POTENTIOMETRIC MEASUREMENTS 29 



to have a toxic effect on oxidation enzymes (Quastel and Wheatley, 1931). Perdrau 

 and Todd (1933) describe the photodynamic oxygen-sensitising effect of methylene 

 blue on viruses and bacteriophage. 



It must not be forgotten that dyes differ not only in the range of potential 

 in which they change colour, but also in chemical constitution. Dyes containing 

 a sulphonic acid group have been found to behave differently in respect to living 

 cells from dyes without that group. The well-known " proteiti " and " salt errors " 

 in colorimetric pH determinations may well have their counterpart in indicator 

 methods of Eh measurement. 



Thus, although dye experiments may yield valuable results, particularly in 

 comparative experiments, data obtained by this means must be accepted with 

 caution. Perhaps negative results, i.e., when the dye is not reduced, should be 

 examined more critically. The ideal to be aimed at in selecting a dye for the 

 determination of oxidation-reduction potential should therefore be one : — 



(1) Becoming reduced at a suitable range of E^. 



(2) Of distinctive colour, preferably blue, to avoid confusion with the 

 natural colour of the system studied. 



(3) Not changing colour with pH changes. 



(4) Of intense colour so that very low concentrations may be used, thus 

 not over-poising the system studied. 



(5) With no direct participation in the system studied, i.e., it should not 

 catalyse biological oxidations or have toxic effects on cells or tissues, and 

 should not combine with components of the system. 



Unfortunately no approach to ideal oxidation-reduction potential indicators 

 has been made. Despite the careful and continued work of Clark and his collaborators, 

 no simple reliable indicator method has been worked out which will bear comparison 

 with the colorimetric methods of pH determination. Electrometric methods must 

 remain, at present at all events, of much greater importance in the study of 

 oxidation-reduction processes than in the case of hydrogen ion studies. 



DIRECT POTENTIOMETRIC MEASUREMENT OF E^ 



The precautions in technique and interpretation of electrode potential measure- 

 ments, when using dyes as indicators, make the direct potentiometric method of 

 measurement very necessary for dependable results. It appears doubtful if colori- 

 metric methods can ever attain the dependability and accuracy attained in colori- 

 metric methods of pH determination. 



It is remarkable how comparatively simple the direct potentiometric method 

 of E,, measurement can be made with suitable equipment. To measure the potential 

 of the half-cell — unattackable electrode/oxidation-reduction system (X) — the 

 cell must be completed by another half-cell (St.). According to definition, the E^ 

 is the potential referred to the normal hydrogen electrode, but as this half-cell is 

 not a practical possibility any convenient standard half-cell, of known potential 

 referred to the normal hydrogen electrode, may be used. The theoretical circuit used 



