26 Afid’st, January, 1968, Vol. 93, pj5. 26-27 An Improved Silver Reagent for Use in the Walden Silver Reductor BY M. A. SALAM KHAN AND W. I. STEPHEN (Chemistry De$avtment, The University, P.O. Box 363, Birmingham 15) The preparation and use of silver-impregnated pumice granules in the silver reductor is described. This material possesses advantages over the conventional chemically precipitated silver and is equally effective for the reduction of iron(II1) and copper(I1) in solution. WHILE investigating the formation of hydrogen peroxide in metallic reductors,l some difficulty was experienced with the conventional silver reductor containing chemically precipitated silver metaL2 The finely divided silver readily forms a compact mass in the column; this impedes the flow of solution through the column, and the application of reduced pressure is necessary for the attainment of practically useful flow-rates.Smith and CagleS recognised this difficulty in the operation and regeneration of the silver reductor, and recommended the use of an electro-deposited silver in the form of “silver-tree” type aggregates. Although their method is effective in producing a highly crystalline silver of low apparent density that does not readily compact in the column, it is not practicable in that it requires unusually high electrical energies (60 to 80 amp2res at 6 volts) and pure silver anodes and platinum cathodes. A simpler method has been the use of a granular material as an inert substrate on which silver metal can be deposited. Granular pumice, which is inert to reducing substances and resistant to attack by hydrochloric acid, has been found effective.The porous nature of the material allows it to absorb an appreciable volume of silver nitrate solution which, after drying, can be decomposed to metallic silver and oxides of nitrogen, by heating to a tem- perature of about 500” C. In the preparation described below, the granulated material was found to contain between 50 and 55 per cent. of silver metal (by extraction with nitric acid and precipitation of silver chloride). When filled into a conventional glass reductor column, the silverised pumice allows a free flow of solution through the bed of reductant, the rate of which can now be controlled by means of the stop cock on the reductor. The column is first activated by passing 2 N hydrochloric acid through the bed of reductant.The efficacy of the silver - pumice reductant has been compared with that of the normal silver reductant in two typical reductor experiments, the determinations of iron(II1) and copper(I1). Provided that the same pre- cautions to avoid formation of hydrogen peroxide and re-oxidation of the reduced solutions are observed, results from the two types of reductor are in close agreement (maximum difference is 0.2 per cent.). This modified silver reductant possesses other advantages over the copper-precipitated material. It appears to need less frequent regeneration (about 40 determinations against the 20 to 25 determinations with the conventional reductor), probably because the silver chloride formed from the reduction is less firmly held on the granules of pumice than on the surface of silver metal, which in the precipitated form is soon covered by a layer of silver chloride.Regeneration of the reductant is best effected by passage of chromium( 11) chloride solution through the column (Huffmann’s process) .* It is easier to prepare and handle than the copper-precipitated material, and, as it contains little more than 50 per cent. of silver, it is obviously a cheaper reagent. Although the new reagent has not been exhaustively tested as a reductant for cations other than Fe3+ and Cu2+, there is no reason to suppose that it cannot be used for the reduction of V5+, UO,a+ and MOO?-, for which ions the conventional Walden reductor has been successfully applied.0 SAC and the authors.SALAM KHAN AND STEPHEN 27 PREPARATION OF SILVER - PUMICE REDUCTANT- Dissolve 200 g of silver nitrate in 200 ml of distilled water contained in a 400-ml beaker, and add, in small amounts, 100 g of thoroughly washed pumice, granulated to a size of about 10 mesh. Pour off the surplus liquid, transfer the pumice granules to an evaporating basin and dry them in an air-oven at 110" C. Return the still hot granules to the beaker and pour the remaining silver nitrate solution over them. Remove the granules and dry, as before. Repeat the process until all of the silver solution has been absorbed. Heat the dry granules at 500" C in a muffle furnace, which has a suitable means for the extraction of fumes, until the evolution of nitrous fumes has ceased and all the silver nitrate has decomposed (about 1 to 13 hours).Allow the granules to cool to room temperature and wash them on a suitable sintered-glass filter funnel with distilled water to remove any soluble silver salts. Fill the 2-cm diameter reductor column to a height of 25 cm with the granular material and keep the column of reductant covered with distilled water. A sample of the dried material should be assayed for its silver content, by boiling it with dilute nitric acid, and extracting the granules with distilled water until free from silver ions. The silver solution is then treated with an excess of chloride ions and the precipitate of silver chloride is collected, dried and weighed in the usual way. The material should contain between 45 and 65 per cent. of silver. We are grateful to Professor R. Belcher for his encouragement with this work. REFERENCES 1. 2. Salam Khan, M. A., and Stephen, W. I., Analytica Chim. Acta, 1968, in the press. Walden, G. H., Hammett, L. P., and Edmonds, S. M., J. Amer. Chem. Soc., 1934,56, 67; see I. M. Kolthoff and R. Belcher, "Volumetric Analysis," Volume 111, Interscience Publishers Inc., New York, 1967, p. 14. Smith, G. F., and Gagle, F. W., Analyt. Ckem., 1948, 20, 183. Huffman, E. H., I d . Engng Chem. Analyt. Edn, 1946, 18. 278. 3. 4. Received June 22nd, 1967