LOCAL OXYGEN TENSION 407 



i = AE/yR 



where y represents the voltage gain of the amplifier and aE the 

 change in output voltage resulting from the change iR in input volt- 

 age. The grid current of the input tube of the amplifier also passes 

 through resistance R but does not affect the measurement appreci- 

 ably. The very small currents make it imperative to avoid leaks to 

 the ground, and for this reason the switch and current-measuring 

 instrument are put on the platinum electrode side of the circut. A 

 coaxial cable with polystyrene bead spacers is used as the shielded 

 lead from the electrode and the end of the glass electrode shank is 

 coated with Petrowax (Gulf Oil Co.). In this fashion leaks can be 

 held down to the negligible magnitude of 1 X lO"^-'' amp. 



Recessed Electrodes. The recessed electrodes are prepared by 

 sealing a platinum wire in a soft-glass tube of comparable inside 

 diameter in such a fashion that the tube extends beyond the end of 

 the wire to form a recess of the chosen length (recesses of 0.6-1.6 

 mm. have been used by Davies and Brink). To avoid gas bubbles in 

 the seal it is essential to degas the platinum before the sealing by 

 flaming it to white heat. The recess should be a uniform cylinder 

 whose axis coincides with that of the wire. The completed electrode 

 is annealed at 425° to prevent formation of cracks, which would 

 cause electrical leakages. In the preparation of small electrodes, 

 e.g., with a recess of 0.6 mm. length and 25 fi inside diameter, greater 

 control in the sealing requires the use of electrically heated platinum 

 loops. One loop, 0.3 mm. diameter, is used for sealing the wire into 

 a small glass tube, while another loop, 4 mm. diameter, is used to 

 seal the small tube unit into a larger glass shank. A low-power 

 microscope aids in the observation of the sealing and allows for 

 greater control of the process. The current-potential curves for 

 these small electrodes have more poorly defined plateaus than those 

 obtained with larger electrodes. 



In testing, the electrode is placed in the circuit shown in Figure 

 145, and sufficient time is allowed for the oxygen in the recess to 

 attain equilibrium with that dissolved in the solution. After setting 

 the potential the switch is closed, and the current falls as the concen- 

 tration gradient spreads into the solution. After a fixed number of 

 sec. from the time the switch was closed the current reading is taken. 

 Then the switch is opened and sufficient time is allowed for the 



