liOCAL OXYGEN TENSION 



405 



animal tissues can be determined with a spatial resolution of 25 /x. 

 In one electrode the end of a platinum wire is recessed inside a 

 cylindrical glass tip; this instrument may be used to measure abso- 

 lute oxygen tensions as often as once every 5 min. In the other elec- 

 trode the end of the wire is directly exposed to the outer medium; 

 with this electrode relative oxygen tensions may be recorded con- 

 tinuously. An idea of the applicability of the method may be gained 

 from the work of Davies and Brink, who employed the electrodes for 

 the measurement of the oxygen tension at the surface of superficial 

 arterioles and venules of the cat cerebral cortex, in the cortical 

 substance, at the surface of muscle cells, and at chosen distances 

 from the surface of unicellular organisms. 



The principle of the method is based on the reduction at a plati- 

 num electrode of the dissolved oxygen according to the following 

 equation: 



2H+ + O2 + 2e -> H2O2 



and the measurement of the currents developed when suitable 

 potentials are applied, the currents being proportional to the oxygen 

 tension. 



The variation in electrode current with applied potentials is 

 illustrated in Figure 144. The currents in the plateau region of the 



Fig. 144. Current-voltage curve for 

 recessed electrode no. 11 in air-satu- 

 rated 0.15 M NaCl. Temperature, 

 37°C. Recess length, 1.0 mm. Bore, 

 0.176 mm. Each point is the value of 

 current 20 sec. after closing the cir- 

 cuit. Re-equilibration time, 20 min. 

 between measurements. Fro?n Davies 

 and Brink (19^2) 



-02 -0.4 -06 -08 -1.0 

 POTENTIAL iVOLTS: us. 15 M CALOMEL 



curve are limited by the maximum rate at which oxygen can diffuse 

 to the cathode, and this rate is proportional to the oxygen tension. 

 Also, the effect of change in the applied potential is a minimum in 

 the plateau region, and, accordingly, it is in this region that the 

 potentials are chosen for the measurement. The increase in the 



