MEASUREMENTS WITH MICROCAPILLARY ELECTRODES 



remove cyanide. The silver is chloridized at the anode, often in a bath 

 containing isotonic sodium chloride or Ringer's fluid. However to obtain 

 the most stable electrode a current density of approximately 5 mA/cm^ of 

 surface is passed in a solution of 0-1 M hydrochloric acid for 1-3 hours. By 

 reversing the current several times the effective surface can be increased. 

 Pairs of electrodes are short-circuited together and their tips immersed in 

 Ringer's fluid. After some hours or days any initial difference of potential 

 is eliminated. Silver chloride is soluble in concentrated potassium chloride 

 so that for the most stringent condition the latter is not advisable as a direct 

 connection. In order to reduce fluid junction potentials an intermediate 

 potassium chloride bridge may be used if the electrode surface is placed in 

 a weak chloride solution. Another possibility is the use of ammonium 

 nitrate. 



If direct sunlight be avoided the potential fluctuates less than 1 mV with 

 variations in the ambient light. The potential will vary in the presence of 

 some dissolved constituents such as chloride, bromide, iodide, or oxygen, 

 when the electrode will tend to become more negative. 



Fluid short-circuits — It may be necessary sometimes to use a fluid short- 

 circuiting arrangement to examine the properties of certain electrodes or of 

 artificial membranes. For this purpose a 3 M potassium chloride electrolyte 

 bridge serves best, either connected between the fluid of the electrodes or 

 between one electrode and the intermediate test or bath solution. It is not 

 advisable to operate such a fluid switch by means of a glass tap, as the resis- 

 tance of a greased tap with 3 M potassium chloride is often only a few 

 megohms when closed. A convenient bridge to test electrode properties was 

 used by del Castillo and Katz^* in which a short arm filled with 3 M potassium 

 chloride projected from the tube supporting the electrode, and could be 

 connected to the bath fluid simply by lowering the electrode further. 



Some common arrangements for measuring tissue voltages — It is often 

 unnecessary to eliminate all sources of e.m.f. in a measuring circuit in order 

 to measure d.c. voltages within tissues, as the voltages measured are essen- 

 tially differences of potential between two points, such as outside and inside 

 a cell or between a crushed and a functional portion of tissue. The amplifier 

 can be brought back to its operating point by 'backing off', i.e. applying a 

 voltage of the opposite sense in series by means of a calibrator. The most 

 common cause of such potentials is that due to the use of dissimilar electrodes. 

 Connection with a microelectrode is sometimes made with a chlorided silver 

 wire inserted into its 3 M potassium chloride electrolyte, while the indifferent 

 or bath electrode is formed by Ag-AgCl in Ringer's or tissue fluid. The result- 

 ing difference in electrode potentials has to be 'backed off'. While this can 

 be readily done with a fluid bath some precautions are necessary when work- 

 ing with whole animals. A calibrator of low output resistance can be placed 

 between earth and the indifferent electrode, usually a large silver plate, while 

 care is taken to insulate the rest of the preparation from accidental earthing 

 by insulating all clamps and supports. 



An alternative point for 'backing off' is actually in the input lead. How- 

 ever this has disadvantages in that the whole apparatus must be well insu- 

 lated, and can introduce distortion when recording rapid signals. A better 

 method for re-balancing an amplifier is to include a voltage shift device 



549 



