198 PRINCIPLES OF GENERAL PHYSIOLOGY 



found to be that the solution has an acid reaction. .Such a case is that of aniline 

 hydrochloride. The degree of hydrolytic dissociation may be determined by 

 methods involving the estimation of the concentration of hydrogen or hydroxyl 

 ions, such as the hydrogen electrode, rate of hydrolysis of esters, etc. 



The treatment of tin- subject given above is that of Philip (1910, p. 260). The U>ok of 

 Nernst (1911, pp. .~>.SO-fi.'W| may also In- consulted with advantage. It will In- noticed that the 

 process essentially depends <m the slight electrolytic dissociation of weak acids and weak l>u-i---. 



From the equation for the reaction constant of hydrolysis given l>y Nernst (1911, p. 531). 

 which is 



where K 4 is the dissociation constant of water, K 2 that of the acid, and K :! that of the base, 

 we see that the degree of hydrolysis can be calculated when the strengths of the acid and lia*e 

 are known, and that it may have the same value with very various relative values of K. 2 and 

 K 3 , being greatest when both are low. Moreover, if the one or the other of the non-dissociated 

 components is insoluble, it may happen that nearly the whole of the solute is hydrolysed. An 

 instructive case, where the process of Ii3'drolytic dissociation is visible, is that of mercuric 

 acetate; a fresh solution is clear, but gradually becomes more and more turbid and red oxide 

 is deposited. 



The fact is sometimes overlooked that this process of hydrolysis in water rarely 

 amounts to more than 3 to 5 per cent, of the total content of solute. When both acid 

 and base are weak, as in aniline acetate, the hydrolysis may amount to 28 per cent. 

 (Bayliss, 1909, 2, p. 359). But, as a rule, it is a small thing compared with 

 electrolytic dissociation, and indeed is not always to be found when it might be 

 expected. For example, it appears that sodium stearate is considerably hydrolysed, 

 sodium palmitate is not. Congo-red is not so to any appreciable degree, neither is 

 the sodium salt of caseinogen. The acids in these cases are insoluble in water, so 

 that it is a matter of much difficulty to know a priori what are to be reckoned as 

 strong acids. 



We may now return to the consideration of the phosphate system. 



In a solution of NaH 9 PO 4 , which has an acid reaction, the only source of H- 

 ions is the stage of dissociation numbered (4) in the list above. (2) must precede 

 this, so that, combining the two, we have : 



NaH 2 PO 4 = Na- + H- + HPO 4 ". 



In a solution of Na.,HPO 4 we have also HPO 4 " ions from (1) and (3) : 

 Na 2 HPO 4 = Na- + Na' + HPO 4 ". 



If we add Na.,HPO 4 to a solution of NaH 2 PO 4 , we add an excess of HPO 4 " 

 ions. Therefore, since these solutions, as weak acids and bases, obey the law of 

 mass action, we reverse the dissociation of equation (4) 



and the H 1 ion concentration of the acid phosphate is reduced. 



Similarly, the alkalinity of a solution of Na.,HPO 4 is due to the OH' ions 

 derived from hydrolysis of HPO 4 " ions, according to equation (6). Perhaps it 

 would be more correctly expressed by saying that the HPO 4 " ion combines with 

 H- ions of water to form H 2 PO 4 ' ions, in a way analogous to that in which acetic 

 anions combine with hydrogen ions to form non-dissociated acetic acid. In any 

 case the result is an excess of OH' ions. If, then, NaH.,PO 4 is added to Na.,HPO 4 , 

 the excess of H.,PO 4 ' ions throws back equation (6), and the alkalinity is reduced. 



The mono sodium phosphate, as a weak acid, gives off very few H' and HPO 4 " 

 ions by (2) and (4), go that a very small amount of the di-sodium salt, which, 

 as a sodium salt, gives many HPO 4 " ions by (1) and (3), has considerable power 

 of diminishing the acidity of the former. Again, the di-sodium salt as a weak 

 base gives rise to very few OH' ions by (1), (3), and (6). Hence a very small 

 amount of NaH PO 4 , which, in its character as a sodium salt, dissociates with 

 the production of many H.,PO 4 ' ions, diminishes considerably the hydroxyl ion 

 concentration of the di-sodiura salt by throwing back equation (6). 



These considerations show that phosphate mixtures vary comparatively little 

 from neutrality, even with considerable excess of the acid or alkaline constituent. 



