SULPHUR. 215 



ing agent. For example, sulphur dioxide is reduced by it to sulphur, 

 but is not affected by free hydrogen gas : 



2H 2 S + SO 2 = 2H 2 O + 38. 



It is probable that native sulphur found in volcanic regions is produced 

 in this way. Because of the instability of the gas, its sulphur often 

 acts like sulphur in the free state. Thus, the metals from potassium 

 to silver inclusive in the electrochemical series (see page 198) soon 

 become tarnished with a layer of sulphide when exposed to the gas : 



2Ag + H 2 S = Ag 2 S + 2H. 



The solution of hydrogen sulphide is slowly affected by oxygen of the 

 air with precipitation of sulphur, H 2 S -f O H 2 O -f S. Hence, 

 it does not keep except in full bottles. In solution it is a dibasic 

 acid of extremely weak character, only 0.07 per cent, of the molecules 

 being dissociated, mainly according to this equation : 



H 2 S 7 H- + HS'. 



The ion HS' also dissociates, but to a less degree even than water : 



HS'^H- + S". 



Hydrogen sulphide, like any dibasic acid, can give two kinds of salts, 

 acid and normal ; for example, sodium hydrosulphide, NaHS, and 

 sodium sulphide, Na 2 S. The acid salt is obtained in solution, when 

 hydrogen sulphide is passed into a solution of sodium hydroxide to 

 saturation. It has a neutral reaction : 



NaOH + H 2 S = NaHS + H 2 O. 

 In solution it dissociates, thus : 



NaHS 7 Na- + HS'. 



The normal salt, Na 2 S, does not exist in solution, but can be obtained 

 in the dry state by adding to the acid salt an amount of alkali equal 

 to that used in making the same, and evaporating to dry ness : 



NaHS 4- NaOH = Na 2 S -f H 2 O. 



When the dry salt is dissolved in water, it is completely hydrolyzed 

 into the acid salt and free alkali, and, therefore, has a strong alkaline 



reaction : 



Na 2 S + H 2 = NaHS + NaOH. 



Hydrogen sulphide gas and its solution in water are frequently 

 used as reagents in analytical chemistry for precipitating and recog- 

 nizing metals. This use depends on the property of the sulphur to 



