162 



OXIDATION-REDUCTION POTENTIALS 



stages of reduction are in agreement with results obtained by the measurement 

 of the oxidation-reduction potentials of the systems. Anomalous waves often 

 encountered in polarograms have been attributed to adsorption effects at the 

 electrode which are sometimes removed by adding surface active substances to the 

 system or varying the mercury-dropping time. In the case of pyruvic acid some of 

 the effects seen have been attributed to keto-enol tautomerism and polymerisation. 



ANTIBIOTICS 



In an investigation of penicillin derivatives du Vigneaud and Melville (1949) 

 studied acylated and non-acylated thiohydantoins polarographically. Acylation 

 has no effect at the anode, both types of compounds giving a small wave between 

 and — 0-2 volt. At the cathodically polarised dropping mercury electrode the non- 

 acylated thiohydantoins have no effect, but the acylated 2-thiohydantoins have a 

 pronounced double wave with half- wave potentials at — 1-55 and — 1-7 volts respec- 

 tively, in an ammoniacal buffer at pH 9-2 ; neither type of compound produces a 

 catalytic cobalt wave like cysteine. 



Penicillin has been assayed polarographically (Scudi and Woodruff, 1949). 

 Cook, Hall, Heilbron and Roberts find that when penicillin is inactivated by alkali 

 there is a marked SH wave in the polarograph proportional to the original biological 

 activity. Hens, Page and Robinson use the catalytic wave in the presence of cobalt 

 to assay penicillin after alkaline degradation. 



MOULD PRODUCTS AND QUINONES 



The half- wave potentials of a number of mould metabolites and related quinones 

 have been measured by Page and Robinson (1943). The results are summarised in 

 Table 30. The measurements were made at pH 6-24, except in the case of citrinin 

 and penicillic acid which were examined at pH 4-63. 



TABLE 31 

 Half-wave Potentials of Quinones 



t 3 : 6-dihydroxy-4 -methoxy-2 : 5-toluquinone 



