158 OXIDATION-REDUCTION POTENTIALS • 



motor, the voltage is recorded and the current passing is also recorded. The record 

 is in the form of a voltage current curve. Apart from the rapidity of measurement 

 with an automatic polaro graph, the results are obtained in the form of a permanent 

 record and as a continuous curve on which small waves may be seen which would 

 possibly be missed with a manually-operated apparatus. In most polarographs the 

 recording is photographic {e.g., in the instrument of the Heyrovsky type made by the 

 Cambridge Instrument Company of London, the Nedjedly Company of Prague and the 

 E. H. Sargent Company of Chicago), but in the instruments made by Tinsley in this 

 country and by Leeds and Northrup in the U.S.A., the recording is by a mechanical 

 pen. The cathode ray oscillograph is being applied to polarographic measurements 

 with interesting results in the case of slow electrode reactions. 



Fig. 42 

 Suppression of " maximum wave " by dye 



When the metal after reduction is absorbed at the dropping mercury electrode 

 there is a peak on the polarographic curve. If surface active substances such as dyes, 

 protein, starch, etc., are added in small amounts the peak disappears. The pheno- 

 menon has been used for determining traces of surface active compounds. In the 

 figure is shown the effect of fuchsine in abolishing the peak on the polarographic 

 curve of copper. 



It is essential in polarographic work to ensure that the solution is adequately 

 buffered otherwise the hydrogen-ion concentration near the electrode will not be that 

 of the bulk of the solution owing, probably, to diffusion of hydrogen or hydroxylions 

 away from the immediate vicinity of the drop of mercury. Unstable potentials and 

 variation of the half-wave potential may be due to inadequate buffering and much of 

 the earlier work on the subject has been criticised for this fault. It is suggested that 

 the buffer concentration should be one hundred times that of the reactant investigated. 



It i s important to note that in many papers on polarographic methods potentials 

 are given not with reference to the normal hydrogen electrode as in oxidation- 

 reduction potential measurements, but are referred to the saturated calomel electrode 

 (S.C.E.) which is 0-25 v. more positive at 20°. 



SYSTEMS OF BIOLOGICAL INTEREST 



Although much of the work with the ])olarogra]ih has been applied to the 

 detection of traces of metals there has been a considerable volume of work on 

 other biologically interesting substances. 



