5o8 APPENDIX 



above-mentioned normal solutions, exactly the same quantity 

 of caustic soda will be required, namely, 40 grams :— 



HCl + NaOH = NaCl + Hp 

 36-5 40 



CH3COOH + NaOH = CHsCOONa + H^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 -001 N solution of hydrochloric acid is ionized and only 

 about 84 per cent in a N solution, whilst in -ooi N 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 deter- 

 mined 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 neutraUzation 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 

 take the place of those which have been neutralized, and so on 

 until all have been satisfied. 



In practice it is the ionized hydrogen only which is respon- 

 sible for the acidity of a solution at any given moment and so 

 it comes about that the hydrogen ion concentration for a 

 solution is, for biochemical purposes, a much more valuable 

 criterion of the actual conditions prevailing in any given 

 circumstances than is the potential alkali neutralizing power. 



The concentration of hydrogen ions may be expressed as 

 follows : In a decinormal solution of hydrochloric acid there 

 would be o-i gram in 1000 c.c, presuming it to be completely 

 ionized. In actual fact, however, a decinormal solution of 

 hydrochloric acid is only ionized to the extent of 97 per cent, 

 consequently the concentration is only 0-1x97 or 97x10 . 



