CONDUCTIVITY OF SODIUM HYDROXIDE IN AQUEOUS SOLUTION. 287 



1 000 ohms and as low as 5 ohms. The resistance of the leads was reduced as far as 

 jxwsible by employing stout copper rods to connect the bridge with the vessel, and by 

 using copper enclosed in glass, rather than platinum, for the leads inside the conduc- 

 tivity vessel ; there was, however, some loss of accuracy when the resistance to be 

 measured was much below 10 ohms. To measure resistances above 500 ohms it was 

 necessary to introduce a correction for the slight electrostatic capacity of the coils of 

 the resistance box ; the correction was determined by stretching a thin wire resistance 

 of 2000 ohms or 3000 ohms over the ceiling in such a way as to be free from capacity 

 and then measuring its apparent resistance (1) by a direct current and galvanometer, 

 and (2) by an alternating current and telephone ; the difference gave the correction 

 to be applied to the readings of the telephone, and this correction was found to be 

 proportional to the resistance measured. 



Taking into account the various sources of error to which the measurement of the 

 electrolytic conductivity of solutions of varying concentrations is liable, we consider 

 the general accuracy of our tabulated results for 18 C. may be taken to be within 

 1 part in 1000, although an inspection of Diagram VII., in which all the observations 

 are set out, shows that there are some individual readings which are below this 

 standard. 



The results obtained in measuring the conductivity of aqueous solutions of sodium 

 hydroxide at 18 C. are shown in Tables XII. and XIII., and are plotted out in 

 Diagram VII., together with the values given by KOHLRAUSCH. The maximum 

 conductivity, * 18 = 0'3490, is at about 15 per cent. NaOH, but there is no substantial 

 alteration of conductivity between 14 -9 per cent, and 15 '6 per cent. KOHLRAUSCH'S 

 value for 15 per cent, is 0'3463. For a normal solution LOOMIS gives /3i 8 = T0418, 

 K 18 = 0'145, whilst our values are p lg = 1 '04172, * 18 = g 1575. 



The values given by KOHLRAUSCH are shown on Diagram VII. by open circles, 

 plotted out in reference to the percentages given by him. If the position of these 

 circles be shifted to the percentages deduced from the densities given by .KOHL- 

 RAUSCH, most of the observations come very nearly on to our own curve, but the 

 unconnected values differ by as much as 0'0053 on 0*1560 at 35 per cent. NaOH. 

 KOHLRAUSCH'S value for the maximum, which should not be affected by errors of 

 concentration, differs from ours by nearly 1 per cent., and it is therefore probable that 

 his solutions contained appreciable quantities of neutral impurities.* 



* Prior to the introduction of metallic sodium as a source of purified caustic soda this alkali was made 

 by the action of lime upon sodium carbonate. In concentrated solutions the interaction 



2NaOH + CaCO, 



is probably reversible and the solution would therefore contain a definite proportion of sodium carbonate, 

 in addition to traces of calcium salts. In this way it is possible to account for the wide difference that 

 exists between the densities tabulated by SCHIFF, KOHLRAUSCH and HAGEN, and the later observations, 

 tabulated by PICKERING, and those in the present paper, in which the alkali was prepared from metallic 

 sodium. 



