Sept., 19501 MILLER AND CURRIE 467 The Micro-Gravimetric Determination of Lead in White Metals, Fusible Alloys and Tin-Free Copper-Base Alloys BY CHRISTINA C. MILLER AND LESLIE R. CURRIE SYNoPsIs-Lead (0.2 to 6 mg.) is separated from other alloy components, except copper in excess of 1 mg., by precipitating it twice as the lead nitrate - thiourea complex. The latter is dissolved in water, lead sulphide precipitated, separated, and dissolved in hydrochloric acid, and, finally, lead nitrate formed, which is dissolved in an aqueous solution of ammonium acetate, and filtered. The solution is acidified with acetic acid and lead separated and weighed as lead chromate, which is dried at 140' C. The method is applied to 5 mg. amounts of white metals and a fusible alloy, which are dissolved in hydrochloric acid and nitric acid.When more than 1 mg. of copper is present with lead, it must first be removed by electro-deposition. The method is then applicable to tin-free copper-base alloys, 5 to 10 mg. quantities of which are dissolved in nitric acid. ON the micro-scale lead has been determined as lead sulphate, picrolonate, phosphate and dioxide,l but there have been few applications in alloy analysis. Strebinge? determined lead in lead - tin solders as lead sulphate. Lindsey3 and his collaborators are working out a comprehensive scheme of analysis for alloys, based on micro-electrolytic depositions, but so far no results for the determination of lead in alloys have been published. Our objective was to devise a procedure for lead that could be applied to the determination of 2 to 90 per cent.in 5 to 10 mg. quantities of some non-ferrous alloys. For white metals and a fusible alloy we have adapted to the micro-scale and suitably modified the procedure used by Mahr and O€~le,~ whereby lead is precipitated as the lead nitrate - thiourea complex and, after appropriate treatment, converted into lead chromate as the weighing form. In the analysis of copper-base alloys containing little or no tin, we have removed copper by electro-deposition before applying the method. EXPERIMENTAL Mahr and Ohle showed that 20 to 50 mg. of lead contained in about 100 ml. of 1 to 2 N nitric acid could be quantitatively separated from comparable amounts of nearly all the common metals if the solution were saturated with thiourea and maintained at a temperature of 0" C.In the presence of bismuth a second precipitation was necessary, and in the presence of copper and silver, an adequate excess of thiourea. Aqua regia could be used without objection for the solution of alloys containing antimony and tin. The precipitates were separated, washed and dissolved in hot water, and in the resultant, almost neutral solution, after the addition of ammonium acetate, lead was precipitated as lead chromate, They applied the method to a lead - tin solder, a fusible alloy, a brass and a bronze. The simplicity of the method was attractive for micro-analysis because it seemed that, by using centrifugal methods of separation and the filter-stick technique, one might be able to carry out all the operations in one vessel. In preliminary experiments many difficulties arose when we tried to apply the method microchemically.As solubility losses were easily incurred with lead nitrate - thiourea, the volume of the solution in which its precipitation was effected had to be made a minimum, partly in order to counterbalance the effect of having to use proportionately more wash solution on the micro-scale. In consequence of this the tendency for the precipitate to be contaminated with other metals was greater. Karaoglanov and Michov5 effected the separa- tion of lead from moderate amounts of other metals by precipitating lead chromate from solutions 0.06 N in nitric acid. When we applied the method to lead in the solutions obtained from contaminated lead nitrate - thiourea precipitates difficulties arose because, in adjusting the acidity by neutralising the solution with ammonium hydroxide and adding a calculated amount of nitric acid, a precipitate of metallic sulphides separated.When this precipitate was dissolved, the resulting solution was contaminated by sulphur, which made lead chromate468 MILLER AND CURRIE : MICRO-GRAVIMETRIC DETERMINATION OF LEAD I N [VOl. 75 difficult to filter. Reduction of excess chromate also occurred during the digestion that preceded filtration. Lastly, no satisfactory results could be obtained with small amounts of lead and the method was ultimately abandoned in favour of precipitation of lead chromate in solutions containing acetic acid and ammonium acetate, as was used by Mahr and Ohle.This meant that lead had first of all to be separated completely from other alloy components, as a rule by double precipitation with thiourea. Since partial decomposition of thiourea caused contamination of lead chromate, it was necessary to eliminate it before precipitating the latter. This was achieved by precipitating lead as sulphide, dissolving it in hydrochloric acid, expelling hydrogen sulphide and then converting lead chloride into lead nitrate by evaporation with nitric acid. As some lead sulphate was invariably produced it was essential to dissolve the residue obtained after evaporation in a concentrated solution of ammonium acetate. Filtration of its solution was still required in order to remove a slight turbidity, and, in alloy analysis, to remove a little hydrated stannic oxide.The filtrate was collected in the vessel in which the lead was to be determined as lead chromate. Guzelj6 has shown that lead sulphate dissolved in ammonium acetate solution can be satisfactorily determined as lead chromate. The reagents were as far as possible AnalaR. Thiourea was recrystallised from water. PROCEDURE FOR THE SEPARATION OF LEAD AS LEAD NITRATE - THIOUREA AND ITS DETERMINATION AS LEAD CHROMATE Place the solution containing 0.2 to 5 mg. of lead as lead nitrate in a test tube (75 mm. x 12 mm.) and make up to a volume of 1.5 ml. that is 2 N in nitric acid. Add 0-2 g. of thiourea, shake the tube until precipitation starts, then cool it for half an hour at -5" C., shaking vigorously at intervals. Centrifuge, while maintaining the temperature below 0" C.by placing the tube within a wider container of cooling liquid, that fits into the metal bucket of the centrifuge. Withdraw the centrifugate by suction through a sintered porcelain filter-stick and wash the precipitate once with 0-5 ml. of 2 N nitric acid that has been saturated with thiourea at room temperature and cooled to -5" C. Next dissolve the precipitate by placing the test tube containing it within a suitable suction apparatus and drawing 1.5ml. of hot water through the filter into the tube.' Add sufficient 7 N nitric acid to the solution to make it 1 to 2 N, add a small amount of thiourea, and repeat the above treatment, washing the precipitate thrice with 0-5-ml. portions of the prescribed solution before dissolving it.In order to precipitate lead sulphide, add to the solution 0.5 ml. of 15 N ammonium hydroxide, saturate with hydrogen sulphide, cool to room temperature and centrifuge and withdraw the supernatant liquid as before. Wash the precipitate twice with 0.25 ml. of a saturated solution of hydrogen sulphide, then dissolve it by drawing 1.5 ml. of boiling 5 AT hydrochloric acid through the filter-stick into the test tube. Heat to effect complete solution and evaporate the solution to dryness on the steam-bath, accelerating evaporation by means of a current of purified air. Dissolve the residue in 1 ml. of 7 N nitric acid, re-evaporate to dryness and dissolve again in 0.25 ml. of a hot, 20 per cent. ammonium acetate solution. Transfer the solution through a clean, porcelain filter-stick into a 5-ml.beaker that has been weighed together with a sintered glass (porosity No. 4) or asbestos-packed Emich type filter- stick. Wash the test tube, etc., twice with 0.25 ml. of hot water followed by 0.1 ml. of ammonium acetate solution, and finally twice with 0.25 ml. of hot water. If the amount of lead is small (0-2 mg.) reduce the volume to less than half by evaporation. Whatever the final volume, make the solution 0-25 to 0-7 N in acetic acid, heat to boiling and precipitate lead chromate by the dropwise addition of a 0.5 per cent. solution of potassium dichromate, in amount ranging from 1.5 ml. for 5 mg. of lead, to 0-1 ml. for 0.2 mg. Digets the precipitate at 90" C. for 10 minutes then let it cool for 1 hour and filter. Wash a large precipitate once with 0.5 ml.of hot water and five times with 0-25 ml., and a small precipitate once with 0.3 ml. and then three times with 0.25 inl. Dry the beaker and its contents at 140" C. in a Benedetti - Pichler drying block, cool and weigh, with strict adherence to the programme outlined in the next section. THE TREATMENT OF BEAKERS AND FILTER-STICKS BEFORE WEIGHING Trouble caused by the production of electrostatic charges on glassware that had been wiped, prior to weighing, in accordance with Pregl's well-known procedure, led us to abandon this in favour of another routine, which also compensated for changes in barometric pressure. The tare for each beaker and filter-stick was another beaker of the same glass, adjusted toSept., 19501 JVHITE METALS, FUSIBLE ALLOYS AND TXN-FREE COPPER-BASE ALLOYS 469 weight by the addition of short lengths of glass rod.The outside of the prepared beaker and its counterpoise were wiped once with a pair of very damp flannelette cloths, one held in each hand, a rotary motion being imparted to the wiping, and then similarly with a pair of slightly damp pieces of chamois leather. Each beaker, held by forceps, was next passed four times through a non-luminous flame in order to dispose of any electrostatic charge, and then placed in the drying apparatus at 140” C. for 10 minutes. After removal, the beakers were cooled for 5 minutes on metal blocks, and the tip of the filter, which had been connected by rubber tubing to the suction line, was wiped clean. Both beakers were then placed for 15 minutes in a desiccator containing a saturated solution of calcium nitrate, which allowed the glass surfaces to assume rapidly a reproducible film of moisture.8 Finally, they were transferred simultaneously to the balance case and weighed after 5 minutes.A reliable duplicate of the first weighing was obtained by merely reheating (no suction) and cooling both beakers as before. After use the beaker and filter-stick were submitted to the full treatment, but the counterpoise, which had been kept in the desiccator, was only given the heating at 140” C. All weighings must be duplicated. THE DETERMINATION OF LEAD IN ALLOYS Method 1. For white metals and fzfisible alloys-Dissolve 5 mg. of the alloy in 0.2 ml. of 5 N hydrochloric acid and the minimum amount of 7 N nitric acid, add 1 ml.of water and sufficient 7 N nitric acid to bring its concentration to about 2 K. Proceed with the deter- mination of lead as already indicated, noting, however, that with fusible alloys both precipitates of lead nitrate - thiourea must be washed thrice. For coeper-base alloys containing Little or no tin-Copper, which in excess of 1 mg. accompanied lead nitrate - thiourea, was removed initially by electro-deposition as follows. Dissolve 5 to 10mg. of the alloy, contained in the usual test tube, in 0.2 ml. of 7 N nitric acid, add 2.5 ml. of water and boil to expel nitrous acid. Next add 0.1 ml. of a 10 per cent. urea solution and electrolyse for 10 minutes at 2.5 volts, using as a cathode a cylinder (diameter 10 mm., height 15mm.) of platinum gauze (20 strands per cm.), and, as an anode, a platinum wire.Finally remove and rinse the electrodes, carefully protecting the anode on which some lead dioxide deposits. Evaporate the solution to dryness on a steam-bath and take up the residue in 0.2 ml. of 5 N hydrochloric acid. Dissolve the lead dioxide on the anode in dilute nitric acid containing hydrogen peroxide and collect the solution in the test tube, to make a final volume of 1-6 ml. at a concentration of about 2 N in nitric acid. RESULTS (i) The direct determinatiopi of lend as lead chromate-I!eighed portions of a standard solution of lead nitrate, prepared from “British Chemical Standards” lead, gave the following results, when the theoretical factor of 0.6411 was used for converting lead chromate into lead.TABLE I Method 2. Precipitate lead nitrate - thiourea once only and continue as described above. JVeight of Xumber of Maximum range lead taken, csperirnents in error, -4 \-erage error, mg. PF$ w 3 4 +6 t o - 5 - 1 1 9 +9 to -5 - 3 0.2 10 +5 t o -7 nil No significant difference was noted when the amount of ammonium acetate present was varied between 20 and 100mg., or when the amount of acetic acid ranged between 0.25 and 0.7 N , provided that the washing of the lead chromate was very carefully done. A negative error averaging 4 to 5 pg. was incurred when the smallest quantity of lead was precipitated with chromate in the same volume of solution as the larger quantities. (ii) The determination of lead as lead chromate aftev $first separating it as lead nitrate - thioztrea-Weighed portions of the above standard solution of lead nitrate were submitted to the full scheme of separations previously outlined and the following results obtained. In a “blank” run on the reagents no lead was found.Error on 5 mg. of lead, -22, -20, +13 and +7 pg. Error on 0-2 mg. of lead, -2, -19, +4 and -11 pg. Inadequate precautions for keeping the lead nitrate - thiourea precipitates cool during470 MILLER AND CURRIE : MICRO-GRAVIMETRIC DETERMINATION OF LEAD [Vol. 75 centrifugation caused the rather large negative errors in the first pair of results for 5 mg. of lead, and incomplete mastery of the technique, at this stage, the irregularity in the results for 0-2 mg. Influence of copfier aizd bismutk-The presence of 50 pg. of copper or bismuth did not affect the results for 5 mg.of lead. The maximum amount of copper that could be separated from 1 mg. of lead in 1.5 ml. of solution was 1 mg. Three mg. of bismuth could be separated from 1 mg. of lead if both precipitates of lead nitrate - thiourea were washed thrice with 0.5 ml. of the wash solution. (iii) Lead in alloys-In order to avoid errors due to lack of homogeneity in the samples of prepared alloys, we prepared small amounts for analysis by aliquot partition of a solution made up from 1 g. of the powdered alloy and treated in accordance with the prescribed micro- procedure. The metals used in preparing the synthetic alloy solutions were the purest available; the lead, copper and zinc having a purity in excess of 99.95 per cent.The bismuth and cadmium were examined for lead and none was found, All the results shown in Table I1 refer to 5 mg. of alloy, except for the high-speed brass where 10mg. was taken. In the analyses of the copper-base alloys, copper was removed by electro-deposition before precipitating lead nitrate - thiourea. I Sb 7cZ’hite metal “A”* . . 12.04 White metal “B”* . . 7.31 Lead-base bearing metal? 10.09 Cd Fusible alloy: . . . . 12.3 zn High-speed brasst . . 37.0 - Bearing metal: . . .. TABLE I1 Percentage composition Sn Bi c u 4.64 0.03 0.33 84.0 trace 4-08 10.91 0.06 0.05 25.00 30.00 3*0u * British Chemical Standards sample. -f U.S.A. Bureau of Standards sample. ’+ Synthetic. tj In all except the last experiment the lead nitrate - thiourea had k e n instead of the recommended half hour. Percentage of lead found 82-9, 82.9, 82.7, 82.9 78.7, 78.8 2500, 25-04, 24.79, 24.96 30.10, 30.02, 30.10, 30.00 3-03, 2-92, 2.94, 3.01 3.65, 3.67, 3.65, 3.88$ cooled for 15 niinutes only The results in Table I1 show that the micro-gravimetric determination of lead in the alloys cited is practicable provided that the conditions prescribed on p. 468 are closely followed. A negative error is rather easily incurred in dealing with quantities of lead of the order of quarter of a milligram. We gratefully acknowledge a maintenance grant to one of us (L. R. C.) from the Depart- ment of Scientific and Industrial Research and a grant from the Ritchie Fund for the purchase of apparatus. REFERENCES 1. 2 . 3. 4. 5. 6. 7. 8. Hecht, F., and Donau, J ., ‘‘ASnorganische Mikrogewichtsanalyse,” Edwards Brothers, Inc., Ann Strebinger, R., Chem. Zentr., 1918, 11, 471. Lindsey, A. J., Analyst, 1948, 73, 72. Mahr, C., and Ohle, H., 2’. anorg. Chem., 1937, 234, 224. Karaoglanov, Z., and Michov, M., 2. m a l . (;hem., 1935, 103, 113. Guzelj, L., Ibid., 1936, 104, 107. Hecht, F., and Donau, J ., “4norganische Mikrogefficl.ltsanalyse,” Edwards Brothers, Inc., -4nn Arbor, Michigan, 1940, p. 101. Thiers, R, E., and Beamish, F. E., .-lnal. Chaiir., 1947, 19, 434. Arbor, Michigan, 1940, p. 150. CHEMISTRY DEPARTMENT THE UNIVERSITY, EDINBUKGH May, 1950