LAWS OF ELECTROLYTIC DISSOCIATION 27 



The technic of the concentration chains has been worked out 

 recently in great detail, and it must be considered as the most 

 accurate of the methods. The most reliable measurements were 

 made by S. P. L. S0rensen-^ and yielded the average figure of 



kw = 0.73 X lO-i" = lO-i"!* at 18° 



Michaelis-^ using the same method obtained the figures given 

 in table 7. They represent the results of a large number of experi- 

 ments and of graphic interpolations. 



In the same way by determining the pH of NaOH solutions, whose 

 pOH was calculated from conductivity data, Lewis, Brighton and 

 Sebastian^s found at 25°C. 



kw = 1.012 X 10-1*; pk^ = 13.995 



Hence for pure water at 25° we have the practically exact values 

 [H+] = 1.00 X 10-^ or pH = 7.00. 



The temperature coefficient of kw may be calculated from van't 

 Hoff's equation (called the "reaction isochore" by Nernst) which is 



dlnkw U 



dT RT2 



By substituting the corresponding numerical values and by con- 

 verting into common logarithms, the following expression is obtained 

 for 20°C.: 



dlnkw 13700 



dT 4.571 • (293)2 



in which we replace U, the heat of the reaction H+ + 0H~ = H2O, 



" S. P. L. S0rensen, Biochem. Zeitschr. 21, 131 (1909). 

 " See first edition (1914) of this book. 



25 Lewis, G. N., Brighton, T. B. and Sebastian, R. L., Journ. of the Amer. 

 Chem. Soc. 39, 2245 (1917). 



