ISSN:0003-2654    

DOI:10.1039/AN95782FX009

出版商:RSC    

年代:1957

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95782BX011

出版商:RSC    

年代:1957

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95782FP029

出版商:RSC    

年代:1957

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95782BP039

出版商:RSC    

年代:1957

数据来源: RSC

摘要:

MARCH, 1957 THE ANALYST Vol. 82, No. 972 PROCEEDINGS OF THE SOCIETY FOR ANALYTICAL CHEMISTRY NEW MEMBERS ORDINARY MEMBERS Lawrence Thomas Coleman; Leslie Arthur Cook, BSc. (Lond.), Dip. Chem. (Wigan Mining and Tech. Coll.) ; Denis Crowther, B.Sc. (Manc.) ; Sydney Farthing; Peter David Foulkes, B.Sc. (Reading) ; Hendrik Jan Hardon, D.Sc. (Utrecht) ; Douglas Frederick Inkpin; John Charles Kerridge, B.Sc. (Lond.) ; Kenneth Geoffrey Langley, B.Sc. (Lond.), A.R.I.C. ; Colin Macfarlane, A.R.I.C. ; Haydn Alonzo Morris; Herbert Guy Nicholl, B.Sc. (Lond.), A.R.I.C. ; Irmgard Karla Helmtraud Otter, Dr.Phi1. (Graz) ; Michael Allan Owen; John Arthur Paine. JUNIOR MEMBERS Terence Brian Featherstone; Grieves Harnby, B.Sc. (Birm.), A.R.I.C. ; Raymond Arthur Jones. DEATHS WE record with regret the deaths of Andrew Dargie Henry John Davis Thomas Macara Archibald Steele Whamond.NORTH OF ENGLAND SECTION THE thirty-second Annual General Meeting of the Section was held at 2.15 p.m. on Saturday, January 26th, 1957, at the Engineers’ Club, Albert Square, Manchester. The President was among the 35 members present, over whom the Chairman of the Section, Mr. J. R. Walmsley, A.M.C.T., F.R.I.C., F.P.S., presided. The following appointments were made for the ensuing year :-Chairman-Mr. A. N. Leather. Vice-Chairman-Dr. J. R. Edisbury. Hon. Secretary and Treasurer-Mr. A. C. Wiggins, J. Lyons & Co. Ltd., 5 Laurel Road, Liverpool, 7. Members of Committee-Messrs. A. C. Bushnell, J. F. Clark, A. A, D. Comrie, C. J. House, B. Hulme and R. Mallinder. Messrs.I?. Dixon and T. W. Lovett were appointed Hon. Auditors. The Annual General Meeting was followed by an Ordinary Meeting of the Section, at which a paper entitled “Recent Advances in the Analysis of Fertilisers” was given by H. N. Wilson, F.R.I.C. SCOTTISH SECTION THE twenty-second Annual General Meeting of the Section was held at 1.30 p.m. on Thursday, January 17th, 1957, at the Rhul Restaurant, 123 Sauchiehall Street, Glasgow. The Chairman of the Section, Dr. F. J. Elliott, F.R.I.C., F.R.S.E., presided. The following office bearers were elected for the forthcoming year :-Chairman-Dr. Magnus Pyke. Vice-Chuirmun- Mr. A. N. Harrow. Hon. Secretary and Treasurer-Mr. J. A. Eggleston, Boots Pure Drug Co. Ltd., Airdrie Works, Airdrie, Lanarkshire. Members of Committee-Messrs.D. M. W. Anderson, J. Brooks, R. A. Chalmers, J. W. Gray, H. C. Moir and J. W. Murfin. Messrs. J. Andrews and J. McL. Malcolm were re-appointed as Hon. Auditors. 137138 PROCEEDINGS [Vol. 82 The Annual General Meeting was followed by an Ordinary Meeting of the Section, at which Dr. F. J. Elliott, F.R.I.C., F.R.S.E., reviewed the activities of the Section during his two year term of office. Dr. Magnus Pyke, F.R.I.C., F.R.S.E., gave an address on present conditions in the analytical world, the developments leading up to them and the changes that are likely to occur in the future. WESTERN SECTION THE Section participated in a meeting of the South-Western Counties Section of ,the Royal Institute of Chemistry at 5.30 p.m. on Friday, ,January 18th, 1957, in the Technical College, Tavistock Road, Plymouth.The Chair was taken by the Chairman of the South-Western Counties Section, Dr. F. D. M. Hocking, L.R.C.P., M.R.C.S., A.C.G.F.C., F.R.I.C. A lecture on “Silicosis” was given by Professor E. J. King, M.A., Ph.D., D.Sc., F.R.I.C. MIDLANDS SECTION THE second Annual General Meeting of the Section was held at 7 p.m. on Thursday, January 24th, 1957, in the Gas Showrooms, Nottingham. The Chair was taken by the Chairman of the Section, Mr. J. R. Leech, J.P. The following appointments were made for the ensuing year :-Chairman-Dr. R. Belcher. Vice-Chairman-Dr. S. H. Jenkins. Hon. Secretary- Mr. G. W. Cherry, 48 George Frederick Road, Sutton Coldfield, Warwickshire. Hon. Treasurer -Mr. F. C. J. Poulton. Members of Committee-Dr.Bella B. Bauminger, Messrs. H. E. Brookes, H. J. G. Challis, J. R. Leech, R. Sinar, J. H. Thompson, W. H. Stephenson and T. S. West. Miss M. E. Tunnicliffe and Mr. H. .J. Alcock were re-appointed as Hon. Auditors. The Annual General Meeting was followed by an Ordinary Meeting of the Section, at which the Chair was taken by the Vice-chairman, Dr. S. H. Jenkins, F.R.I.C. A talk on “Some Contradictions and Discrepancies Concerning a Classical Method of Analysis” was given by R. Belcher, Ph.D., D.Sc., F.R.I.C. BIOLOGICAL METHODS GROUP THE twelfth Annual General Meeting of the Group was held at 6.30 p.m. on Wednesday, January 23rd, 1957, in the restaurant room of “The Feathers,” Tudor Street, London, E.C.4. The Vice-chairman of the Group, Mr. S. A. Price, BSc., presided.The following Officers and Committee Members were elected for the forthcoming year :-Chairman-Dr. S. K. Kon. Vice-Chairman-Dr. J. I. M. Jones. Hon. Secretary and Treaswer-Mr. K. L. Smith, Standards Department, Boots Pure Drug Co. Ltd., Nottingham. Members of Committee- Miss A. Jones, Miss J. Stephens, Messrs. L. J. H(arris, s. A. Price, K. C. Sellers and J. Simpson. Mr. J. W. Lightbown was re-appointed Hon. Recorder and Messrs. D. M. Freeland and J. H. Hamence were re-appointed as Hon. Auditors. The Annual General Meeting was followed by an Ordinary Meeting of the Group, at which the Chair was taken by Mr. S. A. Price. A discussion on “The Relationship Between Statistics and Microbiological Assay” was opened by J. P. R. Tootill, B.Sc. JOINT COMMITTEE ON METHODS OF ASSAY OF CRUDE DRUGS THE Joint Committee of the Pharmaceutical Society and the Society for Analytical Chemist;y, which, as reported in The Analyst, 1956, 81, :322, has been appointed to prepare standard methods of assay for crude drugs and kindred.materials used in commerce for which there are no official methods, has appointed a number of small working panels. Three panels are already engaged in collaborative experimental work ; their consitutions and terms of reference are given below. PANEL 1 : Digitalis purpurea: CHEMICAL METHOD- Dr. J. M. Rowson, Mr. K. L. Smith and Professor J. P. Todd. preparations and to attempt to correlate them with biological methods of assay.” Constitution: Professor H. Brindle (Chairman), Dr. G. E. Foster, Mr. G. J. Kigby, Terms of reference: “To investigate chemical methods for the assay of digitalis and itsMarch, 19571 PROCEEDINGS 139 PANEL 2 : CAPSICUM : CAPSAICIN CONTENT- Constitution: Mr. H. B. Heath (Chairman), Mr. E. A. Elsbury, Mr. C. A. MacDonald, Mr. G. R. A. Short and Mr. D. 0. Singleton. Terms of reference: “To investigate methods of assay of capsicum and capsicum products with particular reference to the determination of the capsaicin content .” PANEL 3 : ANTHRAQUINONE DRUGS- Constitzttion: Dr. J. M. Rowson (Chairman), Dr. J. W. Fairbairn, Mr. C. A. Johnson, Dr. W. Mitchell, Mr. H. A. Ryan and Mr. W. Smith. Terms of reference: “To investigate methods for estimating the purgative activity of drugs and of preparations of drugs containing anthraquinone derivatives with a view to recommending standard methods of assay.”

ISSN:0003-2654    

DOI:10.1039/AN9578200137

出版商:RSC    

年代:1957

数据来源: RSC

摘要:

March, 19571 PROCEEDINGS 139 Analytical Applications of the Barker Square-wave Polarograph Part III.* Orthophosphoric Acid as a Solvent and Base Electrolyte in Direct Inorganic Polarographic Analysis BY G. W. C. MILNER AND L. J. SLEE Orthophosphoric acid is a versatile solvent for diverse materials and the resulting solutions may be analysed polarographically. The polarographic behaviour of several elements in a A4 phosphoric acid base solution has therefore been investigated, and half-wave potentials and diffusion-current constants are reported. With use of the square-wave polarograph, simple and direct procedures are described for the determination of small amounts of lead in copper, tin, zinc and aluminium-base alloys, respectively. The determination of lead (a few tenths of a per cent.) in monazite and in various miscellaneous materials is also described.With no samples were chemical separations necessary after their dissolution in phosphoric acid. Direct procedures are also described for the determination of the copper content of lead, tin, and aluminium-base alloys, the zinc content of copper, tin, and aluminium-base alloys, and the cadmium content of tin and lead-base alloys. All the procedures outlined provide a considerable improvement on those in which use is made of conventional polarography. ALTHOUGH the polarographic behaviour of many elements has been investigated in several mineral-acid base electrolytes, including hydrochloric, sulphuric, nitric and perchloric acids, very little consideration has been given to the use of orthophosphoric acid as a base elec- tro1yte.l It appears that molybdenum is the only element to have been studied in such a base solution, Holtje and Geyer2 reporting a doublet wave with half-wave potential values of -0.33 and -0-90 V against the saturated-calomel electrode (S.C.E.) in M phosphoric acid. The use of phosphoric acid in analytical chemistry has assumed greater importance since the realisation of its ability to effect the complete solution of materials that dissolve only with difficulty by conventional procedures.It is, for example, very convenient for effecting the complete solution of several types of mineral^,^ and its powerful solvent properties are most probably due to the formation of soluble complexes at the high temperatures that can be attained by heating.In conjunction with other mineral acids, such as hydrochloric, nitric and hydrofluoric, the solution of a wide range of materials can be effected, with subsequent removal of the other acids, simply by evaporation until only phosphoric acid remains. After such a solution procedure, phosphoric acid is potentially useful as a base electrolyte for the direct determination of constituents in these solutions. Consequently * Presented a t the XVth International Congress on Pure and ,4pplied Chemistry (Analytical Chemistry), Lisbon, September 8th to 16th, 1956. For particulars of Parts I and I1 of this series, see reference list, p. 151.140 MILXER AND SLEE ANALYTICAL APPLICATIONS OF [Vol. 82 the polarographic characteristics of several elements have been determined in this base solution.Values are given for half-wave potentials in volts against the saturated-calomel electrode and for diffusion-current constants ( I ) . With use of the square-wave polarograph, the phosphoric acid base solution has been applied to the direct determination of copper, lead, cadmium and zinc in metallurgical and other types of sample. EXPERIMENTAL APPARATUS- Conventional polarograms, used for the determination of diffusion-current-constant data, were recorded with an automatic pen-recording polarograph incorporating a Brown Electronik potentiometer constructed in this l a b ~ r a t o r y . ~ The equipment used for recording square-wave polarograms was designed by Dr. G. C. Barker and has already been de~cribed.~ A STUDY OF THE POLAROGRAPHIC BEHAVIOUR OF VARIOUS ELEMENTS I N ORTHOPHOSPHORIC ACID- Initially the polarographic behaviour of several elements of importance in inorganic and metallurgical analysis was studied in a M phosphoric acid base solution prepared from the reagent of specific gravity of 1.75.The elements included iron, copper, bismuth, lead, thallium, cadmium, zinc, nickel, cobalt, antimony, tin and indium. Suitable solutions were prepared by taking convenient aliquots of a solution of the metal in hydrochloric or nitric acids and evaporating almost to dryness in a 50-ml beaker. Then 643 ml of the phosphoric acid were added and the solution was warmed gently at Arst to remove water and the remainder of the more volatile acids. The solution was then heated to the maximum temperature of an efficieni: electric hot-plate for about 15 minutes; the temperature reached at this stage will be in the region of 220" C.No fuming takes place and these conditions can be realised only by experience. After cooling, each solution was diluted with distilled water to a volume of 3L00ml in a calibrated flask. Suitable aliquots of the solutions were then de-aerated with nitrogen, and the polarograms were recorded with the square-wave polarograph. For ease of comparison a concentration of 50 pg per ml was used for each element in conjunction with a selected low sensitivity of the instrument. Details of the half-wave potential values and peak heights for these elements are given in Table I. TABLE I POLAROGRAPHIC DATA FOR THE REDUCTION OF METAL IONS IN A hf PHOSPHORIC ACID BASE SOLUTION Peak height, Metal ion mm Fe3+ 55 CU2f 188 Bi3+ 8.5 pb? + 60 T1+ 28 Cd2+ 105 Zn2+ 111 &Ti?+ - CO2+ - Sbj+ - Sn4+ - 1n3+ - Reversibility reversible reversible reversible reversible reversible reversible reversible irreversible no apparent reduction no apparent reduction no app,xrent reduction no apparent reduction a, * volts against S.C.E.+ 0.10 + 0.01 - 0.085 - 0.395 - 0.455 - 0.585 - 0.995 N - 1.01 - * Half-wave potential values against the S.C.E. were obtained with the square- wave polarograph by measuring the potential against the mercury - mercurous phosphate anode and applying a correcion amounting to +0.404 V against S.C.E. for the potential of the mercury - mcrcurous phosphate electrode against the S.C.E.Several of these figures were checked by the accurate method of plotting 2 log - against the potential of the dropping electrode. Zd - 2 From the results in Table I it can be seen that the lowest limit of detection is obtained Iron, lead, thallium, cadmium and zinc are also reversibly reduced and the The reduction of bismuth is less reversible, for copper. peaks should be suitable for analytical purposes.March, 19571 THE BARKER SQUARE-WAVE POLAROGKAPH. PART 111 141 but the peak is usable. Elements that are very irreversibly reduced include antimony, tin, indium and cobalt. The peak for copper cannot be used indiscriminately in the presence of iron, however, because of the fairly close proximity of the peaks for these two elements. The copper determination is limited to samples in which the ratio of ferric iron to copper is about 1 to 1 or less.A hydrochloric acid base solution is more suitable for the deter- mination of copper in the presence of ferric iron. Nevertheless, there are some instances in analytical chemistry when the copper peak in a phosphoric acid base solution could be used with advantage, especially, for example, in several types of non-ferrous alloy systems. The zinc peak likewise cannot be used indiscriminately, owing to the irreversible reduction of nickel at a slightly more negative potential. The interference from nickel is, however, negligible for ratios of nickel to zinc of 2 to 1 or less. As cobalt is not reduced in this base solution, there is no interference from this element in the determination of zinc.Similarly, the extreme irreversibility of indium permits the direct determination of cadmium in the presence of indium, a determination that has presented difficulty to polarographic workers for many years. It is possible to determine cadmium in the presence of a hundredfold excess of indium without interference. For example, a M phosphoric acid solution containing 200pg of indium per ml and 2 pg of cadmium per ml gave a peak for cadmium identical with that for a solution containing 2 pg of cadmium per ml. The extremely irreversible reduction of stannic ions in phosphoric acid solutions permits the determination of lead to be accomplished directly in the presence of tin. The procedure previously adopted for this determination involved the removal of the tin as its volatile halide before the lead peak is recorded from a chloride base s ~ l u t i o n .~ Although the separation of the tin is unnecessary with a phosphoric acid base electrolyte, a slight disadvantage arises from the smaller value of the diffusion-current constant for lead. The value is 3.03 in this medium compared with a value of 3-86 in M hydrochloric acid (see Table 11). Nevertheless, there is sufficient sensitivity with a square-wave polarograph for the determination of lead concentrations down to 0.025 pg per ml. EVALUATION OF DIFFUSION-CURRENT CONSTANTS- In order that a quantitative correlation of the behaviour of various elements in phosphoric acid solutions could be made with reference to other base electrolytes, a knowledge of the diffusion-current constants for the various elements was required.This constant is based on the IlkoviC equation and is defined by the expression- where id is the diffusion current in pA, c is the concentration of the reducible ion in millimoles per litre, m is the mass of mercury flowing in mg per second and t is the drop time in seconds. All these variables are easily measured by using conventional polarographic equipment. Unfortunately the IlkoviC equation is not strictly applicable in square-wave polarography by virtue of the dependence of the diffusion current on the capacity of the mercury - solution interface. Nevertheless, a knowledge of diffusion-current constants is extremely useful as an approximate quantitative guide to the magnitude of diffusion currents obtained with the square-wave polarograph.The polarographic behaviour of several elements in a M phosphoric acid base electrolyte was investigated with a conventional polarograph. The elements included most of those examined with the square-wave polarograph, namely copper, lead, thallium, cadmium, zinc, antimony, indium and tin, and the results obtained showed similar behaviour. For example, copper, lead, thallium, cadmium and zinc were reversibly reduced and, with the exception of zinc, the polarographic steps were well developed. Antimony, tin and indium did not produce reduction steps, which also confirms the results obtained with the square-wave polarograph. As the reversibly reduced ions appeared to possess favourable characteristics in the phosphoric acid solutions, the diffusion-current constants for these elements were determined by using millimolar solutions in the presence and absence of a maximum sup- pressor. Diffusion currents were measured with a calibrated microammeter and the capillary characteristics “m” and “15” were determined by the usual procedures. The results obtained are given in Table 11.The diffusion-current constant values in columii 4 of Table I1 are taken from Kolthoff and Lingane.l 2 per cent. These figures were checked and our results agreed to within142 MILNER AXD SLEE: ANALYTICAL APPLICATIOXS OF [Vol. 82 The values in column 3 for the phosphoric acid base solution should be accurate to 2 2 per cent., apart from that for zinc. With some elements in the phosphoric acid solution, the limiting current developed more slowly in the presence of gelatin and this caused a rounding off at the top of the polarographic step.This effect was particularly noticeable in the reduction of cadmium and zinc, making it difficult to obtain an accurate measurement of the diffusion current. Therefore any procedure for these two elements, in which phosphoric acid is used as a base electrolyte precludes the addition of gelatin. As copper is the only element among those studied producing a maximum in this base solution, the presence of gelatin would only be necessary for its determination. TABLE I1 DIFFUSION-CURRENT CONSTANTS FOR METALLIC IONS IN M PHOSPHORIC ACID Capillary characteristics: m = 1.510 mg per second; h = 50.6 cm Gelatin Zd Metal ion concentration, 06 cmt i t cu2+ nil 0.002 Pb2+ nil 0.01 Ti+ nil 0.01 Cd2+ nil 0.01 Zn2+ nil 0.01 maximum produced 2.97 3.03 3.03 2.59 2-59 2.86 poorly developed step poorly developed step 2.8" Value in A4 hydrochloric acid + 0-01 per cent.of gelatin, for comparison - 3-39 3.86 not quoted - 3.58 Not quoted, owing to masking by discharge of H+ * Diffusion current not well developed owing to Hf discharge. A comparison of diffusion-current constants in M phosphoric acid and M hydrochloric acid shows higher values in the latter medium. This difference is probably due to the higher viscosity of phosphoric acid, since an increase in viscosity results in a decrease of the diffusion coefficient for the element in solution. However, the difference is never greater than 20 per cent., which indicates a loss in sensitivity of this order for determinations in phosphoric acid solutions.The diffusion current for the reduction of zinc could not be measured accurately, because of interference from the discharge of hydrogen ions. With the square-wave polaro- graph, however, the zinc peak is well formed before the increase in current due to the reduction of hydrogen ions takes place and it is possible to use this peak for analytical purposes. Although an investigation of the analytical applications of phosphoric acid in con- ventional polarography was not carried out in this work, it would appear to be potentially useful for the determination of copper, lead, thallium and cadmium. Also many of the advantages of the phosphoric acid solution in analysis, which are illustrated later with reference to square-wave polarography, should apply in conventional polarography. ANALYTICAL APPLICATIONS OF THE ORTHOPHOSPHORIC ACID BASE ELECTROLYTE WITH THE SQUARE-WAVE POLAROGRAPH DETERMINATION OF LEAD- The value of the square-wave polarograph for the determination of lead in ferrous and non-ferrous metallurgical alloys has already been shown by Ferrett and Milner.5 $6 These workers used a chloride base solution and effected the removal of the interfering element tin by volatilisation as its volatile bromide.As previously mentioned, the very irreversible behaviour of stannic tin in the phosphoric acid base solution obviates the need for its removal. This improvement is slightly offset, however, by the decrease in sensitivity for lead compared with the chloride media, but this is only of importance for the determination of trace amounts of this element.Details of the procedures developed for the determination of the lead content of various materials and the results obtained follow. Lead in tin-base materials-Although the tin peak is completely suppressed in a M phos- phoric acid base solution, this is not so with solutions containing trace amounts of chloride ions. Under such conditions chlorostannic complex ions are present in solution and they produce an interfering peak. This behavioiir unfortunately precluded the use of hydro- chloric acid for effecting the initial solution of the sample, as, although most of the hydrochloricMarch, 19571 THE BARKER SQUARE-WAVE POLAROGRAPH.PART I11 143 acid is removed on strong heating with phosphoric acid, its complete removal is not easy to ensure. In experiments with prepared solutions to examine the possibility of determining small amounts of lead in the presence of tin, quantities of Specpure tin metal were attacked with lead-free nitric acid and the resulting solutions were carefully evaporated almost to dryness. Phosphoric acid was then added and the solution was heated at the maximum temperature of an efficient hot-plate to dissolve the metastannic acid, followed by heating for a further 10 minutes. After cooling, the solution was diluted with cold water and made up to volume to give a final solution molar in phosphoric acid. Polarograms obtained with solutions containing 0.1 and 1.0 pg of lead per ml, respectively, both in the absence and presence of a thousandfold excess of tin, showed negligible interference from tin. The values for the peak heights are given in Table 111, and a typical polarogram is shown in Fig.1. TABLE I11 PEAK HEIGHTS FOR LEAD IN THE PRESENCE AND ABSENCE OF TIN Lead concentration Tin concentration Peak height, Instrument && divisions sensitivity 6 x 10-7 0.1 nil nil 19 maximum 6 x 10-7 0.1 1 x 10-3 100 22 maximum 5 x 10-6 1.0 nil nil 20 1/10 maximum 8 x 10-6 1.0 1 x 10-2 1000 20 1 / 10 maximum M pg per ml M p g per ml 50 40 30 20 10 0 -0.5 -0.6 -0.7 -0.8 Potential against mercury-pool anode, volts Fig. 1. Square-wave polarogram for a 1 p g per ml solution of lead in the presence of tin; the ratio of tin to lead is 1000 t o 1 r ' O t O b -0.4 -0.5 -0.6 -07-08 Potential against mercury-pool anode, volts Fig.2. Square-wave polarogram for a 1 pg per ml solution of lead in the presence of copper; the ratio of copper to lead is 1000 to 1 The satisfactory results in Table I11 indicated the possibility of being able to determine directly the lead content of pure tin metal and tin-base alloys. No difficulties were envisaged in alloy analysis, since of the other constituents antimony is very irreversibly reduced and the peak for copper is well separated from that for lead. Details of the procedure finally used in alloy analysis are as follows- Attack 100 mg of sample (white metal, Babbitt metal or refined tin) with 3 ml of lead-free nitric acid in a small beaker and evaporate the solution almost to dryness.Add 6.8ml of phosphoric acid, sp.gr. 1.75, and heat strongly on an efficient hot-plate to dissolve metastannic acid, and then heat for about 10 minutes more. Cool the solution and dilute it to 100 ml with cold water. Place a 5-ml aliquot in a polarograph cell, de-aerate for a few minutes and then record the lead peak. Finally determine the lead content of the sample by the standard-addition technique.144 MILNER AND SLEE: ANALYTICAL APPLICATIONS OF [Vol. 82 Typical results obtained in the analysis of tin-base samples are given in Table IV, the agreement between the polarographic and the chemical values being reasonably satisfactory. The chemical results were obtained by taking several grams of sample and precipitating the lead as sulphate after volatilisation of tin and antimony as their bromides.The deter- minations were completed gravimetrically in the usual way as lead molybdate. In the analysis of the pure tin sample, there was no interference from the major constituent and this corresponded to a tin to lead ratio as high as about 2000 to 1. However, this is probably the limiting ratio for such a determination. The concentration of lead in the solution from this sample was about 0-5 pg per ml, but it is possible to analyse solutions containing 0.1 pg of lead per ml, provided that the tin to lead ratio is not greater than 1000 to 1. The results for the Babbitt and white-metal samples confirmed the non-interference of both antimony and copper. The cadmium in the white metals produced a peak, following, but well separated from, the lead peak.TABLE IV DETERMINATION OF LEA:D IN TIN-BASE ALLOYS Lead /------Ap- 7 Square-wave Chemical polarographic Alloy type Composition, value, value, O / Y O YJ /O \$'bite metal Cu, 3-12; Sb, 7.3; Cd, 0.9, remainder Sn 0.11 0-09 Babbitt metal Cu, 3.6; Sb, 8.6; remainder Sn 0.22 0.22 White metal Cu, 1.27; Sb, 7.9; Cd, 0.44; remainder Sn 0.39 0.42 Babbitt metal Cu, 3.75; Sb, 8.5; remainder Sn 0.24 0.14 Refined tin Cu, 0.03; Bi, 0.03; In, < 0.005 0.055 0.055 Lead in copper-base alloys-Although no interference is incurred from copper in the determination of lead for moderate ratios of copper to lead, Ferrett and Milner6 were unable to determine directly small amounts of lead in copper-base axe-head samples with use of a chloride base solution.When the applicability of the phosphoric acid base solution to this problem was investigated, gelatin was included in the final solution at the suggestion of Dr. G. C. Barker.’ The addition of gelatin did not affect the square-wave polarogram for lead and under these conditions a well defined peak was obtained for a ratio of copper to lead of 1000 to 1, spurious instrumental respo:nses obtained in the absenke of gelatin being eliminated. In this work polarograms were recorded on solutions molar in phosphoric acid containing 1 pg of lead per ml both in the absence and presence of 100 pg of copper per ml. To 5-ml aliquots of solution, 0-2 ml of 0.05 per cent. gelatin solution was added, and the lead peak was recorded with the square-wave polarograph.The peak for lead in the presence of a thousandfold excess of copper is given in Fig. 2, a peak height of 30 divisions being obtained for 1 pg per ml solution of lead on a sensitivity setting of 1/10 of the maximum, both in the absence and in the presence of copper. Several types of copper-base alloys conta:in tin as an alloying constituent in amounts up to approximately 10 per cent. The ratio of tin to lead in these materials is therefore much lower than the ratios encountered in the analysis of tin-base alloys. Zinc may be another major constituent, but should cause no difficulty in the determination of lead, since its reduction occurs at a more negative potential. Further considerations indicated that none of the usual minor alloying constituents cbf this type of alloy should cause interference and the square-wave polarograph should be applicable to the direct determination of the lead content of all types of copper-base alloys. In the analysis of representative samples, the solution of 100 mg of material was effected by the same method as that employed for tin-base alloys.A 5-ml aliquot from the 100 ml of solution was transferred to the polarograph cell, and 0.2 ml of a 0.05 per cent. gelatin solution was added. After the solution had been de-aerated for a few minutes, the lead peak was recorded and the lead content of tlhe sample was determined by the standard- addit ion technique. The results obtained in the analysis of several typical alloys are shown in Table V. Good agreement was obtained between the polarographic figures and those by standard chemical methods on much larger sample weights.It can be seen from the results that noMarch, 19571 THE BARKER SQUARE-WAVE POLAROGRAPH. PART 111 145 difficulty was encountered in the determination of lead, even for copper to lead ratios as high as 3000 to 1. The lowest concentration of lead was encountered with the standard brass samples and amounted to about 0.2 pg of lead per ml. This value represents the lowest limit for this determination and the accuracy here is only approximately 30 per cent. TABLE V DETERMINATION OF LEAD IN COPPER-BASE ALLOYS Lead Sq u are-wave Chemical polarographic Alloy type Composition, value, value, Y O Y O Y O B.C.S. bronze A Sn, 9.96; Sb, 0.24; Zn, 1.86; remainder Cu 1-83 1-78 B.C.S.bronze C Sn, 9.80; Zn, 2.53; Xi, 0.09; Fe, 0.06; remainder 0.41 0.40 Phosphor bronze Sn, 7.95; Zn, 4.2; Ni, 0-02; remainder Cu 0.28 0.3 1 c u B.C.S. manganese brass Zn, 33.9; Mn, 1.03; Al, 1.62; Sn, 1.75; Ni, 1.01; 0.78 0.80 Cartridge brass Bi, 0.015; Sb, 0.01; As, 0.018; Ni, 0.02; Sn, 0.03; 0.06 0.058 Fe, 0.91 ; remainder Cu Fe, 0.025; Zn, 30; remainder Cu remainder Zn Standard brass Sn, 1-34; Ni, 2.85; Fe, 0.95; Al, 0.46; Cu, 55.45; 0.02* 0.013 * Spectrographic result. Aluminium-base alloys-The lead and tin contents of aluminium alloys are generally quite small, but nevertheless a knowledge of the lead content is frequently required. The polarographic method was applied to solutions prepared from typical alloy samples, the final solutions containing 1 mg of sample per ml as before.The solution procedure was the same as that used for the tin-base alloys, with the exception that 5 ml of nitric acid were required. No interferences were expected in the analysis of this type of sample and this is substantiated by the good agreement between the chemical and polarographic figures for the lead contents reported in Table VI. The lowest concentration of lead determined in this work was in the B.C.S. alloy and amounted to 0-15pg of lead per ml. * TABLE VI DETERMINATION OF LEAD IN ALUMINIUM ALLOYS Lead r 1 Square-wave Chemical polarographic value, B.C.S. aluminium alloy KO. 216 Cu, 4.15; Fe, 0.81; Sn, 0.02; Ni, 0.22 0.01 0.015 N.B.S. aluminium alloy No. 86C Cu, 7.92; Zn, 1.50; Fe, 0.90 0.03 1 0.027 Duralumin Cu, 4.07; Fe, 0.45; Mn, 0.54; Si, 0.51; 0.03 0.039 Mg, 0.58; Ni, 0.05; Zn, 0.06 N.B.S.aluminium - silicon alloy Si, 6.21; Xi, 0.59; Fe, 0.46; Cu, 0-30; 0.068 0.076 Mn, 0.30; Sn, 0.063 Pure aluminium Cu, 0.02; Fe, 0.37; Si, 0.030; other ele- 0.02 0.026 ments, < 0.03 Alloy type Composition, value, Y O Y O % No. 87 Zinc-base alloy-The Mazak type of alloy contains several elements in trace amounts, including lead, tin and copper. In earlier work on the investigation of a chloride base electrolyte, the copper and cadmium contents of tEese alloys could be determined directly, but the direct determination of the lead content proved impossible because of interference from tin. With use of the phosphoric acid base solution for the determination of the lead content, the interference from tin was eliminated, but a decreased sensitivity resulted, For this reason the lowest lead content determinable in Mazak alloys was about 0.005 per cent.In the analysis of suitable alloy samples, solutions were prepared as previously described on p. 143, and the standard-addition technique was employed to determine the lead content.[Vol. 82 The results on several standard samples are reported in Table VII, and for all samples reason- able agreement was obtained between the polarographic and chemical results. The lowest concentration of lead determined here was 0.06 pg per ml in the National Bureau of Standards sample and the accuracy of such a determination is better than 10 per cent. TABLE VII 146 MILNER AND SLEE: ANALYTICAL APPLICATIONS OF DETERMINATION OF LEAD I N ZINC ALLOYS Lead Alloy type Composition, % Square-wave Chemical polarographic value, value, % % N.B.S.alloy No. 94B Al, 4.07; Cu, 1.01; Fe, 0.017; Mn, 0.014; 0.006 0.007 Standard Mazak alloy, A9 Cd, 0.0067; Sn, 0.0045; CU, 0.05; Mg, 0.04 0.007 0.008 B.C.S. zinc metal No. 194b Fe, 0.001; Cd, 0.006; Cu, 0.002; 0.027 0.023 Mg, 0.042; Sn, 0.005 Standard Mazak alloy, A10 Cd, 0.01; Sn, 0.0065; Cu, 0.095; Mg, 0.04 0.010 0.010 Zn, 99.964 (by difference) Ferrous alloys-Although the potentialities of the square-wave polarograph have already been shown for the determination of the lead content of steels with use of a chloride base solution,6 the application of the phosphoric acid base solution should result in the extension of the method to samples containing tin in addition to lead.The procedure used in the analysis of samples involved the solution of 100 rng in 7 to 8 ml of lead-free nitric acid, diluted with a little water. After the evaporation of the solution almost to dryness, 6.8 ml of phos- phoric acid were added and the solution was strongly heated. Then the determination was completed as described previously. Experimental work was confined to the B.C.S. steel No. 212 to study the effect of tin. Sufficient tin was added to a solution of the sample to make it approximately 12 per cent. in tin. The square-wave polarographic value for the lead content of this solution was found to be 0.265 per cent., which compares favourably with the chemical value of 0.28 per cent. and confirms the non-interference of tin.Minerals-A knowledge of the lead content of a monazite mineral is very important in age-determination work on the earths crust, lead being the end-product of the decay scheme of the uranium and thorium in this type of mineral. A polarographic method is available for this determination, involving a preliminary separation of the lead as sulphate from other sample constituents, strontium sulphate acting as a carrier for the small lead sulphate precipitate.8 Unfortunately a fairly long standing period is needed in this method to ensure complete precipitation of the lead sulphate. An absorptiometric procedure em- ployed at the Chemical Research Laboratory, Teddington, is based on the coloured complex formed by lead with dithizone. Again some preliminary separation of the lead is necessary and it is precipitated as its insoluble sulphide in the presence of silver sulphide as a carrier.When the applicability of the phosphoric acid base solution to this determination was examined, it was found that solution of the saniple could be effected in hot phosphoric acid and the lead content could be determined directly on the resulting solution. Further details of the method are as follows- Attack 100 mg of the finely ground sample in a small platinum dish with 10 ml of hot lead-free nitric acid and 2 ml of hydrofluoric acid. Digest hot for a period up to about 60 minutes, when a large portion of the sample should have gone into solution. Then evaporate just to dryness, add 6.8 ml of phosphoric acid and heat strongly on an efficient hot-plate for about 30 minutes to obtain complete solution of the sample.After cooling, dilute the solution with water and make up to a volume of 100ml with water. Record the lead peak on a 5-ml aliquot of this solution in the usual way and determine the lead content of the sample by the standard-addition technique. Results obtained by the above method 011 various samples of monazite are given in Table VIII, together with the chemical or conventional polarographic values or both. The agreement between the results for the various methods is good, if account is taken of the complex nature of the sample material. Therle is a tendency for the square-wave polaro- graphic results to be slightly higher than those by the other methods, and this is possiblyMarch, 19571 THE BARKER SQUARE-WAVE POLAROGRAPH.PART I11 147 explained by the fact that this new method is the only one that avoids any chemical separation of the lead. The precision of the square-wave polarographic method was determined on the sample of monazite CRLlSl/54 shown in Table VIII and found to be quite high. Six deter- minations by two different operators gave an average value for the lead content of 0.237 If: 0.007 per cent. This method takes about 2 hours for a single determination, compared with 2 days at least by the conventional polarographic method. TABLE VIII RESULTS FOR LEAD IN MONAZITE SAMPLES Lead Sample type composition, % r Chemical value, YO Monazite N.R.S. 2601 Tho,, 9.65; U,O,, 0.38; remainder 0.23 Monazite CRL 181/54 Tho,, 10.38; U,O,, 0.17 0.26 rare-earth oxides and phosphates Monazite CRL 398/53 Tho,, 6.04; U,O,, 0.23 0.17 Monazite CRL 398/53 Tho,, 5.24; U,O,, 0.18 - Monazite (ebonite) - - 7 Conventional Square-wave polarographic polarographic value, value, % % 0.225 0-25 - 0-237 - 0.19 0.22 0.24 0-22 0.22 (k 0-006) ( & 0.007) (k 0.005) ( & 0.005) (i 0.003) Miscellaneous samples-Miscellaneous materials successfully examined by this procedure include an opal glass and a sample of fluorspar.With each material 100 mg of sample were attacked with lead-free nitric acid and 2 ml of hydrofluoric acid in a small platinum dish. After evaporation of the solution to dryness, 6-8 ml of phosphoric acid were added, and the mixture was heated strongly to effect complete solution of t.he sample. Then the lead peak was recorded and evaluated as before.Results for National Bureau of Standards samples are given in Table IX. The satisfactory agreement between the polarographic and chemical figures serve as an illustration of the possibilities for the direct determination of lead in materials other than alloys. TABLE IX DETERMINATION OF LEAD IN MISCELLANEOUS MATERIALS Lead r Square-wave Chemical polarographic Sample Composition, value, value, Y O 70 0 ’ /O Opal glass N.B.S. No. 19 Fluorspar N.B.S. No. 79 SiO,, 1-88; Zn, 0.35; Fe,O,, 0.15; remainder 0.23 0.25 A1,0,, 6.01; CaO, 10.48; F, 5.72; As, 0.1; 0-097 (PbO) 0.099 (PbO) remainder SiO, CaF, DETERMINATION OF COPPER- The determination of copper with the square-wave polarograph has previously been studied in a chloride base solution and found to possess many advantages in the analysis of such materials as steels, zinc, magnesium and aluminium-base al10ys.~ j6 With samples containing antimony the chloride base electrolyte could not be used, however, without the previous removal of the antimony, which gives an interfering peak.For example, in the analysis of tin-base white-metal samples containing 7 to 8 per cent. of antimony, the antimony and tin were removed as their volatile bromides before the copper peak was recorded from the chloride base solution. As antimony is not reduced at the dropping-mercury electrode from a phosphoric acid base solution, this type of base solution should be more satisfactory for the determination of copper in the presence of antimony. Unfortunately, iron can cause difficulties in this base solution in certain circumstances.Direct methods have, however,148 MILNER AND SLEE : ANALYTICAL APPLICATIONS OF [Vol. 82 been developed for determining the copper content (up to about 4 per cent.) of lead, tin and aluminium-base alloys, provided that the ferric iron to copper ratio in the sample is not greater than 1 to 1. In experiments to investigate the behaviour of copper in the M phosphoric acid base electrolyte, solutions were prepared by evaporating suitable aliquots of a standard copper solution in nitric acid with 6.8 ml of phosphoric acid, sp.gr. 1.75. On cooling, each solution was diluted to a volume of approximately 50 ml with water and then boiled for a few minutes. After the solution had cooled to room tempera-ture, it was diluted to 100ml with water.A linear relationship was observed between peak height and concentration for the range of copper concentrations up to about 30 pg per ml. For higher concentrations, however, this relationship was not followed. It was found that slight variations in the anode-pool potential from sample to sample sometimes resulted in difficulty in the measurement of the height of the copper peak, owing to the incomplete formation of the peak near zero applied potential. To circumvent this the anode was covered with a thin layer of mercurous phosphate, thereby fixing the potential of the mercury-pool anode at + 0-404 V against the S.C.E. For the analysis of the alloys, sample weights were taken to give a final copper concentration in the range 0 to 30 pg per ml.Several determinations were carried out on each sample to ascertain the precision of the method, which was usually found to be better than 2 per cent. Details of the application to alloys follow. Lead-base alloys-In the analysis of this type of alloy, 100 mg of sample were dissolved in nitric acid. Then the solution was evaporated almost to dryness and heated as before in the presence of 6-8 ml of phosphoric acid. After cooling slightly, the solution was diluted to about 50 ml with water and then boiled for a few minutes. After cooling to room tem- perature, the solution was diluted to 100 ml with water, and the copper peak was recorded on a small aliquot, Some typical results obtained by this method are given in Table X. Only one B.C.S.sample was available for aria-lysis and for this excellent agreement was obtained with the chemical value. Two lead-base standards prepared for spectrographic work were available and the polarographic results for copper were found to be in reasonable agreement with the known value for this constituent. In this work the lowest concentration of copper determined in the presence of a 7500-fold excess of lead was 0.13 pg per ml, and this represents about the limiting concentratioii for the determination. TABLE X DETERMINATION OF COPPER I N LEAD-BASE ALLOYS Copper Alloy type B.C.S. white-metal A Lead-base standard (1) Le’ad-base standard (2) Composition, Y O Sb, 12.04; Sn, 4.64; Pb, 82.6; Bi, 0.03; Fe, 0.06; Bi, Zn, Sb, Cd, As, Mg, h l , Cu, Ag, Bi, Zn, Sb, Cd, As, Mg, Al, Cu, Ag, As, 0.06; Zn, 0.08 Sn, Fe Sn, Fe 0.01 each [ ,“a: * Known from preparation of alloy.r 1 Sq uare-wave Chemical polarographic value, value, 0.33 0-32 O/ % /O 0.080 0.10 0.078 0-01” 0.013 Tzn-.base and aluminium-base alloys-The solution procedure described above for lead- base alloys was employed for these alloys, but the weight of sample taken was such as to give a final concentration of copper in the range 0 to 30 pg per ml. The peak for copper in a white-metal alloy (“Babbitt metal” in Table XI:) is shown in Fig. 3. The results for various alloys are given in Table XI and for all there was good agreement with the chemical values. With the tin-base samples the precision of the determination was better than 2 per cent. for copper contents ranging from 0-03 to 4 per cent:.DETERMINATION OF ZINC- Ferrett and Milner6 have already applied the square-wave polarograph to the direct determination of the zinc content of copper-base alloys by using an ammonia - ammonium chloride base electrolyte. This base solution is clearly limited to alloys in which the majorMarch, 19571 THE BARKER SQUARE-FVAVE POLAROGRAPH. PART 111 149 constituent is not precipitated by ammonia. An acidic base electrolyte is potentially more useful for the analysis of several different types of alloys, but has so far proved useless in conventional polarographic analysis, owing to the masking of the zinc step by the diffusion current from the reduction of hydrogen ions. With the square-wave polarograph this inter- ference is less troublesome; for example, a well developed peak is produced for zinc in a 2kf phosphoric acid base solution before the discharge of the hydrogen ions (see Fig.4). The possibility of employing this base electrolyte for the determination of zinc was therefore examined in some detail. 50 40 30 20 10 0 1 I 1 I I -0.2 -0.3 -0.4 -0.5 Potential against rnercury-pool anode, volts -1.2 -1.3 -1.4 -1.5 -1-6 Potential against rnercury-pool anode, volts Fig. 3. Square-wave polarogram Fig. 4. Square-wave polarogram for copper in a white-metal sample for zinc in M phosphoric acid TABLE XI DETERMINATION OF COPPER IN TIN-BASE AXD ALUMIKIUM-BASE ALLOYS Copper r Square-wave Chemical polarographic Alloy type Composition, value , value, White metal Cu, 1-27; Sb, 7-9; Cd, 0.44; remainder Sn 1.27 1*25* White metal Cu, 3.12; Sb, 7.3; Cd, 0.9; remainder Sn 3.13 3.20 Refined tin Cu, 0.03; Bi, 0.03; In, < 0.005 0.03 0.029 R.C.S.aluminium alloy Fe, 0.81; Sn, 0-02; Ni, 0.22; Pb, 0.01 4.15 4.19* % % O f /O Babbitt metal Cu, 3.75; Sb, 8-5; remainder Sn 3.75 3.75* No. 216 * These results are the mean of four separate analyses, with a precision of better than 2 per cent. Difficulties occurred at once when calibration graphs were produced with solutions prepared by heating with phosphoric acid, and then diluting with water in the cold. The peak height - concentration relationship was not linear, and it proved impossible to reproduce the peak height for a given concentration of zinc. It was observed that the peak height was dependent on the time taken for heating in the presence of phosphoric acid, the height being smaller the longer the heating time.These results indicated that quantitative reduction of zinc ions was not occurring, owing to complex formation with solvent molecules, and also that the extent of complex formation with the solvent depended on the time of heating. However, the complex was easily decomposed by boiling the sample solution before recording the polarogram. The procedure finally adopted involved heating with phosphoric acid for about 15 minutes, and then dilution with about 50 ml of water; the solution was boiled for about 10 minutes, cooled to room temperature and finally diluted to 100ml with water. With use of this technique, reproducible peak heights were produced and linear calibration150 MILNER AND SLEE: ANALYTICAL APPLICATIONS OF [Vol.82 graphs were obtained in the concentration range from 0 to 30pg of zinc per ml. Higher concentrations were not investigated. The fact that nickel is reduced at only a slightly more negative potential than zinc in a phosphoric acid base electrolyte presents a limitation of this method applied to the determination of zinc in alloy systems. Fortunately the reduction of nickel is irreversible and causes no interference in the determination of zinc for nickel to zinc ratios of 2 to 1 or less. For the analysis of copper, tin, lead and aluminium-base alloys 100 mg of sample were dissolved in the minimum amount of Polaritan nitric acid and heated in the presence of 6-8 ml of phosphoric acid in the usual way.After slight cooling, 50 ml of water were added and the solution was boiled for 10 minutes. Then the solution was cooled to room temperature and diluted to 100 ml with water. 'The zinc peak was recorded on a 5-ml aliquot of this solution. Results obtained by this procedure for the determination of the zinc content of several types of analysed metallurgical samples are given in Table XII. The agreement with the reported chemical values is good for all the alloy systems studied with the exception of the white-metal samples. The chemical values for zinc in the B.C.S. white-metal samples A and B has now been shown to be in error by different workers. Hunter and Millerg obtained figures of 0.03 per cent. for standard A and 0.44 per cent. for standard B. They used an ion-exchange method for the separation of the zinc, followed by titration with ethylenediaminetetra-acetic acid.In earlier work Milnerlo reported figures of 0.023 and 0.43 per cent., respectively, for these standard samples by a procedure involving the separation of the zinc as its sulphide, followed by titration with potassium ferrocyanicle, with naphthidine as the indicator. The agreement between the polarographic results and the above revised chemical values is reason- ably satisfactory and the precision is normally better than 2 per cent. The lowest concen- tration of zinc determined in the analysis of these samples occurred with the lead-base'standard and amounted to 0-1Opg per ml. However, it is estimated that it would be possible to detect 0.02 pg of zinc per ml if use is made of the maximum sensitivity of the instrument.TABLE XI1 DETERMINATION OF ZINC IN VARIOUS TYPES OF NON-FERROUS ALLOYS Zinc -7 Square-wave Chemical polarographic Alloy type B.C.S. aluminium alloy No. 216 N.B.S. aluminium alloy No. 86C Duralumin B.C. S. white- metal A B.C.S. white- metal B Composition, % Cu, 4-15; Fe, 0.81; Sn, 0.02; Ni, 0.22 Cu, 7.92; Fe, 0.90; Si, 0.68; NI, 0.03; Mn, 0.04; Cu, 4-07; Fe, 0.45; Mn, 0.54; 5, 0.51; Mg, 0.58; Sb, 12.04; Sn, 4-64; Pb, 82.6; 1 5 , 0-03; Fe, 0.06; Pb, 3.86; Sn, 84.0; Sb, 7-51; C:u, 4.08; Fe, 0.05; Cr, 0.029 Ni, 0.05; Zn, 0.06 As, 0.06; Zn, 0.08 AS, 0.04 Bi, Zn, Sb, Cd, As, Mg, Al, Cu, Ag, Sn, Fe{ ;$ Lead-base B.C.S. bronze A Sn, 9.96; Sb, 0-24; Zn, 1.86; remainder Cu standard (1) value, 0.26 1.50 % 0.06 0.08 0.40 0.014 1 -86 - * value, 0.26 1.45 0.052 0.018 0.44 ( * 0.015) 0.01 1 1.83 (k 0.015) % ( * 0.008) ( 5 0.01) ( & 0.002) ( * 0.002) (k O*OOO) DETERMINATION OF CADMIUM- Although cadmium possesses reasonably fa.vourable polarographic characteristics in a phosphoric acid base solution ( I = 2-86 compared with 3.41 in M hydrochloric acid), analytical applications in metallurgical analysis are limited to the very few alloys requiring an analysis for this constituent.The determination of cadmium is sometimes needed in lead, tin and copper-base alloys, and the possibilities of using the square-wave polarograph for this type of determination were therefore examined. Solutions containing 1 pg of cadmium per ml and 500 pg of lead, tin or copper per ml, respectively, in M phosphoric acid were prepared.Well developed peaks were obtained for cadmium in all experiments, although the peak in the presence of lead was only just resolved.March, 19571 THE BARKER SQUARE-WAVE POLAROGRAPH. PART I11 151 Some typical lead and tin-base alloys were analysed for cadmium with use of the square- wave polarograph, the solution procedure employed being identical with that for the deter- mination of lead in white metals. The cadmium results for the various samples are reported in Table XIII. No difficulties occurred with the tin-base samples owing to the apparent non-reducibility of the major constituent. However, with lead-base alloys, although the determination of the cadmium content (0.07 per cent.) of the lead-base standard was accomplished without difficulty, the interference from lead became quite serious for samples with lower cadmium contents, and the limiting ratio of lead to cadmium is about 1500 to 1.Good agreement was obtained with the chemical values for all alloys. TABLE XI11 DETERMINATION OF CADMIUM IN TIN AND LEAD-BASE ALLOYS Cadmium Alloy type Composition, % White metal Cu, 1.27; Sb, 7.9; Cd, 0.44; Pb, 0.39; remainder Sn White metal Cu, 3.12; Sb, 7.3; Cd, 0.9; Pb, 0.11; remainder Sn Lead-base Bi, Zn, Sb, Cd, As, Mg, All CU, Ag, Sn, Fe { ?$ standard (1) CONCLUSIONS r 1 Square-wave Chemical polarographic value, value, 0.44 0.44 0.90 0.92 0.07 0.07 1 % % The use of a phosphoric acid base electrolyte in conjunction with the square-wave polaro- graph has resulted in a considerable improvement in the analytical methods for the deter- mination of copper, lead, cadmium and zinc contents of various materials, particularly metallurgical alloys, including copper, tin, lead, aluminium and zinc-base alloys. The major advantages of this new base electrolyte in polarographic analysis arises from such factors as the non-interference from tin in the determination of lead and similarly of indium in the determination of cadmium. Further, the polarographic methods described represent a con- siderable improvement on existing methods in simplicity and speed. For example, the polaro- graphic determination of a single constituent of an alloy takes approximately 30 minutes and it is possible to determine the copper, lead, cadmium and zinc contents of a tin-base alloy in less than 2 hours. This new base solution, however, results in a slight loss of sensitivity compared with a M hydrochloric acid solution. The diffusion-current constants of the elements studied were found to be approximately 20 per cent. smaller in the phosphoric acid solution. The applicability of phosphoric acid as a solvent has been shown by the variety of materials that can be dissolved by it and maintained in solution. Monazites were the only mineral samples examined in this work. Phosphoric acid should, however, be applicable to the solution of a wide range of rocks and minerals, which may then be examined polarographically for various constituents. Several of the samples of monazite were kindly supplied by the Chemical Research Laboratory , Teddingt on. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. NOTE-References 5 and 6 constitute Parts I and 11, respectively, of this series. ATOMIC ENERGY RESEARCH ESTABLISHMENT Kolthoff, I. M., and Lingane, J. J., “Polarography,” Second Edition, Interscience Publishers Inc., Holtje, R., and Geyer, R., Z. anorg. Chem., 1941, 246, 258. Strickland, E. H., AnaZyst, 1955, 80, 548. Whittem, R. N., ’and Milner, G. W. C., Atomic Energy Research Establishment, Memorandum Ferrett, D. J., and Milner, G. W. C., Analyst, 1955, 80, 132. -,- , Ibid., 1956, 81, 193. Barker, G. C., private communication. Smales, A. A., Proc. Roy. Soc. Edinb., 1948, 63, 125. Hunter, J. A., and Miller, C. C., Analyst, 1956, 81, 79. Milner, G. W. C., Anal. Cham. Acta, 1952, 6, 226. New York, 1952. C/M 185, 1953. ANALYTICAL CHEMISTRY GROUP HARWELL, NR. DIDCOT, BERKS. September 21st, 1956

ISSN:0003-2654    

DOI:10.1039/AN9578200139

出版商:RSC    

年代:1957

数据来源: RSC

摘要:

152 ABSON AND LIPSCOMB: THE DETERMINATION OF LEAD AND COPPER [VOl. 82 The Determination of Lead and Copper in Organic Materials (Foodstuffs) by ,a Dry-ashing Procedure BY D. ABSON AND .4. G. LIPSCOMB A method of dry-ashing described has been found suitable for the pre- liminary destruction of the organic matter of a wide range of food materials before the determination of trace amounts of lead and copper. The sample material is sulphated, which permits ashing to be carried out up t o a tem- perature of 550' C without loss of lead by volatilisation. An aqueous sus- pension of light magnesium carbonate is used as an ashing-aid when the amount of ash is small, being added t o the sulphated sample after i t has been charred and crushed to a powder. After solution of the ash, lead and copper are determined by measurement of the colours of the complexes formed in chloroform solution with diphenylthiocarbazone and diethyl- ammonium diethyldithiocarbamate, respectively.WHEN ashing is used as a means of destroying organic matter before the determination of lead, a temperature below 550" C is usually recommended in order to prevent losses due to volatilisation,l~2,3 and frequently a temperature not greater than 500" C is refer red.^ Various workers recommend additions to the material before ashing in order to improve recovery. Sulphuric acid is added by Schmidt5 t o prevent the loss of volatile lead compounds. Seiser, Necke and Muller6 consider that blood can be ashed without loss of lead only if sulphuric acid is first added. They recommend a temperature between 500" and 530" C and state that lead sulphate is not volatile below 550" C.Tompsett and Anderson' favour the addition of phosphate on similar grounds. Complete recovery of lead may also be prevented by the formation of insoluble compounds during ashing; for example, the formation of lead silicates owing to reaction with silicates in the sample or with the silica basin in which the sample is ashed. In the latter event the losses can be reduced by mixing with the sample a solution of a suitable metal salt so that on ashing the bulk of the ash is increased and the proportion of lead in contact with the dish is decreased. Lockwoods mixes the sample with calcium nitrate solution before ashing it at 500" C and finds an 80 to 95 per cent. recovery of added lead by this technique.Other ashing-aids frequently used include solutions of calcium acetate, calcium hydroxide, magnesium acetate and magnesium nitrate. Dry-ashing as a means of destroying orga.nic matter has found wider acceptance for copper determinations than for lead. If the ash is sulphated and the silica dishes used are not worn, there is no loss of copper on a~hing.~ When old worn dishes are used, the copper may be tenaciously held at the surface of the dislh and give rise to low results. ComrielO uses magnesium nitrate to keep the ash bulky and to reduce contact with the dish. The method of dry-ashing described below is the outcome of an investigation into the suitability of ashing as an alternative to wet-oxidation with sulphuric and nitric acids before the colorimetric determination of lead and copper.I t has been successfully used for some time in our laboratory for routine determinations of metals in a wide variety of raw materials and finished confectionery goods, and possesses certain advantages over the wet-oxidation procedure previously used, notably that there is a considerable saving of reagents, especially nitric and sulphuric acids, and in that dry-ashing does not require constant attention, as does wet-oxidation. The procedures for the extraction of the metals as coloured complexes (lead with dithizone from alkaline citrate - cyanide solution and copper with diethyldithio- carbamate from acid solution) are adapted from the scheme presented by Strafford, Wyatt and Kershaw.11 The dry-ashing stage can, of course, be linked with other existing methods for the determination of these metals. EXPEIUM ENTAL PRELIMINARY ASHING, EXPERIMENTS- Recovery experiments were initially confined to lead, as it was considered that losses during ashing would be more likely with this metal than with copper.For the purpose of these preliminary experiments a bulk sample of granulated sugar was used to provide the basic material, this substance being chosen for its uniform nature and because it is easilyMarch, 19571 153 destroyed by ashing to yield a light soluble ash virtually free from trace metals, calcium, phosphates and other. substances liable to interfere with the subsequent extraction processes. In3uence of variom additions before ashing on the recovery of Zead-Amounts of lead up to the equivalent of 4 p.p.m.were added as a standard solution to a series of 5-g samples, which, after various additions had been made, were ashed at a temperature not greater than 500 “C. The ash so obtained was dissolved in boiling dilute hydrochloric acid, 4 ml of 50 per cent. citric acid solution were added, the solution was neutralised with ammonium hydroxide, sp.gr. 0.880, and made alkaline by adding 0-5 ml in excess. Then 5 ml of 10 per cent. potassium cyanide solution were added and, when cool, the solution was transferred to a separating funnel and extracted with dithizone solution and the lead determined as described under “Method” (p. 157). Recovery of lead was incomplete when the sugar was simply charred over a burner and ashed.Sulphating the ash, by adding 1 ml of sulphuric acid to the samples before ashing, and increasing the bulk of the ash, by adding an ashing-aid (achieved by dissolving the samples in 5 ml of 5 per cent. magnesium nitrate solution before charring), both led to improved recovery of lead, as shown in Table I. It was thought that by adding to the material both sulphuric acid and the ashing-aid recovery should be further improved. In practice, however, this procedure proved unsatisfactory owing to the retention of an un- oxidised residue of carbon, which gave a greyish brown colour to the ash. The same difficulty was encountered when other ashing-aids (solutions of magnesium acetate, calcium nitrate and calcium acetate) were tried in conjunction with sulphuric acid.In any event calcium salts were unsuitable, owing to the formation of insoluble calcium sulphate. I t was possible to get rid of the residual unoxidised carbon by moistening the ash with water or nitric acid and re-ashing, but this had the effect of breaking down the bulk of the ash and thereby largely vitiated the efficacy of the ashing-aid. The recovery of added lead from samples subjected to this treatment was found to be variable and incomplete. I t was assumed that the incomplete oxidation of carbon that was observed, especially when magnesium nitrate solution was added to the sulphated material, was probably due to the formation of a protective layer of magnesium sulphate around some of the organic matter, and it was thought that this could probably be overcome by first sulphating the material and then removing excess of sulphuric acid by heating before adding the ashing-aid.A sample of sugar was therefore sulphated as before and heated over a burner until the black spongy mass first formed became dry and brittle, when it was powdered by being crushed with a pestle. After being heated further, the crushed char was saturated with 5 m l of the magnesium nitrate solution, dried and ashed. The material so treated ashed quickly to give a carbon-free ash, which was, however, only small in bulk, as the magnesium nitrate solution had largely dried out in a layer on the bottom of the dish, giving little protection to the ash formed. From these observations it appeared that a more satisfactory result could be obtained if the ashing-aid were added in such a form that it entirely covered and supported the crushed char and for this purpose an undissolved powder was considered.Light magnesium carbonate was selected as being potentially suitable, as it is extremely light in relation to bulk and being insoluble could be added as a suspension in water. Pre- liminary trials showed that 7 g of light magnesium carbonate shaken with 100 ml of water gave a suitable suspension and 10 ml of this suspension gave adequate coverage of the crushed char obtained from 5 or 10 g samples. On drying the mixture before ashing, it was observed that the individual particles of charred material were dispersed in a matrix of magnesium IN ORGANIC MATERIALS (FOODSTUFFS) BY A DRY-ASHING PROCEDURE TABLE I INFLUENCE OF SULPHURIC ACID AND MAGNESIUM NITRATE SOLUTION ON THE RECOVERY O F LEAD FROM SUGAR SAMPLES ASHED BELOW 500°C Sample alone 7- Lead Average Average added, lead found, recovery, 0.5 0.47 94 1.0 0-95 95 2.0 1.56 78 4.0 3-15 79 p.p.m.p.p.m. % Sample with sulphuric acid added 7*- 7 Average Average lead found, recovery, p.p.m. % 0.52 104 0.97 97 1.86 93 3.82 95 Sample with magnesium nitrate added 7- Average Average lead found, recovery, p.p.m. % 0-48 96 0-93 93 1.78 89 3.58 89154 ABSON AND LIPSCOMB: THE DETERMINATION OF LEAD AND COPPER [VOl. 82 carbonate, which also formed a coating on the inside of the dish. a finely divided state, ashed quickly to give a bulky carbon-free ash. The material, being in TABLE I1 RECOVERIES OF ADDED LEAD AND COPPER FROM SUGAR SAMPLES TREATED WITH SULPHURIC ACID AND MAGNESIUM CARBONATE AND ASHED BELOW 550°C Amount of metal .added, p.p.m.0.5 2.0 3.0 4.0 Lead- Coppev- 5.0 20.0 Average metal found, Average recovery, p.p.m. % 0.50 2.01 3.04 3.93 5.0 19.8 100 101 101 98 100 99 A series of sugar samples containing known. amounts of added lead were treated in this manner and ashed at a temperature between 500" and 550" C (it had been established in a separate experiment that lead was not lost from sulphated material ashed below approxi- mately 550" C). Full recovery of the added lead was attained under these conditions, as shown by the results in Table 11. The procedure was then checked for the recovery of copper, which was added to a separate series of test samples, the copper content of the ashes being determined by extraction of the acid solution with diethylammonium diethyldithio- carbamate as described under "Method" (see p.157). Recovery was also complete with this metal, as indicated in Table 11. Efect of temperature of ashing on recovery 0.f Zead-In order that routine determinations of lead and copper by an ashing procedure could be conducted as rapidly as possible, it was important to know the effect of the temperature of ashing on the recovery. The equivalent of 1 p.p.m. of lead was added to a series of sugar samples, which were sulphated, charred and treated with magnesium carbonate suspension and then ashed at various temperatures between 470" and 675" C. The samples were kept in a muffle furnace for a fixed time (2 hours) and the temperature was kept at a steady value (within 2 5" C) by adjusting the flue of the furnace. At each particular temperature similar samples without the addition of sulphuric acid were ashed, in order to compare the recovery of lead with that from sulphated material.Duplicate sulphated and non-sulphated samples were placed alternately in the chamber of the muffle. Table 111 shows the results of this experiment. They confirm that materials to which sulphuric acid has been added can be ashed without loss of lead at higher temperatures than can materials that have not been sulphated. The approximate temperatures above which lead is lost from sulphated and non-sulphated materials are 550" C and 500" C, respectively. TABLE I11 EFFECT OF TEMPERATURE OF ASHING ON THE RECOVERY OF 1 p.p.m. OF LEAD FROM SUGAR SAMPLES, I N PRESENCE AND I N ABSENCE OF SULPHURIC ACID Temperature of muffle furnace, "C 470 520 560 580 610 675 Sulphated samples w - - l Average Average lead found, recovery, 1.04 104 1.05 105 1.00 100 0-60 60 0.38 38 0.27 27 p.p.m.,Yo Xon-sulphated samples -7 Average Average lead found, recovery, 1.05 105 0.72 72 0.71 71 0.25 25 0.30 30 0.14 14 p.p.m. % APPLICATION OF ASHING METHOD TO VARIOUS IMATERIALS- Having established an ashing method that gave good recoveries of added lead and copper The lead and from test samples, we applied the procedure to a variety of food materials.March, 19571 155 copper contents of the raw materials listed in Table IV were determined after destruction of organic matter by the following methods- (a) sulphating and charring the sample, crushing the char, adding 10 ml of magnesium carbonate suspension and ashing at 500" to 550" C, (b) as (a), but with omission of the ashing-aid, and (c) wet oxidation with sulphuric and nitric acids.IN ORGANIC MATERIALS (FOODSTUFFS) BY A DRY-ASHING PROCEDURE Duplicate determinations were made on 10-g samples, the solutions obtained being divided into two portions, each equivalent to 5 g of material, for separate lead and copper determina- tions. To prevent possible interference from iron, hydroxylamine solution was added to the lead aliquot before extraction. TABLE IV RESULTS OF DUPLICATE DETERMINATIONS OF LEAD AND COPPER CONTENTS OF VARIOUS FOODSTUFFS BY DRY-ASHING AND WET-OXIDATION Material Honey . , . . .. Jam .. . . . . Milk powder . . .. Condensed milk . . .. Milk crumb . . .. Cocoa powder . . .. Gelatin . . . . .. Glyceryl monostearate . . Wax .. .. .. Malt . . . . . . Glac6 cherries . . .. Lead found with- 7---* 7 Sulphated, then magnesium Sulphated carbonate Wet- sample, added, oxidation, p.p.m. p.p.m. p.p.m. 0.25 0.33 0.23 0.20 0.15 0.17 0.65 0.75 0-70 0.20 0.17 0-17 0.10 - 0.10 0.13 0.13 0.15 0.25 0.30 0.33 0.27 0-23 0.25 0.35 0.43 0.38 1.05 1-10 1-15 0.48 0.55 0.55 Copper found with- f Sulphated sample, p.p.m. 0.4 8-6 1.6 0.4 3-6 0.7 2.1 33.0 6-7 0-5 0-5 Sulphated, then magnesium carbonate added, p.p.m. 0.5 8.9 1.5 0.4 0.7 2.3 32.5 6.8 0.7 0.6 - Wet- oxidation, p.p.m. 0-5 8.7 1.5 0.4 3.7 0.8 2.1 32-5 6.7 0.9 0.6 Chewing-gum base- Material A . . .. 1.10 1.45 1.45 1-4 1.8 1.8 Material B .. . . 1-68 1-78 1.83 0.3 0.3 0.3 Material C . . . . 0.63 0.95 1.00 0.1 0.1 0.1 Efect of size of ash-Reference to Table IV shows good agreement between the lead and copper contents determined with the different methods of destruction (ashing with and without ashing-aid and wet-oxidation) with the exception of certain materials having much smaller ashes than the other materials examined, namely the chewing-gum base-materials and wax. With these the lead and copper recovery was significantly lower when the materials were ashed without an ashing-aid than when ashed with an ashing-aid or wet-oxidised. Ash-aided ashing and wet-oxidation showed good agreement for lead and copper contents for all materials regardless of ash size. These results confirm that an ashing-aid is necessary for full recoveries of lead and copper with materials having a low ash content, but is not necessary otherwise.COMPARISON OF SPEED AND CONVENIENCE OF ASHING AND WET-OXIDATION- Although the over-all time taken for ashing of any particular material was usually somewhat longer than the time taken for wet-oxidation, constant attention was not necessary, and so it was possible to carry on with other operations such as extracting one batch of solutions while another series of samples was being ashed. The majority of materials examined were ashed to apparent completion in 1 to 2 hours and some were completely ashed in a much shorter time than this. It was observed generally that the most difficult materials to destroy by wet-oxidation were those that were completely ashed in the shortest time.(a) Precipitation of phosphates from alkaline solution-When the acid solution derived from materials containing appreciable amounts of alkaline-earth phosphates is made alkaline with ammonium hydroxide before the extraction of lead with dithizone solution, a precipita- tion of calcium or magnesium ammonium phosphate occurs, which makes the subsequent INTERFERENCE FROM ALKALINE-EARTH METALS AND PHOSPHATES-156 ABSON AND LIPSCOMB: THE DETERMINATION OF LEAD AND COPPER [vol. 82 extraction procedure difficult or impossible and gives rise to errors due to the occlusion of lead in the precipitate. Various means of overcoming this difficulty have been put forward, including the following- (a) the addition of excessively large amounts of citrate before the solution is made alkaline,12 (b) the separation of the calcium and phosphorus components by chemical manipulation, and their separate extraction with ditlhizone,2 and (c) the separation of the lead by extraction from acid solution with sodium diethyl- dithiocarbamate' or with diethylammonium diethyldithiocarbamate,l3 followed by evaporation of the extract, wet-oxidation of the residue and dithizone extraction of the clear solution so obtained.Recently, Johnson and Polhill14 have introduced the use of sodium hexametaphosphate, which, added to the acid solution before it is made alkaline prevents or delays phosphate - - precipitation. Of the raw materials listed in Table IV tht: acid solutions of the ashes from the milk products, cocoa powder and gelatin showed the formation of a phosphate precipitate when neutralised with ammonium hydroxide.With the solution derived from milk powder the precipitation was immediate and heavy even when no magnesium carbonate was present in the ash, whereas with the other materials the extent of precipitation was considerable when the ashing-aid was present, but only gradual and slight when it was not present. Increasing the amount of citrate normally added before neutralisation to 10ml (5 g) had Some temporary effect on keeping the phospha-te in solution, but a gradual precipitation occurred. To investigate the effect of sodium hexametaphosphate on solutions of ashes containing phosphate and magnesium, 5 samples of condensed milk, milk crumb, cocoa powder and gelatin were ashed with the incorporation of the magnesium carbonate ashing-aid, and the ashes were dissolved as for the-determination of lead, including the addition of citrate, but before neutralisation with ammonium hydroxide 10 ml of 10 per cent.sodium hexameta- phosphate solution were added. In each case immediate precipitation of phosphates was prevented, but a granular deposit formed slowly as the solutions cooled. A rapid extraction of such solutions was possible, but unsatisfactory, as precipitation tended to occur in the separating funnel before extraction was complete. The precipitation of phosphates from the solution of the unaugmented ash of milk powder was completely prevented by the addition of 10 ml of sodium hexametaphosphate solution.I t was concluded from these observations that the addition of sodium hexametaphosphate to the solutions obtained from food materials rich in phosphates satisfactorily prevented precipitation on neutralisation, provided the magnesium ashing-aid had not been incorporated. (AS materials containing appreciable amounts of phosphates generally have a sufficiently bulky ash, the addition of an ashing-aid to such materials is not necessary.) TABLE V INFLUENCE OF CALCIUM SULPHATE ON RECOVERIES OF LEAD AND COPPER Calcium content of sample High ( = 2% of CaO) Moderate (f 1% of CaO) Nil High; ash treated with sodium carbonate solution { Metal added, p.p.m. 1.0 of lead 2.5 of lead 4.0 of copper 10-0 of copper 1.0 of lead 2.5 of lead 4.0 of copper 10-0 of copper 2.5 of lead 10.0 of copper 2-5 of lead 10.0 of copper Recovery of metal after ash had been treated with- 7 7 Cold acid, Warm acid, Boiling acid, A % % % 64 62 49 50 74 89 79 78 103 98 68 78 63 64 85 89 79 95 99 101 Mean = 98 Mean = 103 80 87 91 86 100 97 95 97 103 99 (b) Formation of calcium su@hate-When materials containing calcium salts are wet oxidised, or ashed by a method involving the addition of sulphuric acid, a residue of calcium sulphate is formed that is generally held to adsorb traces of lead and copper from solution,March, 19571 157 so that its formation during ashing is likely to lead to low recoveries of these metals.To investigate the extent to which such adsorption might occur a series of experiments was carried out in which lead and copper were added to sugar samples containing added calcium, the samples were sulphated with 1 ml of sulphuric acid and ashed at 500" to 550" C, and the proportion of the metals recovered from the filtered solutions of the ashes was determined.Calcium was added as 4 per cent. calcium chloride solution and the amount added was varied to give the equivalent of a high calcium content (10 ml, equivalent to 2 per cent. of CaO in sample) or a moderate calcium content (5 ml), in relation to the calcium contents of the materials we deal with, of which milk powder (maximum CaO content about 1.6 per cent.) is richest in calcium. The temperature of the dilute acid in which the ashes were dissolved was also varied, being either cold (room temperature), warm (10 minutes in an oven at 70°C) or boiling.Recoveries of the metals were calculated as percentages of the colorirneter readings for control solutions prepared by ashing samples as for the reagent blank determinations and afterwards adding the same amounts of lead and copper as originally added to the test samples. From the results, shown in Table V, it is evident that losses of lead and copper by adsorption are of importance only when relatively large amounts of calcium sulphate are formed, provided the ash is taken up in boiling hydrochloric acid solution. The results indicate that low recoveries might be experienced with certain food materials rich in calcium. Preliminary trials with the ash of milk powder showed that by digesting it with hot sodium carbonate solution a decomposition was effected, with the formation of calcium carbonate, so that on subsequent acidification the ash was completely soluble.When a further series of ashes, prepared from samples of sugar containing 10 ml of calcium chloride solution and added lead and copper, were digested with 20ml of hot 5 per cent. sodium carbonate solution for about 18 hours before acidification, the recoveries of lead and copper from the solutions obtained were complete, as is shown in Table V. After this treatment had been applied to the ash of milk powder, the solution, when prepared for the determination of lead, deposited considerable amounts of phosphates on neutralisation, despite the addition of citrate and 10 ml of 10 per cent. sodium hexametaphosphate solution, and it was found necessary to increase the amount of sodium hexametaphosphate solution to 20 ml in order to prevent precipitation completely.METHOD REAGENTS- All reagents used should be obtained substantially free from lead and copper by selection or by purification as described. Sulphuric acid, 20 per cent. v/v-Add 1 volume of sulphuric acid, sp.gr. 1.84, to 4 volumes of water. Hydrochloric acid, diluted (1 + 1)-Dilute concentrated hydrochloric acid with an equal volume of water. Magnesium carbonate sus@ension-Well wash 80 g of light magnesium carbonate with boiling water and then add water to a total volume of 1 litre. Potassium cyanide solution, 10 per cent. w/v-Dissolve 50 g of potassium cyanide in water and dilute to 100 ml. Extract with successive small portions of dithizone solution until the last extract is green.Remove the excess of dithizone from the aqueous solution by repeated extractions with chloroform, add 10 ml of 20-volume hydrogen peroxide and set aside for at least 1 day before diluting to a volume of 500ml. IN ORGANIC MATERIALS (FOODSTUFFS) BY A DRY-ASHING PROCEDURE Shake well before use. Citric acid solution-A 50 per cent. w/v solution in water. Hydroxylamine hydrochloride solution-A 20 per cent. w/v solution in water. Sodium hexametaphosphate solution, 10 per cent. w/v-Dissolve 50 g of sodium hexa- metaphosphate in water and dilute to 500ml. Add thymol blue indicator and then am- monium hydroxide, sp.gr. 04380, to a blue-green colour. Extract with successive portions of dithizone solution until the extracts are green.Make slightly acid with dilute hydro- chloric acid, remove excess of dithizone by extraction with chloroform and finally make alkaline as before with ammonium hydroxide. Ammonium hydroxide, sp.gr. 0.880. Dithixone solution, 0-02 per cent. w/v-Dissolve 0.01 g of dithizone in 50 ml of chloroform. Transfer to a 100-ml separating funnel and shake vigorously for 1 minute with 50 ml of water containing 1 ml of ammonium hydroxide, sp.gr. 0.880. Reject the chloroform layer, make158 ABSON AND LIPSCOMB: THE DETERMINATION OF LEAD AND COPPER [VOl. 82 slightly acid with hydrochloric acid and extract with two successive 25-ml portions of chloroform. Wash the combined chloroform extracts with two successive 10-ml portions of water. Prepare freshly as required.Ammonium hydroxide - cyanide wash solutiow-Dilute 40 ml of ammonium hydroxide, sp.gr. 0.880, and 20ml of 10 per cent. potassium cyanide solution to 1 litre with water. Standard lead solution, 1 ml = 10 pg of lead--Dissolve 1-60 g of lead nitrate in water, add 10 ml of concentrated nitric acid and dilute to 1 litre. Dilute 1 ml of this solution to 100ml with water when required. Potassium iodide solution, 20 per cent. w/v-IDissolve 100 g of potassium iodide in water and dilute to 500 ml. Add 1 ml of ammonium hydroxide, sp.gr. 0.880, transfer to a separating funnel and shake vigorously for 30 seconds with1 25 ml of diethylammonium diethyldithio- carbamate working solution. Reject the chloroform layer and wash the aqueous layer with two successive 10-ml portions of chloroform, rejecting the washings.Combine the chloroform extracts and reject the acid layer. Sodium metabisulphite solution-A 5 per cent. filtered solution in water. Diethylammonium diethyldithiocarbamate solutions-(a) Stock solution, prepared weekly. Dilute 3.0 ml of redistilled diethylamine to 10.0 ml with chloroform, and add slowly with stirring 1.0 ml of redistilled carbon disulphide previously diluted to 10.0 ml with chloroform. Cool and preserve in a dark coloured, glass-stoppered bottle. (b) Working solution, prepared daily, as required. Standard copper solution, 1 ml = 10 pg of co$per-Djssolve 3.93 g of crystalline copper sulphate, CuS0,.5H20, in water and dilute to 1 Ilitre. Dilute 1 ml of this solution to 100 ml with water when required. Dilute 5.0 ml of stock solution to 100ml with chloroform.Sodium carbonate solution-A 5 per cent. slolution in water. Chloro form-Redistil over lime in an all-glalss apparatus. Water-Prepare by distillation in all-glass apparatus. (NOTE-We have recently found that the use of a proprietary ion-exchange purifier provides suitable water without distillation.) PROCEDURE FOR THE DESTRUCTION OF ORGANIC MATTER- Materials having low ash contents-Well mix the finely divided sample (10 g) with 5 ml of 20 per cent. sulphuric acid in a silica dish having a diameter of 10 cm, and heat over an Argand burner, gently at first to avoid possible spurting or frothing, and then more strongly until a brittle char remains. (With certain materials that do not mix well with the sulphuric acid, it is often advantageous to add one drop of Teepol before heating; the Teepol should also be added to the blank.) The charring stage is speeded up by breaking and turning over the partly charred cake of material with a platinum wire.Crush the dry char to a powder with a clean dry porcelain pestle, and heat again over the burner to remove traces of free sulphuric acid. If the char is particularly hard, transfer it to a porcelain mortar to crush it and then brush it back into the dish. Add by pipette 10 ml of the suspen- sion of magnesium carbonate, covering all the particles of charred material, and dry the contents of the dish over the burner at a low heat. Transfer the dish to an electrically heated muffle furnace at a temperature not exceeding 560" C and ash the contents as completely as possible.Other materials-Proceed as described above, but omit the magnesium carbonate suspension. PROCEDURE FOR EFFECTING SOLUTION OF THE ASH- When the magnesium carbonate ashing-aid has been used, allow the dish to cool and then moisten the ash with water to reduce effervescence on the addition of acid. To the ash in the silica dish add 10 ml of diluted hydrochloric acid (1 + l), bring the solution to boiling point over a gauze, and filter it througlh an acid-washed filter-paper into a 50-ml graduated cylinder. Add a further 10 ml of dilute hydrochloric acid to the dish, boil, swirl round and filter into the cylinder and repeat this step with about 10ml of water. Dilute the volume of filtrate in the cylinder to 50 ml with water, drain into a 100-ml conical flask, swirl round to ensure thorough mixing of the contents, pour back into the cylinder, then back into the flask and mix again.Divide the solution into two equal portions by returning 25 ml to the cylinder and thence to another 100-ml conical flask, washing out the cylinder into the second flask with a little water. Use one portion for the determination of lead and the other for the determination of copper.March, 19571 159 Considerable amount of calcium sulphate present-Transfer the ash as completely as possible to a 100-ml beaker, using a glass rod to loosen the ash. Wash traces of ash remaining in the dish into the beaker with two 10-ml portions of 5 per cent. sodium carbonate solution, each being brought to boiling point before being poured into the beaker.Cover the beaker with a clock-glass and gently simmer the contents on a gauze over a low flame or on a hot- plate for at least 14 hours, stirring frequently with the glass rod. Then add a small piece of litmus paper and make just acid by the careful addition of dilute hydrochloric acid. Add a further 10ml of acid, boil the solution and filter it through an acid washed filter-paper into a 50-ml graduated cylinder. Rinse out the beaker with 10 ml of dilute hydrochloric acid and finally with water, filtering into the cylinder, and dilute the filtrate to 50 ml with water. Divide the solution into two equal portions, as described above, for the separate determinations of lead and copper. PROCEDURE FOR DETERMINING LEAD- To the lead aliquot of the ash solution add 4 ml of citric acid solution and 10 ml of 10 per cent.sodium hexametaphosphate solution. (After treating the ash with sodium carbonate, it may be necessary to increase the amount of sodium hexametaphosphate solution to 20 ml.) Add a small piece of litmus paper to the solution and neutralise by adding ammonium hydroxide, sp.gr. 0.880, from a burette, adding 0-5 ml in excess. Add 5 ml of 10 per cent. potassium cyanide solution and 1 ml of 20 per cent. hydroxylamine hydrochloride solution, cool the solution and transfer it to a 100-ml separating funnel, washing in with 10 ml of water. Add 2 ml of chloroform and 1 ml of 0.02 per cent. dithizone solution, shake vigorously for 30 seconds and allow the layers to separate. If the chloroform layer is coloured pink, add further 0.5-ml increments of dithizone solution, shaking for 30 seconds after each addition, until the lower layer becomes purple or blue, indicating the presence of excess of dithizone.Run off the chloroform layer into a second separating funnel containing 50 ml of ammonium hydroxide - cyanide wash solution, washing through with 2 ml of chloroform. Add 2 ml of chloroform and 0.5 ml of dithizone solution to the solution in the first separating funnel, shake for 30 seconds, separate and run the lower layer into the second funnel, again washing through with 2 ml of chloroform. Shake the second funnel containing the combined extracts for 30 seconds and allow the layers to separate. Run off the lower layer into a dry 25-ml graduated cylinder and wash through with one or two small amounts of chloroform.Make the volume of extract in the cylinder to 15 ml with chloroform, drain into a 50-ml conical flask, swirl to mix and stopper the flask. Measure the optical density of the solution against chloroform, using an EEL colorimeter (Evans Electroselenium Ltd.) or other suitable instrument with a green filter. Determine also the optical density of the reagent blank, subtract it from the test reading and find the lead content of the sample by reference to a calibration curve. PROCEDURE FOR CONSTRUCTING A CALIBRATION CURVE FOR LEAD- Measure 0,0.5, 1.0, 1.5, 2.0 and 2.5-ml portions of standard lead solution ( 1 ml = 2 p.p.m. an 5 g) into 100-ml conical flasks, add 15 ml of dilute hydrochloric acid and 10 ml of water and continue as described for the determination of lead in the test solution.Determine the optical density of each extract against that containing no added lead as solution blank and plot the readings against amount of lead in p.p.m. in 5 g of sample. PROCEDURE FOR DETERMINING COPPER- To the copper aliquot of the ash solution add 10 ml of dilute hydrochloric acid and 5 ml of 20 per cent. potassium iodide solution and warm to about 40" C. Reduce the liberated iodine by adding 2 ml of 5 per cent. sodium metabisulphite solution, cool and transfer to a 100-ml separating funnel, washing in with 10 ml of water. Add 10 ml of diethylammonium diethyldithiocarbamate working solution, shake vigorously for 30 second's and allow the layers to separate. Run off the lower layer into a second separating funnel containing 50 ml of water and wash through with 2 ml of chloroform.Repeat the extraction with 10 ml of diethylammonium diethyldithiocarbamate working solution, shaking for 30 seconds and transfer the extract to the second funnel, washing through with 2 ml of chloroform. Shake the second funnel vigorously for 30 seconds, allow the layers to separate and run the lower layer into a dry 50-ml graduated cylinder, washing through with one or two small volumes of chloroform. Dilute the extract to 30 ml with chloroform and pour through a dry filter- paper into a 100-ml conical flask. IN ORGANIC MATERIALS (FOODSTUFFS) BY A DRY-ASHING PROCEDURE160 ABSON AND IJPSCOMB [Vol. 82 Measure the optical density of the solution against chloroform, using a blue filter with the EEL colorimeter, and find the copper content of the sample by reference to a calibration curve, allowing for the reading of the blank solution.PROCEDURE FOR CONSTRUCTING A CALIBRATION CURVE FOR COPPER- Measure 0, 2.5, 5.0, 7-5, 10.0 and 12-5-ml portions of standard copper solution (1 ml = 2 p.p.m. on 5 g) into 100-ml conical flasks and add sufficient water to make the volume in each flask approximately 15 ml. Add 25 ml of dilute hydrochloric acid and proceed as described for the determination of copper in the test solution. Determine the optical densities of the extracts against that containing no added copper and plot the instrument readings against amount of copper in p.p,m. in 5 g of sample, B LAN K D ETE RM I NATIONS- sample.PRECAUTIONS TO BE OBSERVED IN THE METHOD- Contamina- tion of samples by air-borne dust during ashing can be reduced by covering the dishes with clock-glasses between stages in the procedure. Apparatus should be thoroughly cleaned with hot dilute hydrochloric acid and water before use. Glassware and reagent bottles should be of Pyrex or similar glass. Filled etchings on glassware may give up lead and should not be allowed to come into contact with solutions. Carry out blank determinations exactly as described throughout, omitting only the Care should be taken to prevent the adventitious gain of lead and copper. SUMMARY A method is described for the dry ashing of food materials and the colorimetric deter- mination of lead and copper in the solution of the ash obtained. Initial sulphation of the sample before ashing permits the ashing to be carried out up to a temperature of 550” C without loss of lead or copper. Ashing is further speeded up by crushing the charred material to a powder, and a suspension of magnesium Carbonate is used to increase the bulk of small ashes. Interference during the determination of lead caused by the alkaline-earth phosphates normally present in food materials is overcome by the addition of sodium hexametaphosphate solution before neutralisation. When large amounts of calcium sulphate are formed, the ash is rendered completely soluble by a preliminary treatment with hot sodium carbonate solution. REFERENCES 1. 2. 3. 4. 5. Schmidt, P., Dtsch. Med. Wsch~., 1928, 54, 520. 6. 7. 8. 9. 10. 11. 12. 13. 14. Fairhall, L. T., J . Ind. Hyg., 1922-23, 4, 9. Roche-Lynch, G., Slater, R. H., and Osler, T. G., Analyst, 1934, 59, 787. Weyrauch, F., and Muller, H., 2. Hyg. Infekt Kr., 1933, 115, 216. “Official Methods of Analysis,” Eighth Edition, The Association of Official Agricultural Chemists, Washington, D.C., 1955, p. 427. Seiser, A., Necke, A., and Muller, H., 2. angew. Chem., 1929, 42, 96. Tompsett, S. L., and Anderson, A. B., Biochein. J . , 1935, 29, 1851. Lockwood, H. C., AIzal.yst, 1954, 79, 143. Monier-Williams, G. W., “Trace Elements in Food,” Chapman & Hall Ltd., London, 1949, p. 38. Comrie, A. A. D., Analyst, 1935, 60, 532. Strafford, N., Wyatt, P. F., and Kershaw, F. G., Ibid., 1945, 70, 232. Kent-Jones, D. W., and Herd, C. W., Ibid., 1933, 58, 152. Analytical Methods Committee, “The Determination of Lead in Foodstuffs,” Ibid., 1954, 79, 397. Johnson, B. I., and Polhill, R. D. A., Ibid., 1955, 80, 364. RESEARCH DEPARTMENT JOHN MACKINTOSH & SONS LTD. First submitted, April loth, 1956 Amended, November 26th, 1956 HALIFAX, YORKSHIRE

ISSN:0003-2654    

DOI:10.1039/AN9578200152

出版商:RSC    

年代:1957

数据来源: RSC

摘要:

March, 19571 COULSON 161 Polarographic Determination of Arsenic in Zinc-smelting Residuals and Zinc Metal BY R. E. COULSON ,4 simple and rapid method has been developed for the determination of 0-1 to 5 per cent. of arsenic in residuals arising from the production of zinc from sulphide ores. With use of a base solution of N sulphuric acid con- taining 0.01 per cent. w/v of gelatin, the only metal that interfered was cadmium. Arsenic was separated when required by co-precipitation as arsenate with ferric hydroxide. The absolute accuracies, with and without separating the arsenic, were 0-01 and f 0.04 per cent., respectively. Arsenic in zinc can be determined down to 0.0001 per cent. after separa- tion by the iron-collection technique. *ARSENIC is a normal impurity in the sulphide ores used for the production of zinc, and the elimination of this impurity is an important part of the process.The residuals produced at various stages in the process, which must be treated to recover the lead and zinc that they contain, have arsenic contents in the range 0.1 to 5 per cent. Determination of arsenic in these residuals by the usual chemical methods is a lengthy process compared with the determination of other constituents such as lead or zinc, and a reduction in the time required was sought by the application of polarographic methods. EXPERIMENTAL The most useful base solution described for the determination of arsenic was N sulphuric acid containing 0-01 per cent. of gelatin as a maximum suppressor,l in which tervalent arsenic gives a well defined reduction wave.Use of such a base solution makes possible a method with a small number of steps, as will be seen below. During the progress of this work, it was reported2 that gelatin was not a suitable maximum suppressor, as the height of the arsenic wave is reduced by the addition of gelatin. This effect had been observed, but with a nominal gelatin concentration of 0.01 per cent. no variation in step height was caused by the small differences in gelatin concentration that occurred in practice. All measurements were made with a Tinsley pen-recording polarograph, with open cells 2 inches high and 1 inch in diameter. The solutions were de-aerated with cylinder nitrogen and the temperature was maintained at 25" & 0.1" C. All voltage measurements were made with reference to the anode pool.In the base solution used, satisfactory polarograms were recorded with arsenic concen- trations from 0-002 to 0-15 g per litre with no significant change in the diffusion-current constant. Below 0.002 g per litre the slope of the top of the wave becomes excessive and the accuracy of the results is greatly reduced. Variations of & 10 per cent. in the sulphuric acid concentration in the base solution produced no measurable change in the wave height obtained, while similar variations in the gelatin concentration produced changes of 4 per cent., the wave height decreasing with a rise in gelatin concentration. Variations occurring in practice do not cause any measurable errors in the wave height produced. The composition range of the residuals analysed was as follows: arsenic, 0.1 to 5%; cadmium, 0.1 to 06%; calcium, 1 to 5%; iron, 0.1 to 4%; lead, 15 to 60%; zinc, 20 to 70%.Above 0.15 g per litre the diffusion-current constant decreases. EFFECT OF CADMIUM- The wave height for cadmium is only one-fifth that of an equal concentration (g per litre) of arsenic. As the accuracy aimed at in the determination of arsenic was 0.05 per cent. absolute, and in most samples the cadmium content was less than 0.1 per cent., it was expected that, for the majority, it would be unnecessary to separate the arsenic from the cadmium. For samples containing more than 0.1 per cent. of cadmium, the arsenic was separated by co-precipitation with ferric hydro~ide.~ In N sulphuric acid, cadmium and arsenic give a combined wave.162 COULSON POLAROGRAPHIC DETERMINATION OF ARSENIC [vol.82 To bring the arsenic in the drosses into solution, an initial attack by nitric acid, followed by fuming with sulphuric acid is satisfactory. The lead can then be removed as lead sulphate. Zinc is then the only metal present in major quantities. In concentrations up to 40 g per litre of zinc, zinc sulphate has no effect on the wave height. Ammonium sulphate may be present in considerable amounts from the neutralisation of sulphuric acid. The wave height of arsenic is decreased by ammonium sulphate concentrations in excess of 20 g per litre, and therefore concentrations of this salt must be kept low. EFFECT OF ZINC AND AMMONIUM SULPHATES- METHOD FOR DETERMINING ARSENIC I N RESIDUALS REAGENTS- All reagents should be of recognised analytical quality.SuZPhuric acid, approximately 5 N-Add 140 ml of concentrated sulphuric acid to 800 ml Gelatin solution, 0.25 per cent. w/v-Dissolve 2.5 g of leaf gelatin plus 1 g of phenol in Standard arsenic solution, 0.4 g per Zitre-Dissolve 0.528 g of arsenious oxide in the Add Cool, and dilute Ferric ammonium sulphate solution, 20 per cent. w/v-Dissolve 20 g of the hydrated salt of water,*cool, and dilute to 1 litre. hot water, cool, and dilute to 1 litre. minimum amount of dilute sodium hydroxide solution and dilute to about 500 ml. a small piece of litmus paper and just acidify with 5 N sulphuric acid. to 1 litre. in water and dilute to 100ml. PROCEDURE- Digest 10 g of sample (see note) with 20 ml of concentrated nitric acid and then evaporate the solution to a small bulk.Add 20 ml of concentrated sulphuric acid, evaporate to fumes of this acid, and continue fuming for 5 minutes. Cool, and wash down the sides of the beaker with a jet of water; then repeat the fuming for 5 minutes. Cool, add about 150 ml of water, boil for 5 minutes and cool to room temperature. Filter the solution through a Postlip 633B or equivalent filter-paper into a 250-ml calibrated flask. Wash the residue well with water and discard it. By pipette put 10ml of this solution into a 400-ml beaker and dilute to a volume of about 200 ml. For samples containing less than 0.1 per cent. of cadmium continue by Procedure A. For other samples continue by Procedure B. NoTE-The samples, which contain both oxidised and metallic material, cannot be reduced to a fine state of division; 10 g is the minimum weight required to obtain a repre- sentative sample.Procedure A : no separation of arsenic-Pass sulphur dioxide through the solution for 10 minutes and then boil off excess and cool. Transfer the solution to a 250-ml calibrated flask. Add ammonia solution until a permanent precipitate of ferrous hydroxide is formed, and then immediately add 50 ml of 5 N sulphuric acid. Add 10 ml of 0-25 per cent. gelatin solution, dilute the solution to volume and mix well. Prepare a standard solution from 10 ml of standard arsenic solution with 50 ml of 5 N sulphuric acid and 10ml of 0.25 per cent. w/v gelatin diluted to 250ml. Transfer about 15 ml of each solution to a polarograph cell, pass nitrogen for 5 minutes and then record polarograms, commencing at - 0-5 V.For samples containing less than 1.4 per cent. of arsenic a sensitivity of 10,uA full scale is suitable. Procedure B: separation of arsenic-Add Ei ml of 20 per cent, w/v ferric ammonium sulphate solution, Bring the solution to the boil and add ammonia solution slowly while the solution is gently boiled, until the iron is completely precipitated, and there is a slight excess of ammonia present. Collect the precipitate on a Whatman No. 54 or equivalent filter-paper and wash it once with water containing one or two drops of ammonia solution. Discard the filtrate. Wash the precipitate back into the precipitation beaker with hot water. Wash the paper with 10ml of diluted sulphuric acid (1 .+ 9) and then with hot water.Dilute the solution to a volume of about 200ml and conlhue as described above. Dilute the filtrate to volume and mix well.March, 19571 I N ZINC-SMELTING RESIDUALS AND ZINC METAL 163 CHEMICAL METHOD- The results obtained by the polarographic methods were compared with results obtained by the chemical method previously employed, which was as follows- A 10-g sample was brought into solution, and lead sulphate was removed as in the polarographic method. The arsenic in the solution was then reduced with sulphur dioxide, separated as arsenious sulphide from 8 N hydrochloric acid, and then converted to arsenite and titrated with standard iodine solution in the usual way. RESULTS- 0.1 per cent. of cadmium were as follows- Typical results for arsenic obtained by Procedure A on residuals containing less than Arsenic by chemical method, yo .. 1.32 1.40 1.44 1-63 2.21 3.10 Arsenic by Procedure A, % . . . . 1.30 1.40 1-40 1-58 2-14 3.15 The following results are for arsenic with and without its separation, in residuals containing more than 0.1 per cent. of cadmium- Cadmium content, yo .. . . 0.19 0.33 0.22 0.43 0-17 Arsenic by chemical method, yo . . 0.14 0.18 0.19 0-28 0-52 Arsenic by Procedure A, % . . . . 0.13 0.17 0.15 0.33 0-50 Arsenic by Procedure B, yo . . . . 0.13 0.18 0.18 0.29 0.52 The mean absolute deviations for the results obtained by Procedure A, i e . , no separation of cadmium, given above, are 0-04 and & 0-03 per cent.; therefore the higher cadmium content does not introduce any appreciable error.The polarograms obtained with the latter samples showed irregularities in the slope at the foot of the arsenic wave, but as the results obtained showed a maximum absolute deviation of 4 0-07 per cent. compared with that sought of & 0-05 per cent., these irregularities were not investigated. They are apparently due to a shift in half-wave potentials. When a greater accuracy is required in the lower range of arsenic contents Procedure B gives results with a mean absolute deviation of less than 0.01 per cent. The time required for the polarographic method is about 3 hours compared with about 8 hours for the chemical method. METHOD FOR DETERMINING ARSENIC IN ZINC The method was mainly required for the determination of 0.01 per cent. or less of arsenic, but occasionally for samples encountered in experimental work in which arsenic contents of 0-1 per cent.or more occurred. The arsenic content of zinc can be determined directly by dissolving a sample in nitric acid, fuming with sulphuric acid and reducing the arsenic with sulphur dioxide. The whole solution may then be treated as already described. The limit imposed by the maximum tolerable zinc content of the final solution, 40g per litre, restricts this method to arsenic contents of not less than 0.05 per cent. By applying the iron-collection technique already described, as much as 25 g of zinc may be taken and the final volume be as little as 25 ml, which extends the range down to 0.0001 per cent. of arsenic. Appropriate sample weights and final volumes of solution for various arsenic contents are as follows- Arsenic content, yo .. . . 0*0001 t o 0.001 0.001 t o 0.01 0.01 to 0.1 Sample weight, g . . .. 25 5 5 Final volume, ml .. .. 25 25 250 PROCEDURE- Dissolve a suitable sample weight in nitric acid and dilute to a volume of about 600 ml. Add 5 ml of 20 per cent. w/v ferric ammonium sulphate. Bring to the boil; continue boiling gently and add ammonia solution until a permanent precipitate forms. Then add more ammonia solution, a few drops at a time until all the iron is precipitated and any zinc precipitate has re-dissolved. Allow the precipitate to settle and collect it on a Whatman No. 54 or equivalent filter-paper. Wash the precipitate twice with hot water containing a few drops of ammonia solution.Discard the filtrate. This can be judged by the colour of the precipitate.164 RAY AND BHATTACHARAYYA : DETERMINATION OF URANIUM [Vol. 82 Wash the precipitate back into the precipitation beaker with hot water. Wash the paper with 10 ml of diluted sulphuric acid (1 $. 9) and then with hot water. Dilute the solution to about 150ml and pass in sulphur dioxide for 10 minutes. Remove the excess of sulphur dioxide by boiling the solution. If a final volume of 25 ml is required, evaporate the filtrate to a volume of less than 20 ml. Cool the solution and then transfer it to a calibrated flask of appropriate size. Add the requisite volumes of 5 N sulphuric acid and 0.25 per cent. w/v gelatin solution to give a final solution of N sulphuric acid and 0.01 per cent. w/v of gelatin. De-aerate the solution and record polarograms as already described, the same standard solution being required. RE s ULTS- Results within the range 0.1 to 0.001 per cent. of arsenic showed a mean relative deviation of & 2 rer cent. Below 0.001 per cent. the accuracy of the method diminishes as the arsenic content approaches 0.0001 per cent., at which concentration an absolute accuracy of & 0.00002 per cent. was the best that could be obtained. REFERENCES Dilute the solution to volume and mix well. 1. 2. 3. Lingane, J. J., Ind. Eng. Chem., Anal. Ed., 1943, 15, 583. Everest, D. A., and Finch, G. W., J . Chem. SGC., 1955, 704. Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoffmann, J. I., “Applied Inorganic Analysis,” Second Edition, Chapman & Hall Ltd., London, 1953, p. 265. RESEARCH LABORATORY IMPERIAL SMELTING CORPORATION LIMITED AVONMOUTH, BRISTOL September 27th, 1956

ISSN:0003-2654    

DOI:10.1039/AN9578200161

出版商:RSC    

年代:1957

数据来源: RSC

摘要:

164 RAY AND BHATTACHARAYYA : DETERMINATION OF URANIUM [Vol. 82 Determination of Uranium by Ammonium Thiosulphate and Sodium Hypophosphite BY H. N. RAY AND N. P. BHATTACHARAYYA Uranium is determined gravimetrically by precipitation as the greenish phosphate from dilute mineral-acid solution by means of sodium hypophos- phite and ammonium thiosulphate; the precipitate is ignited a t 900" C, the sulphur present being removed during the ignition. Elements forming sulphides insoluble in dilute mineral acids are removed by treatment with hydrogen sulphide, and zirconium and titanium, the only interfering elements, are removed by treatment with sodium hypophosphite alone. The method has been successfully applied to the determination of uranium in steel. MOLYBDENUM has been determined by precipitation as the sulphide from acid solution by treatment with ammonium thiosulphate and sodium hypophosphite, which react in presence of acid to produce hydrogen su1phide.l Copper, mercury, lead, bismuth, arsenic, tin, antimony and cadmium, whose sulphides are insoluble in acid, interfere, whereas those elements with sulphides that are soluble in acid or water do not.Uranium, however, is an exception; although its sulphide is soluble, when a solution of a sexavalent uranium salt is boiled with ammonium thiosulphate or sulphurous acid and sodium hypophosphite in presence of dilute mineral acid, a precipitate is formed. This pre- cipitate is a phosphate, although it is not uranyl ammonium phosphate, UO,NH,PO,, which is precipitated from neutral solutions of uranium salts, or uranyl hydrogen phosphate, UO,HPO,, a white precipitate formed in acetic acid solutions of uranium salts in presence of a reducing agent, such as sodium thiosulphate or sulphurous acid.This reaction only occurs when the uranium is in the sexavalent state, uraniumIV and lower valency forms being unaffected; hence, if the uranium solution is treated in a Jones reductor and then oxidised to the quadrivalent state by exposure to air, treatment with ammonium thiosulphate and sodium hypophosphite gives a precipitate of sulphur only. The only interfering elements are zirconiurn and titanium, but they can be removed by modifying the procedure. If the acid solution is first boiled with sodium hypophosphite,March, 19571 BY AMMONIUM THIOSULPHATE AND SODIUM HYPOPHOSPHITE 165 these elements are separated as white crystalline precipitates, the uranium remaining in solution, from which it can be precipitated by boiling with ammonium thiosulphate or sulphurous acid.Uranium can be easily separated from vanadium and other elements that generally interfere in the ordinary method of determination. Very few elements form phosphates that are insoluble in dilute mineral acids. Those that would interfere, e.g., zirconium, phosphate, can be easily removed. Elements forming sulphides insoluble in dilute mineral acids are removed by treatment with hydrogen sulphide before addition of the hypophosphite and ammonium thiosulphate. Moreover, this characteristic green precipitate is a sure indication of the presence of uranium in the solution, as no other element produces this type of precipitate in acid solution. METHOD REAGENTS- Ammonium thiosulphate soEution-A 20 per cent.filtered aqueous solution of ammonium t hiosulphate. Sodium hypophosphite solution-A %O per cent. solution of crystalline sodium hypo- phosphite, acidified with 2 ml of concentrated hydrochloric acid and allowed to stand for 2 to 3 minutes before use. PROCEDURE- Place a suitable aliquot (say, 50 ml) of the uranium solution in a 600-ml beaker and add about 150 ml of distilled water and 8 to 10 ml of concentrated hydrochloric acid. Warm the solution, add 20 ml of sodium hypophosphite solution, boil, and then add 20 to 25 ml of ammonium thiosulphate solution. Stir the solution vigorously for about 2 minutes ; after a short time a greenish precipitate of uranium phosphate will separate, together with sulphur from the thiosulphate.Set the beaker aside in a warm place, so that the precipitate settles. The supernatant liquid should be colourless, except for some colloidal sulphur, but any green colour indicates incomplete separation of uranium. If this is so, add more sodium hypo- phosphite solution and more ammonium thiosulphate, with vigorous stirring. Collect the precipitate on a Whatman No. 41 filter-paper, wash it thoroughly with warm 1 per cent. hydrochloric acid and then with hot water until it is free from chloride. Dry the paper and precipitate in a tared crucible and then ignite them in a muffle furnace, initially at a low temperature until the paper and sulphur have burned away and then at 900" C for 9 hour.Weigh the residual white ash. RESULTS- Extra-pure uranium nitrate (obtained from Chemical & Dye Corporation, New York) was dried in a vacuum-desiccator. A solution containing 1 g of solid in 250 ml of water was prepared, aliquots of this solution were taken and uranium phosphate was precipitated and ignited as described above; it contains 53.60 per cent. of uranium, corresponding to the formula UO,P,O,. This reaction may, to some extent, be considered as specific for uranium. The results were as follows- Uranium nitrate solution taken, ml .. . . 10 20 20 30 50 Uranium phosphate found, g . . .. . . 0.0355 0.0712 0.0709 0.1058 0.1704 Theoretical amount of uranium phosphate, g . . 0.0359 0-07078 0.07078 0.10617 0.17695 DETERMINATION OF URANIUM IN STEEL Methods for the determination of uranium in steel have been described by Kelly, Meyers and Illingworth,2 by Johnson3 and by Little.4 As the percentage of uranium present is small, there is a possibility of a large error in separating it from the iron.The usual procedure involves precipitating the iron as ferric hydroxide and extracting the precipitate with ammonium carbonate solution, a troublesome procedure, owing to the gelatinous nature of the hydroxide. In the proposed method the precipitation is quickly carried out, the pre- cipitate being compact and coagulating very easily. The determination can be made in acid medium in presence of a large amount of iron. A slightly modified procedure was applied to determine the recovery of uranium from samples of various steels, as follows- About 1 to 2 g of steel drillings were dissolved in diluted hydrochloric acid (1 + 1) and a suitable volume of standard uranium solution (4 g of uranium nitrate per litre)166 HOLNESS AND STONE: A SYSTEMATIC SCHEME OF SEMI-MICRO [Vol.82 was added. The solution was diluted to about 200ml and the acidity was adjusted so that it contained about 3 per cent. of hydrochloric acid. The solution was treated with hydrogen sulphide to precipitate sulphides of copper, molybdenum, etc., which were filtered off, and the filtrate was boiled to remove hydrogen sulphide, a few drops of nitric acid being added to oxidise any ferrous iron. An acidified solution of sodium hypophosphite containing 3 to 4 g was added, with vigorous stirring, and any precipitate formed was removed by filtration.About E;g of ammonium thiosulphate were added to the filtrate, which was then warmed and stirred vigorously for a few minutes. The precipitate was allowed to settle for 3 hour and then collected on a Whatman No. 41 filter-paper, washed free from iron and chloride and ignited as described under “Procedure.” The recoveries of uranium, a-dded as uranium nitrate, for a number of steels are shown in Table I. TABLE I RECOVERY OF URANIUM FROM STEEL Uranium nitrate ml B.C.S. steel No. 215 .. .. 10 15 Plain carbon steel . . .. .. 20 40 50 30 Sample solution added, Nickel - chromium - molybdenum steel Uranium phosphate found, g 0.0356 0.0620 0.0709 0.1405 0-1770 0.1063 Theoretical amount of uranium phosphate, g 0.0353 0.0529 0.0707 0.1415 0.1769 0.1061 REFERENCES 1. 2. 3. 4. R&y, H. N., Analyst, 1953, 78, 217. Kelly, G. L., Meyers, F. B., and Illingworth, C. B., J . Ind. EBg. Chem., 1919, 11, 316. Johnson, C . M., Chem. & Met. Eng., 1919, 20, !523 and 588. Little, S., Chemist Analyst, 1922, 38, 22. INDIAN ORDNANCE DEPARTMENT ICHAPORE, W. BENGAL, INDIA June 20th, 1966

ISSN:0003-2654    

DOI:10.1039/AN9578200164

出版商:RSC    

年代:1957

数据来源: RSC

摘要:

166 HOLNESS AND STONE: A SYSTEMATIC SCHEME OF SEMI-MICRO [Vol. 82 A Systematic Scheme of Semi-micro Qualitative Analysis for Anionic Surf ace-active Agents BY H. HOLNESS ANII W. R. STONE The systematic scheme of semi-micro qualitative analysis presented allows the identification of an anionic surface-active agent as a member of one of the twenty-one groups of commoner anionic surface-active agents com- mercially available. Two novel reagents are described; they may be used to indicate the presence of fatty acids, sulphated or sulphonated anionic surface-active agents, quaternary ammonium or pyridinium salts and cation-active amines when present as single substances. THE development in recent years of a large variety of materials that are used either as wetting agents or detergents has created a need for a systematic scheme for their qualitative analysis.During the past fifteen years, several schemes have been published1 to 11, but all of these have been designed for the macro scale of working, requiring substantial quantities of the test material and a considerable time for theiir operation. The work described in this paper requires only milligram quantities of the active material and the analysis is often completed in less than an hour. Many of the published schemes4~6~7~8~10~11 rely on the precipitation of the anion-active compound by a cation-active compound as a mea:ns of differentiation, but this use of amphi- philic ions of opposite charge to bring about precipitation is open to criticism on the grounds that solubilisation may occur.The formation of chloroform-soluble complexes of anionicMarch, 19571 QUALITATIVE ANALYSIS FOR ANIONIC SURFACE-ACTIVE AGENTS 167 and cationic surface-active agents with dye-stuffs of opposite charge has often received mention in the literature in both the q~antitativel~1~~,~4,~6,~6,~7,18 and qualitative*,SJ1,19 analysis of surface-active agents. Gobe120 reported the use of an aqueous solution containing equal amounts of methylene blue and fluorescein as an indicator for anionic and cationic materials, the change in colour and fluorescence of the solution on the addition of a surface- active agent indicating the type of material present. This method can be criticised in that with aqueous solutions a certain minimum concentration of surface-active agent is required for the formation of micelles, with the consequpt change in colour of the dye-stuff solution.In the work described below use is made of a mixture of two dye-stuffs, which, when used at two different pH values, give coloured chloroform extracts in accordance with the type of surface-active agent present. Successful results were obtained with individual samples drawn from some fifty different members of the twenty-one classes of anion-active compounds covered by this scheme. As yet, the scheme has not been extended to deal with mixtures of differing classes of detergents, but preliminary work suggests that it can be applied to some mixtures with little or no modification. Many of the commercial products used as surface-active agents contain much “inactive” material, partly owing to the method of manufacture and partly to the addition of “builders.” In either event it is advisable to extract the active material from a commercial product before attempting its analysis.Miller, Bann and Ponsford21 have used n-butanol in a continuous liquid - liquid extraction process for the extraction and concentration of detergents from their aqueous solutions. In the proposed semi-micro scheme it was found that three successive extractions with n-butanol gave too small a yield of the active material to be of value, and an alternative extractant involving use of tert.-butanol was devised. Anionic materials are often marketed as salts of nitrogen-containing cations, and it is therefore necessary to be able to differentiate between nitrogen in the anion or in the associated cation.Bergeron, Derenemesnil, Ripert and Moniers precipitated the anionic material from solution as the insoluble benzidine salt, which was removed and converted to the sodium salt by treatment with sodium hydroxide solution. The free benzidine base was filtered off and a semi-micro Kjeldahl determination was carried out on the prepared sodium salt. Bergeron et aL6 found that it is not possible to obtain a clean separation by this method and that a nitrogen content of less than 0.5 per cent. should be regarded as a negative test. A semi-micro Kjeldahl determination was too time-consuming to find place in a scheme of qualitative analysis and so a simpler procedure was evolved. This entailed the precipita- tion of the anionic material with a phosphonium salt, followed by its isolation and examination for nitrogen.EXPERIMENTAL APPARATUS AND TECHNIQUE- The procedure closely resembled that previously described by one of us (H.H.).22 The entire analysis was carried out with use only of 3-inch x #-inch test-tubes, 2-inch x &inch boiling-tubes and 1-inch x $-inch beakers. All of these held the same volume of liquid, roughly 3 ml. Separations were made by using a centrifuge designed to carry the test-tubes, and the required layer was removed by means of a pipette with a rubber teat. All vessels were heated in a heating block and those whose contents required prolonged periods of boiling were equipped with a length of capillary tubing sealed about 5mm from its lower end to prevent bumping.In all this work great care was taken to clean all apparatus thoroughly after use. The sensitivity of many of the tests and the small quantities of material used made the risk of contamination very real. REAGENTS- In addition to the usual reagents normally found in the laboratory the following special solutions are needed. Mixed indicator solution at pH 1.99-Dissolve 0.01 g of Disulphine blue V 200 (obtainable from Imperial Chemical Industries Ltd.), 0.04 g of Dimidium bromide (obtainable from Burroughs Wellcome Ltd.) and 16.4g of sodium acetate in distilled water, add 210ml of N hydrochloric acid and dilute to 1 litre.168 HOLNESS AND STONE: A SYSTEMATIC SCHEME OF SEMI-MICRO [Vol. 82 Mixed indicator sohtion at pH 8.6-Dissolve 0-01 g of Disulphine blue V 200, 0.04 g of Dimidium bromide, 3.09 g of boric acid and 3.72 g of potassium chloride in distilled water, add 12ml of N sodium hydroxide and dilute to 1 litre.Ammoniwn co baltothiocyanate ~eagent~~-Dis~solve 200 g of ammonium thiocyanate and 30g of cobalt nitrate in distilled water and dilute to 1 litre. Phenol test reagents-(a) Dissolve 0.1 g of Genochrome (obtainable from May & Baker Ltd.) and 6.0 g of sodium metabisulphite in distilled water and dilute to 100 ml. (b) A 0.5 per cent. aqueous solution of sodium periodate. (c) A 15 per cent. aqueous solution of sodium carbonate. Diaxotised sulphanilic acid reagent~~~-(a) A 0-5 per cent. solution of sulphanilic acid in 2 per cent. hydrochloric acid. Nitrogen test reagent-A saturated aqueous solution of Eulan NK (obtainable from M.W. Hardy & Co.). Bufer solution at p H 7.0-Mix 250ml of 0.2 M potassium dihydrogen phosphate and 147.7 ml of 0.2 M sodium hydroxide. EXTRACTION OF THE ACTIVE MATERIAL- Dissolve sufficient of the test sample estimated to contain 20 mg of the active material in 1 ml of water in a test-tube. Make just acid to Congo red paper, add 0.5 ml of light petroleum, boiling range 40" to 60" C, and diethyl ether mixture (1 + 1), shake well and spin in a centrifuge. If difficulty is experienced owing to the formation of an emulsion, add a few drops of tert.-butanol and then mix gently to ensure a clean separation on centri- fuging. Repeat this extraction twice more and combine the extracts. Wash the combined ether extracts three times with 50 per cent.ethanol, combine the washings, evaporate to dryness in a beaker and unite with the original aqueous solution. The washed ether extracts are evaporated to dryness in a beaker and examined separately as indicated in the qualitative scheme. Make the extracted aqueous solution alkaline to litmus with 15 per cent. sodium carbonate solution, add 0.5 ml of tert.-butanol, shake well and then add sufficient sodium chloride to saturate the solution. Remove the alcohol layer with a pipette and repeat the extraction with two further 0.5-ml portions of tert.-butanol. Combine the alcohol extracts, dry by shaking with anhydrous sodium sulphate, and evaporate to dryness in a beaker. Remove the final traces of alcohol by the addition of a little n-hexane, followed by evaporation.The residue is the extracted active material used in the systematic scheme described later. SPECIAL TESTS- The tests described below are used several times in the systematic scheme and to avoid repetition they are described in detail here and reference is then made back to this section. 1. Test with mixed indicator solution at pH 1-99-Put into a test-tube 0.5 ml of the mixed indicator solution at pH 1.99, add 0-2 ml of chloroform and mix thoroughly by means of a clean pipette, Spin in a centrifuge, and examine the chloroform layer for the complete absence of colour, which indicates that the test-tube, reagents and pipette are free from contamination. Add the specified amount of the test solution or material to the tube and again mix the contents and spin it in a centrifuge.Note the colour of the chloroform layer. Test with mixed indicator solution at fiH 8-6-Repeat the test as described above, but using mixed indicator solution at pH 8.6 in place of mixed indicator solution at pH 1.99. Test with ammonium cobaltothiocyanate reagent-Carry out the test as described above for mixed indicator solution at pH 1.99, but using the ammonium cobaltothiocyanate reagent in place of the indicator solution. 4. Hydrolysis with 2 N sodium hydroxide-In a boiling-tube place 5 mg of the extracted active material, add 1.5 ml of 2 N sodium hydroxide solution and a capillary tube sealed near its lower end, and place the tube in the heating block. Bring the solution to the boil and continue boiling for exactly 10 minutes. Cool, make the solution just acid to litmus with diluted hydrochloric acid (I + 1) and then just alkaline with 15 per cent.sodium carbonate solution. Add 0.5 ml of tert.-butanol, shake well and then add sufficient sodium chloride to saturate the solution. Repeat (b) A 0.5 per cent. aqueous solution of sodium nitrite. Remove the ether layer with a pipette. Spin in a centrifuge after shaking thoroughly. 2. 3. Spin in a centrifuge after shaking thoroughly.March , 19571 QUALITATIVE ANALYSIS FOR ANIONIC SURFACE-ACTIVE AGENTS 169 this extraction with two further portions of tert.-butanol (compare with “Extraction of the active material,” p. 168). The indicated amounts of the combined extracts are then used t o carry out the tests specified. Hydrolysis with diluted hydrochloric acid (1 + 1)-Put 5 mg of the active material into a boiling-tube, add 1.5 ml of diluted hydrochloric acid (1 + 1) and a piece of capillary tube sealed near its lower end, and place the boiling-tube in the heating block.Bring to the boil and continue boiling for 10 minutes. Cool, make the solution just acid to Congo red paper, using 40 per cent. sodium hydroxide solution and diluted hydrochloric acid (1 + 1) as necessary, and then extract three times with 0.5-ml portions of light petroleum - diethyl ether mixture (1 + 1). Combine the solvent extracts, wash three times with 50 per cent. ethanol, unite the ethanol extracts, evaporate to dryness and combine with the aqueous solution from the hydrolysis. The combined solvent extracts are evaporated to dryness and the residue is taken up in 0-2 ml of chloroform and used for the tests specified in Tables 111, IV and V.The aqueods solution is extracted with tert.-butanol after saturation with sodium chloride (compare with “Extraction of the active material,” p. 168) and the extract is examined as indicated in Tables 111, IVi and V. Guerbet test4~9~10J1~25-Put 3 mg of the active material at the bottom of a test-tube and add 4 drops of concentrated nitric acid, then place the tube in the heating block and evaporate to dryness. Cool, dissolve the residue in 0.2 ml of 50 per cent. ethanol, and add two drops of concentrated hydrochloric dcid and 15 mg of zinc dust. Warm the mixture gently for 2 minutes, spin in a centrifuge and remove the clear liquid by means of a pipette to a clean test-tube. Cool thoroughly in running water and add two drops of 0.5 per cent.sodium nitrite solution. After a few seconds 10 mg of urea are added to destroy any excess of nitrous acid, and then 3 mg of 2-naphthbl dissolved in 0.1 ml of 50 per cent. ethanol. Then 20 per cent. sodium acetate solution is added until the solution is faintly alkaline to litmus. Note the colour of the solution immediately. 7. Brentamine fast red A L t e ~ t ~ ~ ~ J ~ ~ ~ ~ - D i s s o l v e 6 mg of Brentamine fast red AL salt in 0.3 ml of buffer solution at pH 7 and spih the solution in a centrifuge. Place 3 mg of the active material in a test-tube and dissolve it in 0.2 ml of buffer solution at pH 7, and then add 0.2 ml of the prepared clear solution of Brentamine fast red AL.Note the formation of any colour. Then add 0.1 ml of chloroform to the solution, and shake the tube and spin it in a centrifuge. As an additional confirmation of this test, place 3 mg of the active material in a test-tube and dissolve it in 4 drops of 2 N sodium hydroxide. Evaporate the contents to dryness and gently heat the tube over a semi-micro burner until the white melt becomes tinged with grey. Cool the tube, dissolve the melt1 in two drops of water, and make the solution acid with diluted hydrochloric acid (1 + 1). Mix two drops of 0.5 per cent. sulphanilic acid solution and two drops of 0.5 per cent. sodium nitrite solution in a separate test-tube, adding 5mg of urea within a few seconds. Add the resulting mixture to the solution from the sodium hydroxide fusion, and then add 20 per cent.sodium acetate solution until the solution is faintly alkaline to litmus. Alkylnaphthalenesulphonates give a red colour, whereas alkyl- benzenesulphonat es and alkyltoluenesulphonates give a yellow to orange colour. Test for $resence of monoglyceride-Place 3 mg of the active material at the bottom of an ignition tube and cover it with 10 mg of potassium bisulphate powder. Cover the end of the tube with a small circle of filter-paper moistened with a freshly prepared solution of equal volumes of 1 per cent. sodium nitroprusside and diethanolamine. Invert a small glass cup over the paper and gently heat the base of the tube until the contents have become molten and signs of charring appear. Set the tube aside and examine the filter-paper for the formation of a strong cornflower-blue colour within 5 minutes.5. 6. Note the colour of the bhloroform layer. 8. CLASSIFICATION OF THE TYPE OF ACTIVE MATERIAL PRESENT- Types of anion-active materials capable of identification in the present scheme are- 1 . 2. 3. Alkanesulphonates. 4. “Sulphonated” castor oil. 5. Highly sulphated oil. 6. Fatty-acid monoglyceride sulphate. 7. Fatty-acid isethionate. Fatty acids and their salts. Primary and secondary alkyl sulphates.170 HOLNESS AND STONE: A SYSTEMATIC SCHEME OF SEMI-MICRO Greew. Carry out test a t pH 8.6, using 4 drops of solution. [Vol. 82 Colourless. Carry out the test a t pH 8.6 (test 2), using 4 drops of the solution. 8. 9. 0. 1. .2. 13. 14. 15. 16. 17. 18. 19. 20. 21. Ptnk or colour- less.Cation- active amine or amidc. Dialkyl esters of sulphosuccinic acid. Sulphated alkylphenol- ethylene oxide condensates. Alkylphenylphenolsulphonate. Alkyldiphenylsulphonate. Alkylbenzenesulphonates and alkyltoluenesulphonates. Alkylnaphthalenesulphonates. Water-soluble petroleum sulphonates. Oil-soluble petroleum sulphonates. Fatty-acid monoethanolamide sulphate. Fatty-acid amide of methyltaurine. Fatty-acid arylamidesulphonate. Substituted (long-chain) benzimidazolesulphonate. Monosulphostearylsuccinamide. Mono- and di-benzylsulphanilates. Green. Quatern- Pink. Free fatty ary cationic acid present. In addition, the use of the indicator reagents will confirm the presence of cation-active amines, amidoamines, quaternary cationic salts and non-ionic surface-active agents when present as single substances, but as yet the qualitative scheme of analysis has not been extended to include these products.TABLE I Pink. Anionic present. The light petroleum - diethyl ether extract from the initial extraction (p. 168) is evaporated to dryness This solution is used to carry out the Apply the indicator test at pH 1.99 (test l), using two drops of the above solution. Note the colour in a beaker and any residue is dissolved in 0.2 ml of chloroform. following tests. of the chloroform layer. Green. Quaternary cationic salt pre- ~~~~~~ Pink. Extract still contains sulphated or sulphonated an- ionic. Evaporate to dryness. Dissolve in light petroleum - diethyl ether mix- ture and wash three times with 50 per cent. ethanol. Re- peat test.If still pink, strong indi- cation of presence of petroleum sul- phonates. Blue-peen. Non- ionic present. Colourless. Surface- active agent ab- sent, or only those I I ~~ ~~~~~~ Colourless. Carry out test for non-ionic (test 3). using 4 drops of solution. Blue-green. Non-ionic present . - Colourless. Surface-active agent not pre- sent, or only those of low molecular weight. TABLE I1 Dissolve 4mg of the extracted active material (p. 168) in 0.2ml of water and use this solution to Apply the indicator test a t pH 8.6 (test 2), using two test for the type of surface-active agent present. drops of the solution and note the colour of the chloroform layer. sent. Colourless. Apply test a t pH 1.99 (test l), using 4 drops of the solution. Green. Cation- active amines or amides.-- Colourless. Apply test for non-ionic (test 3), using 4 drops of the solution. EXAMINATION OF ANION-ACTIVE AGENTS- For the sake of convenience this group of materials is first divided into two parts, those containing nitrogen in the anion and those that do not. Before this division can be made, however, it is necessary to remove interference from the possible presence of nitrogen in the associated cation, e.g., from ammonium, ethanolamine and similar nitrogenous bases.March, 19571 QUALITATIVE ANALYSIS FOR ANIONIC SURFACE-ACTIVE AGENTS 171 To accomplish this separation dissolve 3 mg of the extracted active material (p. 168) in 0.5 ml of water. Add a solution of Eulan NK until no further precipitation occurs (on shaking the precipitate should flocculate).Extract the solution three times with 0.5-ml portions of chloroform, combine the extracts and wash them three times with 0-3-ml portions of water. Dry the chloroform extracts containing the “anion - Eulan NK” complex with anhydrous sodium sulphate and evaporate to dryness in a 2-inch x #-inch ignition tube. Carry out a sodium fusion on this residue in the normal way and test for the presence or absence of nitrogen, using the copper acetate - benzidine acetate method.26 The result determines whether the method in Table 111, IV or V is to be followed. MateriaL not containing nitrogen in the anion-Carry out the Guerbet test as indicated (test 6). If the resultant colour is yellow, indicating the absence of aromatic nuclei in the molecule, refer to Table 111. The formation of an orange to red colour indicates the presence of aromatic nuclei in the molecple and reference should be made to Table IV.TABLE I11 ,4nion-active material not containing an aromatic nucleus in the molecule. Hydrolyse 5 mg of the Apply the indicator test a t pH 1.99 active material with 2 N sodium hydroxide as in test 4 (p. 168). (test 1) to two drops of the alcohol extract. Colourless. Apply indicator test a t pH 8-6 (test 2) to 4 drops of the alcohol extract. Colourless. Esters of Pink. Apply test sulphosuccinic acid. for glyceride (test 8). Negative result, acyl isethionate. Positive result, Fatty-acid mono- glyceride sul- phate. Pink. Hydrolyse 5mg of the active material with diluted hydrochloric acid (1 + 1) as in test 5. Apply the indicator test at pH 8.6 (test 2), using 4 drops of the solution from the light petroleum - diethyl ether extract.Pink. Dissolve 3 mg of active material in 0.1 rnl of water and add 1 ml of 2 N acetic acid. Heat in water bath for 5 minutes. Cloudy result, “Sulphonated” castor oil. Cleav solufion, Highly sulphated oil. Colourless. Apply indicator test at pH 1.99 (test 1) to two drops of the tert.-butanol extract. Colourless. Primary Pink. Alkanesul- and secondary phonates. alkyl sulphates. DISCUSSION OF THE PROCEDURES EXTRACTION PROCEDURE- The extraction procedure used permits the active material to be obtained relatively free from inactive organic and inorganic materials. It affords a convenient means of con- centrating the more dilute powders, pastes and liquids without having to resort to drying.I t gives the active material in a standard form convenient for analysis. The initial extraction from acid solution with light petroleum - diethyl ether mixture (1 + 1) removes inactive fatty material, fatty acids, perfumes and so on, when present. A subsequent extraction with tert.-butanol after making alkaline with sodium carbonate solution and saturation with sodium chloride effects the final separation of the active material. Although tert.-butanol is normally completely miscible with water in all proportions, it has only a limited solubility in saturated sodium chloride solutions. I t has been found to give satisfactory extractions of the active material from aqueous solutions saturated with sodium chloride. As a solvent, it has the advantage of a low boiling-point, 82-5” C, which permits the extract to be evaporated to dryness rapidly without the decomposition some- times experienced with the higher boiling solvents; also, the final traces of alcohol may be removed by the addition of a small quantity of n-hexane, with which it forms a low-boiling azeotrope (63.7” C).Extractions with tert.-butanol are carried out from solutions that have been brought from acidity to alkalinity with a sodium carbonate solution and then saturated with sodium chloride. This procedure eliminates the possibility of mineral acids or caustic alkalis being extracted by the alcohol, with the resulting interference in the indicator tests subsequently applied,172 HOLNESS AND STONE: A SYSTEMATIC SCHEME OF SEMI-MICRO TABLE IV [Vol.82 Apply the ammonium cobaltothiocyanate test (test 3), using 4 drops of the solution of the active agent @. 168). Note the appearance of the chloroform layer. Green colour. Apply the diluted hydro- chloric acid (1 + 1) hydrolysis (test 5) to 3mg of the active material. Apply in- dicator test a t pH 1.99 (test 1) to 4 drops of the alcohol extract. ,4 colour- less chloroform layer confirms the pres- ence of a sulphated alkylphenol - poly- glycol ether. I I - Colourless. Place 2mg of the active material in a test-tube and dissolve it in 0.1 ml of Genochrome solution, add an excess of 15 per cent. sodium carbonate solution followed by 0.1 ml of 0-5 per cent. sodium periodate solution. Observe the formation of a colour within 1 minute.Blue-gveen. Alkyl- phenylphenolsul- phonate. i INDICATOR REAGENTS- Red to brown. Refer to Table I for behaviour of the light petroleum - diethyl ether extract on original material. Extract gives positive indicator test a t pH 1-99 (test 1) after 6 washes. Take 2 mg of the active material and dis- solve it in 0.1 ml of water. Add 0.2 ml of 5 per cent. cal- cium chloride solu- tion and extract with 0.2 ml of diethyl ether. Apply indi- cator test at pH 1-99 to extract (test 1) * Colourless OY very fuint pink, water soluble petroleurn sulphon- ate. Strongly pin?,., oil soluble petroleum sulphonate. Extract gives negative indicator test a t pH 1-99 (test 1) after 6 washes. Apply test 7 (p. 169). 3range to red. Al- kylnaphthalene- sulphonate. Pale yellow. Dis- solve 3 mg of the active material in 0.1 ml of chloro- form in a test- tube.Sprinkle a few crystals of fused aluminium chloride on the side of the tube above the liquid, tilt the tube to just moisten the chloride and ob- serve the forma- tion oi colour 011 the crystals. Red-brow?a, alkyl- diphenylsulphon- ate. E'ello w, a1 kylbenz- enesulphonate or alkyltoluenesul- phona.te. In combining two dye-stuffs of differing colour and opposite charge in solution at two values of pH, one acid and the other alkaline, a pair of reagents is produced that between them are capable of differentiating between four classes of active material when present as single substances. These represent the main four classes of the scheme, namely, fatty-acid soaps, sulphated and sulphonated anionics, cation-active amines, amides and amidoamines and, lastly, quaternary ammonium, pyridinium and quinolinium salts.In preparing these solutions it was necessary to select a pair of dye-stuffs that, when shaken with chloroform, were not themselves extracted at either pH value in the absence of active material. Of the cationic dye-stuffs as yet mentioned in the literature, none proved entirely satisfactory, and it seemed probable that only dyes containing a true quaternary nitrogen atom in the molecule would prove to be of value. Samples were therefore obtained of Alcian blue 8GS, pinacyanol chloride, pinacryptol green, pinacryptol yellow, pinaverdol and Acronol yellow TC180. Some of these showed promise, but were not satisfactory on all counts. Owing to the large number of sulphonated dye-stuffs of all colours commercially available, no difficulty was experienced in the selection of a suitable anionic dye-stuff.The success of the indicator method therefore centred round the search for a suitable cationic dye-stuff. The formula of Dimidium bromide (2 : 7-diamino-lO-methyl-9-phenylphenanthridinium bromide) as given in a Note on its analysis2' clearly indicated the presence of a true quaternary nitrogen atom in the molecule and in addition the text stated that the colour of its aqueous solution was pink. A sample of Dirnidium bromide was obtained and when made up in solution with Disulphine blue V200 at two values of pH it was found in the absence of activeMarch, 19571 QUALITATIVE ANALYSIS FOR ANIONIC SURFACE-ACTIVE AGENTS 173 material to give a colourless extract at both pH 1.99 and pH 8.6.In the presence of active material it gave the expected coloured chloroform extracts, correctly indicating the type of active material present. The solutions at both pH values had very good stability and no decomposition took place over a period of 6 months. Active material with nitrogen in the anion- TABLE V Place 3 mg of the extracted active material in a test-tube and dissolve it in 0.2 mI of water, warming Add two drops of concentrated hydrochloric if necessary. acid and look for the formation of a strong cloud or precipitate. The solution is cooled and should remain clear. Orange-red. Acyl- arylamine- sulphonate. Immediate cloud or precipitate. Sub- stituted benzimid- azolesulphonate. Yellow colour only.Gently fuse 2 mg of the active material with 5 mg of potassium bisulphate and examine for the odour of benzaldehyde. ! Green. Fatty- acid ethanol- amide sulphate. Solution remains clear. Carry out the phosphoric acid hydrolysis (test 9) on the original extracted material. Apply the indicator test a t pH 1.99 (test 1) to 4 drops of the Colourless. Acylamide- sulphonate. ether extract. Green. Sulpho- stearylsuccin- amide. Odour of benz- aldehyde. Benz- anilates. ylsulph- At pH 1.99 fatty-acid soaps gave a colourless extract, whereas sulphated and sulphonated anionics gave a pink colour. Cation-active amines, amides and quaternary nitrogen salts gave a blue-green colour. At pH 8-6 fatty-acid soaps, sulphated and sulphonated anionics gave a pink colour, but only quaternary nitrogen salts gave a blue-green extract.The cation- active amines and amides gave colourless extracts at pH 8.6 (but pink in the presence of fatty acid). Non-ionic amphiphilic compounds gave colourless extracts at both pH values. The use of the ammonium cobaltothiocyanate reagent as a confirmatory test for the presence of polyethylene glycol condensates has received mention in previous schemes of detergent analysis.4y7y8J0 This reagent also gives a positive test with quaternary nitrogen salts. When single substances are being examined, the prior application of the other indicator tests that are not prone to interference by non-ionic surface-active agents allows the cobalto- thiocyanate to be used in this scheme. A recent appli~ation~~ of the ammonium cobaltothiocyanate reagent in the colorimetric determination of non-ionic esters makes use of an extraction procedure in which the photo- metric absorption of a chloroform extract is related to the concentration of non-ionic material in the solution tested.This forms the basis of the qualitative test for the presence of non-ionic surface-active agents. TEST FOR ALKYLPHENYLPHENOLSULPHONATES- $-Diethylaminoaniline reacts with phenols and naphthols possessing an unsubstituted para position in alkaline solution in the presence of an oxidising agent with the formation of a blue to green col0ur.2~~~~ This reagent is commercially available as its crystalline complex with sulphur dioxide under the name of Genochrome.30 Aqueous solutions of this material were found to have only moderate stability.It was found that the addition of a small174 amount of sodium metabisulphite gave a reagent of considerably improved stability. advisable to carry out a blank test if the reagent has been prepared for some time. TEST FOR THE PRESENCE OF NITROGEN IN THE ANION- A method was sought whereby the anion could be precipitated by a non-nitrogenous cation and separated from the possible nitrogen-containing cations in solution. Attention was directed to sulphonium and phosphonium compounds, which, by analogy with the quaternary ammonium salts, could be expected to give precipitates with anion-active materials. Of these two types of material only a sample of Eulan NK (triphenyl-3:4-dichlorobenzyl- phosphonium chloride) ,31 a phosphonium salt manufactured as a moth-proofing agent was available.This compound precipitates anion-active material from aqueous solution and the resulting "anion - Eulan NK" complex can be extracted by chloroform. The extract is washed with water to remove any nitrogenous cations (ethanolamine, etc.), which may also have been extracted, it is dried with anhydrous sodium sulphate and evaporated to dryness in an ignition tube. A sodium fusion carried out on the residue is then used to test for nitrogen. The use of an extraction procedure for the separation of the precipitated material eliminates the possibility of solubilisation by the surface-active agent, which might otherwise give incomplete separation from nitrogenous cations, It is interesting to note that the Eulan NK as marketed and used in the reagent contained urea as a diluent, but in spite of this no nitrogen was found in a blank test carried out as described.HOLNESS AND STONE: A SYSTEMATIC SCHEME OF SEMI-MICRO ["Ol. 82 I t is The procedure is simple and can be completed within 10 minutes. TEST FOR ESTERS OF GLYCEROL- One class of material included in this scheme is an ester of glycerol and its presence may be confirmed by utilising a test for glycerol in the molecule. The test used is a modification of that given by FeigP2 and by Gilby and H~dgson,~ in which the active material is heated with potassium bisulphate and the acrolein liberated is identified by means of filter-paper moistened with a freshly prepared mixture of equal volumes of 1 per cent. sodium nitro- prusside and piperidine.In the present application, improved results were obtained by replacing the piperidine in the reagent by diethanolamine. This may well be due to the low volatility and rather hygroscopic nature of diethanolamine. HYDROLYSIS WITH 2 N SODIUM HYDROXIDE- Boiling 5 mg of the active material with 1.5 ml of 2 N sodium hydroxide for 10 minutes is effective in bringing about the hydrolysis of any completely organic ester groups that may be present in the molecule, although it will not completely hydrolyse organic sulphuric acid monoesters. It is found that, when the long chain of the molecule is linked with the sulphate or sulphonate group by means of an intermediate organic ester group, the sodium hydroxide hydrolysis results in the loss of anionic properties conferred by the sulphate or sulphonate group.In some instances, however, this loss is replaced by the anionic properties of the fatty acids resulting from the hydrolysis. Loss of anionic properties due to sulphate or sulphonate groups is indicated by colourless chloroform extracts in the indicator tests at pH 1-99 and pH 8.6, while the presence of fatty acid is indicated by a colourless extract at pH 1.99 and a pink extract at pH 8.6. HYDROLYSIS WITH DILUTED HYDROCHLORIC ACID (1 + 1)- Boiling with diluted hydrochloric acid (1 + 1) effects the hydrolysis of sulphuric acid monoesters without leading to the complete hydrolysis of true sulphonates or fatty-acid amides. Thus fatty-alcohol sulphates are hydrolysed with the complete loss of anionic properties as is shown by negative indicator tests at both pH values.Fatty-acid ester sulphates in which the sulphate group is attached to the oleophilic acid portion of the molecule give rise to free fatty acid, which is indicated by a colourless test at pH 1.99 and a pink colour at pH 8.6. Fatty-acid ethanolamide sulphates are mainly hydrolysed to the free fatty ethanolamide, which gives a blue-green indicator test at pH 1.99 and generally a pink colour at pH 8.6. This pink colour is probably due to the partial hydrolysis of the fatty ethanolamide with the resultant liberation of fatty acid. True sulphonates and fatty amidesulphonates retainMarch, 19571 QUALITATIVE ANALYSIS FOR ANIONIC SURFACE-ACTIVE AGENTS 175 the anionic properties of the sulphonate group as shown by the pink colour produced in the indicator test at pH 1.99.HYDROLYSIS WITH SYRUPY PHOSPHORIC ACID- Hydrolysis with syrupy phosphoric acid is used in the scheme to effect the conversion of amidosulphates and amidosulphonates to the corresponding free acid and amine. In carrying out this test it is of great importance to make certain that none of the active material adheres to the side of the tube and so avoids contact with the acid. Should this happen, an incorrect result will be obtained on applying the indicator tests. In the hydrolysis tests previously described the solution is actively boiling while hydrolysis procedes and any active material on the sides of the tube is continually returned to the solution by vapour condensation on the side of the tube.With phosphoric acid, however, the liquid is not actively boiling during the hydrolysis and any active material adhering to the side of the tube and not initially covered by the acid remains unattacked. I t is also advisable to transfer the solution to a second clean test-tube by means of a pipette before dilution. This treatment is effective in hydrolysing an amide linkage with the liberation of free acid and amine. The presence of free fatty acid is indicated by a negative indicator test at pH 1.99 and a pink colour at pH 8.6. If a fatty amine is liberated in the hydrolysis, then the indicator test at pH 1.99 gives a blue-green colour. An aromatic primary amine giving a negative indicator test at pH 1.99 can be detected by diazotising and coupling with 2-naphthol to form a coloured dye-stuff.TEST FOR STABILITY TO 2 N ACETIC ACID- between classes of anionic materials. “sulphonated” castor oil and acyl isethionates. THE GUERBET TEST- This test was originally described as a means of detecting the presence of the benzoyl group in cocaine and other alkaloids that give rise to benzoic acid on oxidation. It has been u ~ e d * ~ ~ y l ~ 911925 to indicate the presence of aromatic nuclei in surface-active agents. We found it to give satisfactory results with all the alkylarylsulphonates tested and also with sulphated alkylphenol - polyglycol condensates. As the quantity of material used is small, care should be taken to adhere to the volume of solution stated. BRENTAMINE FAST RED AL TEST- Brentamine fast red AL is a stabilised diazonium salt of 1-aminoanthraquinone (Imperial Chemical Industries Ltd.).I t is used in the present scheme to differentiate between the presence of benzene and naphthalene rings in alkylarylsulphonates. The procedure of van der Hoeve has been modified for use on the semi-micro scale of working. As the extracted active material should be essentially neutral in reaction, we have found that the correct conditions of pH can be simply obtained by dissolving both the active material under test and the diazonium salt in a buffer solution at pH 7. In the presence of alkylnaphthalene- sulphonates an orange-red colQur or precipitate is formed. Alkylarylsulphonatesnot containing a naphthalene nucleus give no more than a yellow colour. The colour produced can be extracted with chloroforni.SODIUM HYDROXIDE FUSION TEST- It is of value to have additional confirmation of the nature of the alkylarylsulphonate when present, and for this purpose use is made of the conversion of the alkylarylsulphonate to alkylphenol or alkylnaphthol on evaporation with 2 N sodium hydroxide followed by gentle fusion, The melt is dissolved in two drops of water, made acid and then coupled with the diazotised sulphanilic acid reagent. The colour formed depends on the nature of the aromatic nucleus in the original sulphonate. I t is strongly advised that the analyst should become familiar with the colours obtained in this test by carrying out the procedure with materials of known composition. THE ALUMINIUM CHLORIDE TEST- sulphonates or alkyltoluenesulphonates. This test has found place in several previous schemes1y5y6 as a means of differentiating In the present scheme it is used to differentiate between This test is used to differentiate between alkyldiphenylsulphonates and alkylbenzene- The procedure is that given by Bergeron et aLs176 HOLNESS AND STOSE [Vol.82 adapted to the semi-micro scale. calcium chloride. DIFFERENTIATION OF PETROLEUM SULPHONATES- The relative solubility of the calcium salts of petroleum sulphonates in diethyl ether is used as a means of differentiating between oil and water-soluble products of this nature. The test is based on the original observation of Von Pilat and Sereda.33 We thank the following Firms for their co-operation in supplying samples of their products, together with details of their composition : Allied Colloids (Manufacturing) Co.Ltd., The Clayton Aniline Co. Ltd., Fine Dyestuffs & Chemicals Ltd., The Geigy Co. Ltd., The Imperial Chemical Industries Ltd., The Manchester Oil Refinery Ltd., Lankro Chemicals Ltd., Leda Chemicals Ltd., Messrs. M. W. Hardy Sr Co. and T. Swan & Co. Ltd. I t is advisable to dry the chloroform over anhydrous 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. REFERENCES Linsenmeier, K., Mellialzd Textilber., 1940, 21, 468. Skinkle, J. H., Amev. Dyestuff Reportev, 1946, 35, 449. Goldstein, H. B., Ibid., 1947, 36, 629. Van der Hoeve, J. A., Rec. Tvav. Chim. Pays-Bas, 1948, 67, 649. Gilby, J. A., and Hodgson, H. W., Mfg Chem., 1050, 21, 371 and 423. Bergeron, J., Derenemesnil, R., Ripert, J., and Monier, G., BuZl. Mens. I , Wurzschmidt, B., 2. anal. Chem., 1950, 130, 8. Simmons, W. H., Analyst, 1951, 76, 279. Kortland, C., and Dammers, H. F., Chem. Weekblad, 1953, 49, 341. Van der Hoeve, J. A., J . SOC. Dyers 65 CoZ., 1954, 70, 145. Kortland, C., and Dammers, H. F., J . Amer. Oil Chem. SOG., 1955, 32, 58. Auerbach, M. E., I n d . Eng. Chem., Anal. Ed., 1943, 15, 492. Epton, S. R., Nature, 1947, 160, 795. Barr, T., Oliver, J., and Stubbings, W. V., J . SOC. Chem. Ind., 1948, 67, 45. Epton, S. R., T r a m . Faraday SOG., 1948, 44, 226. Wijga, P. W. O., Chem. Weekblad, 1949, 45, 447. Klevens, H. B., Anal. Chem., 1950, 22, 175 and 1141. Miller, D. D., and Elliker, P. R., J . Dairy Sci., 1951, 34, 273. Koot, W., Ing.-chim., 1954, 36, 12. Gobel, E. F., Rev. Quim. Ind., Bio de Janeiro, 1946, 15, 16. Miller, M. A., Bann, B., and Ponsford, A. P., J . AppZ. Chem., 1951, 1, 523. Holness, H., “Inorganic Qualitative Analysis by Semi-Micro Methods,” Sir Isaac Pitman & Sons Brown, E. G., and Hayes, T. J., Analyst, 1955, 80, 755. Feigl, F., “Spot Tests,” Elsevier Publishing Co. Inc., -4msterdam and New York, 1954, Volume 11, Guerbet, M. Ri., Compt. Rend., 1920, 171, 40. Feigl, F., op. cit., p. 76. Foster, G. E., and Grove, W. F., Analyst, 1946, 71, 287. Warfield, P. F., Anal. Chem., 1952, 24, 890. Camber, B., Nature, 1955, 175, 1085. Anon., M a y 6% Baker Lab. BuZl., 1956, 2, 9. B.I.O.S. Final Report No. 259, H.M. Stationery Office, London. Feigl, F., op. cii., p. 283. Von Pilat, S., and Sereda, J., PetroZ. Z . , 1933, 29, 1. 50, 4, 118. Ltd., London, 1954. p. 108. CHEMISTRY DEPARTMENT SOUTH-WEST ESSEX TECHNICAL COLLEGE WALTHAMSTOW, LONDON, E.17 August lst, 1966

ISSN:0003-2654    

DOI:10.1039/AN9578200166

出版商:RSC    

年代:1957

数据来源: RSC