possibly higher derivatives).A calibration should be carried out in each particular instance. Acknowledgement is due to Dr. M. Uhlif of this Institute for the synthesis of pure DLTP and its oxidation products. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 24. 15. 16. Dickson, D. M., Hercules Powder Company, German Patent 1,108,427, 1958. Neureither, N. P., and Bown, D. E., I n d . Engng Chem., Prod. Res. Dev., 1962, 1, 236. ICI Technical Information, R 42, Dyestuffs Division, Blackley, Manchester. Wheeler, D. A., Talanta, 1968, 15, 1315. Crompton, T. R., Eur. Polym. J . , 1968, 4, 473. _- , “Chemical Analysis of -4dditives in Plastics,” Pergamon Press, Oxford, 1971. Granatelli, L., Analyt. Chew., 1959, 31, 434. Slanina, J., Agterdebos, H., and Grieping, B. F. A., Microchim. Acta., 1970, 1225. Coupek, J., Pokornjl, S., ProtivovA, J., HolCik, J., KarvaS, M., and PospiSil, J., J . Chromat., 1972, Majer, J , and KocmanovA, V., Chem. Prdm., 1967, 17, 372. McCaulley, D., J . Ass. Off. Analyt. Chem., 1967, 50, 243. Kirkbright, G. F., Narayanaswamy, R., and West, T. S., Analytica Chim. Acta, 1970, 52, 237. Lappin, G. R., and Zannucci, J . S., Analyt. Chem., 1969, 41, 2076. “Houben-Weyl’s Methoden der Organischen Chemie,” Band VIII, Georg Thieme Verlag, Stuttgart, Knoll, R., J . prakt. Chem., 1926, 113, 40. Rheinboldt, H , and Giesbrecht, E., J . Amer. Chem. SOG., 1946, 68, 973. 65, 279. 1952, p. 522. Received June lst, 1973 Accepted August 2nd, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900050

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

54 Analyst, January, 1974, Vol. 99, p p . 54-57 Gas-chromatographic Analysis of Polyurethane Polyethers by Using a Mixed Anhydride Reagent for the Cleavage of Ether Linkages BY KAZURO TSU JI AND KAZUO KONISHI (Industrial Research Laboratories, I<ao Soap Co., Ltd., 1334 Minatoyakushubata, Wakayawta-shi Japan) The polyethers formed from propylene glycol, glycerol, trimethylol- propane [ 1, 1, 1-tris(hydroxymethyl)propane], pentaerythritol, sorbitol, triethanolarnine, 1,2-diaminoethane and 2,2'-diaminodiethylamine, and which are used in the manufacture of polyurethane foams, have been quali- tatively analysed by gas chromatography after conversion of the base com- pounds into their corresponding acetates by using the mixed anhydride of acetic and toluene-p-sulphonic acids as a reagent for cleavage of the ether linkages.The polyethers are allowed to react with the reagent a t 120 "C for 2 hours. The reaction products are extracted with diethyl ether and the concentrated extract is subjected to gas chromatography. As, under appro- priate conditions, the peaks of these acetates appear a t different positions, they were easily distinguished. The proportions of oxyethylene and oxypropylene groups in the poly- ethers can also be determined by gas-chromatographic analysis of the acetyla- tion products. ETHYLENE oxide and propylene oxide adducts of polyhydric alcohols and amines are widely used as polyethers in the production of polyurethane foams by reaction with diisocyanates. The physical properties of the foams depend to a certain extent on the chemical structure of these polyethers, so it is very important to establish a method for the identification of the base compounds and determination of the proportions of their oxyethylene and oxypropylene groups.Mathias and Mellorl split the polyethers with hydrobromic acid - acetic acid to give bromides, which were analysed by gas chromatography. In this way, the content of oxy- ethylene groups, and, therefore, the original polyliydric alcohols, were determined. Stead and Hindley2 modified this method and obtained good results for the determination of the oxyethylene group contents of ethylene oxide - propylene oxide copolymers. (The deter- mination of the content of oxyethylene groups in these copolymers can easily be carried out by nuclear magnetic resonance ~pectrometryl~~ without chemical splitting of the ether linkages.However, it is difficult to identify the base compounds by this method.) A number of methods for the cleavage of ethers have been studied, but few were applied to the identification of the base compounds of the polyurethane polyethers. Several mixed anhydrides of carboxylic and sulplionic acids, as proposed by Karger and I l l a z ~ r , ~ act as reagents for the cleavage of ether linkages, particularly that of acetic and toluene-$-sulplionic acids, which is not only a powerful reagent for the cleavage of ether linkages but is also an active acetylating agent. For example, when the propylene oxide adduct of glycerol is treated with this reagent, the polyether is split, thus giving glycerol triacetate and propylene glycol diacetates, which are easily identified by gas ~hromatography.~ In this paper, this method is extended to the identification of eight base compounds and the determination of their oxyethylene and oxypropylene group contents.EXPERIMENTAL CLEAVAGE REL4GENT- Acetic anhydride (80 g) was added dropwise to 120 g of toluene-9-sulphonic acid con- tained in a 300-ml round-bottomed flask at room temperature and the mixture was refluxed at 120 "C for 30 minutes. The product obtained was used as the reagent without removal of the ace5c acid produced and the excess of acetic anhydride. @ SAC and the authors.TSUJI AND KONISHI 55 SAMPLES- Polypropylene glycols based on propylene glycol, glycerol, trimeth ylolpropane [ l , l , 1- t ris (hydrox ymethyl) propane], pent aerythrit 01, sorbit 01, triethanolamine, 1,2-diaminoethane and 2,2'-diaminodiethylamine were used for the identification of the base compounds.For the purpose of determining their oxyethylene contents, ethylene oxide - propylene oxide block copolymers and polyols (glycerol and trimethylolpropane) , and 1,2-diaminoethane that had been first oxypropylated and then oxyethylated, were used. All of these samples are commercially available. APPARATUS- The separation of acetates was carried out on a Hitachi gas chromatograph, Model 063, equipped with a thermal conductivity detector. The column consisted of a 1-m length of 3 mm i.d. stainless-steel tubing packed with 15 per cent. m/m FFAP (free fatty acid phase) (or 10 per cent. m/m SE-30 silicone) coated on 60 to SO-mesh Uniport B (Gas Chromatography Co., Ltd., Japan).The sample was injected with a 10-pl Jintan Terumo syringe. The determination of the content of oxyethylene groups was ascertained by the combined use of the nuclear magnetic resonance instrument, Model JNM- 3H-60 (Japan Electron Optics Laboratories), equipped with a JES-1D-2 integrator. PROCEDURE- A 100-mg amount of sample and 2 ml of reagent were mixed in a 20-ml round-bottomed flask and the mixture was refluxed on an oil-bath at 120 "C for 2 hours. The contents of the flask were cooled to room temperature and then neutralised with 50 per cent. aqueous sodium carbonate solution, followed by extraction with about 20 ml of diethyl ether. The ether layer was washed with de-ionised water and concentrated to a small volume on a steam-bath; the concentrate was then injected into the gas chromatograph. The FFAP column was opera- ted isothermally at the appropriate temperature for the purpose of both identifying the base compounds and determining their oxyethylene group contents.For the analysis of the polyethers based on 1,2-diaminoethane or 2,2'-diaminodiethylamine the SE-30 column was used. The samples for measurement by nuclear magnetic resonance were prepared as 10 per cent. solutions in chloroform, with tetramethylsilane as an internal standard. Helium gas was used as the carrier gas. RESULTS IDENTIFICATION OF BASE COMPOUNDS- The reaction products of the mixture of polyethers based on propylene glycol, glycerol, trimethylolpropane, pentaerythritol and triethanolamine were analysed. The column was operated at 170 "C and the flow-rate of the carrier gas was regulated at 60 ml min-l.A typical gas chromatogram is shown in Fig. 1. The acetate peaks were satisfactorily separated from each other and the peak of propylene glycol diacetate produced by the cleavage of the polyoxypropylene groups did not overlap those of the derivatives from polyol base compounds except that for the polyether based on propylene glycol. These base compounds could there- fore be easily distinguished and identified. By decreasing the temperature of the gas-chro- matographic column, the peak for propylene glycol diacetate can be accurately identified. Polyethers based on sorbitol yielded complex products that consisted of the acetates of sorbitans and sorbides produced by dehydration. However, the gas chromatograms always showed similar patterns, so that the base compound (sorbitol) could be identified from the chromat ogram.In order to obtain the peaks of the reaction products of polyethers based on 1,2-diamino- ethane and 2,2'-diaminodiethylamine, the SE-30 column was operated isothermally at 230 "C and the flow-rate of the carrier gas was maintained at 60 ml min-l. A typical gas chromato- gram is shown in Fig. 2. These base compounds can also be distinguished and identified. DETERMINATION OF THE CONTENT OF OXYETHYLENE GROUPS- The ethylene oxide - propylene oxide copolymers, and polyols that were first oxypropy- lated and then oxyethylated, were decomposed as described above for the determination of the polyol base compounds.The FFAP column was operated isothermally at 65 "C and the flow-rate of the carrier gas was regulated at 60 ml min-1.56 1 2 TSU JI AND KONISHI : GAS-CHROMATOGRAPHIC [Analyst, VOl. 99 4 A 5 Time/minutes Fig. 1. Gas chromatogram of reaction products of polyurethane polyethers: 1, propylene glycol di- acetate; 2, glycerol triacetate; 3, tri- methylolpropane triacetate ; 4, tri- ethanolamine triacetate ; and 5, penta- erythritol tetraacetate. Gas-chrom- atographic conditions: column, 1 m x 3 mm, packed with 15 per cent. FFAP on Uniport B; oven, 170 " C ; and chart speed, 1 cm min-l .2 5 10 Timelminutes Fig. 2 . Gas chromatogram of reaction products of polyurethane polyethers based on arnines: 1, pro- pylene glycol diacetate; 2, derivative from 1,Z-diaminoethane; and 3, deri- vative from 2,2'-diaminodiethylamine.Gas-chromatographic conditions : col- umn, 1 m x 3mm, packed with 15 per cent. SE-30 on Uniport B; oven, 230 "C; and chart speed, 1 cm min-' A typical chromatogram for the reaction products of the 1,2-diaminoethane ethylene oxide - propylene oxide adduct is shown in Fig. 3. The ethylene glycol diacetate and propy- lene glycol diacetate peaks, produced from polyoxyethylene and polyoxypropylene groups, respectively, were completely separated. The proportions of ethylene and propylene oxides were determined by measuring the two peak areas (i.e., of the ethylene glycol and propylene glycol diacetates) and applying the appropriate calculations. The derivative from the base I Time/minutes Fig.3. Gas chromatogram of reaction products of polyether based on 1,Z-diamino- ethane: 1, propylene glycol diacetate; and 2, ethylene glycol diacetate. Gas-chromato- graphic conditions: column, 1 m x 3 mm, packed with 15 per cent. FFAP on Uniport B; oven, 65 "C; and chart speed, 1 cm min-lJanuary, 19741 ANALYSIS OF POLYURETHANE POLYETHERS 57 compound (1,2-diaminoethane) did not appear in the chromatogram and so did not interfere in the determination of the ratio of oxyethylene to oxypropylene groups. The ethylene oxide - propylene oxide adducts of glycerol and trimethylolpropane were also analysed by a similar procedure. The contents of the oxyethylene groups in these polyethers are given in Table I. The determination of these contents was carried out by the nuclear magnetic resonance method, and the results obtained are also included in Table I.The values obtained by the cleavage method are in fair agreement with those obtained by nuclear magnetic resonance. TABLE I RESULTS FOR THE CONTENTS OF OXYETHYLENE GROUPS per cent. Oxyethylene + oxypropylene groups ' mlm' by Oxyethylene groups f h > Sample Base compound gas chromatography nuclear magnetic resonance 1 Ethylene oxide - propylene oxide copolymerl3.8 2 24.4 3 4 5 6 7 8 9 10 11 Glycerol Trirnethylolpropane 1,2-Diaminoethane 35.8 45.4 48.8 11.4 27.5 10.3 21.7 26.1 48.3 13.8 24.6 36-0 45.5 50-0 11-3 27.0 10.0 19.9 26-2 49.2 A11 values are the means of three determinations. CONCLUSION An analytical method for the identification of the base compounds of polyurethane polyethers and the determination of the contents of their ethylene and oxypropylene groups has been established. All of the derivatives from these base compounds are relatively stable and, under appro- priate conditions, appeared at different positions in the gas chromatogram and were therefore easily distinguished and determined. The fact that the boiling-points of the ethylene glycol and propylene glycol diacetates are almost identical (190 to 191 "C) at a pressure of 1 atmos- phere is a favourable feature of the cleavage method. The method is practical because of the simplicity of the procedure and the apparatus required and can be widely applied to the analysis of a series of non-ionic surfactants. REFERENCES 1. 2. 3. 4. 5 . Mathias, A., and Mellor, N., Analyt. Chem., 1966, 38, 472. Stead, J. B., and Hindley, A. H., J. Chromat., 1969, 42, 470. Konishi, K., and Kanoh, Y., Japan Analyst, 1966, 15, 1110. Karger, M. H., and Mazur, Y., J. Amer. Chem. SOL, 1968, 90, 3878. Tsuji, K., and Konishi, I<., Analyst, 1971, 96, 457. Received April 18th, 1973 Accepted July 19th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9749900054

出版商:RSC    

年代:1974

数据来源: RSC