406 



GASOMETRIC-POLAROGRAPHIC METHODS 



furrent above 0.8 volt is due to a second reaction, i.e., reduction of 

 ionic to molecular hydrogen. 



Calihration and Measuring Instruments. The set-up for cali- 

 bration is illustrated in Figure 145. The platinum electrode {A) 



Fig. 145. Diagram of apparatus used for calibrating 



a stationary platinum electrode. 



Frov^ Davics and Brink (1942) 



dips into a 0.15 ilf sodium chloride solution and mixtures of oxygen 

 and nitrogen may be bubbled through the solution as indicated. The 

 calomel half cell is filled with the same solution and the potential 

 is supplied by a voltage divider. The potential is measured on the 

 voltmeter (T) and the current on the galvanometer (G). 



When the electrode is used on tissue the surrounding tissue fluid 

 serves as the indifferent electrolyte and the calomel half cell is 

 placed as near the electrode as possible, touching either the exposed 

 tissue or a salt pad on another area of the organism. 



With an air-saturated solution and electrodes made with wire of 

 0.2 mm. diameter or larger, currents of the order of 1 X 10"^ amp. 

 or greater are obtained. A galvanometer with a sensitivity of 5 X 

 10"^" amp. per mm. may be used. With electrodes of smaller di- 

 ameter, e.g., 25 /i., the currents are of the order of 1 X 10^^" amp. 

 for air-saturated solution and may be measured using a direct - 

 coupled amplifier. In the latter instance the galvanometer is replaced 

 by a well-shielded resistance whose value (R) is chosen to effect a 

 l)otential drop of about 1 millivolt applied to the input of the am- 

 I)lifier. The current (i) is given by the expression: 



