164 PHYSIOLOGY OF MICROORGANISMS 



where 



PH the term adopted by S. P. L. Sorensen to express the exponent 



of gm.- equivalent of hydrogen ions per liter. 

 P the total E. M. F. of the chain. It can be determined by the 

 following equation, having the apparatus arranged as it is 

 shown in the diagram: 



P = - ' - - in which RI the bridge reading for the chain against 



an accumulator. 



R the bridge reading for the ac- 

 cumulator against the normal ele- 

 ment. 

 1.0189 the voltage of the normal element at 



1 8 (standard). 



0.3377 the sum of potential of calomel electrode (N/io KC1) and 

 hydrogen electrode in a solution where the hydrogen 

 concentration is normal (H) = i or PH = o. 



0.0577 thermodynamical factor at 18 which is influenced by tem- 

 perature, 0.0002 for each degree centigrade, or it changes 

 as follows : 



0.0577 + 0.0002 (t 1 8), of which t equals temperature 

 at the time of determination. 



After PH is determined it is necessary to understand the value of 

 H-ion concentration, although the experimental results are generally 

 expressed in P H . It will be shown at the end of an example, illustrating 

 the application of the formula as well. 



Example. 



t = i9.2C (constant during the experiment). 

 RI = 307.0 (constant reading on the bridge at five minute 



interval). 



R = 500.2. (as above). 

 M. F. of the normal element = 1.0189. 



