DETERMINATION OF ACIDITY AND ALKALINITY 103 



of such a concentration chain is according to Nernst's theory (the 

 details of which will be discussed later) 



E.M.F. = RT In - 

 C2 



in which formula Ci and C2 are the concentrations of the current-pro- 

 ducing ions in the two solutions. All other ionic species but those 

 that can be yielded by the electrodes (i.e., in this case all ions other 

 than H-ions) are of no significance. R is the gas constant, T the 

 absolute temperature, and In the natural logarithm. If the E.M.F. 

 is to be given in volts and the logarithm converted from the natural 

 into the common system then the value of R becomes 0.000,198,3. 

 Ci being = 1 and Ca the desired [H+] in this case, we have for any 

 temperature T in our concentration chain the relationship: 



E. M. F. = -0.000,198,3 (t + 273) log [H+] 



or for 18°C. 



E. M. F. = -0.0577 X log [H+] 



hence 



, , ^, E.M.F. 



log H+ = 



^ 0.0577 



and 



E.M.F. 



pH = 



0.0577 



Thus the measurement of the E.M.F. gives us the pH after mere 

 division by a constant (for a given temperature) factor. The details 

 of this method will be given in the part on experimental technic. 



Another method for the determination of the hydrion concentra- 

 tion is the so-called indicator method. The use of the indicators in 

 this case is quite different from that in the case of titration. The 

 indicator is simply added to the solution in question and its pH is 

 determined by the shade of color produced. This can be done in two 

 ways. One method which was carefully worked out by S0rensen^ 

 consists of matching the shade of the indicator color in the unknown 

 against a known buffer solution showing the same shade of indicator 

 color. The buffer solution is prepared by mLxing certain known 



' S. P. L. S0renseii, Biochem. Zeitschr. 21, 131 (1909). 



