74 



MANUAL OF MICROBIOLOGICAL METHODS 



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

 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 Eq. (2), 



ApH 



= 0.000,198,322^ 



(2) 



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

 ent 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 page 75. 



Standardization of the saturated calomel half-cell. For ordinary meas- 

 urements, 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 

 and contamination, and it is advisable to check its value regularly as a routine 

 procedure. 



The precise standardization of the calomel half-cell is discussed in detail by Clark 

 (1928). It consists in measuring the potential of this half-cell against the hydrogen 

 electrode in a solution of known hydrogen-ion activity or against other carefully con- 

 structed half-cells of reproducible, known potential. For measurements of ordinary 

 precision, the quinhydrone electrode (see below) in O.IA^ HCl can serve for standardi- 

 zation of the calomel half-cell. 



The quinhydrone electrode. Ignoring refinements and minor details, 

 we may state that the potential of a noble-metal electrode in an acid or 

 neutral solution saturated with quinhydrone varies linearly with the pH 

 of the solution, and this so-called quinhydrone electrode may, therefore, 

 be used to measure the pH of such solutions. 



The linear relationship of potential to pH holds only for acid and neutral 

 solutions to about pH 8. In more alkaline solutions two effects disturb 

 this regularity. One is the ionization of the reductant, and the other is 

 deterioration of the components of the system. 



The quinhydrone electrode, within its range of usefulness, may often 

 be employed in cases where the hydrogen electrode cannot be applied. 



^ See footnote on page 73. 



