WATER, ITS PROPERTIES AND FUNCTIONS 239 



although they exist only in very small amount in water, their action must not 

 be neglected. 



Hydroxyl ions are not catalysts, at all events in the saponification of esters, since they are 

 used up in the reaction, thus : 



CH 3 CO 2 CH 3 + OH' = CH.,COO' + CH 3 OH. 



The result of this reaction is to increase the hydrogen ions and to diminish the hydroxyl 

 ions. There is, then, a double process, the details of which may be found in Nernst's book 

 (1911, p. 567). 



Most oxidation processes were supposed, up to recent times, to be simply 

 explained by the direct union of oxygen with the substance to be oxidised ; but 

 it has been shown conclusively, chiefly by the work of H. B. Dixon and 

 H. B. Baker, that the presence of water is necessary. This fact will require 

 further discussion in our chapter on oxidation, so that attention is directed to it 

 here as another case in which water acts as a catalyst. 



It should be mentioned that Armstrong does not admit that it is water itself which acts in 

 these cases, but the impurities contained in it, acting as conducting systems to bring the other 

 components into reaction. A striking case, which seems to support this view, is that described 

 by Brereton Baker (1902). It had been already shown by Dixon that water vapour is necessary 

 for the explosion of a mixture of oxygen and hydrogen gases. Baker showed that if the gases 

 are almost completely dried, a slow combination occurs on heating ; but although more than 

 sufficient water is formed to bring about an explosion, none happens. The explanation, accord- 

 ing to Armstrong, is that the water formed is too pure to allow the necessary conducting 

 system between the reacting gases to be produced. 



WATER IN REVERSIBLE REACTIONS 



A large number of the reactions occurring in living organisms are those in 

 which water is removed or added. The addition of water, hydrolysis, results in 

 the splitting up of a complex molecule into smaller ones, and plays a large part in 

 the phenomena of digestion, where certain agents, enzymes, are present whose 

 function it is to hasten the process catalytically. As a simple instance, we might 

 take glycyl-glycine : 



COOH CH 2 - NH CO CH 2 NH 2 . 



By the entrance of a molecule of water at the arrow, the compound is split into two 

 molecules of glycine : 



COOH CH 2 NH 2 HOOC CH 2 NH 2 . 



If two molecules of glycine be taken and a molecule of water removed, that is, H 

 from the one, and OH from the other, synthesis of glycyl-glycine occurs. 



Consider, further, the equilibrium in a mixture of methyl acetate and water. 

 Here, when water is added to methyl acetate in the proportion of one molecule to 

 each molecule of the ester, part of the water hydrolyses part of the ester similarly to 

 the previous case. But, when a certain fraction of the ester is hydrolysed, the process 

 comes to an end, owing to the increase of the opposite synthetic reaction by mass action 

 of the products of hydrolysis. Expressed in the usual way, we have in equilibrium : 



Kf\ f\ f\ f~i /->> TT 



.Ce S ter.CH0 = Calcohol.Caeid, Or K = 



'-'ester 



- 



'-'alcohol * t-'acid 



Suppose that we now increase the concentration of the water. It is plain that the 

 only way K can remain constant is by diminution of C es ter, which involves, at the 

 same time, increase of the components of the denominator. Similarly, 

 decrease of water means increase of ester, or synthesis. It is clear that, in 

 this way, by alteration of the actual or effective concentration of water, the 

 living cell has the possibility of changing the position of equilibrium in such 

 reversible reactions, and thus causing the preponderance of hydrolysis or synthesis. 

 It seems most probable that mechanisms of such a kind are active in the 

 protoplasmic system, and that the taking up or giving off of water by colloidal 

 substances is the chief one. In any case, we see the importance of the presence of 

 water, not merely as a solvent to allow the reagents to come together, but also as 

 an actual component of the chemical reactions themselves. 



In pure water, the process of attainment of equilibrium is extraordinarily slow, 



