THE MEASUREMENT OF pH 1x48-3 



normal atmosphere and the hydrogen ions are at unit activity. The potential differ- 

 ence between electrode and solution in the normal hydrogen half-cell is assumed to be 

 zero at all temperatures. 



In standardizing the pH scale by means of measurements with a cell composed of a 

 hydrogen half-cell and a saturated KCl calomel half-cell, it is customary to ignore 

 the small and indeterminate liquid junction potential between the saturated solution 

 of KCl and the solution in the hydrogen half-cell. 



The combination of the two half-cells to make an electric cell is indicated schema- 

 tically as follows: 



(Pt)H2; H+ in solution X I Sat. KCl I Sat. KCl; HgaClj; Hg (Pt) 



Hydrogen KCl Sat. calomel (reference) 



electrode bridge electrode 



For a pH determination, purified hydrogen is bubbled through the 

 test solution to saturate it and the platinized platinum electrode until 

 equilibrium is attained as indicated by constancy of the emf deter- 

 mined potentiometrically between the metal terminals of the hydrogen 

 and the calomel half-cells. The observed emf, in volts^, is converted 

 to pH by the following equation, where T is the absolute temperature. 



Observed emf - Emf of calomel cell Eh 



pH = = (1) 



0.000,198,322 T 0.000,198,322 T 



For this equation to be applicable, the temperature must be constant. For precis- 

 measurements, a correction must be made for any departure of the hydrogen partial 

 pressure from one atmosphere. The correction seldom exceeds 0.001 volt (0.017 unit 

 of pH) for the ordinary ranges of barometric pressure and vapor pressures of solutions. 



As indicated by equation 2, 



-AEh 



= 0.000,198,322 T (2> 



ApH 



the slope of the straight line relating potential to pH is a constant 

 dependent on the absolute temperature. For example, at 25°, the 

 potential of the hydrogen electrode becomes more negative by 0.0591 

 volt- for each unit increase in pH. Values of this constant at certain 

 temperatures are shown as constant "A" on p. iX48-4. 



Standardization of the saturated calomel half-cell. For ordinary 

 measurements, the values at different temperatures of the saturated 

 calomel half-cell, referred to the normal hydrogen half-cell, are as 

 follows : 



The potential of this half-cell after continued use may change as a result of dilution 



-The electrical units employed in this leaflet are based on the "international" system 

 in which, according to the National Bureau of Standards, 1 international volt (U. S.) 

 equals 1.00033 absolute volts. The Bureau has announced that, as of January 1, 1948, 

 absolute electrical units will supersede international units. 



However, the efifect of this new convention for potentiometry is to introduce changes 

 which may be regarded as negligibly small in ordinary measurements of pH and oxida- 

 tion-reduction potentials. For example, in equation 2, -AEh 'ApH equals 0.05912 

 international volt and 0.05914 absolute volt, at 25°C (298.1° absolute). 



