40 



OXIDATION-REDUCTION POTENTIALS 



required in biological work glass electrodes are stated to be usable up to a temperature 

 of 70°C. 



Temperature Effect and Equation Constant 



In many of the electrode potential equations the term 2-303 RT -— F occurs and 

 at 30°C this approximates to 0-06. The change of potential with pH varies, however, 

 with temperature and for convenience in calculations the values of this term for 

 other temperatures are given in Table 8. 



TABLE 8 

 Values of 2-303 RT -^ F fob Various Tempbratttrhs 



If the pH value is required to be accurate to the nearest 0-02 pH the potentio- 

 meter must be accurate to the nearest millivolt, the galvanometer must be sensitive 

 to a millivolt potential difference and the temperature must be controlled to the 

 nearest 1°C. In table 9 are summarised the maximum degrees of tolerance which 

 should be allowed to attain various degrees of accuracy. 



TABLE 9 

 Accuracy Tolerances 



pH METERS 



There are now many pH meters available and the prospective buyer is faced with 

 an embarrassment of choice. There are, however, several factors which should govern 

 the type to be chosen since the number of refinements must be reflected in the price 

 and simplicity of standardisation and manipulation. 



(1) Potential and pH measurements. — If the meter is required for potential 

 measurements in millivolts as well as pH determinations a suitable calibration must be 

 incorporated. 



