G BACTERIOLOGICAL CHEMISTRY 



Using the more familiar symbol, H+, for the hydrogen 

 ion this becomes 



(a.H+) (a.A-) 

 ^^' ^ (a. HA) .... (3) 



For a base dissolved in water the equilibrium is 

 expressed by the equation 



B + H.OH ^=i BH+ + OH- 



(Base) 



The dissociation constant, K'b, for the base is then given 

 by the expression 



(a.BH+) (a.OH-) 



K'b 



(a.B) (a.HOH^ 



Since in this case, too, the activity of the A\'ater can be 

 regarded as constant this becomes, with the new 

 constant, Kb, 



(a.BH+) (a.OH-) ' 



^^ = OTBJ • • • ^^\ 



Water is capable of either giving up or accepting a 

 proton. Therefore in pure water and in all aqueous 

 solutions the equilibrium 



H.OH + H.OH ^=^ H.OH2+ + OH- 



niust exist and the dissociation constant is given by 



_ (a . H.OH,+) (a . 0H-) 

 "" ~ (a.H.0H)2 



The activity, (a . H.OH), of un-ionised water can be 

 regarded as constant and therefore 



Kw = (a . H.OH2+) (a . 0H-) ... (5) 



The product of the activities of the acidic and basic ions 

 of a solvent is also loiown as the ionic product of the 

 solvent. Replacing H.OH^+by the familiar symbol for 

 tlie hydrogen ion, equation (5) becomes 



Kw = (a . H+) (a . OH") .... (0) 



