LOCAL OXYGEN TENSION 



409 



no. of electrons used per molecule of oxyge nelectrolyzed, Fy = 1 

 faraday (96,500 coulombs), A = area of a platinum surface (cm.^). 

 From the preceding relationship the current is inversely propor- 

 tional to the square root of the time (t). When the time is increased 

 to the degree that the gradient extends into the solution outside the 

 recess, the oxygen diffuses to the tip of the electrode from all direc- 

 tions instead of from only one, and this results in a concentration 

 at the orifice higher than that for the linear diffusion. Thus the 

 current is increased beyond that expected from the l\/[ relation. 

 This is illustrated in Figure 147, which shows the linear relationship 



Fig. 147. Current-time curve for 

 electrode no. 11. Air-saturated 0.15 M 

 NaCl. Temperature, 25 °C. Potential, 

 —0.60 V. vs. 0.15 M calomel. From 

 Davies and Brink (1942) 



14 



12 - 



E 8 

 S' 6 



UJ 



cc 



§ 4 

 o 



2 - 



01 0.2 0.3 



TIME, l/vT 



0.4 



up to 40 sec. Although the profile of the current-time curve is in- 

 fluenced by the shape of the recess in the electrode, the current at a 

 given time after closing the circuit is still proportional to the oxygen 

 tension as long as there is no change in T>. Thus, if the concentration 

 gradient is confined to the recess when the current is measured, only 

 the length of the recess will be important and it will not matter 

 whether the cross-sectional area of the recess is uniform. 



The distance from the platinum surface to which the concentra- 

 tion gradient should extend at times after the onset of electrolysis 

 is shown in Figure 148. Up to 20 sec. the gradient extends to 1 mm. 

 from the platinum surface. 



In Figure 149, the electrode current at intervals after the start of 

 diffusion is shown as a function of the initial uniform oxygen tension 

 of the solution in recess after equilibration with known gas mix- 

 tures. While the electrodes are calibrated at the temperature of the 



