Recovery of Silver and Cobalt from Laboratory Wastes Donald F. Foust Union College, Schenectady, NY 12308
The cost of reagents is freauentlv the determinine factor in selecting student laborator; experiments. A portion of the cost often can be offset by reverting products obtained in these rapid, inexpensive, experiments into useful material~.~Sim~le, and safe procedures for the generation of silver nitrate from silver chloride residues and the reclamation of cohalt(I1) chloride hexahydrate from cobalt(II1) ammine chlorides are reported. In our General Chemistry laboratories, students convert CoClzSHzO into a number of cohalt(lIl) complexes, including [Co(NH3)6]C13and [Co(NHs)5Cl]Clz.One method utilized in analyzing these complexes is chloride precipitation by the addition of silver nitrate ( I ) . Large quantities of cobalt complexes and silver chloride residues accumulate over the course of a year. Converting these products into starting materials minimizes the expense of such a costly experiment. A variety of procedures for recovering silver from silver halide residues have been reported; however, these methods usually suffer from serious drawbacks, limiting their general application. The use of high temperature reactions (5001000°C) (2, 3) or hazardous materials (cyanides, silverammonia solutions) (4, 5) is undesirable. The following method is based on previous procedures (6,7) in which common, inexpensive laboratory reagents are utilized. Large quantities of silver nitrate may be recovered easily in a relatively short time while encountering only minimal hazards. Sliver Recovery Silver chloride was reduced by granular zinc in dilute sulfuric acid solutions to elemental silver. The reaction was complete within five minutes. The sulfuric acid oxidized excess zinc. The crude silver was isolated and then oxidized to silver nitrate by addition of concentrated nitric acid. Silver nitrate was isolated in a 76%yield following recrystallization (8).
Procedure for the Recovery of Sliver (Note: Allpanipulations were performed in a well-ventilated hood.) A sus~ensionof 50 a of silver chloride in 125 mL of water containing 10 mL of 1 8 HzS04 ~ was stirred vigorously. Twelve grams of 30-mesh granular zinc was carefully added to the reaction mixture. Zinc was added until only a flocculent gray solid remained in the reaction vessel. Once gas evolution had ceased, the mixture was filtered and the gray silver residue was washed with an aqueous solution of 25% sulfuric acid. The crude silver was washed with water, air dried, and collected. Fifty milliliters of 16 M HN03 was carefully added to the residue. The resultimg slurry was diluted with 100 mL of water and filtered. The filtrate was concentrated on a water bath until the solution was saturated. A volume of 16 M HN03, twice that of the concentrate, was added and the mixture cooled. The resulting white crystals were collected by filtration, washed with cold 16 M HN03, and air dried. The crystals were then dried a t llO°C for 1h, giving 45 g (76%) of white crystalline AgN03. 924
Journal of Chemical Education
A Volhard titration for silver ion revealed the recovered material to he greater than 98% silver nitrate. An increased yield of AgN03 may be obtained by precipitation of the silver nitrate mother liquors with chloride ion followed by recycling the AgCl through the method described. Scale-up of this procedure is limited only by the size of the available apparatus. Cobalt Recovery The recovery of cobalt was realized by the thermolysis of a t 320°C (9).The either [Co(NH3)6]C13 or [CO(NHB)~CI]CIZ cobalt(II1) complexes were reduced to anhydrous cobalt(I1) chloride. Cohaltous chloride hexahydrate was isolated in quantitative yield following crystallization of CoC12 in water.
Procedure for the Recovery of CobaR In a typical run, 70 g of [Co(NH3)6]C13was loaded in a 10 cm porcelain evaporating dish and placed in a 320°C muffle furnace located in a well-ventilated hood. The furnace door was left slightly ajar to facilitate the escape of gaseous ammonium chloride. The s a m ~ l eturned hlue. melted, and resolidified. After 2112hof heiting thedark bl"esolid was removed from the furnace and Dulverized. The crushed s a m ~ l e was returned to the furnace and heated for an additional ill2 h. resultine in ~ o w d ehlue r CoCla. h he anhidrous cobalt(11)chloride was added to 125 mL of water. The resultinr! aarnet red solution was filtered and the filtrate was transferred to an evaporating dish. The solution was allowed to evaporate to drvness at room temperature. The moist, dark red cr.ystals obtained were crushed and allowed to air dry com~letelv.Cohaltous chloride hexahydrace (6:lgJ was obtained in quantitative yield. Substitution of [Co(NH&,Cl]CIz for [Co(NH3)6]C13also afforded CoC12. The size of the sample used in this procedure is limited only by the size of the muffle furnace. Acknowledgment I wish to thank R. Dubs for his initial literature search for methods of recovering cobalt. Literature Cited
(7) Mellm, J. W., "A Comprehensive Treatise on Inorganic and Theoretical Chemiafry," VOI. L ~G ~and ~ company, ~ ~~ NW ~ yark, ~. 1923,314. ~ ~ , (8) Garin, D.L.and Henderson,K. 0..J. CHeM. Eouc.,47,741 (1970). (9) Meuor, J. W.. "A Comprehensive Treatise on Inorganic and T h e m e t i d Chemishy," Vol. XN, Longmans. Green, and Company, London, 1935,~~. 654,663.
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