ELECTRODES 31 



cracked or faulty seal probably due to direct electrical contact of mercury witb. 

 the solution studied. Cannan, Cohen and Clark (1926) in one case used platinised 

 platinum electrodes in biological work, but there is no apparent advantage 

 in this for electrode potential studies, except in unstable systems (Ball and Chen, 

 1933). 



(2) Gold. — Gold-plated electrodes have been used, but other workers 

 claim that solid gold is of greater value (Dixon and Quastel, 1923). The author 

 used gold foil and gold wire electrodes, but in neither case did he obtain the 

 reliable, reproducible values observed with platinum. Gold has been recom- 

 mended for use with the quinhydrone half-cell but platinum seems very satis- 

 factory. The chemical inertness of platinum should make it preferable for an 

 imattackable electrode. Lepper and Martin (1931) found that gold did not 

 give the low potential observed with iridium in the presence of hydrogen gas. 

 Longsworth and Maclnnes (1935, 1936) are exceptional in preferring gold to 

 platinum electrodes for the investigation of bacterial cultures. 



(3) Iridium. — Lepper and Martin (1930) used electrodes of iridium, which 

 is a very inert metal similar in many respects to platinum. 



(4) Tungsten. — Tungsten electrodes were found unsuitable by Flexner 

 and Barron (1930). 



(5) Graphite. — Impure graphite electrodes were found to be quite useless, 

 and pure graphite electrodes had to be replaced after each experiment (Flexner 

 and Barron, 1930), but Tuttle and Huddleson (1934) used special Acheson 

 graphite electrodes successfully. 



(6) MercMry.— Dropping mercury electrodes, in which the surface can 

 be constantly renewed (Clark, Cohen and Gibbs, 1925), have been used, but 

 mercury appears to participate readily in many biological systems (Michaelis 

 and Flexner, 1928). Mercury electrodes give different results from platinum in 

 many cases and probably do not behave as " unattackable " electrodes. If a 

 platinum electrode in a culture be cracked and the mercury come into direct 

 contact with the culture medium the potential is entirely different from that 

 of the platinum electrode (Hewitt, 1930, i.). The dropping mercury electrode 

 is used largely in the polarograph (Chapter V). 



(7) Hakamori (1931), investigating hydrogen peroxide solutions, preferred 

 palladium electrodes to gold or platinum. 



(8) Various. — Clark and Cohen (1923, 2) mention the use of rhotanium alloy, 

 burnt-on platinum, and amalgamated burnt-on platinum electrodes in dye 

 systems, and found general agreement with other electrodes. 



It is concluded therefore that no more suitable electrode than bright platinum 

 has been described, and these are readily prepared and cleaned, and yield steady, 

 reproducible readings. After use, the author immerses the electrodes in hot nitric 

 acid. Platinum wire electrodes sealed into glass tubing of about 3 mm. internal bore 

 may be held in test-tubes with cotton-wool plugs for hot-air sterilisation, and trans- 

 ferred from the test-tube to the apparatus when required. It is convenient to 

 select test-tubes for this purpose of the same bore as the orifice in the apparatus 

 into which the electrode is eventually to be placed. When an air-tight apparatus is 

 to be used, the electrode, etc., must be placed in a varnished rubber bung. 



