214 NON-METALS AND THEIR COMBINATIONS. 



3. Barium chloride solution gives a white precipitate of barium 

 thiosulphate, BaS 2 O 3 - Calcium chloride, however, gives no precipi- 

 tate, whereas with a sulphite a precipitate is formed. 



Persulplmric acid, H 2 S 2 8 , is obtained by passing an electric current 

 through sulphuric acid of a specific gravity 1.3 to 1.5. The reaction is 

 2H 2 S0 4 = 2H + H 2 S 2 8 . 



The ammonium or potassium salts of this acid are obtained by the electrol- 

 ysis of saturated solutions of the bisulphates of the metals, thus : 

 2KHS0 4 = K 2 S 2 8 + H 2 . 



Persulphuric acid and its salts act as strong oxidizing agents, liberating, 

 for instance, chlorine from hydrochloric acid or from chlorides. 



Hydrogen sulphide, H 2 S (Sulphuretted hydrogen). This compound 

 has been mentioned as being liberated by the decomposition of organic 

 matter (putrefaction) and as a constituent of some spring waters. It 

 is formed also during the destructive distillation of organic matter 

 containing sulphur. The best mode of obtaining it is the decomposi- 

 tion of metallic sulphides by diluted sulphuric or hydrochloric acid. 

 Ferrous sulphide is usually selected for decomposition : 

 FeS + H 2 SO, = FeS0 4 + HJ3. 



Experiment 15. Use apparatus shown in Fig. 42, page 207. Place about 20 

 grammes of ferrous sulphide in the flask, cover the pieces with water, and add 

 sulphuric or hydrochloric acid. Pass a portion of the washed gas into water, 

 another portion into ammonia water. Use the solutions for the tests mentioned 

 below. Ignite the gas at the delivery tube and notice that sulphur is deposited 

 upon the surface of a cold plate held in the flame. Place the apparatus in the 

 fume chamber during the operation. How much ferrous sulphide is required 

 to liberate a quantity of hydrogen sulphide sufficient to convert 1000 grammes 

 of 10 per cent, ammonia water into ammonium sulphide solution? The reac- 

 tion taking place is this : 



2NH 3 + H 2 S = (NHJ 2 S. 



Hydrogen sulphide is a colorless gas, having an exceedingly offen- 

 sive odor and a disgusting taste. Water absorbs about three volumes 

 of the gas, and this solution is feebly acid. It is highly combustible 

 in air, burning with a blue flame, and forming sulphur dioxide and 

 water. It is directly poisonous when inhaled, its action depending 

 chiefly on its power of reducing, and combining with, the blood- 

 coloring matter. Plenty of fresh air, or air containing a very little 

 chlorine, should be used as an antidote. 



Hydrogen sulphide can be driven completely from its aqueous solu- 

 tion by heating. It is a rather unstable compound, being easily 

 broken up into its constituents. For this reason it is a good reduc- 



