44 



OXIDATION-REDUCTION POTENTIALS 



In Fig. 13 are plotted the extinction coefficients for A = 550 m/x. of a series of 

 buffers of different pH containing phenol red. 



It is extremely simple to measure pH spectrophotometrically even at the extreme 

 end of the range where matching by eye is often difficult. 



Fig. 13 

 Extinction coefficients (A. 550 m/x) of phenol red bufifer tubes 



For measuring pH values of drops of fluid, e.g., cultures, the use of stronger 

 indicator solutions than usual enables colour matching by eye to be carried out in 

 capillary tubes. 



Indicator papers (other than the familiar litmus paper) are coming into increased 

 use, but, of course, great accuracy is difficult to attain especially with mixed indicators 

 owing to " chromatographic absorption " effects and the development of bands of 

 colour. 



The " protein error " of indicators is well known and a word of caution should be 

 given regarding the interpretation of colorimetric pH determinations, especially in 

 very acid solution. On the acid side of the isoelectric point protein-dye compounds 

 are formed (Hewitt, 1927) which do not change colour at the same pH as the dyes 

 alone. 



Summary of Chapter III 



The applications of various oxidation-reduction potential systems to pH deter- 

 minations are commented upon and some practical aspects of the selection of glass 

 electrode apparatus and colorimetric pH methods are discussed briefly. 



