JULY. 1953
RECLAIMING SILVER FROM SILVER CHLORIDE RESIDUES OTTO L. WILLBANKS University of Texas, Austin, Texas
IN MANY universities budget considerations make it desirable to have available a simple and economical method for the reclaiming of silver from silver chloride precipitates' in the quantitative analysis course. In some cases three or four analyses requiring the use of silver nitrate are run by each student, and often the cost of this reagent alone exceeds the income provided by student fees. A convenient method for accomplishing the conversion of silver chloride to silver is proposed which takes advantage of the fact that silver oxide is less soluble in concentrat,ed base than is silver chloride. The solubility product constants for silver chloride and silver hydroxide are 1.0 X 10-'0 and 1.0 X respectively. From this it follows that the concentration of silver ion in equilibrium with a precipitate of silver oxide in a solution five molar in hydroxyl ion is 2.0 X mols/liter, while the concentration furnished by a precipitate of silver chloride in the same mols/liter. Hence under these solution is 1.0 X conditions silver chloride furnishes 5000 times the silver ion concentration required for precipitation of silver oxide, and this reaction can be used to convert silver chloride to silver oxide.
-+ +-
AgCl OH2 AgOH Ag20
+
AgOH CIH 2 0 (spontaneous)
Student precipitates collected in the laboratory are filtered, washed with dilute nitric acid, and transferred to a large beaker where they are covered with about three times their apparent volume of five-molar sodium hydroxide. The mixture is boiled for 15 minutes with stirring to break up clumps of silver chloride, filtered, and washed with tenth-molar sodium hydroxide. The
resulting residue is treated again as above and then transferred to a crucible or ~asseroleand ignited at 500°C. for 15 minutes, converting the silver oxide to metallic silver. After cooling, the spongy solid is washed several times in the crncible with concentrated ammonium hydroxide to remove traces of silver chloride which were not converted, and then heated until the silver melts. A large Biichner funnel is convenient for the filtrations and a muffle furnace is desirable-but not necessary for the ignitions. The purity of silver obtained in this manner is primarily dependent upon the purity of the silver chloride used as starting material. In one case where specially prepared silver chloride was used, the silver obtained was 99.2 per cent pure. In most cases silver obtained from student precipitates is in excess of 96 per cent pure, the principal impurities being silver chloride and sodium hydroxide. By dissolving in nitric acid, filtering to remove traces of silver chloride, and diluting, an acceptable reagent is had for the determination of chloride gravimetrically or, if standardized, volumetrically. This method may also be used with equally satisfactory results on the chromate and thiocyanate of silver or on mixtures of the chloride, chromate, and thiocvanate. Due to the explosive nature of silver ammino compounds, homogeneous precipitation from ammoniacal solution is not practical. I t should be pointed out that other methods of effecting this conversion exist. Among those used are electrolysis, fusions with sodium hydroxide and sodium carbonate, and procedures involving organic reductants.
