32 



OXIDATION-REDUCTION POTENTIALS 



Salt Bridges 



A salt bridge is used in order to connect the experimental half-cell with the 

 standard half-cell. A convenient bridge consists of an inverted glass U-tube of 

 internal bore about 3 mm. One limb of the bridge is turned up at the tip, in order 

 to minimise downward diffusion of KCl from the agar to the culture, and this limb is 

 plugged into a test-tube with cotton wool. The tip of the other limb is provided 

 with a temporary cotton-wool plug during hot-air sterilisation. Sterile 2 per cent, 

 agar saturated with potassium chloride is melted in a steamer, and the agar is then 

 drawn into the bridge by suction until it is nearly full. Bubbles in the turned-up 

 tip are expelled by momentary immersion in boiling water, and the tip of the other 

 limb is filled with saturated potassium chloride. In use the agar-filled limb with 

 the turned-up tip is immersed in the oxidation-reduction system studied, and the 

 other limb makes contact with a vessel containing saturated KCl solution. The 

 standard half-cell also is connected to the KCl reservoir. It would appear that the 

 use of sintered glass tips for agar bridges would be useful. 



Electrode Vessel 



The design of the vessel containing the oxidation-reduction system studied 

 depends, of course, on the purpose and conditions of the experiment. If it is desired 

 to maintain anaerobic conditions by bubbling purified nitrogen through the systems 

 a vessel provided with a rubber bung, such as that described by Clark and Cohen 



Connection to 

 \ Ridley's apparatus 



Fig. 7 



Eh OF Bacterial Culttjhes 

 (Arrangement of apparatus in incubator) 



(1923, 2), may be used. In the author's experiments aerobic bacterial culture, 

 have been studied, and for this purpose glass double-limbed cells of 12 mm. internal 

 bore proved most satisfactory (see fig. 7). These are hot-air sterilised and, when 

 the culture medium has been pipetted in and inoculated, the cotton-wool plugs are 

 replaced by the sterilised electrode in one limb and the salt bridge in the other. 

 Hollow wooden blocks, preferably provided with masts, support the cells. In some 

 experiments the cultures are kept aerated and agitated by Ridley's apparatus (1928) 



