282 Mechanism of Electrolytic Conduction. 



Tiinnermann (1827). A revised table was published by Hager in 

 1883, but the first table in which even the second place of decimals 

 is correctly given is that of Pickering (1894). Pickering's solutions 

 were prepared from purified material, but were standardised by titra- 

 tion, and can therefore be relied on only to the third place of decimals. 

 The re-determination of the densities was undertaken in order to 

 obtain a method of standardisation that should be more accurate than 

 titration. Quantities of sodium, amounting to about 150 grammes at 

 a time, were weighed, and converted quantitatively into concentrated 

 solutions of sodium hydroxide by the action of steam in a platinum 

 vessel. Eleven determinations, made with six different standard 

 solutions, gave, as the density of a 50-per-cent. solution, the value 

 1-5268, with an average error of O'OOOl. Solutions of known concen- 

 trations having been prepared by dilution, their densities were deter- 

 mined ; it was then possible, by measuring the density, to determine 

 the percentage of sodium hydroxide in any pure solution, with an 

 error only one-tenth as great as that introduced in standardising by 

 titration. This method contributed greatly to the accuracy of the 

 electrical measurements ; in the absence of such a method, the measure- 

 ments of some of the earlier investigators have been vitiated by errors 

 in the concentration, amounting to as much as 2 per cent, on the total 

 weight of alkali in the solution. 



(4) The determination of the conductivity at 18 C. of solutions 

 prepared in this way led to results which, although slightly different 

 from the earlier observations of Kohlrausch, gave a curve of similar 

 form. The maximum conductivity at 18 C. is 0*3490 in a 15-per-cent. 

 solution, the value given by Kohlrausch being 0-3462. At higher tem- 

 peratures the maximum conductivity is considerably greater, rising to 

 over 1-4 at 100, and occurs in solutions of greater concentration. 



(5) The viscosity of a 50-per-cent. solution of sodium hydroxide is 

 approximately seventy times as great as that of water. This increase 

 of viscosity must produce a large effect on the ionic mobility; the 

 influence of this factor may be to some extent eliminated by dividing 

 the molecular conductivity by the fluidity, and this ratio we have 

 called the "intrinsic conductivity" of the solution. Whilst the 

 molecular conductivity of sodium hydroxide solutions decreases 

 steadily as the concentration is increased, the intrinsic conductivity 

 falls to a minimum at about 8 per cent. NaOH, and then rises, until 

 at 50 per cent. NaOH, the value is considerably greater than in the 

 most dilute solutions. It is believed that this increase is due to the 

 fact that liquid soda is an electrolyte, per se, and that, in concentrated 

 solutions, the current is conveyed partly by the soda alone, as if it 

 were in the fused state. 



(6) A study of the influence of temperature on the density revealed 

 the fact that the addition of soda produces a simplification in behaviour 



