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VIII. The lonization of Dilute Solutions at the Freezing Point. 



By W. C. D. WIIKTHAM, M.A., Fellow of Trinity College, Cambridge. 



Communicated by E. H. GRIFFITHS, F.R.S. 



Received February 14, Read February 22, 1900. 



INTRODUCTION. 



THE existence of a relation between the depression of the freezing point, produced 

 by dissolving an acid or a salt in water, and the electrolytic conductivity of the 

 solution thus obtained was pointed out by ARRHENIUS in 1887, and has been the 

 subject of much experiment and discussion since that date. 



As is well known, the facts of electrolysis indicate that an electric current, when 

 passing through a solution, is associated with a passage in opposite directions of the 

 constituents of the salt. FARADAY called these mobile parts ions. The number of 

 the ions depends on the chemical nature of the salt, and is usually indicated by its 

 formula. Thus for one molecule of potassium chloride we have two ions, the 

 potassium travelling in one direction and the chlorine in the other. For barium 

 chloride or sulphuric acid we have three ions, and, since the electric charge of an ion 

 is proportional to its valency, the electrically equivalent weights of these substances 

 are represented by BaCl 8 and H 2 So 4 , respectively. 



The freezing point of water is depressed by equal amounts when molecularly 

 equivalent weights of various non-electrolytes, such as cane-sugar or alcohol, are 

 dissolved in it. Equivalent quantities of electrolytes, however, produce greater 

 effects. Thus a gramme-molecule of potassium chloride causes nearly twice as much 

 depression as a gramme-molecule of sugar, while the effect of barium chloride or 

 sulphuric acid is almost three times as great as that of a non-electrolyte. It will be 

 noticed that the molecular depression of the freezing point is approximately propor- 

 tional to the number of ions into which we must suppose the salt resolved in order to 

 explain its electrical properties. 



Again, if the concentration ot a solution be diminished by continual dilution, it is 

 found that the conductivity decreases at a slower rate, so that a rise occurs in the 

 value of the equivalent conductivity, a quantity which is defined as the conductivity 

 of the solution divided by its concentration expressed in electrical gramme-equivalents. 

 This rise continues till the dilution has reduced the concentration to a value of about 

 the ten-thousandth of a gramme-equivalent of the solute per litre of solution, after 



VOL. CXCIV. A 259. 2 T 20.7.1900 



