194 PRINCIPLES OF GENERAL PHYSIOLOGY 



In a later paper Walpole (1914, 1) describes improvements in this electrode. Peters 

 (1914) uses another excellent form. 



In the case of blood, or other solution containing haemoglobin, there is another difficulty. 

 Platinum takes up oxygen as well as hydrogen, and, in pure oxygen, it serves as a hydmxyl 

 ion electrode, although not so accurately defined as the hydrogen one, owing to its sensibility 

 to various disturbing conditions. When in use as a hydrogen electrode, it is obvious that the 

 potential which it assumes in a solution of given hydrogen ion concentration will not be the 

 same if the gas in contact with it contains oxygen, as must be the case if shaken with a 

 solution of oxyhfemoglobin. At present it seems impossible to devise a method of removing 

 oxygen without producing other changes in the blood. Perhaps carbon monoxide would srrvr. 



The general method of determining the concentration of particular ions in 

 a solution by the use of appropriate electrodes is probably capable of wider 

 application in physiology than it has yet received. Thus the changes in the con- 

 centration of chlorine ions due to separation and dissociation of chlorides and 

 changes in the tension of oxygen can be investigated on these lines. These are pro- 

 cesses which occur in physiological activity, and Roaf (1913) has already obtained 

 valuable information with regard to changes in contracting muscle by these 

 methods. Reference will be made to these results later. 



In the description of the Nernst theory of the metallic electrode, it must not be forgotten 

 that the process is not the same as that of ordinary solution. Owing to the forces of 

 electrostatic attraction, the ions given off from the metal cannot actually pass beyond the 

 immediate proximity of the electrode itself, thus giving rise to a Helmholtz double layer. 

 The case of a solution enclosed by a membrane permeable only to one of the ions into which 

 the solute dissociates is a completely analogous one. The surface of the metal itself 

 in the Nernst electrode may be regarded as permeable to its own positively charged ions, 

 but not to the oppositely charged mass of metal. The former ions, however, are held 

 fast by electrostatic attraction until the circuit of the battery is completed, when they 

 are able to pass out from the one electrode, which is dissolved, and are deposited on the 

 opposite one, losing their charges and increasing the mass of the metal. 



There is one point in connection with the Nernst formula which may have struck the 

 reader, although it is not alluded to in the usual descriptions of the theory. It had, however, 

 not escaped the notice of the original author (Nernst, 1911, p. 139). If p l in the expression : 



becomes zero, i.e., if the liquid in one electrode is in infinite dilution, or, in other words, 

 is water, the value of the potential difference becomes infinite. Nernst points out thai, 

 theoretically, the diffusion of any substance into a space which is, for it, a vacuum, should take 

 place with infinite velocity. In the case of a gas this condition would last only for an 

 infinitesimally short time. Water is practically never a vacuum for electrolytic diffusion, 

 since there are always ions in it. There are, moreover, other reasons connected with the 

 conditions at the surface, which make measurements with solutions of less than O'OOl molar 

 strength unreliable as indicating the state of the solution as a whole (see the remarks by 

 Nernst referred to above). 



Certain other methods of estimating the hydrogen ion concentration of a solution 

 require a brief account.- These are of a more chemical nature, and are occasionally 

 useful. As a rule they necessitate a previous knowledge of the composition of 

 the solution apart from its concentration in hydrogen ions. 



Hydrolysis of Enters. The rate at which methyl or ethyl acetic esters are 

 hydrolysed in water is found to be proportional to the hydrogen ion concentration 

 present. It may be used as a convenient method for the comparison of fairly 

 high concentrations of these ions, but with weak acids the rate is too slow 

 to be of much practical value. The presence of neutral salts affects the rate 

 of the reaction in an anomalous way. If we return for a moment to the equation 

 for the dissociation of a weak acid in equilibrium with its ions, viz. : 



K(0)e4XC)i. x (C) H . or K = ^g^ 5 > 



it will be seen that any increase in the concentration of the acetic ion leads to 

 diminution in that of the hydrogen ion, in order that K may remain constant. 

 This increase may be produced by the addition of a salt of the weak acid, in 

 our case say sodium acetate, which dissociates into acetic and sodium ions. 

 Experimentally this is found to be the case. It is, indeed, a deduction from 

 the law of mass action. But it does not apply to strong acids and their salts. 

 In fact, the addition of sodium chloride to a solution of hydrochloric acid increases, 



