OTHER ELECTRODES 



The degassed wire is fused into a soft glass capillary tube so that the tube 

 projects beyond the end of the wire in a regular cylinder for a known 

 distance; the glass then being annealed to prevent cracking. Alternatively 

 the platinum can be fused into the tube so that it is flush with the end, or 

 even projects, after which the recess is produced by dissolving the metal 

 to the required depth with aqua regia. These electrodes are very delicate 

 and can only be used for acute experiments. They are more suitable for 

 measurements at surfaces than in the depths of tissues. At a given oxygen 

 tension the current passing through a recessed electrode remains constant 

 over a range of applied potentials of approximately 0-2 to 0-8 V, but the 

 open-ended type do not behave in this convenient way so that the current 

 varies with the voltage without any simple relationship between the two. 

 Oxygen tension electrodes sometime suffer from so-called 'poisoning' 

 after being exposed to the tissues for several hours. This appears as a 

 progressive fall in current when the oxygen tension remains constant. It 

 appears to be due to tissue fluid proteins or bacterial activity on the surface 

 of the metal. If electrodes are carefully cleaned and sterilized by boiling 

 before insertion, and are implanted with aseptic precautions 'poisoning' 

 can usually be avoided. Reference electrodes for use in oxygen tension 

 measurements can be silver/silver chloride for imbedding or calomel half-cell 

 with a bridge for surface contact. 



Electrodes similar to the open-ended type described above can be used 

 for measurements of oxidation-reduction potentials in animal tissues, with 

 silver/silver chloride as the reference electrode. For a balanced electrometer 

 valve circuit suitable for multi-channel measurement of redox potentials 

 see Cater, PhiUips and Silver^^-^^. 



Many other types of electrodes are available for measuring oxygen 

 tension, the most accurate of which is the dropping mercury electrode, 

 but this is unsuitable for use on biological material except for suspensions 

 of cells, and even here the toxicity of the mercury renders results obtained 

 with living material rather open to criticism (Petering and Daniels^^). 



GLASS ELECTRODES FOR MEASURING pH IN TISSUES 



Several types of electrodes have been used in an attempt to measure intra- 

 and extracellular levels of pH in living preparations. A micro-tungsten 

 electrode described by CaldwelP^ was affected by factors other than pH, 

 such as cystein, HgS, calcium ions and oxidizing agents. Antimony 

 electrodes are sensitive to changes of oxidation-reduction potential and 

 oxygen tension. Glass electrodes however appear to be affected only by 

 changes in pH in the range pH to pH 9 (Dole^^) so that these are the 

 electrodes of choice for pH work despite the disadvantage of extreme 

 fragility. 



For work on animal tissues a glass electrode must be sufficiently small 

 to be entirely imbedded in the tissue concerned, of suitable shape for 

 insertion and must have extremely good insulation near its tip. It should 

 give a range of at least 50 mV per pH unit and have a resistance in the 

 range of 50 to 1,000 MQ. depending on the type of electrode. The smaller 

 the tip the greater the impedance. 



578 



