NORMAL SOLUTIONS 67 



Reaction may, therefcre, be expressed in terms of p^ or of j^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 jDj^. 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^^ 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 concentration 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 hvdrogen as H + . The concentration of acid commonlv used for 



N 

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



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

 (H+) of 1 X 10-1 OP p^ of I and ^^g of 13. 



Water of p^ = Pos = '^ i^ t^uis, at 23° C, N/10,000,000 acid and 

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

 pletely dissociated {i.e. a weak acid), then, of course, the degree of 

 dissociation must be taken into account. A deeinormal solution of 

 acetic acid, for instance, at 23° is dissociated 1-36 per cent. There- 



1-36 

 fore its (H+) would be equal to "— x lO'^ = 1-36 X 10~^ or p^ of 



2-86. 



Normal solutions of acid are all equal as regards the amount 

 of alkali they can neutralise. 1 c.c. of any N/10 acid is exactly 

 neutralised by 1 c.c. of any N/10 alkali. That is, they have the 

 same titratable acidity. They differ in their concentration of 

 hydrogen ions. 



As we have seen 



— HCl in water at 23° = j^b. 1 »r ^'h = 1 X lO"!, 



N 

 10 



N 



— CH3COOH „ = Ph 2-86 or Cjj = 1-36 x lO-^. 



5 — 2 



