REACTIONS IN HOMOGENEOUS SYSTEMS 45 



MEASUREMENT OF THE CONCENTRATION OF IONS OTHER THAN 



HYDROGEN 



Electrochemical methods can be used to determine the 

 concentrations of ions other than hydrogen ions. The 

 electrical potential at the surface of a metal depends on the 

 concentration of the ions of the metal in the solution. This is 

 called an electrode of the first order. This form of electrode 

 is exemplified by the one described for the measurement of 

 hydrogen ions by platinum saturated with hydrogen gas. 



Another form of electrode is called an electrode of the 

 second order. The principle on which it is based is as follows : 



A solution of an " insoluble " salt gives a constant figure 

 for the product of its constituent ions (solubility constant). 

 By adding a soluble salt with one ion the same as one of the 

 ions of the " insoluble " salt the concentration of the other ion 

 of the " insoluble " salt must be inversely decreased if the 

 product of the ions of the " insoluble " salt is to remain 

 constant. 



To make use of this type of reaction an " insoluble " salt of 

 a metal is used with an electrode of the metal ; the anion 

 (acidic ion) of the insoluble salt is the same as the ion to be 

 measured. Variations in the concentration of the soluble 

 salt produce variations in concentration of the anion with 

 inverse changes in the concentration of the cation (metallic ion) . 

 As has been pointed out above the potential at the metal varies 

 with the concentration of the metallic ion in contact with it, 

 therefore variation in concentration of the anion produces 

 changes in potential at the metal electrode. 



In the case of the calomel electrode the potential at the 

 mercury surface is kept constant by the presence of the 

 insoluble mercurous chloride so long as the concentration of 

 chloride ions in the solution is kept constant. A standard 

 potassium chloride solution is used when the calomel electrode 

 is used as a constant electrode but the concentration of chlorine 

 ions can be measured by this electrode owing to variations in 

 potential when chlorine ions are varied in the solution as 

 increase in chlorine ions must cause changes in the con- 

 centration of mercurous ions. 



C Hg X Cc]- = K . COT- = ~- 



W g . 



Electrodes of the second order may be used to measure 

 changes in the concentrations of ions in tissues.* 



* H. E. Roaf, Proc. Roy. Soc., 1913, B. vol. 86, p. 215. 



