32'2 MR. W. C. D. WHKTHAM ON THE IONIZATION OF 



which the equivalent conductivity reaches a limit. Beyond this point further dilution 

 causes no corresponding rise, and in the case of acids and alkalies eventually leads 

 to a sudden decrease in the equivalent conductivity. 



These variations are usually referred to changes in the percentage amount of the 

 dissolved substance which is actively concerned in conveying the electric current, 

 and it is easy to calculate what proportion of the whole quantity of such substance 

 must be active in any given solution, for, if no other condition changes, it is 

 measured by the ratio of the equivalent conductivity of the solution to its maximum 

 value. This ratio is therefore known as the coefficient of ionization. 



In examining the freezing point phenomena, very similar variations are noticed. 

 The molecular depression is relatively greater in those solutions where the ionization 

 is complete, or nearly complete, and relatively less where the conductivity shows that 

 more of the salt is electrically inactive. 



Whether this relation between the electrical ionization and the depression of 

 freezing point is exact or only approximate is one of the most important questions 

 in the present state of the theory of solution, and, although an enormous amount of 

 experimental work has been devoted to the subject, it is still a question the answer 

 to which remains uncertain. 



This uncertainty arises from two causes. The first is the difficulty of determining 

 the extremely small freezing-point depressions of very dilute solutions, and the 

 second is the fact that most of the comparisons have been made with the ionization 

 of solutions measured by KOHLBAUSCH, OSTWALD and others at temperatures of 

 18 or 25 Centigrade. Now, unless the temperature coefficients of conductivity are 

 the same for different concentrations of a solution, the ionization at 18 will not be 

 the same for a definite concentration as the ionization at 0, with which it is evident 

 that the freezing point phenomena should be compared. 



As far as is known to the present writer, the only measurements of the ionization of 

 electrolytic solutions which have been made at are those of R W. WOOD.* He 

 finds that the ionization of solutions of potassium chloride, sodium chloride, dichlor- 

 acetic acid, and trichloracetic acid is, as long as the dilution is great, sensibly the 

 same at as it is at 18, or, at all events, that the difference between the values at 

 these temperatures is not enough to bring the conductivity measurements into 

 harmony with the freezing point values. The observations were only carried to a 

 dilution such that one gramme-equivalent weight would be contained in 1024 litres. 

 This is insufficient to give the limiting values of the molecular conductivity directly, 

 so " the value expressing the conductivity of infinitely dilute solutions was calculated 

 by means of the temperature coefficient from the values found by KOHLBAUSCH and 

 OSTWALD for 18 and 25." 



Apparently this process must involve a certain amount of extrapolation, for the 



* 'Zeitschrift fur Phys. Chemie,' vol. 18, p. 3, 1895. Translated in the 'Phil. Mag.,' vol. 41, p. 117, 

 1896. 



