THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 



75 



It comes to equilibrium rapidly, and its manipulation is simple and con- 

 venient. Consult Clark (1928) for fuller details. 



Its utilization may be illustrated in the standardization of the satu- 

 rated calomel half-cell. The potential Ecai of this half-cell is to be deter- 

 mined relative to that of a standard solution of fixed pH and saturated 

 with quinhydrone, e.g., 0.1 M HCl, the pH of which is 1.082 at 38°. This 

 is done with purified quinhydrone and accurately prepared HCl solution 

 as follows. Place about 5 ml of the standard HCl solution in a suitable 

 electrode vessel. Add 50-100 mg of quinhydrone crystals to saturate the 

 solution; some quinhydrone in the solid phase must be present. Insert a 

 clean platinum or gold electrode preferably in contact with the solid 

 phase at the bottom of the vessel. Then join this half-cell with the 

 calomel half-cell by means of a siphon containing saturated KCl solution, 

 bring the system to constant temperature, and measure the potential 

 which should reach a constant value in a few minutes. 



The observed potential, Fobs, is related to the potential of the calomel 

 cell, Ecai, as follows : 



Er.nl = E, 



Ec 



A-pTl 



(3) 



Eg and A are constants at any given temperature, and have the following 

 values : 



For example, at 38°, with a quinhydrone electrode in 0.1 M HCl, 



E 



cal 



0.6896 - £o6, - (0.0617 X 1.082) 



(4) 



from which the value of Ecai can b6 calculated after substitution of the 

 experimentally determined value of Eobs- 



To determine the pH of an unknown solution, proceed as above 

 except that the unknown solution is substituted for the standard HCl. 



The *'glass electrode/' Under suitable conditions, a properly pre- 

 pared thin membrane of special glass separating two solutions of different 

 pH exhibits an electric potential that is proportional to the difference in 

 pH of the solutions. Based on this property, a device called the glass 

 electrode is now widely used for the comparative determination of pH. 



