CHAPTER XXV COPPER; MERCURY; SILVER 189. Cuprous chloride. Powder 5 g. of cupric sulfate and place it in a large test tube or a small beaker. Add 20 cc. of concentrated hydrochloric acid and about 10 g. of copper, preferably in a form that has a large surface. What ions are now present? What action should metallic copper have on the Cu++ ion? Boil the mixture vigorously for about ten minutes (hood), and then pour it into a beaker containing at least 200 cc. of water. The white precipitate is cuprous chloride (CuCl) (R). Decant the solution and wash the precipitate once with water. Boil a little of the cuprous chloride suspended in water (R). Treat a small portion with a solution of sodium hydroxide (R). Warm a third portion with aqua regia (R). 190. Cuprous iodide. To 2 or 3 cc. of a solution of cupric sulfate add about 1 cc. of a solution of potassium iodide (R). How do you account for the separation of iodine? How can you remove the latter from the insoluble cuprous iodide? Which is the more easily prepared, cuprous chloride or cuprous iodide ? 191. Reactions of cupric salts. a. Recall the action of sulfuric and of nitric acid on copper (R). Dip a nail into a solution of cupric sulfate (R). What other metals would act like iron? b. To a cold solution of cupric sulfate add one half its volume of a solution of sodium hydroxide (R). Heat the solution to boiling and account for the change in the color of the precipitate (R). Is cupric hydroxide soluble in sodium hydroxide? c. Try the action of a solution of hydrogen sulfide on cupric sulfate (R); of a solution of ammonium sulfide (R). Add a drop of ammonium hydroxide to a dilute solution of cupric sulfate (R). Continue to add the ammonium hydroxide, drop by drop, until the precipitate dissolves. How does the color of the solution compare with that of the original cupric sulfate? Can there be any considerable number of cupric ions in the solution? Why? d. To 5 cc. of a solution of cupric sulfate add 1 cc. of hydrochloric acid; then add an equal volume of a solution of hydrogen sulfide (R). Repeat with a solution of zinc sulfate. Try the action of ammonium sulfide upon zinc sulfate. How can you detect copper and zinc in the presence of each other? 4 e. From the results obtained in § 37 calculate the number of molecules of water of hydration present in cupric sulfate. 192. Tetraminocuprisulfate (Cu(NH ̧)SO̟ ̧· H2O). Powder about 10 g. of cupric sulfate and dissolve it in 20 co. of concentrated ammonium hydroxide and 10 cc. of water. While vigorously stirring the solution, add, drop by drop, about 25 cc. of alcohol. Collect the purple-blue crystalline precipitate on a filter. Can you prove that the compound contains copper? that it contains ammonia? that it is a sulfate? 193. Analysis of brass. To detect the presence of copper and zinc in brass, place 0.5 g. of brass in an evaporating-dish (hood) and dissolve it in as little nitric acid as possible. Note the color of the solution. What does it indicate? Evaporate the solution just to dryness, add 5 cc. of hydrochloric acid and 5 cc. of water, and warm gently. Transfer the clear liquid to a beaker, dilute to 100 cc. with hot water, and pass a slow current of hydrogen sulfide through the solution as long as a precipitate forms. What is the precipitate? Filter, and test the filtrate with hydrogen sulfide in order to be certain that the precipitation is complete. If no more precipitate forms, evaporate the filtrate to half its volume, add a few drops of nitric acid, and again heat to boiling. When the solution is cool, add ammonium hydroxide until it is alkaline, then warm gently and set aside for a few minutes. A slight precipitate |