THE PRODUCTION OF SULPHURETTED HYDROGEN 29 



sulphide is formed, and the sand is black. With dehydration 

 the reaction is reversed and more FeCOg than FeS is produced ; 

 and it is this condition that results in a lightened colour. 



The Equilibrium of Hydrogen Sulphide in Water. 



Sulphur bacteria grow as readily in sea water as in fresh 

 water, since they are not affected by the degree of salinity 

 of the water. When, however, they multiply in sea water, 

 the multiplication takes place in a hard water in which the 

 concentration of the hydrogen ions is low owing to the high 

 buffer value of calcium and magnesium salts. The equilibrium 

 of the sulphide in such a medium has been studied by Baas- 

 Becking (i). 



The H2S will first be dissociated into hydrosulphide ions 

 and hydrogen ions. 



Ki[H3S] = [H^[HS-]. 



First dissociation constant, Kj = 0-91 X io~' at 18° C. 

 In the second dissociation we have 



K,[HS-] - [H+][S--]. 



Second dissociation constant, K., = 10 ~^^ at laboratory tem- 

 perature, also 



Ka>= [H^-j[OH-] = 10- 1*. 



Hence, as the sum of the positive charges is equal to the 

 sum of the negative charges, 



[H+] = [HS-] + [S--] + [0H-] 

 _ KJH2S] KJHS-] Kg. 

 [H+] ^ [H^-] ^[H+] 

 _ K,[H,S] KiK,[H,S] Ko^ 

 ^ J [H+] ^ [H+j^ "^ [H-*-] 



[U^Y = K/[H-^][H,S] + KiK,[H,S] + Ka>[H+]. . .(i) 

 [H+]3 - Ka>[H+] 



and [H2S] = 



K,[H-*-] +K,K2* 



* Baas-Becking's paper shows 2KjK, in place of KjKj, but this does 

 not affect the conclusions. 



