CH. I.] HYDROGEN ION CONCENTRATION. 19 



We can therefore write equation (2) as 



K ( acid ) / 



(Sodium salt) 



Since the sodium salt is not fully dissociated, except 

 in high dilutions, it is more correct to write it 



(U) - K ( acid ) ( A \ 



a (Sodium salt) 



where a is the degree of dissociation of the salt. 



It follows from this that the (H) of such a mixture is 

 mainly conditioned by the relative concentrations of the 

 acid and of its salt, and is only very slightly affected by 

 dilution, which does not alter the relative concentrations. 

 This is of considerable importance, since a large number of 

 physiological fluids can be regarded as mixtures of weak 

 acids with their sodium or potassium salts, and so suffer 

 little change in (H) on dilution. 



The determination of the hydrogen ion concentration. 



The most accurate method is an electrical one, involving 

 expensive and intricate apparatus. It is too complicated 

 to be described here. A valuable method that does not 

 require elaborate apparatus is the " indicator," or " colori- 

 metric " method. 



An indicator is a substance that varies in colour tone or 

 in depth of colour with the P H of the solution. Each 

 indicator shows a colour change over a certain range of P H . 

 At some particular P H the indicator may show an inter- 

 mediate or faint tint. The solution is then said to be 

 " neutral " to this indicator. It does not follow that the 

 solution is neutral in the strict sense, i.e. (H) = (OH). 

 The P H of a solution " neutral to phenol phthalein " is 

 about 9 ; that of a solution " neutral to methyl orange " is 

 about 4, the (H) in the latter case being 100,000 times 

 greater than in the former. 



The method adopted for the determination of P H by 

 indicators is to take standard solutions of certain substances 



