THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 95 



tabulations are given by Hewitt (1936). Fuller details can be found in 

 Clark, Cohen, et aL (1928) and Cohen (1933, 1935). Table 10 gives the 

 names of the indicators, listed in the order of their E'o values at pH 7.0, 



Table 10. A Selection of Oxidation-reduction Indicators E'o at 



pH 7 (30°) 

 (Values of E'o between pH 5 and 9 will be found in Table 1 1 ) 



Compound E'^, volts 



a. Phenol-m-sulfonate-indo-2,6-dibromophenol 0.273 



h. m-Chlorophenol-indo-2,6-dichlorophenol 0.254 



c. o-Chlorophenol-indophenol . 233 



d. 2,6-Dichlorophenol-indophenol 0.217 



e. 2,6-Dichlorophenol-indo-o-cresol 0.181 



/. l-Naphthol-2-sulfonate-indo-2,6-dichloroplienol 0.119 



g. Lauth's violet (Thionin) . 062 



h. Cresyl blue . 047 



i. Methylene blue +0 . Oil 



j. Indigo tetrasulfonate —0 .046 



k. Methyl Capri blue — . 061 



I. Indigo trisulfonate — . 081 



m. Indigo disulfonate — . 125 



n. Gallophenine — . 142* 



o. Brilliant alizarine blue — . 173* 



p. Phenosafranine — . 252 



q. Tetramethyl-phenosafranine —0.273 



r. Safranin T -0 . 285 



s. Induline scarlet —0.299 



t. Neutral redf -0.324 



u. Rosindone sulfonate No. 6 — . 385. 



(hydrogen at 1 atm) ( — . 421) 



* At 25°. 



t See the footnote on p. 94 in text. 



and Table 11 gives the corresponding E'o values at successive levels 

 between pH 5.0 and 9.0. The magnitude of the salt and protein errors of 

 these compounds has not been established. 



Each indicator system listed in Tables 10 and 1 1 involves a two-electron 

 transfer, and the relation of E'o to other factors at fixed pH is given bv 

 Eq. (9). 



E, = £'. - If In [■;£ductant] 



2F [oxidant] ^ ^ 



Converted to ordinary logarithms after insertion of numerical values, this 

 equation becomes, at 30°C, 



i?,. = £'„ - 0.0.30 log [£?d^^ (10) 



[oxidant] ^ 



