HYDROGEN ION CONCENTRATION 7 



In dilute solutions, that is when the ionic strength is 

 small, the activity coefficients are very nearly unity and 

 the activities (a.H+) and (a. OH") of the hydrogen and 

 hydroxyl ions can be replaced by their concentrations 

 (C.H+) and (COH") respectively. Equation (6) can 

 now be written 



Ku' = (C.H-) (C.OH-) .... (7) 



In an exactly neutral solution, obviously, the concentration 

 of the acidic (hydrogen) ions must be equal to the 

 concentration of the basic (hydroxyl) ions. That is 

 (C.A+) = (C.OH~) and, from equation (7), each must be 

 equal to VKw. 



The ionic product, Kw, can be determined experi- 

 mentally from conductivity measurements since the 

 conduction of electricity through a liquid depends on the 

 number of ions available to carry the current. The 

 value for pure water at room temperature has been 

 found to be 10"^*. Consequently in neutral solution the 

 concentration of both hydrogen and hydroxyl ions must 

 be V10~^* or 10^' gram ions per litre. If more hydrogen 

 ions than this amount are present the solution is acid 

 and if there are less hydrogen ions the solution is alkaline, 

 but whatever the state of the solution the ionic j^roduct is 

 constant and equal to 10-^*. In other words a greater 

 amount of hydrogen ions means a smaller amount of 

 hydroxyl ions and vice versa. Consequently the strength 

 of an alkali, as well as that of an acid, can be expressed 

 in terms of hydrogen ion concentration. 



A normal solution of a strong acid will contain about 

 1 gram ion per litre of hydrogen ions, the exact amount 

 depending upon the degree of dissociation of the particular 

 acid and the activity of the ions. A strong solution of an 

 alkali will contain about 10-^^ gram ions per litre of 

 hydrogen ions, derived from the ionisation of the water. 

 The degree of ionisation of a solution can be measured by 

 the conductivity of the solution, which depends on the 



