HYDROGEN ION CONCENTRATION i) 



or H+) and a base is a substance which tends to acquire 

 a proton. This may be expressed b}^ the equation 



A ^=^ H+ + B 



where A represents an acid and B a base. This means 

 that every acid must be associated with a corresponding 

 or " conjugate " base and vice versa. Generally the 

 conjugate acid or base is the solvent in which the substance 

 is dissolved. In aqueous solutions water can act as the 

 conjugate base of an acid or as the conjugate acid of a 

 base since it is capable of either taking up or giving up a 

 proton. For an acid in water the ecpiilibrium is : — 



HA + H.OH ^=^ H.OH,+ + A- 

 (acid) (base) 



H.OH2+ is what is usually known as the hydrogen ion 

 for which the symbol H+ is commonly used. 

 Applying the Law of Mass Action 



(a . H.OHo-) (a . A-) 

 (a. H.OH) (a. HA) 



(1] 



where K'a is the dissociation constant of the acid and 

 (a . H.OH0+), (a . A-), (a . H.OH) and (a . HA) are the 

 " activities " of the hydrogen ion, conjugate l^ase, 

 un-ionised water and un-ionised acid respectivel}^ The 

 " activity " of an ion is the product of its concentration 

 and its " activity coefficient " which is a measure of the 

 influence of surrounding ions upon it and which accordingly 

 depends upon the dilution of the solution. Since the 

 activity of un-ionised water may be regarded as constant, 

 because its amount is virtually unaffected by the very 

 small degree of ionisation which it undergoes, equation (1) 

 can be rewritten with the new constant K a 



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

 ^'-^ = i^TRA) ... (2) 



