HYDROGEN ION CONCENTRATION AND PROTEOLYSIS. 149 



solution, iust gram molecule. Thus the reaction is usually 



10,000,000 



expressed in terms of hydrogen ion concentration unless it is indicated 

 otherwise. 



From the above discussions it is readily seen that if the ionization con- 

 stant is known, and the hydrogen ion concentration is determined ex- 

 peiimentally, then the hydroxyl ion concentration can be calculated. 

 The determmation of hydrogen ion concentration is accomplished by the 

 use of the gas cell, of which the principle is based upon the potential of 

 the chain. This chain, as described in phj^sical chemistry, consists of — 



Hg-HgCl ! n/lO KCl I cone. KCl 1 solution ] Ft Hz 



calomel electrode concentr. (unknown) platinum elec- 

 potassium trode saturated 



chloride with hydrogen 



in a dish. gas. 



The potential of such a chain can be determined by the usual physical 

 method. Then the relation between the measurement of potential and 

 hydrogen ion concentration can be calculated by the following equation : — 



p. P -0.3377 



Ph = 



0.0577+0.0002 (t°-18°) 

 where — 



Ph — the term adopted by S. P. L. Sorensen to express the exponent of gm. — 

 equivalent of hydrogen ions per liter. ^ 

 P — the total E. M. F. of the chain. It can be determined by the following 

 equation, having the apparatus arranged as it is shown in the dia- 

 gram : — 



P = — ' • , in which Ri — the bridge reading for the chain against an accumu- 



^ lator. 



R — the bridge reading for the accumulator against the 

 normal element. 

 1.0189 — the voltage of the normal element at 18°C. (stand- 

 ard). 



0.3377 2 — the sum of potential of calomel electrode (N/10 KCl) and hydrogen 

 electrode in a solution where the hydrogen concentration is normal 

 (H) = 1 or Ph = 0. 



0.0577^ — thermodjTiamical factor at 18° C. which is influenced by temperature, 

 0.0002 for each degree centigrade, or it changes as follows: — 



0.0577+0.0002 (t°-18°), of which t° equals temperature at the 

 time of determination. 



After Ph is determined it is necessary to understand the value of H ion 

 concentration, although the experimental results are generally expressed 

 in Ph. It will be shown at the end of an example, illustrating the appli- 

 cation of the formula as well. 



1 Sorensen, S. P. L. Ergebnisse d. Physiologie, 12, 416, 417, 1912. 



2 This will be explained further by an illustration on the following page. 



3 Bjerrum. Ibid., 53, 428, 1905. 



* \V. Nernst. Zeitschr. physik. Chem., 4, 129, 1889. 



