GLASS MICROCAPILLARY ELECTRODES 



(Philpot*^, Fricke'*^ and GrahamC*^). The electrochemical polarization 

 capacitance is not constant and decreases as the frequency increases. This is 

 probably because time is required for the electrochemical action to take place. 



Some methods of constructing metal-filled electrodes 



To construct a metal-filled electrode the metal may be introduced after 

 the capillary is formed or the electrode drawn from pre-fiUed tubing. An 

 example of the first method is that described by Weale^^. After the capillary 

 is drawn a stout silver wire is jammed into the stem. The electrode is filled 

 with aqueous silver nitrate solution and a chain of silver crystals deposited 

 in the shank up to the tip, by means of electro-deposition. The process 

 can be observed under a microscope. Such electrodes should possess 

 non-polarizable surfaces in physiological fluids however their resistance 

 rises in use, possibly due to the deposition of a resistant layer (Svaetichin^"). 



The method of drawing a metal-filled glass capillary was described by 

 Taylor*^. The metal and glass used are such that the softening point of the 

 glass lies between the melting and boiling points of the metal. A simple 

 combination is that of soft solder and soda glass. A piece of metal is placed 

 in the bottom of a tube closed at one end, and heated. When the metal 

 melts an oxide film forms which it is advisable to remove by pinching (with 

 pliers) the bottom of the glass tube repeatedly — when the film will stick to 

 the glass — until most of the oxygen at the bottom of the tube is used up. 

 Capillary tubes can now be drawn in one or more stages containing metal 

 with a bright surface, which should be inspected for cracks. 



Electrodes with a silver alloy conductor can be made and those described 

 by Svaetichin*'' have been used to study single cells. Their properties have 

 been examined by Gray and Svaetichin*''. Tips of under 1 /i diameter can 

 be formed under microscopic control from capillaries of approximately 

 20 fj, ; the alloy is then electrolytically coated with rhodium to form a 

 hemispherical cap, and finally platinum black is deposited. The latter 

 forms a spongy layer with a relatively large effective surface which can be 

 cleaned in acid when necessary, and reformed. 



Electrodes such as these have a low noise level which was measured in 

 many electrodes by Svaetichin^", e.g. a tip of 5 /^ diameter had a peak-to-peak 

 (sic) noise of 10 /^V. The impedance of these electrodes measured at 1 kc is 

 approximately 0-2-1 MQ. for tips 3 to 10 /i. Their high frequency response 

 is found to be very good. However the potential level of the electrode is 

 not stable. Gray and Svaetichin^' found large drifts on first introducing 

 the electrode into an electrolyte solution, and after 15 minutes the drift 

 declined to approximately 1 mV per minute. These electrodes have also 

 been employed for intracellular recording. A comparison of the membrane 

 potential recorded with these and with saline-filled electrodes would be 

 of value. 



Another form of electrode has been described by Dowben and Rose*^ 

 whose preparation is relatively simple and certain. It embodies a novel 

 construction in that an alloy of indium — a glass-wetting metal of low 

 melting point — is used. A microcapillary is drawn from a tube containing 

 some indium, and when gently heated the metal can be pushed down to 

 fill the electrode. The indium alloy at the tip is electroplated first with gold 



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