GLASS ELECTRODES FOR MEASURING pH IN TISSUES 



Glass electrodes for physiological use are usually based on the capillary 

 electrode designed by Voegtlin, Kahler and Fitch^®, and modified for the 

 particular purpose required. An extracellular electrode suitable for most 

 purposes where pH changes in small blocks of tissue are to be measured 

 is described by Sonnenschein, Walker and Stein^'^. A piece of Corning 015 

 glass capillary tubing is cleaned in nitric acid and washed in water. It is 

 drawn down in a microflame to an outside diameter of about 2 mm. This 

 narrow tip is re-drawn to 0-2 to 0-5 mm and the end is sealed very carefully 

 so that no globule of glass forms, and no pinpricks are left in the seal. 



""v^ 



^ 



m 



Silver wire 



Rubber bung 



.Electrolyte 



Capillary wall 

 Copper gauze 

 sheathing 



Sealing wax 



Resin 



Uninsulated tip 



Figure 36.6 Extracellular pH electrode 



The tube is then half filled with either a saturated solution of silver acetate 

 in 50 per cent acetic acid or with 0-1 N hydrochloric acid. With the first 

 solution a pure silver wire is needed as the inner electrode but with 

 the second electrolytically plated silver/silver chloride is necessary. Air 

 bubbles must be ehminated and then the electrode is left in boiling water 

 for 2 hours as this equilibrates the asymmetry potentials of the glass more 

 quickly. After boiling the tube is placed in warm air at 150°C so that the 

 outside of the glass becomes heated. Apply a good insulating resin which 

 will harden immediately on cooling. Sonnenschein et al}"^ recommend 

 de Khotensky cement for the critical region which consists of the 2 mm 

 diameter part of the electrode and the first part of the main tube {Figure 36.6). 

 Fill the tube with the rest of the chosen electrolyte and seal it with a rubber 

 bung through which projects the internal electrode of silver wire. The 

 main part of the glass is then covered with a layer of sealing wax which is 

 continued over the rubber bung. A final coat of paraffin wax is added to 

 everything except the very fine tip of the electrode, and the whole of the 

 insulated part is then covered with fine copper gauze sheathing. The 

 complete electrode should be kept in distilled water or buffer at pH 6. 

 The resistance of this type of electrode is about 400 MQ and it should 



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