42 



OXIDATION-REDUCTION POTENTIALS 



use of saturated potassium chloride as a liquid junction may be undesirable. To avoid 

 these difficulties the author has used a special arrangement, shown diagrammatically 

 in Fig. 11. 



to NaC! 

 haif-celi 



Fig. 11 

 Unconventional Micro-biological Electrode 



The usual small-bulbed glass electrode is protected by being sheathed with a 

 piece of glass tubing of sHghtly larger diameter than the bulb. The sheath tube is 

 chipped away one side or perforated higher up to give access to fluid. The connection 

 with the standard half-cell is by means of a capillary pipette opening just above the 

 bulb and also encased in the sheath tube. The standard half -cell may be the usual 

 simple calomel or silver-chloride type, but the fluid is physiological saHne instead of 

 saturated potassium chloride, so that the saUne can be used to flush and clean the 

 glass electrode. The theoretical objection to this is that liquid : liquid junction poten- 

 tials are not as low as with saturated potassium chloride solution, but in practice, by 

 using for the initial calibration of the meter a solution similar to that of the test 

 solution, difficulties have not been encountered. The advantages of this theoretically 

 indefensible electrode are : — 



(1) A very small amount of test solution is required. 



(2) The sheathing tube eliminates bulb breakage. 



(3) By using saUne in the half-cell much time is saved in flushing and cleaning 

 the electrode and KCl does not contaminate the sample. 



(4) The apparatus is readily sterilised after use with cultures. 



(5) The set-up is simple, cheap and robust. 



COLORIMETRIC pH METHODS 



The technique of colorimetric pH measurements is so general that little needs to 

 be said. The greatly extended availabiUty of photoelectric colorimeters and 

 spectrophotometers may lead to their use in pH determinations especially by those 

 who are colour-blind and have difficulty in colour matching. Fig. 12 gives the 

 absorption curves of phenol red in acid and alkaline solution measured with 5 cc. of 

 solution in a diffraction grating spectrometer (Unicam) and, for comparison, in a 

 photoelectric colorimeter with Ilford colour filters. 



