On the supposition that the zinc used in the reduction of the silver is the source of the lead in the American coin, it is easy to calculate the amount of lead which would thus find its way into the coin, since the quantity of zinc used in reducing a given weight of silver, and the per cent of lead which that zinc may be expected to contain, are both known quantities. Professor Booth* says that an excess of zinc is required to insure total and rapid reduction, and Wilson † states, that two equivalents of zinc are used, in practice, for each equivalent of silver. Our memoir, already cited, gives the per cents of lead found in two specimens of Vieille Montagne zinc. The standard of the American silver coin is silver and copper, and the weight of fifty cents' worth of this alloy, in either half-dimes, dimes, quarters, or a half-dollar, has been 192 grains 12.433 grammes, since the year 1853.‡ Fine silver in the half-dollar, Zinc used in reducing 11.19 gram. silver, contains 0.292 per cent of lead, § If zinc of the best quality (containing 0.292 per cent. of lead) had been used, the silver coin would have contained 0.158 per cent of lead; if the second quality (containing 0.494 per cent of lead) has been employed, the coin may contain 0.268 per cent of lead. Between these two limits all our determinations of lead in American silver will be found to lie. In offering this explanation of the occurrence of lead in American silver coin, we would by no means affirm that the zinc is the exclusive source of this impurity, for it is not at all improbable that a portion of the lead is derived from the leaden vats in which the reduction of the determined, exhibited a slightly yellowish-light-brown color, nothing similar to which occurred in any of the other experiments. A slight black residue remained when this silver was dissolved in nitric acid, and a trace of gold was detected in the residue described on p. 59. * Loc. cit. † Loc. cit. Brightly's Dig. Laws U. S., for Standard, Title Coinage, § 3; for Weights, Title Coinage, § 13. See our Memoir in Mem. Amer. Acad. [N. s.], VIII. 61, Table I. chloride of silver is effected, or from the sulphuric acid which is used to excite the reaction. The method of analysis employed was as follows. The coin having been dissolved in moderately strong, pure, nitric acid, the cooled solution was treated with an excess of a solution of pure caustic ammonia, added by small portions, no notice being taken of the small, darkcolored residue (AgS, etc.) insoluble in nitric acid. The precipitated oxide of silver was re-dissolved in nitric acid, enough of the latter being added to render the solution distinctly acid. The whole was then heated in a water-bath, and a strong solution of pure chloride of ammonium added, until chloride of silver was no longer precipitated, care being taken to avoid adding an unnecessary excess of chloride of ammonium. It will be seen, that the chloride of silver was thus precipitated in presence of a large excess of nitrate of ammonia; and since chloride of lead is readily soluble in a solution of nitrate of ammonia,* any lead which the coin may have contained would remain in solution. It may be mentioned in passing, that the small amount of chloride of silver which, as has already been stated by Mulder,† is retained in solution by nitrate of ammonia, especially by hot solutions, did not interfere in the least with the subsequent steps of the analysis, or in any way to influence the amount of lead obtained. The precipitated chloride of silver was collected upon a filter, and thoroughly washed with hot water, the lumps of chloride of silver being broken down as much as possible with a glass rod. The filtrate and wash waters were evaporated in a porcelain dish to the consistence of a thin syrup, and then transferred to a flask of hard German glass, in which the evaporation was continued until the nitrate of ammonia had been entirely destroyed. This operation requires care in the application of the heat, lest violent decomposition of the nitrate of ammonia ensue, and portions of the substance be projected from the flask. When properly conducted, the process goes on tranquilly, and no loss occurs. The residue, consisting of nitrate of copper, together with nitrate of lead, and more or less gold, should be washed with hot water into a porcelain dish, where it is mixed with a slight excess of pure sulphuric acid. It may be necessary to use a little nitric acid to dissolve the last portions of the substance in the flask. The mixture * Bolley, Ann. Ch. u. Pharm., 1854, XCI. 115. is then evaporated over the water-bath until it has become almost entirely dry, or until it no longer exhibits any tendency to effervesce; it is then somewhat more strongly heated on a sand-bath, until all the free sulphuric acid has been driven off, that is, until fumes of sulphuric acid are no longer perceptible. The necessity of thoroughly evaporating this mixture cannot be too strongly insisted upon, for a solution of gold in sulphuric acid is formed when the mixed nitrates, etc. are treated with sulphuric acid, and unless this compound be entirely destroyed by heating, it will be subsequently decomposed when water is added, and give rise to a precipitate of metallic gold, in a condition so finely divided that it cannot be separated by filtering. The occurrence of this precipitate would ruin the analysis. We at first sought to prevent the formation of this solution of gold in sulphuric acid, by heating very strongly the residue left in the flask after all the nitrate of ammonia had been decomposed, until the escape of nitrous fumes had entirely ceased, and the nitrate of copper was decomposed. We then treated the mass with pure dilute nitric acid, and filtered off the solution from the gold which had been deposited and from any chloride of silver which had been dissolved by the nitrate of ammonia, with the intention of determining the lead directly, by precipitating it as sulphate from this solution after evaporation with sulphuric acid. It was found, however, that a large portion of the gold-salt escaped decomposition in the flask, the yellow solution of sulphate of gold being formed almost as abundantly after this treatment as when the flask was not heated so strongly; and since in this method there is great risk of fracturing the vessel from drops of acid condensing in its neck and flowing down upon the hot portion, it can in no wise be recommended. A perfectly dry mixture of sulphate of copper, sulphate of lead, and metallic gold having been obtained, it is treated with distilled water, a considerable quantity of the latter being added at once, and the mixture quickly stirred to prevent the sulphate of copper from forming a hard cake as it becomes hydrated. As soon as the sulphate of copper is completely dissolved, a small quantity of pure sulphuric acid is to be added to the solution, and the latter set aside for at least forty-eight hours. The precipitate, consisting of sulphate of lead and metallic gold, was then collected upon a small Swedish filter and washed, first with dilute sulphuric acid to remove the sulphate of cop * Compare Pelletier, Ann. Ch. et Phys. [2], XV. 12. per, and subsequently with alcohol, until no trace of free sulphuric acid remained. After drying, the precipitate of sulphate of lead and gold, with the filter cut into small pieces, was transferred to a beaker glass, and the whole was then digested with pure bicarbonate of soda * (prepared from the oxalate), during at least forty-eight hours. The sulphate of soda formed by this operation was then filtered off from the carbonate of lead and metallic gold, and the amount of sulphuric acid which it contained determined by precipitation with chloride of barium in the usual way. From the amounts of sulphate of baryta thus obtained, recorded in column (3) of the table, the amounts of lead (col. 4) in the mixed precipitates of sulphate of lead and gold were calculated. The carbonate of lead, mixed with bits of paper and gold, above mentioned, was now dissolved in dilute nitric acid, the solution evaporated with sulphuric acid until all the nitric acid had been expelled, subsequently treated with water acidified with sulphuric acid, and, after standing for forty-eight hours, collected upon a tared filter, washed with dilute sulphuric acid and alcohol, dried at 100°, and weighed. The results may be found in column (a) of the table. The re-agents used in the foregoing operations were all chemically pure. A special experiment was moreover made, in which portions. of the nitric acid, ammonia, chloride of ammonium, and sulphuric acid used, twice as large as any which were actually employed in the analysis of either of our samples of coin, were evaporated together, and subjected to a course of treatment identical with that which the solutions of coin were forced to undergo; but not a trace of sulphate of lead, or of any precipitate, other than a few light and utterly insignificant floating flocks, probably alumina, could be detected as the result of this trial. That our process is sufficiently accurate to exhibit all the lead which the samples of coin examined really contained, we do not think probable. All of the sources of error to which the process, when properly conducted, is subject, tend to cause a slight loss of lead. Thus, in the .first place, it is scarcely possible that the last traces of chloride of lead can be removed by washing from the caseous precipitate of chloride * See Fresenius, Anleitung zur Quantitativen Analyse, (Braunschweig, 4te Aufl. 1858,) p. 286, § 132, II. b. ß. of silver,* which precipitate is in enormous mass as compared with the amount of chloride of lead; and again, the first precipitate of sulphate of lead, mixed with gold, which is obtained after evaporation with sulphuric acid, occurs in so finely divided a state, that it is exceedingly difficult to prevent loss by the passing of a portion of it through the pores of the filter upon which the precipitate is collected. To one unused to the process, it might seem as if the last traces of sulphuric acid could hardly be removed from this precipitate by washing with alcohol. We have satisfied ourselves, however, that this is easily effected, and that this seeming source of error is, in reality, entirely insignificant; at any rate, the results given in columns (a), (b), and (c) are in no way influenced by this imaginary error. It may not be amiss to observe, that we have been particularly scrupulous with regard to the treatment of the above-mentioned precipitate, having in every instance continued to wash it with alcohol long after any reaction of sulphuric acid could be detected. It should also here be stated, that the results in which we ourselves place the most confidence are those obtained by determining the amount of sulphuric acid in the mixed precipitate of sulphate of lead and gold (see cols. (3), (4), and (5) of the table), for columns (a), (b), and (c) contain the results of a secondary process liable to all the accumulated errors of a long series of operations; moreover, the final precipitate of sulphate of lead is very finely divided, and, like the first precipitate, liable to pass through the pores of the filter, as has been previously explained. This second series of experiments has been carried out, not from any expectation that the results would closely agree with those of the first series, but merely to control the latter to a certain extent, and to remove any objection which might be made to the indirect method by which they were obtained. Since the possible inaccuracies of the process tend to a loss of lead, we are confident that the amounts of lead we have obtained are in no instance greater than those really existing in the coin. The precipitates of pure sulphate of lead (column (a) of the table) were, in every instance, reduced on charcoal with carbonate of soda, and the button of lead obtained carefully tested for silver, of which none could be detected in any case; nor did the metal exhibit any reactions, other than those of pure lead. The residue of gold and filter paper left after dissolving the car * Compare Mulder, op. cit., p. 217. |