Sept., 195Oj MILLER AND CURRIE 47 1 The Micro-Gravimetric Determination of Lead in Bronzes BY CHRISTINA C. MILLER AND LESLIE R. CURRIE SyNoPsIs-Lead in amounts of 0.2 to 1 mg. is quantitatkrely separated under carefully controlled conditions from 5 to 10 mg. of copper and 1 mg. of tin, by depositing it electrolytically as lead dioxide from solutions containing hydrofluoric acid and nitric acid (cf. McCayl). Lead dioxide is dissolved and lead precipitated and weighed as lead chromate. The method is applied to 5 and 10mg. amounts of two bronzes con- taining, respectively, 20 and 1.83 per cent. of lead. WHEN the method described in the preceding paper, see p. 467, for the determination of lead in tin-free, copper-base alloys, was applied to those containing an appreciable amount of tin, no satisfactory results could be obtained.The average result for a bronze containing 20.00 per cent. of lead was 19.37, and for another containing 1.83 per cent., 1-69 per cent. In the final filtration that preceded the precipitation of lead chromate, hydrated stannic oxide was invariably withheld on the filter and it contained adsorbed lead. Although present in the original bronze solutions, stannic oxide dissolved in the hydrochloric acid subsequently added, but /I-stannic salts apparently contaminated the lead nitrate - thiourea complex and lead sulphide, and gave rise again to hydrated stannic oxide in the final treatment with nitric acid. Stannic oxide did not separate initially if tartaric or citric acid was included in the nitric acid solvent for the bronze, nevertheless it still appeared in the final solution.All efforts made to eliminate tin by volatilising its halides were unsuccessful. Attempts made to remove copper by depositing it electrolytically from hydrochloric acid solutions2 of the alloys, in which /?-stannic salts were not produced, unaccountably failed. The recovery of adsorbed lead from impure stannic oxide, by fusing the latter with sodium carbonate and sulphur and separating soluble sodium thiostannate from insoluble lead sulphide, was associated with complications of procedure that led to other negative errors of undetermined origin. The object of our investigation was therefore to devise an alternative method for the determination of lead in bronzes. McCayl showed that 60 to 400 mg.of lead could be separated from up to 500 mg. of tin by depositing it anodically as lead dioxide from a solution containing nitric and hydrofluoric acids. We have evolved comparable micro-procedures for the separa- tion of 1 and 0.2 mg. of lead from 1 mg. of tin and 5 mg. or more of copper, and applied them to the bronzes mentioned above, the lead being finally weighed as chromate. EXPERIMENTaL ELECTROLYTIC APPARATUS FOR THE ANODIC DEPOSITION OF LEAD DIOXIDE- Electrolyses were conducted in a platinum crucible (height 26 mm., diameter at the top, 20 mm., and at the bottom, 13 mm.) as the anode, with a platinum gauze (10 strands per cm.) cylinder (height 13 mm., diameter 10mm.) as the cathode. Several strands of platinum wire were placed across the bottom of the cylinder so that it formed a kind of basket.The effect of this was to induce the deposition of lead dioxide on the bottom of the crucible, which did not occur without this modification. To the upper edge of the cylinder was welded a platinum wire that was bent at right angles above the crucible and sealed into a glass support. The crucible rested in a triangle of platinum wire, which was attached to an insulated ring, and connected to the positive terminal of a 4-volt battery. The electrical circuit contained a %-ohm, variable resistance and a milliammeter, and across the electrodes was a &volt voltmeter of 50,000-ohm resistance. The electrically driven stirrer was a platinum wire, slightly flattened at the lower end, and sealed into a glass rod at the upper.Attached toit just above the crucible, and rotating with it, was a protective celluloid cover. A moderate rate of stirring only was required. Vigorous stirring caused depositions to be incomplete. In order to obtain adherent deposits of lead dioxide the crucible was etched by brief treatment with aqua regia.472 MILLER -4ND CURRIE THE MICRO-GRAVIMETRIC [Vol. 75 CONDITIONS FOR THE DEPOSITION OF LEAD DIOXIDE- Quantitative deposition, especially of a small amount of lead, occurred under carefully controlled conditions that were deduced from the results of a long series of experiments, in which electrolyses were done in the presence of 5 to 10 mg. of copper and 1 mg. of tin. The method is as follows: Evaporate the solution containing nitrates and fluorides of the metals to dryness in the crucible to be used for the electrolysis and, if 1 mg.of lead is present, take up the residue in 2.5 ml. of a solution that is 1-5 N in nitric acid and 1.0 IV in hydrofluoric acid. Add 5 mg. of urea, set up the electrolysis outfit and start the stirrer. Begin the electrolysis at room temperature, with the full resistance in the circuit, and, after 1 to 2 minutes, increase the voltage to 2.50 Maintain this value for 30 minutes, then remove the stirrer, withdraw the electrolyte by suction and rapidly replace it with water. Remove and replace the water until no current is registered. For 0.2 mg. of lead reduce the concentration of both acids to 0.5 AT. 0.05. THE DETERMINATION OF LEAD AS LEAD DIOXIDE OR AS LEAD CHROMATE- If the determination of lead as lead dioxide is required, proceed as follows: Dry the crucible containing lead dioxide for 10 minutes in the platinum-lined cavity of a metal drying block maintained at 230" C., then cool for 5 minutes on a metal block and 5 minutes in the balance case, and weigh against a similar crucible as a counterpoise.Next dissolve the lead dioxide in 0-5 ml. of hot 7 N nitric acid and 0.1 ml. of 3 per cent. hydrogen peroxide solution, and, if the further determination of lead as lead chromate is required, transfer the solution to a 5 ml. glass beaker that has been previously weighed together with a sintered glass filter- stick (porosity No. 4). Wash the crucible first with half the above volumes of nitric acid and hydrogen peroxide and then five times with 0.25ml.of hot water. Dry and weigh the crucible. For the determination of leadas lead chromate evaporate the solution in the beaker to dryness, dissolve the residue in 1.5 ml. of a 2 per cent. solution of ammonium acetate and 0.1 ml. of 7 N acetic acid, and proceed exactly as is described in the preceding paper. THE DETERMINATION OF LEAD IN BRONZE- In the crucible to be used for the electrolysis, dissolve 5 or 10 nig. of bronze in a mixture of 4 drops of water, 2 of hydrofluoric acid (40 per cent. w/w) and 3 of concentrated nitric acid. Evaporate the resulting solution to dryness and dissolve the residue in 2.5 ml. of a solution containing the amounts of nitric acid and hydrofluoric acid appropriate for the anticipated amount of lead. If a little antimony is present, oxidise it with a drop of a 0-5 per cent.solution of potassium dichromate (cf. McCay), then add 5 mg. of urea and proceed as already outlined. RESULTS The purity of the lead, tin and copper used in the following experiments exceeded 99.95 per cent. AnalaR reagents were employed throughout and examined when necessary for traces of lead. Stock solutions containing hydrofluoric acid were prepared and kept in platinum vessels. Lead as lead dioxide-All depositions were effected in the presence of 5 or 10 mg. of copper and the amounts of nitric acid and hydrofluoric acid appropriate for the weight of lead. Weighed portions of a standard solution prepared by dissolving lead in nitric acid were used. Error on 1 mg. of lead, (a) Tin absent: +B, +20, +17, +2l and +17 pg.(b) 1 mg. of tin present : +36 and +28 pg. (c) 5 mg. of tin present: +13 and +28 pg. Error on 0.2 mg. of lead, (a) Tin absent : +8, + 1, +9 and +6 pg. (b) 1 mg. of tin present: +4, +4, 0, +5 and + 2 pg. These errors, which are much in excess of what could be caused by our use of the theoretical factor for the conversion of lead dioxide to lead (cf. Sand3), are in accord with positive errors of about 2 per cent. found by McCay, who attributed them to fluoride contamination of the lead dioxide. He eliminated them by dissolving the lead dioxide and redepositing it in the absence of hydrofluoric acid. We inexplicably failed to improve our results by repeating the electrolysis in the presence of 5 mg. of copper and nitric acid only. The copper was added in the second electrolysis in order to prevent cathodic deposition of lead.v I.lzJEuence of other eZements-The addition of 50 pg.of phosphorus added as potassium dihydrogen phosphate had no significant adverse effect on the deposition of 0.2 or 1 mg. of lead as lead dioxide. If 200 pg. of manganese (11) or 100 pg. of antimony (V) were initially present with 1 mg. of lead, the amounts of manganese and antimony that were found in the lead dioxide were less than 1 and 3 pg. respectively. The influence of bismuth was con- siderable, 20 per cent. of that initially added accompanying the lead dioxide. Lead as lead chromate-The following results refer to experiments in which 5 or 10 mg. of copper and 1 mg. of tin were initially present with the lead, which was deposited as lead dioxide and then dissolved and converted into lead chromate.Error on 1 mg. of lead, -3, +2, +12 and -8 pg. Error on 0.2 mg. of lead, +9, - 1, -3, + 10, -4 and +S pg. The excessive positive errors were attributed to the failure to filter the nitric acid solution of the lead dioxide. In the following analyses of bronzes, filtration was carried out through a sintered glass filter-stick (porosity No. 4). Lead in bronzes as lead chromate-(1) High lead bronze. A synthetic solution was prepared by adding lead as lead nitrate to a nitro - hydrofluoric acid solution of British Chemical Standards Bronze “C,” so that the percentage composition of the corresponding alloy was : copper 69.82, tin 7-88, antimony 0.03, bismuth a trace, other (non-interfering) elements 2.27 and lead 20.00. The weighed portions of the solution taken for analysis corresponded to about 5 mg. of alloy. (2) British Chemical Standards Bronze “A.” A standard solution was prepared from 1 g. of bronze in accordance with the micro-procedure and weighed portions equivalent to 10 mg. of alloy were analysed. The percentage composition of the bronze was: copper 85.5, tin 9.7, antimony 0.24, other (non-interfering) elements 2.71 and lead 1.83. Lead found, 20.02, 20.02, 20-31 and 19.94 per cent. Lead found, 1-81, 1-80, 1.84 and 1-81 per cent. We gratefully acknowledge a maintenance grant to one of us (L. K. 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. McCay, L. W., .I. -4wiev. Cheriz. SOC., 1914, 36, 2376. Lindsey, A. J., Analyst, 1938, 63, 159. Sand, H. J . S., “Electrochemistry and Electrochemical Analysis,” Vol. 11, Blackie & Son Ltd., London, 1940, p. 74. CHEMXSTRY DEPARTMENT THE UNIVERSITY, EDIKBURGH May, 1950