ELECTROLYTES AND THEIR ACTION 195 



instead of decreasing, the hydrolysis of an ester by the solution. This difficulty 

 in the electrolytic dissociation theory was noticed by Arrhenius himself (1889, 2, 

 and 1899), and called "neutral salt action." It shows that neutral salts of a 

 strong acid increase the effect of the acid itself in some way not yet clear. 

 Attention has already been called to the anomalous behaviour of salts, strong 

 acids, and strong bases, and the views of Noyes, etc., on the question (see page 182 

 above). Some suggestions made by Senter (1910), at the conclusion of a paper 

 which bears on the subject, may be of interest. The influence of neutral salts 

 may be supposed to be exerted on the water or on the substance being hydrolysed, 

 sugar or ester. In the former case the dissociation may be increased, or the 

 action may be of some unknown kind on the non-dissociated molecules. In the 

 latter case the effect may be due indirectly to an effect on the dissociative force 

 of the medium. Senter himself favours the latter view, but regards it as probable 

 that there may be several causes acting together. Possibly hydration of the ions 

 of sodium chloride may increase the effective concentration of both acid and 

 sugar, but it is doubtful whether the effect would be large enough. 



Of the various hypotheses made in explanation of this effect, those of Caldwell (1906), 

 Snethlage (1913), and Taylor (1914) may be referred to. According to Caldwell, the action of 

 salts in increasing the rate of hydrolysis by acids is to be accounted for by a real increase in 

 concentration of the acid. This takes place in two ways. If volume normal solutions are 

 taken, a part of the water is displaced by the molecules of the salt, in the sense of van der 

 Waals' constant, b. These salts also actually take up water in some way, so that it is rendered 

 unavailable for dilution of the acid ; so that, again, the amount of water really free is less than 

 it appears to be. Determinations of the increased quantity of water required to bring the rate 

 of hydrolysis to the same value as that in the absence of salt leads to values of the amount 

 used in " hydration " of the salt very close to those found in other ways, as will be described in 

 the next chapter. Snethlage's work, in Bredig's laboratory, suggests that the undissociated 

 part of the acid has also a catalytic action in the hydrolysis of esters and cane-sugar. As the 

 affinity constant of the acid rises, so does the catalytic power of the undissociated part. In 

 the weakest acids, that of the undissociated molecules is less than that of the hydrogen ions, 

 but in the strong acids it may actually be greater. The action of chlorides in increasing the 

 rate of hydrolysis of cane-sugar by hydrochloric acid is thus explained by the decrease of 

 dissociation of the acid, as demanded by mass action on the Arrhenius theory. Taylor (1914) 

 comes to conclusions similar to the last, in more detail. He also finds that the catalytic 

 action of the undissociated acid increases with the affinity constant of the acid. If Cj is the 

 concentration of the hydrogen ion, C 2 that of the undissociated acid, & H the catalytic action 

 of the former, k m that of the latter, then 



k m _ Cj 



^-Vc; 



It will be noted that it is not definitely known whether the H' ion concentration is actually 

 raised by neutral salts. 



As regards the part played by this " neutral salt action " in physiological phenomena, see 

 the paper by Hober (1910, 3). 



For very weak acids a sensitive method has been described by Fraenkel (1907). 

 Diazo-acetic-ester is decomposed, with evolution of nitrogen gas, by very low 

 concentrations of hydrogen ions, and is of use even in the case of the very weak 

 amino-acids. 



A method similar to that of hydrolysis of ordinary esters, and, like it, specially 

 useful for the stronger acids, but subject to "neutral salt action," is the inversion 

 of cane-sugar. This consists in the hydrolysis of the disaccharide, with the 

 formation of glucose and fructose and, being associated with a considerable fall in 

 the power of rotating polarised light, can be followed with the polarimeter in a 

 convenient manner. 



PRESERVATION OF NEUTRALITY IN THE ORGANISM. 



We have seen how very sensitive the various processes, both chemical and 

 physical, taking place in the organism are to changes in concentration of hydrogen 

 ions. Now a large number of the reactions going on result in the production of 

 such changes, and it is not to be supposed that it would be desirable that these 

 changes should be entirely neutralised, even if it were possible. For example, the 

 sensitiveness of the respiratory centre to slight increase of hydrogen ion con- 



