38 OXIDATION-REDUCTION POTENTIALS 



QUINHYDRONE ELECTRODE 



In solutions more acid than pH 7-5-8 the quinone : hydroquinone oxidation- 

 reduction system can be used for pH determinations. The oxidant and reductant 

 form a compound or resonance complex which automatically maintains a 50 per cent, 

 oxidised and reduced system. The E„ : pH curve has "0-06 slope " ; that is to say 

 that for every increase of 1 in pH value there is a decrease in potential of 0-06 volt 

 and the electrode potential equation is : — 



Eh = 0-695 — 0-06 pH 

 O H--- 



O — H—- O 



Quinhydrone Resonance Complex 



The value 0-06 applies to 30°C and values for other temperatures are given in Table 8. 



The concentration of quinhydrone has little effect on the potential so that to 

 measure the pH of a solution a little quinhydrone is shaken into it, a platinum (or 

 gold) electrode is inserted and the half-cell is joined by an agar salt bridge to any 

 standard reference half-cell such as calomel or silver chloride electrode. Again, in 

 this case there is no need to attempt absolute potential measurements; it is sufficient 

 to measure the difference in potential of cells containing a standard buffer and the 

 unknown solution. The difference of potentials in the two cases divided by 0-06 

 (at 30°) gives the difference in pH. 



Other quinone systems have been used for pH measurements ; for example 

 chloranil (tetrachloroquinone) has the electrode potential equation : — 



Eh = 0-664 — 0-06 pH 



Although basically simple there are limitations in the use of the quinhydrone 

 electrode for pH determinations. Quinhydrone is unstable at pH values above 7-5 

 to 8-0 so that the electrode cannot be used in alkaline solutions. Also, of course, 

 other oxidising or reducing substances present will interfere with the quinone- 

 hydroquinone oxidation-reduction system and hence will alter the potential indepen- 

 dently of pH changes. 



GLASS ELECTRODE 



The glass electrode has come to be used almost exclusively for electrometric 

 pH measurements in biological work. Essentially this consists, of a thin glass mem- 

 brane which for all practical purposes is permeable only to hydrogen ions so that the 

 difference of potential of electrodes immersed in the solutions on either side of the 



