ioo Mineral Nutrition of Plants 



(RT In a K + + Fk+)i = (RT In a K+ -f Fk+)ii 



(RT In a Gl - + F Ci -)i = {RT In a cl - -f F n Gl -) u 



If we choose to express the activities in both phases in terms of the same 

 standard state, we can write further 



(«K+)l = («K+)ll 



(tfoi-)r = (tfci-)n 



Thus we are led to conclude that, on the basis of the same standard 

 state, the activity of any ion in a soil suspension or gel is the same as 

 in its equilibrium filtrate.* In some quarters it may be contended that 

 this conclusion is contradicted by the well-known observation that the 

 pH, as ordinarily determined with the hydrogen or glass electrode, may 

 be quite different in suspensions of acid clays and in their filtrates. How- 

 ever, it should be pointed out that such measurements invariably in- 

 volve an electrolytic bridge and a liquid-liquid junction. The magni- 

 tude of the junction potential cannot be measured. Conceivably, it may 

 be quite different in a clay suspension and in a dilute salt solution. For 

 these reasons, conclusions regarding H+ activities in systems of this 

 kind based on pH determinations are subject to question. 



The activity of an ion can be expressed in terms of its concentration 

 by means of the relationship 



* According to this treatment, the activity of an individual ion is defined in 

 terms of its electrochemical potential. In a number of treatises (see, for example, 

 J. N. Bronsted, Physical Chemistry, New York, The Chemical Publishing Com- 

 pany, 1938, page 276) the electrochemical potential of an ion is considered as 

 being composed of two parts, one chemical and the other electrical: i.e., u. ; : 

 Hi + z { F i|», where JTj is the electrochemical potential, M-i the chemical potential, 

 z x the valency of the ion, F Faraday's member, and •»!> is the electric potential of 

 the phase. Moreover, some writers using this line of reasoning choose to define 

 the activity of an individual ion in terms of its chemical potential, although they 

 point out that the activity so defined has no thermodynamic significance. 



