160 HYDROGEN ION CONCENTRATION 



realized that we must no longer take it in the sense of that "osmotic 

 pressure" which had been derived from freezing point determinations, 

 but that it would be much better to substitute the term "active mass" 

 for that of "osmotic pressure." 



The active mass is obtained from the concentration by multiplying 

 the latter by the factor fa, the factor being so chosen that the other- 

 wise approximate equation (2) should correspond exactly to the 

 measured E.M.F. Properly speaking, this brings us into a vicious 

 circle. For, an equation such as (2) is set up, it is observed that it 

 is not quite correct, some of its values are altered by means of empiric 

 factors, and then, naturally, it is found that the formula is entirely 

 correct. While there is a grain of truth in this objection, it is not 

 entirely justifiable, for we have shown above, in a quantitative way, 

 the causes for the necessity of these corrections, and, in a few suitable 

 cases, it was also quantitatively possible to arrive at fairly accurate 

 results on the basis of certain verified assumptions. Furthermore, 

 we have seen that even the uncorrected equation is evidently entirely 

 vahd, as a limiting law, for high dilutions. It is beyond doubt that 

 in the near future the activity factor will be derived from other than 

 electromotive force data. Then the reason for the above vicious 

 circle objection will disappear. Considering the newness of the 

 conception of the activity factor, it appeared only proper to point 

 out these difficulties and to forestall this all too easily raised objection. 



If instead of the activity theory we choose the theoiy given on 

 page 131, we could then state even more clearly, the following: 

 The concentration C of the current yielding ions against which the 

 metal electrode has a potential of 0, varies with the amount and the 

 nature of all the ions present in solution (not only the current pro- 

 ducing ions). Therefore, when the concentration of the current- 

 yielding ions is greatly altered, without maintaining the total electro- 

 lyte content of the solution at a constant level by a corresponding 

 change of the non-current-yielding ions, then not only is the value of 



RT C 

 c changed, but also that of C. Hence, in the equation tt = —^ In — 



the value C cannot be strictly considered as being a constant. 



46. The concentration chain 



There is no adequate method for the measurement of any given 

 smgle potential of an electrode, but two electrodes are put together 



