408 



GASOMETRIC-POLAROGRAPHIC METHODS 



restoration of equilibrium before another measurement is made. Tlie 

 operation is repeated at other potentials to obtain a current-voltage 

 curve (Fig. 144). 



The time the circuit must be left open between readings must be 

 determined for each electrode. This is accomplished by plotting cur- 

 rent readings against time between successive closings of the circuit. 

 For each measurement the same number of sec. must be allowed to 

 elapse between the circuit closing and the reading. From Figure 146 



CE 2 



■=> 



" 1 



Recess length 1.6 mm. 



pP°— 



Recess length 0.6 mm. 



10 20 30 



TIME (MIN.) BETWEEN READINGS 



40 



Fig. 146. Twenty-second current 

 as a function of time between readings 

 for two recess lengths. Bore, 25 fi. Air- 

 saturated 0.15 M NaCl. Temperature, 

 37°C. Potential, -0.60 v. vs. 0.15 M 

 calomel. From Davies and Brink 

 (1942) 



it is apparent that the readings can be taken every 20 min. with the 

 electrode having the 1.6 mm. recess, and every 10 min. with the 

 0.6 mm. electrode. 



The current, as a function of the time elapsing between closing 

 the switch and taking the reading, can be predicted on the basis 

 of diffusion theory. If the electrode recess is a true cylinder and if 

 readings are taken before the concentration gradient has extended 

 beyond the orifice, the diffusion will be one dimensional and the 

 following relationships will hold: 



.. - "fJ 



'xi2y/m 



Cr.« — 



exp (-?/2) dy 



it = nYyCAiD/irty/^ 



where Cx.i = concentration of oxygen (moles/ml.) at distance x 

 cm. from the platinum surface at t sec. after the start of diffusion 

 (closing the electrode switch) , C = initial uniform concentration of 

 oxygen, y = a, variable of integration, D = diffusion coefficient of 

 oxygen (cm.Vsec), it = electrode current (amp.) at time, t, n =z 



