372 Scientific Proceedings, Boyal Dublin Society. 



that the potential of the hydrogen electrode changes with the concentration 

 of hydrogen ions as follows : — 



Br dP 



'"^-uF F' 



which on integration becomes 



uF ^ 



where A is an integration constant, R the gas constant, T the absolute 

 temperature, n the valency of the ion, and F the faraday or quantity of 

 electricity carried by one gram equivalent of the ion, and logcZ* is the natural 

 logarithm of the partial pressure due to the hydrogen ions. Now, if two 

 such hydrogen electrodes are connected to form a concentration cell, the 



electromotive force developed is equal to — iog,e 770 where C and C" are 



nF G 



the concentrations of hydrogen ion, since the ratio of the pressures may be 



taken as equal to the ratio of the concentrations. 



In practice such a hydrogen electrode is connected by an inverted U-tube 



arrangement, containing a solution of potassium chloride, with a calomel 



electrode, and the electromotive force of this cell is measured by means of a 



potentiometer, with all due precautions. The constants being evaluated, use 



is then made of the following equation to determine the hydrogen ion 



concentration : — 



E.M.F. (obs.) - E.M.F. (of normal hydrogen and calomel electrode cell) 

 ' 0-0001983 T ■ 



= logpL=pH. 



Using an N/10 KCl-calomel electrode, this becomes at 25° 0. 

 E.M.F. (obs.) - 0-336 = 0-9591 log ~ = 0-0591 pH. 



The fact that the term log ^ft^-i is thus directly determined is another 



reason for expressing hydrogen ion concentrations in - log [H^] or 

 pH values. 



Full proofs of these equations and directions as to the technique may be 

 found in "The Determination of Hydrogen Ions," by Clark. 



The method is used as the fundamental one for determining pH values. 

 By its means buffer solutions of accurately reproducible pH values may be 

 standardized for use in colorimetric determinations by means of indicators, 



