1X48-4 MANUAL OF METHODS FOR PURE CULTURE STUDY 



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

 cedure. 



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 0.1 N 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. 

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

 convenient. Consult Clark (1928) for fuller details. 



Its utilization may be illustrated in the standardization of the 

 saturated calomel half-cell. The potential, Ecai, of this half-cell is to 

 be determined 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 to 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 sys- 

 tem to constant temperature, and measure the potential which 

 should reach a constant value in a few minutes. 



The observed potential, Eobs, is related to the potential of the 

 calomel cell, Ecai, as follows: 



Ecai = Eq-Eobs-A.pH (3) 



Eq and A are constants at any given temperature, and have the following values: 



°C Eq A 



20 0.7029 0.0581 



25 0.6992 0.0591 



30 0.6955 0.0601 



35 0.6918 0.0611 



38 0.6896 0.0617 



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

 Ecal = 0.6896-Eobs-(0.0617X1.082) (4) 



from which the value of Ecai- can be calculated after substitution of 

 the experimentally determined value of Eobs- 



