HYDROGEN ION CONCENTRATION 5 



CH 3 COOH = 60. In the case of a dibasic or tribasic acid, it 

 would be the molecular weight divided by two or by three ; 

 H 2 SO 4 H 3 PO 4 



-^ 4 = 49, -* = 32-6. 



From this reasoning it follows that whilst normal solutions of 

 all these acids contain in the litre different quantities of acid, 

 they all contain the same quantity of hydrogen, namely, I gram 

 per litre. Whilst, however, they all contain potentially the 

 same amount of hydrogen, it does not follow that the whole of 

 this quantity is ionized ; and inasmuch as the actual acidity of 

 a solution at any given moment is measured by the proportion 

 of ionized hydrogen atoms it contains, it follows that the actual 

 acidity of equinormal solutions of these various acids may be 

 very different. This does not mean that treir actual titratable 

 value, as measured by their power of neutralizing alkali, will 

 be different Thus for the complete neutralization of the I 

 gram of hydrogen contained in I litre of each of the above 

 mentioned normal solutions, exactly the same quantity of 

 caustic soda will be required, namely 40 grams. 



HCl + NaOH = NaCl + H 2 O 

 36-5 40 



CH 3 COOH + NaOH = CH 3 COONa + H 2 O. 

 60 40 



As a matter of fact, hydrochloric acid, which is a strong acid, 

 is almost entirely ionized in dilute solutions, whilst acetic acid 

 in solutions of equivalent strengths is ionized to a much smaller 

 degree. In actual figures, about 97 per cent of the hydrogen 

 in a -ooiN solution of hydrochloric acid is ionized and only 

 about 84 per cent in a O'lN solution, whilst in -ooiN acetic 

 acid not more than 13-6 per cent of the hydrogen is ionized. 

 Thus in the case of these two acids of the same normality, 

 although the total amount of titratable hydrogen, as determined 

 by the alkali-neutralizing power, is found to be the same, the 

 actual percentage of ionized hydrogen is seven times as great 

 in the case of the hydrochloric as in that of the acetic acid. 



That the comparatively feebly ionized acetic acid ultimately 

 requires the same amount of alkali for neutralization as the 

 more strongly ionized hydrochloric, is due to the fact that as 

 the ionized hydrogens in the acetic acid are neutralized a fresh 

 quantity of previously un-ionized hydrogens become ionized to 



