1902.] 



Conductivity of Electrolytic Solutions. 



43 



the assumption that the " mobility " of the ions which carry the cur- 

 rent is independent of the dilution, and is probably subject to serious 

 errors when applied to solutions containing more than one equivalent 

 of solute in 100 litres. The second method is based on the determina- 

 tion (usually from the boiling point or freezing point) of the osmotic 

 pressure of the solution. The " active " part of the solute produces 

 an osmotic pressure n times as great as that produced by an equi- 

 molecular quantity of an " inactive " solute, n being the number of 

 ions into which the molecules of the solute would be decomposed on 

 electrolysis. The coefficient of ionisation is deduced from the equa- 

 i — 1 



tion ol = , in which the factor i represents the ratio of the 



71-1 



observed osmotic pressure to that calculated for an inactive solute at 

 equal dilution. This method of measuring a is also valid only in 

 dilute solutions, is probably subject to error when applied to solutions 

 containing more than one equivalent of solute in 10 litres. 



In the majority of cases the magnitude of the coefficient of ionisa- 

 tion is found to decrease as the temperature rises, and the primary 

 effect of an increase of temperature is therefore to reduce the amount 

 of active material in the solution — an effect which may be attributed 

 to the gradual disappearance of the "ionising power " of the solvent. 

 But whilst the coefficient of ionisation usually decreases as the tem- 

 perature rises, the equivalent conductivity at infinite dilution in- 

 variably increases. This is due to an increase in the "mobilities," 

 u and v, of the kathion and anion, which together make up A. x , and is 

 intimately related to the decreasing viscosity of the solution, which 

 allows the migration of the ions to proceed more rapidly as the tem- 

 perature rises. The effect of temperature on the equivalent conduc- 

 tivity of an electrolytic solution is therefore determined by two 



opposing influences, and the temperature coefficient ? C ~ will be 



A ClT 



- or + according as one or other of these influences predominates. 



In the case of aqueous solutions, the temperature coefficient at 18° C. 

 is always positive, and usually amounts to about 2 per cent, of the 

 conductivity at 18° per degree Centigrade. The conductivity tem- 

 perature curves are very flat (compare fig. 1), and have frequently been 

 represented by linear formulae such as A f =A (1 + af), but may often 

 be more accurately represented by a parabolic formula such as 

 X t = a (1 + at + fit' 2 ). If the linear or parabolic curves be produced 

 in the direction of decreasing temperature, they would cut the axis 



* The temperature-coefficient - — of specific conductivity is not identical with 

 k (It 



1 A \^ — § + approximately, where c is the coefficient of expansion 



A dr A. dr k dr 



of the solution, 



