GLASS ELECTRODE 



39 



membrane is governed by the difference in pH of the solutions. In practice the glass 

 electrode is generally a thin glass bulb blown on the end of a tube. Inside the bulb 

 is placed a solution of constant pH and a silver or platinum wire dips into it. When 

 the bulb is immersed in a solution the potential difference across the membrane will 

 be (at 30°C) :— 



Eb = ki — 0-06 (pHi — pHJ 



The significance of the constant kj is that although theoretically there should be 

 zero potential difference across the membrane when the internal and external pH is 

 the same, yet in practice an asymmetry potential is frequently observed. So that 

 experimentally the glass electrode is used as in the cases of other electrodes, by first 

 immersing it in a buffer solution of known pH and then in the test solution when 



J)!! = pH of buffer — Potential difference -^ 0-06 

 A diagrammatic explanation of the glass electrode circuit is given in Fig. 10. 



^^-<Z>^ 



WWVXAAAA- 



^tl_, Potentiometer 



Thin 

 glass 

 bulb ^ 



I 



Standard 



half 

 cell 



orPt\ 

 wire 



\ 



\ Constant j 



\ 



pH 



y 



Test solution pH x 



Fig. 10 

 Diagram of Glass Electrode Circuit 



In very alkaHne solutions the ordinary glass electrode cannot be used so that at 

 pH values above 10 electrodes made of special glass must be used. Although many 

 pH meters have scales reading from pH to 14 readings at the extremes should be 

 examined most critically. The thin glass membranes do not last forever, and it is 

 advisable to renew the electrode periodically if there is a suspicion of ageing. In 

 addition the thin glass is likely to absorb denatured proteins so that careful cleansing 

 is necessary when the electrodes are used for varying purposes. Although infrequently 



