REACTIONS IN HOMOGENEOUS SYSTEMS 33 



method is about 0-005 of a degree corresponding to 45 milli- 

 metres of mercury, hence wherever possible, as for example 

 in measuring the osmotic pressure of protein solutions, the 

 direct method should be employed rather than the indirect. 



IONIC DISSOCIATION 



Certain substances in watery solution show marked depart- 

 ures from the above generalisations. It has been found that 

 although many substances behave normally in regard to 

 osmotic pressure measurements, other substances give either 

 too low or too high results. The low results are ascribed to 

 association, in which two or more molecules unite to form 

 single particles ; thus there are fewer particles in the solution 

 and a lower osmotic pressure than should correspond to the 

 number of molecules dissolved. 



The substances which give too high results are those which 

 aid the passage of the electric current. These are assumed to 

 break up into two or more particles which are electrically 

 charged (ionic dissociation) . These substances vfiiich conduct 

 electricity are called electrolytes, and those which do not 

 conduct electricity are called non-electrolytes. 



The Ionic Dissociation Theory* can be illustrated by the 

 behaviour of sodium chloride solution. Measurements of 

 osmotic pressure show that a certain proportion of the mole- 

 cules break up into two ions, sodium and chlorine, so that 

 there are more particles than correspond to the number of 

 molecules of the salt in the solution but less than twice that 

 of the number of molecules. The sodium chloride dissociates 

 completely into positively charged sodium ions (Na + ) and 

 negatively charged chlorine ions (Cl~~) only in infinitely dilute 

 solution. The total number of molecules being represented 

 by unity the value two is found in very dilute solution. In 

 more concentrated solution the dissociation is partial and 

 it is represented by a. The number of particles formed by 

 complete dissociation of a molecules is 2a (a, sodium ions and 

 a chlorine ions) and there are I a molecules undissociated ; 

 hence the total number of osmotically active particles in 

 the solution will be i a + 2a = i + a. This figure 

 indicates the osmotic pressure of a solution of a binary elec- 

 trolyte which dissociates to a certain extent. f We shall now 

 show that measurements of electrical conductivity give the 

 same value of (a) for the degree of dissociation. 



* Sv. Arrhenius, Zeit. f. physik. Chem., 1887, vol. i, p. 631. 

 f Salts which decompose into more than two ions would give more 

 particles in the solution. 

 3 



