CHAPTER XXXIII

COPPER; MERCURY; SILVER

199. 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).

200. 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 ?

201. 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?

e. From the results obtained in § 37 calculate the number of molecules of water of hydration present in cupric sulfate.

202. Tetraminocuprisulfate (Cu(NH ̧)SO̟· H2O). Powder about 10 g. of cupric sulfate and dissolve it in 20 cc. 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?

203. 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

of ferric hydroxide (Fe(OH),) may form, due to traces of iron in the brass. Filter, and add ammonium sulfide to the clear filtrate. What is the composition of the precipitate?

204. Mercuric compounds. a. Note the physical properties of mercury. Place in a small beaker a globule of mercury as large as a grain of wheat, and add (hood) just enough concentrated nitric acid to dissolve it. Write the equation for the reaction on the supposition that mercuric nitrate is formed. Dilute the product with 10 cc. of water and place a copper cent in the solution. After a few minutes remove the coin and polish it with a piece of cloth. Account for the result.

b. For what purpose have we used mercuric oxide? Place 0.5 g. of it in a test tube and dissolve it in as little nitric acid as possible (R), then add water until the test tube is one fourth full. Divide the solution into three equal parts. To one part add a little hydrochloric acid (R); to a second part add ammonium hydroxide (is the product mercuric hydroxide? how can you prove it?); to the third part add a small piece of zinc or tin, and after a part of the metal has dissolved, add hydrochloric acid (R).

c. Prepare a little mercuric iodide by precipitation. What is its color? Collect it on a filter paper and allow it to dry. Scrape a little of the powder into a test tube and very carefully warm it at some distance above the flame. What change in color takes place? Is this change of color reversed on cooling?

205. Mercurous compounds. Put a small globule of mercury in a test tube and add a little dilute nitric acid, warming gently and taking care that some mercury remains undissolved. Divide the solution into three parts. To one part add hydrochloric acid or a soluble chloride (R),, to a second add ammonium hydroxide (R), to the third add a little concentrated nitric acid (R). When action has ceased, add 1 or 2 cc. of hydrochloric acid (R). How can you distinguish between mercuric and mercurous salts?

206. Compounds of silver. a. Test the reaction of a solution of silver nitrate upon litmus paper. What would you conclude as to the strength of silver hydroxide as a base ?