92 MANUAL OF MICROBIOLOGICAL METHODS 



but this is difficult to clean thoroughly, and there is danger of entrapment 

 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. Second, 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 prac- 

 tical 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 

 page 74). 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, 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 page 74). 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 of KCl per 100 ml of 

 3 per cent 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- 

 sions and bacterial cultures are poorly stabihzed with respect to oxida- 

 tion-reduction potential. Consequently, disturbing polarization may 

 occur if even the small amount of current necessary to operate the usual 



