60 IONISATION 



concentration of OH' must be correspondingly decreased according 

 to the equation, 



[H+] [OH-] = 10- 14 



in -14 



or [OH-] =-J=TC = 10- 14 ( - B) - 10- 9 . 



(H+) 



A y? H of 5 will be accompanied by a p OH of 9. This will be an 

 acid solution. 



Conversely, if the concentration of hydroxyl ions is increased 

 there is a corresponding decrease in hydrogen ions. E.g. 



IQ -14 



if POH - 3 > then (H+) = -^ = 10" 11 = p H of 11, 



an alkaline solution. 



Reaction may, therefore, be expressed in terms of p H or of p OH . 

 Generally the former is used, and alkalinity is expressed as decrease 

 of acidity. The quality as well as the nature of the reaction is 

 expressed by the p H . The greater the concentration of hydrogen 

 ions, the greater is the degree of acidity and the smaller the degree 

 of alkalinity. 



It is rather confusing for the beginner, but he must note : 



(1) that as acidity increases, the exponent or p figure decreases ; 



(2) that as the figures are logarithms, multiplication is done 

 by addition and division by subtraction ; 



(3) that this does not give a measure of the amount of acid 

 present, but of its strength. The p R is not an index of quantity 

 but of intensity. It gives the number of H ions per litre, but of 

 course says nothing of how many litres or c.c. of acid are present. 



The strength of an acid (or alkali) may be expressed as normal 

 or as a fraction of normal. A normal solution contains in one 

 litre, the gram-equivalent weight of the substance. A normal 

 solution of acid, for instance, has in each litre one gram of hydrogen 

 capable of forming hydrogen ions. If the acid is completely 

 dissociated, i.e. if it is a " strong " acid, it will contain one gram 

 of hydrogen as H+. The strength of acid commonly used for 



N 

 laboratory purposes is 1/10 of normal = . The hydrogen ion 



concentration of such a solution would be 1/10 gram per 

 litre = (H+) of 1 x 10- 1 or p H of 1 and p OH of 13. 



Water of p H = p OH = 7 is thus, at 23 C., N/10,000,000 acid and 

 N/10,000,000 alkaline. If the acid added to water is not completely 

 dissociated (i.e. a weak acid), then, of course, the degree of disso- 

 ciation must be taken into account. A decinormal solution of 



