APRIL, 1949
THE RECOVERY OF SILVER FROM A DIME A Laboratory Experiment in General Chemistry HARRY B. rELDMAN Worcester Polytechnic Institute, Worcester, Massachusetts
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observing the formation of a blue color if Cu is present. Uvlally the third decantate is free from Cu. Add 100 ml. of water to the precipitate and 10 ml. of 6 N NaOH and boil the resulting mixture for 15 minutes. The result is a brown suspension of Ag20. Add to this suspension 10 g. of cane sugar and boil 15 minutes. The reduction of the Ag20 to free silver causes the precipitate to darken in color and to appear partly spongy and partly crystalline in appearance. After filtering and mashing twice with cold water, PROCEDURE remove the filter paper and funnel and place in a deWeigh half a dime t o two or three decimal places, de- pression previously prepared in a piece of blowpipe pending upon the type of balances available. Place in a charcoal. Heat with the blowpipe until all the paper 250-ml beaker and add 15 ml. of dilute HN03. After is burned off and the silver has fused together into a covering with a watch glass warn1 gently until all of the single globule. Allow to cool and then weigh. Calculate the per cent recovery of the silver, assuming that metal goes into solution. Wash the liquid from the underside of the watch the original coin was 90 per cent silver and 10 per cent glass into the beaker, dilute t o approsimately 100 ml., copper. The following table lists student results in percentage and add 5 ml. of dilute HC1. Maintain the solution just below the boiling point and stir frequently until recoveries. the precipitate of AgCl has become coagulated and the Number of Stzrdenfs % Recoveru supernatant liquid 1s clear. 16 98. 0 - 1 ~ .O ) .4fter allowing the precipitate to settle, decant the 41 9 6 . 0 4 8 .O supernatant liquid, add 50 ml. of water, stir, let settle, 55 94.0-96 .O 30 decant and repeat the process until the decantate is 92.0-94 0 20 90.0-92.0 Iree from Cu. The test for Cu may be made by adding 9 85.0-90 .O an excess of NHIOH to a few ml. of the decantate and 13 Below 88.0 following experiment has proved very successful in the elementary laboratory. The separation of purc silver from a dime is not only interesting in itself but the method involves several important chemical prinriples. On the average, it requires a little over two hoursfor completion, and in our experience, the majority of freshmen students obtain better than 95 per cent recovery of the metal (assuming the dime to contain 90 per cent Ag and 10 per cent Cu).
