CHAPTER VII 



ELECTROLYTES AND THEIR ACTION 



ELECTROLYTIC DISSOCIATION 



IN the researches of De Vries on plasmolysis (1884, 1885, 1888), it was found 

 that, if sugar in a certain molar concentration was just sufficient to produce a 

 result, a number of other substances, such as mannitol, etc., in the same molar 

 concentration also produced the same effect. On the other hand, another group, 

 sodium chloride, potassium nitrate, etc., produced the effect in a molar 

 concentration which was lower than that of sugar. The relative concentrations 

 of the various substances of the latter group which were plasmolytically, i.e., 

 osmotically, equivalent to that of the former group, were expressed in a series 

 of numerical values, the isotonic coefficients. 



Raoult (1878, etc.) found similar facts in his investigation of the freezing 

 points of the different solutions in question, and they were also expressed in 

 terms of osmotic pressure by van't Hoff (1885). The symbol i will often be 

 met with as expressing the number by which the osmotic pressure of a substance 

 such as cane-sugar must be multiplied in order to obtain that of an equimolar 

 solution of the particular substance in question to which the given value of i 

 refers. The following table given by Philips (1910, p. 132), from the data 

 in the paper by van't Hoff arid Reicher (1889), gives a few values of i calculated 

 (I.) from the depression of the freezing point, (II.) from the plasmolytic experi- 

 ments of De Vries, and (III.) from electrical conductivity. The meaning of the 

 third column will be seen later. 



Now, when we remember that the osmotic pressure of a solution is in direct 

 proportion to the number of molecules of the solute present in unit volume, we 

 see that, apparently, a smaller number of molecules of the second group of 

 substances produces the same effect as a larger number of the first group. If 

 we make a solution by adding 1 gram-molecule of potassium chloride to 

 10 litres of water, we find that its osmotic pressure is about 1'8 times that 

 of a solution made with 1 gram-molecule of cane-sugar. 



It is quite clear, therefore, that there are more osmotically active elements 

 in the first solution (potassium chloride) than in that of cane-sugar. Since 

 that of the latter solution corresponds to the number of molecules taken, it 

 follows that, in the former case, the number of active " molecules " has somehow 

 increased. In other words, the molecules must have been split up so as to 

 make a larger number. 



When the molecules of gases, such as chlorine at a high temperature, are 

 found to be split up into atoms, so that they no longer appear to obey 



169 . 



