Synthetic Inorganic Chemistry

EXPERIMENTS 317

Review the discussion of thio- or sulpho-salts under Prepa-

ration 57. The trisulphides As 2 S 3 and Sb 2 S 3 are oxidized by the

free sulphur to the pentasulphides, As 2 S 5 and Sb 2 S 6 , which react

with the Na 2 S to form the soluble thio-salts, Na 3 AsS4 and

Na 3 SbS4. The addition of HC1 displaces the weak thio-acids,

H 3 AsS 4 and H 3 SbS4, which are too unstable to exist alone and

decompose into H 2 S and the respective pentasulphides. The

color of the pentasulphides seems to be identical with that of the

trisulphides. Bismuth shows no tendency to form a thio-salt.

3. Reducing Action of Phosphorous Acid; Non-Oxidizing
   Property of Phosphoric Acid. Review the discussion of Prepa-
   ration 52, and the test employed for phosphorous acid. Phospho-
   rous acid reduces silver nitrate to metallic silver, it itself being
   oxidized to phosphoric acid.
   Recall that in Experiment 15, page 169, phosphoric acid did
   not oxidize hydrogen bromide or hydrogen iodide, which are excep-
   tionally strong reducing agents.


4. Arsenious and Arsenic Acids. Review Note 1 and Ex-
   periment 3 under Preparation 55. Arsenious acid reduces iodine
   to hydriodic acid in a solution containing sodium bicarbonate, the
   latter neutralizing the acid produced:

+ I 2 + H 2 O *± HsAsO* + 2HI

but in an acid solution the reaction goes in the opposite direction,
the hydrogen iodide being oxidized.
The reaction progressing to the right according to the equation
produces H+ ions. It is natural that the removal of H+ ions
favors the reaction and their presence reverses it.

5. Reduction of Bismuth Salts, (a) Suspend about 2 milli-
   grams of bimuth subnitrate in 10 cc. water, add 5 cc. QN
   NaOH, boil, and note that the white suspension does not
   change color. Add a few drops of dextrose solution, con-
   tinue to boil, and note that the white suspension turns
   black.
   (6) Again suspend about 2 milligrams of bismuth sub-
   nitrate in 10 cc. boiling water. Pour through a filter leaving
   the white Bi(OH) 3 , to which the salt is hydrolyzed, on the
   paper. Make a sodium stannite solution by adding drop by
   drop 6 N NaOH to 2 cc. of SnCl 2 solution with constant shak-