34 



OXIDATION-REDUCTION POTENTIALS 



Therefore, using the quinhydrone electrode with buffer solution, the appropriate 

 values given in the table are to be added to the observed potential to obtain E,,. 

 Quinhydrone electrodes should be re-assembled with fresh buffer solution daily. 



A combined vacuum electrode and half-cell is described by Lehmann (1930). 



The quinhydrone electrode is not a permanent standard and an electrode coming 

 into very common use is the silver : silver chloride electrode which has a potential 

 45 millivolts more negative than the calomel electrode. 



Potentiometer 



The potentiometer to be used should be calibrated to ^ 1 volt, reading to the 

 nearest millivolt. Greater accuracy than this is unnecessary in biological systems. 

 The Cambridge Instrument Company supply a portable instrument which is cali- 

 brated as above and does not require the use of a standard cell. 



Galvanometer 



The choice of a suitable null-point instrument for detecting the point of potentio- 

 metric balance is of the greatest importance in this work. Biological systems are 

 not well poised and measurements are most sensitive to errors due to polarisation. 

 Even a minute current flowing through the cell will seriously disturb the results. 

 Clark and Cohen (1923, 2) use a sensitive high resistance galvanometer for the 

 purpose. 



The author has found the Lindemann electrometer (1924) an excellent null 

 point instrument with which polarisation effects were not observed. The needle 

 can be kept charged whilst potentiometric balance is being obtained, which saves 



-i 



ELECTRODESi 





EARTH 



\/^ J ' — rwAp 



POTENTIOMETER 



x[ xlOO 



WWW^ — QO L 



* R PH MV 



Fig. 8 

 Thermionic valve electrometer circuit 

 (Block lent by Cambridge Instrument Company) 



a great deal of time. Some trouble has to be expended in both the maintenance 

 and use of this electrometer, but the results obtained with it have been of considerable 

 value. 



The rapid development of the thermionic vaRe during recent years has led to 

 the increased use of valve circuits for electrometric purposes. Wurmser (1930) 

 describes a suitable circuit for the measurement of potentials, and Knight (1930) 

 used a Harris (1928) circuit. Modern improved electrometer valves have become 



