THE MEASUREMENT OF 0/R POTENTIALS ix^s-l* 



THE POTENTIOMETRIC METHOD 



Electrode vessel. This may be a test tube with a constriction and 

 bulb at its lower end or a more elaborate container depending on the 

 requirements of the experiment. Such vessels are described by 

 Clark, Cohen, ct al. (1928), Borsook and Schott (1931), Allyn and 

 Baldwin (1932), and Hewitt (1936). 



Electrodes. A "noble" or "unattackable" metal is the electrode of 

 choice. A coil of bright platinum wire has been frequently employed, 

 but this is difficult to clean thoroughly and there is danger of entrap- 

 ment of particulate material during a measurement. Platinum sheet, 

 about 5 mm. square or larger, is preferable. 



Gold-plated platinum electrodes seem to have certain advantages. 

 They can be readily replated to provide a clean, new surface and 

 thereby obviate erratic electrode behavior. Secondly, gold, being 

 relatively impervious to hydrogen, should have less tendency to act 

 as a hydrogen electrode in a culture producing appreciable quantities 

 of molecular hydrogen. However, some observers do not consider 

 this of much practical importance. 



Electrodes should be checked for reliability by measuring the potential of a known 

 oxidation-reduction system (e.g., quinhydrone in 0.1 M HCl, Eh =0.6351 at 25°, see 

 p. 4.) Where possible, duplicate or multiple electrodes should be employed; and one 

 that exhibits persistent erratic behavior should be discarded. Unless the solution or 

 culture under examination is well stirred, the electrode reading may record merely 

 a local oxidation-reduction potential rather than one representative of the solution as 

 a whole. In a heavily growing culture, electrodes may become coated with adherent 

 cell masses, and duplicate electrodes may show widely divergent potentials even when 

 the culture is well stirred. 



The common method of cleaning a platinum electrode involves cautious treatment 

 with aqua regia, or hot concentrated nitric acid, or hot bichromate cleaning mixture, 

 followed by thorough washing in water. For careful oxidation-reduction work, this 

 procedure may not leave the metal surface altogether "inert". A more suitable pro- 

 cedure is to electrolyze a 1:1 solution of concentrated HCl with the electrode to be 

 cleaned as the anode (gold-plated platinum may be deplated in the same way). The 

 well washed electrode may be stored in distilled water. If the metal surface remains 

 dry for any length of time, the electrode may be sluggish in reaching an equilibrium 

 potential. 



Calomel half-cell. (See also p. 3). The "saturated" type of any 

 convenient form is generally suitable, preferably one that permits 

 flushing of the siphon outlet with saturated KCl solution in order to 

 wash away contaminations from liquid junction contacts. Liquid 

 junction between the calomel half-cell and culture should be of a 

 kind which can be made aseptically when desired. For ordinary 

 purposes, this is conveniently accomplished by preparing a glass 

 tube partly sealed at one end over a piece of acid-washed asbestos 

 fiber. This tube is filled by means of a capillary-tipped pipet with 

 melted KCl-agar (40 g. KCl per 100 ml. of 3% agar in water) and 

 autoclaved. The partly sealed end of the tube is inserted into the 

 culture to provide the "liquid" junction, and the open end is placed 

 in bubble-free contact with saturated KCl solution leading to the 

 calomel half-cell. 



Potentiometer and galvanometer. Generally speaking, cell suspen- 



