730 HARMED ART. M 



Further, we mention the cell, 



H2 1 M0H(W2) 1 MxHg I MOH(wi) 1 Ha, 



which measures the transfer corresponding to 



M0H(w2) + H20(mi) -> MOH(wi) + HzOK), 



whence the activity coefficients of alkali metal hydroxides may 

 be measured. By other cells of the same types, alkali metal 

 sulphates and alkaline earth chlorides have been studied. All 

 these data have an important bearing on the theory of electroly- 

 tic solutions.* 



Not only may we obtain these changes in chemical potentials 

 for single electrolytes by these measurements, but also the 

 chemical potentials of one electrolyte in a solution containing 

 another electrolyte may be computed. From the cell, 



Ag I AgX 1 HX{mO, MXim^) \ H2 1 HX(m) \ AgZ 1 Ag, 



we may measure the change of thermodynamic potential of a 

 halide acid from the solution containing the chloride to the 

 pure acid solution, which we represent by 



HX(mi) [MXim^)] -^ HX(m). 



Thus, we may obtain the activity coefficient of the acid at a 

 concentration (wi) in a salt solution of a concentration (wz). 

 Suffice it to say that by similar cells we now know the value of 

 this important quantity for hydrochloric acid, sulphuric acid, 

 and hydrobromic acid in many salt solutions, f Further, cells of 

 the type, 



H2 I MOH(wi), MZ(m2) | MxHg | MOH(w) | H2, 



permit the calculation of the activity coefficients of hydroxides 

 in salt solutions. I 



* Knobel, /. Am. Chem. Soc, 45, 70 (1923). Harned, /. Am. Chem. 

 Soc, 47, 676 (1925). Harned and Swindells, J. Am. Chem. Soc, 48, 126 

 (1926). 



t Harned, /. Am. Chem. Soc, 38, 1986 (1916); 42, 1808 (1920). Harned 

 and Akerlof, Physik. Z., 27, 411 (1926). 



t Harned, /. Am. Chem. Soc, 47, 684 (1925). 



