42 BIOLOGICAL CHEMISTRY 



force at the contact of an electrode with a solution containing 



RT P 



an ion of the metal is n = -log c ^ where n is the electro- 



IN Jr V-/ 



motive force in volts. - is a factor which expresses the 

 NF 



relation of the ions to the absolute temperature, the valency 

 of the ions, etc., P the solution pressure of the metal, i.e. it 

 represents the tendency of the metal to give off ions and C the 

 concentration of the corresponding ion. 



If a series of measurements are made with the same electrode 

 P remains constant and cancels from the equation, thus the 



RT C 



differences of electromotive force are n n t ^^Jog^ 1 and 



NF C 2 



for hydrogen at i8C. log, transferred from natural to Napierian 

 logarithms and using o-iN potassium chloride in the calomel 



T?T 



electrode ~ is equal to 0-0577.* 



CALCULATION OF THE CONCENTRATION OF HYDROGEN IONS 

 FROM MEASUREMENT OF ELECTROMOTIVE FORCE 



By this equation we can calculate the concentration of 

 hydrogen ions in the solution if we measure the electromotive 

 force in the system given above, using a calomel electrode 

 containing o-iN potassium chloride. The electromotive force 

 of a solution in which the molecular concentration of hydrogen 

 ions is equal to if is found to be 0-3377 volts. This substituted 

 in the equation makes it n z 0-3377 = o-o577log 10 1 /C 2 . As 

 log 10 VC is the same as logC we can put n z 0^3377 = 

 0-0577 X lg[H']. In an example in which the electro- 

 motive force is equal to 0-540 volt this substituted in the 

 formula becomes log[H'] xo-o577 = 0^540 0-3377=0-2023, 

 whence log[H']=3'5o and the concentration of hydrogen 

 ions is 3'2Xio~ 4 4 



This method requires great care and a certain amount of 

 apparatus, thus it cannot be carried out away from a 

 laboratory. 



During saturation of the electrode with hydrogen volatile 



* S. P. L. S0rensen, Ergeb. d. PhysioL, 1912, vol. 12, p. 415. 



t The concentration of hydrogen ions can be measured by other 

 methods such as the saponification of esters and where possible the 

 electrometric method should be controlled by some other method. 



t K. A. Hasselbalch, Biochem. Zeit., 1910, vol. 30, p. 317 ; G. S. 

 Walpole, Biochem. Journ., 1913, vol. 7, p. 410, and 1914, vol. 8, p. 



