CONDUCTIVITY OF SODIUM HYDROXIDE IN AQUEOUS SOLUTION. 285 







electrode must have been very slight. The surface of the platinum pot was used as 

 the second electrode.* 



(:{.) Calibration of the Electrode. The majority of the measurements at 18 C. 

 were made with an electrode having the form shown in fig. 5. The required resist- 

 ance capacity was obtained by suitably fixing the length and cross section of the 

 tubes leading outwards from the electrode. The electrode most used was one which 

 had a resistance capacity of 13*74. For calibrating it a normal solution of potassium 

 chloride was used and two series of measurements gave the capacity as 



(I.) 13738 with 200 cub. centims. of solution in the pot, and 

 (2.) 13735 with 300 cub. centims. of solution. 



A very valuable check on the accuracy of the calibration was obtained by com- 

 paring the values for the specific conductivity of the . solutions as deduced from 

 measurements with different electrodes. Using a carefully calibrated electrode of the 

 type described in Part VI. of the paper as a standard, the capacity deduced from 

 measurements of the maximum conductivity of caustic soda was 0'05 per cent, higher, 



* Fouling of the Solutions. (1.) The possibility has been suggested that the electrical measurements might 

 have been vitiated by impurities introduced by the contact of the soda with the glass of the electrodes. 

 Such fouling might have occurred if the electrode figured above had been heated with the solution during 

 several hours. This, however, was not the case, as this electrode was used exclusively for low-temperature 

 measurements, and was removed from the solution when not in use. At atmospheric temperatures the 

 fouling is exceedingly slow, and the experiments quoted on p. 284 may be regarded as affording 

 experimental justification of the view that no appreciable fouling could have taken place in the short 

 period of time during which the electrode was in contact with the solution. 



(2.) The advantages which we believe to have resulted from the use of a platinum vessel with glass- 

 cased electrode over a glass cell of the ordinary kind may be briefly indicated : 



(a) It is reasonable to suppose that under comparable conditions the total fouling is proportional to 

 the surface of glass exposed to the 'solvent action of the soda, but is not directly dependent on 

 the volume of the solution. If this be so, the percentage error due to fouling would be inversely 

 proportional to the volume of the solution, and by using 300 cub. centims. or 400 cub. centims. 

 in place of 10 cub. centims. or 20 cub. centims. the risk of fouling would be proportionately 

 reduced. The electrode was used as a stirrer and was frequently drawn above the surface of the 

 solution so as to be completely emptied ; there was therefore no local fouling. 



(A) Although the fouling at atmospheric temperatures is too slow to be easily detected, the 

 advantages of the platinum vessels were clearly demonstrated in the experiments at higher 

 temperatures, described in Part VII. of the paper. Solutions heated in glass vessels showed a 

 change of conductivity amounting in some instances to as much as 1-6 per cent. (p. 300), but 

 with platinum vessels this source of error was altogether eliminated (p. 300). Thus, in an actual 

 experiment with the electrode figured opposite, the initial resistance at 18 C. of the solution was 

 169-2 ohms; after making eight observations between 18' C. and 100 C. the final resistance 

 was still 169 '2 ohms. This experiment not only shows the superiority of the platinum vessel, 

 but also justifies the conclusion that at low temperatures the fouling of the solution by contact 

 with the glass of the electrode must have been very slight. 



The final experiments quoted in Part VII. were made with the electrode figured on p. 298, fig. 9, which 

 had an even smaller surface of glass. 



