214 Analyst, April, 1968, Vol. 93, @. 21.4-218 Spectrophotometric Determination of Small Amounts of Tin in Lead and Antimonial Lead Alloys BY J. C. H. JONES* Associated Lead Manufacturers, Research Laboratories, 7 Wadsworth Road, Perivale, Greenford, Middlesex) A method is described for the determination of 0.001 to 0.10 per cent. of tin in lead and antimonial lead alloys. The sample (0.25 to 2-45 g) is dis- solved in a mixture of nitric and citric acids. After addition of EDTA and ammonium chloride, the pH of the solution is adjusted to 5.5 with ammonia solution before passing it through a column containing silica gel. Tin is adsorbed on the column, while lead, antimony and other metals pass through. After elution with hydrochloric acid solution the tin is determined spectro- photometrically with gallein.The standard deviation on samples containing 0.0051 and 0-038 per cent. of tin is &-0.0005 and &O.OOZ, respectively. At least eight determinations can be carried out in 1 day. SOME properties of lead, such as resistance to corrosion, fatigue, creep and grain size, are influenced by the presence of small amounts of tin and other metals. An important application of lead is in the manufacture of alloys containing between 1 and 12 per cent. of antimony for accumulator plates, acid pumps, valves and telephone cables. The characteristics of some of these antimonial lead alloys are also dependent on the amount of tin present. It is, therefore, desirable to have a method for determining 0-001 to 0.10 per cent. of tin in lead and antimonial lead alloys suitable for use in a routine laboratory.In most of the methods reported it is necessary to separate the tin from the lead and other metals before the final determination. For example, the tin has been determined polarographically after distillation of tin(1V) bromide1 or after removal of lead as lead sulphate.2 Of the spectrophotometric methods reported, dithio13 has been used after co- precipitation of the tin with manganese dioxide, and phenyMuorone4 after solvent extraction in which cupferron is used. Catechol violet6 has also been used for the spectrophotometric determination of tin in lead after solvent extraction in which tri-(2-ethylhexyl)phosphine oxide is used. All of these methods are lengthy and tedious and not applicable in the presence of large amounts of antimony found in antimonial lead alloys.It was, therefore, decided to investigate methods of separating tin from lead and antimony. Ariel and Kirowa6 reported the anion-exchange separation of tin from lead and antimony and applied the procedure to lead - tin alloy. The method is rather involved, and it is doubtful if it could be applied to the present problem. Although many paper-chromatographic procedures have been published7,* ,gJO for the separation of lead from other metals, these have been mainly concerned with the qualitative separation of Groups I and I1 of the general qualitative scheme. Such separations, although invaluable for qualitative purposes, are not suitable for quantitative analysis. Sulcek, Dolezal, Michal and Sychrall reported a method for the determination of tin in antimony metal, in which column chromatography is used.These workers dissolve the sample in a mixture of hydrochloric acid and bromine, and, after the addition of citric acid, ammonium chloride and EDTA, the pH of the solution is adjusted to 5.5 by the addition of sodium hydroxide solution. When the solution (about 200 ml) is passed through a column * Present address: Science Department, High Wycombe College of Technology and Art, Queen Alexandra Road, High Wycombe, Bucks. 0 SAC and the author.JONES 215 of silica gel, the tin is held quantitatively and the other metals pass through. The tin is determined polarographically after eluting with hydrochloric acid. In this procedure, a 20-g sample of antimony is recommended when the expected tin content is 0*001 per cent.The combination of the separation by using silica gel with a spectrophotometric deter- mination does not appear to have been previously used for the determination of tin. It was hoped that by using a sensitive spectrophotometric reagent it would be possible to determine smaller amounts of tin than those determined by Sulcek, Dolezal, Michal and Sychra. Such a procedure would require a smaller sample and a reduction in the volume of solution to be passed through the column. Woodl2 has compared several different spectrophotometric reagents for tin and concluded that gallein is the most sensitive and simplest to use. This reagent has been used for all of the investigations described in the present paper, which have resulted in a satisfactory method for the determination of tin (0401 to 0.1 per cent.) in lead and antimonial lead alloys.A 2.5-g sample is required for an expected tin content of 0-001 per cent., and the volume of solution to be passed through the column is only 50 ml. EXPERIMENTAL Investigations of the determination of tin with gallein confirmed the findings of Wood that Beer’s law is obeyed up to a concentration of 0.5 pg of tin per ml and that the absorption of the tin - gallein complex at 520 mp reaches a maximum value after 30 minutes and then remains constant for over 1 hour. It was also verified that the absorption at 520mp was constant over the pH range 2 to 3. In all of the subsequent investigations with gallein the conditions were kept as described by Wood, with the pH control achieved by the use of a sodium chloroacetate - chloroacetic acid buffer (pH 2.4 to 2.5).It was also found, as Stanton and McDonald13 have reported, that the results obtained by the method are independent of the oxidation state of the tin in the original solution. Consequently, it is unnecessary to oxidise or reduce the tin in the hydrochloric acid solution before carrying out the determination of tin with gallein. The chromatographic columns were prepared as described by Sulcek, Dolezal , Michal and Sychra. Preliminary investigations showed that the mixture of hydrochloric acid and bromine used for dissolution could be replaced with nitric acid when using samples of lead. Also, it was found satisfactory to use concentrated ammonia solution, instead of sodium hydroxide solution, for adjusting the pH of the solution to 5.5.Experiments were also carried out by using the Procedure described below to determine the effect of varying the amounts of citric acid, EDTA and ammonium chloride. It was found that the amount of citric acid could be varied between 5 and 15 g and that of EDTA between 1 and 3 g without influencing the results. Similar results were obtained by varying the volume of 0.1 M ammonium chloride solution between 5 and 15ml. Other investigations showed that the pH could be varied between 5.4 and 5.6 without affecting the recovery of tin. METHOD APPARATUS- Chromatographic column-This consisted of a glass tube, 1 cm in internal diameter and 50cm in length, provided at the lower end with 10cm of rubber tubing with metal clip attached.A Hilger Uvispek spectrophotometer was used for all of the optical density measurements and a Pye Universal pH meter for pH measurements. REAGENTS- All reagents should be of analytical-reagent grade. Silica gel, M.F.C.-Special grade for chromatography, obtainable from Hopkin and Williams Ltd. Cover a suitable amount of silica gel with distilled water and leave it to stand for 12 hours (preferably overnight). Wash out the fine fraction by decantation, and then repeatedly wash the residue with dilute hydrochloric acid (50 per cent v/v, aqueous) and distilled water until completely free from iron. Nitric acid, 20 per ceutt. v / v , apeow+-Prepare from concentrated nitric acid (sp.gr. 1.42). Citric acid.216 JONES : SPECTROPHOTOMETRIC DETERMINATION OF SMALL [Analyst, Vol.93 Ammonia solution, conceuttrated(sp.gr. 0.88). Ammonium chZoride solution, 0.1 M-Dissolve 5.4 g of ammonium chloride in water and Ethylenediaminetetra-acetic acid, disodium salt (EDTA) . Sodium citrate solution, 0-5 M-Dissolve 121 g of sodium citrate in water and make up to 1 litre. Hydrochloric acid, 50 per cent. v/v, aqueous-Prepare from concentrated hydrochloric acid, sp.gr. 1-18. Hydrochloric acid, 10 per cent. v / v , aqueous-Prepare from 50 per cent. v/v aqueous hydrochloric acid. Bzll$er solution-Dissolve 50 g of chloroacetic acid, 50 g of sodium chloroacetate and 25 g of hydroxylammonium chloride in 500 ml of water. Gallein solution-Warm 5 mg of gallein with 50 ml of ethanol until no more dissolves.Filter through a Whatman No. 540 filter-paper into a 100-ml graduated flask and make up to the mark with ethanol. Wash solation A-Take 200 ml of sodium citrate solution (0-5 M) and adjust the pH to 5.5 by the addition of citric acid crystals. Wash solution B T o 200ml of distilled water add 2 drops of sodium citrate solution (0.6 M). Adjust the pH to 5.5 by adding a few crystals of citric acid. Pure tirt-Obtainable from Associated Lead Manufacturers Ltd. Standard tirt solution A-Dissolve 0.0500g of pure tin metal in 50ml of concentrated hydrochloric acid. Add 50 ml of water and then dilute with hydrochloric acid (50 per cent. v/v, aqueous) to 500 ml. dilute to 1 litre. 1 ml of solution = 0*0001 g of tin (in 50 per cent. v/v hydrochloric acid).Standard tin solution B-Transfer 5 ml of standard tin solution A into a 100-ml graduated flask, add 15 ml of 50 per cent. v/v aqueous hydrochloric acid and dilute to the mark with water. 1 ml of solution = 0-000005 g of tin (in 10 per cent. v/v hydrochloric acid). PREPARATION OF COLUMN- Prepare a slurry of a suitable amount of silica gel in wash solution A and transfer it to a glass column, after placing a plug of glass-wool at the bottom end. The length of the silica-gel column should be 8 to 10cm. Pass 1 O m l of wash solution A through the column. PREPARATION OF CALIBRATION GRAPH- Transfer 1.0, 2.0, 3-0, 4.0 and 5.0ml of standard tin solution B (which contains 10 per cent. v/v of hydrochloric acid) into 50-ml graduated flasks. Introduce into each flask sufficient 10 per cent.hydrochloric acid solution so that the total volume of 10 per cent. hydrochloric acid is 5.0 ml, i.e., add 4*0,3*0,2.0, 1.0 and 0 ml of 10 per cent. hydrochloric acid, respectively, to each flask. Add 20 ml of buffer solution, 5-0 ml of gallein solution and make up to the mark with water. Shake the solution for 30 seconds and, after 45 minutes, measure the optical density in a 4-cm cell at a wavelength of 520 mp, with water as reference solution. Carry out a blank determination by adding the above amounts of gallein and buffer solution to 5-0 ml of 10 per cent. v/v hydrochloric acid solution in a 50-ml graduated flask. Subtract the optical density of the blank from each of the readings and plot the calibration graph. PROCEDURE- Transfer a suitable weight of sample (Note 1) into a 100-ml beaker containing l o g of citric acid. Add 25 ml of 20 per cent.v/v nitric acid solution and heat until dissolution of the sample is complete. Cool the solution, add 12 ml of concentrated ammonia solution and again cool. Add 10 ml of 0.1 M ammonium chloride solution and 2 g of EDTA, and adjust the pH of the solution to 5.5 & 0-1 with citric acid or ammonia solution (Note 2). Transfer the solution immediately (Note 3) on to the prepared column (Note 4) by using a small funnel, and adjust the screw-clip so that the rate of flow does not exceed 3 ml per minute. When the solution has passed through the column, wash the column with 100 ml of wash solution A, followed by 50ml of wash solution B.April, 19681 217 When the solution has reached the upper level of the silica-gel column, add 10.0 ml of 50 per cent.v/v hydrochloric acid solution. Allow this acid to remain on top of the column for 5 minutes before eluting into a 50-ml graduated flask. Wash the column with water and collect the washings in the graduated flask. Transfer, by pipette, 5-0 ml of the solution into another 50-ml graduated flask containing 20 ml of buffer solution. Add 5 ml of gallein reagent, complete the determination and prepare a blank, as described under Preparation of calibration graph. AMOUNTS OF TIN IN LEAD AND ANTIMONIAL LEAD ALLOYS NOTES- 1. It is recommended that a 2.5-g sample is taken for an expected tin content of 0.001 to 0-01 per cent., and a 0-26-g sample for tin contents between 0.01 and 0.10 per cent.2. It is sometimes necessary to stir the solution vigorously for about 2 minutes to dissolve the EDTA. 3. If the sample weighs 2-0 g, or more, a precipitate may appear if the solution is allowed to stand for over 20 minutes before transferring the solution on to the column. For such solutions i t is recommended that they are transferred on to the column immediately after adjustment of the pH to 5.5. It is essential that no precipitate is in the solution when the latter is transferred on to the column, because this would block the column and reduce the rate of flow. 4. One hundred millilitres of wash solution A is passed through the column at the commence- ment of each determination. RESULTS RECOVERY OF TIN IN THE PRESENCE OF LEAD- to which was added a known volume of standard tin solution A.The procedure was studied by using different weights of pure lead samples (0 to 2.5 g), Some typical results are shown in Table I. TABLE I RECOVERY OF TIN IN THE PRESENCE OF LEAD Lead added, g 0 0.26 0.60 0.75 1-5 2-0 2.5 Tin added, mg 0.20 0.20 0.20 0-20 0.20 0.20 0.20 Tin found, mg 0.21 0.20 0.20 0.19 0.19 0.20 0.20 Error, per cent. + 6-0 0.0 0.0 - 6.0 - 6.0 0.0 0.0 RECOVERY OF TIN IN THE PRESENCE OF ANTIMONY AND LEAD- According to Wood, antimony gives a colour with gallein similar to that given'by tin. $ulcek, Dolezal, Michal and Sychra report that at pH 5.5 -f 0.1 the tin is adsorbed on the silica-gel column, while the antimony passes through. In view of the large amounts of antimony present in some antimonial lead alloys it was necessary to verify that this element was completely separated from the tin and, consequently, did not interfere with the spectro- photometric determination.Several determinations were carried out, as described under Procedure, by using synthetic samples prepared from pure antimony and lead. Some typical results are shown in Table 11. TABLE I1 RECOVERY OF TIN IN THE PRESENCE OF ANTIMONY AND LEAD Lead added, Antimony added, Tin added, Tin found, Error, Nil 0.026 0.20 0.20 0 0.25 0.026 0.20 0-20 0 2.25 0.25 0.20 0.21 + 6.0 2-20 0-30 0.20 0.21 + 5.0 2-10 0.40 0.20 0.20 0 g g mg m g per cent. DETERMINATION OF TIN IN THE PRESENCE OF OTHER ELEMENTS- The method was applied to various synthetic samples containing lead and antimony, to which small amounts of other elements that normally occur with lead were added.The results are shown in Table 111.218 Lead, per cent. 96 90 90 88 100 100 100 100 100 JONES TABLE I11 DETERMINATION OF TIN IN THE PRESENCE OF OTHER ELEMENTS Antimony, Impurity added, Form of Tin added, Tin found, per cent. per cent. added impurity per cent. per cent. 4 Zinc, 0.060 Zinc nitrate 0-0050 0.0047 10 Cadmium, 0.035 Cadmium nitrate 0.0060 0.0059 10 Copper, 0-050 Copper(I1) nitrate 0.040 0-043 12 Bismuth, 0.050 Bismuth nitrate 0-040 0440 Nil Arsenic, 0.020 Lead sample con- 0*0020 0-0020 taining 0.020 per cent of arsenic Nil Copper, 0.060 Copper(I1) nitrate 0.0040 0.0041 Nil Zinc, 0.035 Zinc nitrate 0.0085 0.0085 Nil Nickel, 0.040 Nickel chloride 0.040 0.042 Nil Iron, 0-040 Iron(II1) chloride 0.090 0.084 Error, per cent.-6 -2 +3 0 0 +3 0 +5 -7 The reproducibility of the methqd was investigated by determining the tin content of a sample of lead and antimonial lead alloy (Table IV). TABLE IV REPRODUCIBILITY OF THE METHOD WITH A SAMPLE OF LEAD AND ANTIMONIAL LEAD ALLOY Composition, Tin found, Mean Standard Sample per cent. per cent. deviation M91-4 Lead, 91 0.037, 0.038 Antimony, 9 0.040, 0.036 0.037, 0.040 0.038 & 0.002 R2 Lead, 99-99 0-0055, 0-0044 0.0056, 0.0050 0.0052, 0.0048 0.0051 f 0-0005 DISCUSSION The proposed procedure is suitable for the determination of 0.001 to 0-10 per cent. of tin in lead and antimonial lead alloys. A single determination takes about 3 hours, but at least eight determinations can be completed by one analyst in a normal working day.According to Wood, antimony, which itself gives a colour with gallein, is likely to interfere in the spectrophotometric determination of tin by the gallein method. It was found that the chromatographic procedure separated the tin from antimony and so eliminated this inter- ference. No serious interference was observed from other elements investigated (see Table 111). The reproducibility of results by the proposed method is good. For samples containing 04051 and 0.038 per cent. of tin the standard deviation was 5 OWO5 and _+ 0.002, respectively. It is thought that this is the first time that a chromatographic separation, in which silica gel was used, has been combined with a spectrometric determination of tin. It is possible that the method could be applied to the determination of small amounts of tin in other metals and alloys. I thank the Directors of Associated Lead Manufacturers Ltd. for permission to publish this paper, and Dr. E. H. Amstein, Research Manager, and Mr. A. R. Gunningham, Chief Analyst, for their interest and encouragement. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. REFERENCES Kallmann, S., Liu, R., and Oberthin, H., Analyt. Chem., 1958, 30, 485. Cyrankowska, M., Chemia. Analit., 1960, 5, 861. “Chemical Determination of Impurities in Lead,’’ British No%-Ferrous Metals Research Association Gur’ev, S. D., and Saraeva, N. F., Inst. 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