CONDUCTIVITY OF SODIUM HYDROXIDE IN AQUEOUS SOLUTION. 257 



similar influences are at work in the majority of concentrated aqueous salt solutions, 

 but the effects are usually masked by the increased viscosity of these solutions, and 

 very few measurements have been made at concentrations at which the autolytic 

 conductivity of the salt becomes of importance.* 



(4.) Physical Constants of Sodium Hydroxide. The available data with reference 

 to the physical properties of the alkalies are more scanty and less accurate than in 

 the case of the more important acids and salts. This result is mainly due to the 

 exceptional difficulty of preparing and standardising pure solutions of these substances. 

 After a long series of experiments we succeeded in devising a method by which it is 

 possible to prepare concentrated solutions of sodium hydroxide directly from weighed 

 quantities of metallic sodium. We were thus enabled to determine some of the 

 physical properties of the solutions with a closer approach to accuracy than has 

 hitherto been attained. This was notably the case with the densities at 18 C. 

 Data are also given, though with a somewhat larger error, for every 10 C. from C. 

 to 100 C. The electrical conductivity at 18 C. was also determined with an 

 accuracy of about 1 part in 1000, and, as the result of experiments on the influence 

 of temperature on conductivity, we are also able to tabulate the conductivities at 

 0C., 50 C., 100C.f 



PART I. PREPARATION OP CONCENTRATED STANDARD SOLUTIONS OF SODIUM 

 HYDROXIDE FROM METALLIC SODIUM. 



Until a few years ago the best material available for experimental purposes was 

 caustic soda purified by dissolution in alcohol. More recently the commercial pre- 

 paration of metallic sodium has le,d to the introduction of caustic soda, prepared by 

 the cautious addition to one another of drops of water and particles of sodium in a 

 silver dish ; this material contains little impurity except carbon dioxide, and this can 

 readily be removed by means of baryta. Solutions prepared from these materials are 

 necessarily of unknown strength, since the sodium hydroxide is never obtained 

 sufficiently free from moisture and other impurities to be directly weighed out. 



Standardisation is usually effected by titrating against an acid of known strength, 

 but this process is liable to serious errors, and we were soon compelled to abandon 

 the attempt to standardise our solutions in this way. Ultimately we were led to 



* SUTHERLAND (toe. cit.) observed that in the case of zinc sulphate the ratio A^/A^o, corresponding to 

 our I/I., passed through a minimum value, but attributed this result to altogether different causes. 



t The measurements of viscosity given in Part VI. are to be regarded only as approximate deter- 

 minations. The errors were probably of the order of 1 per cent., but the conclusions based upon these 

 measurements would not have been influenced by much larger errors in the viscosity constants of the 

 solutions. It was considered undesirable to describe in detail a type of viscometer that had not been tested 

 with reference to its applicability to exact measurements. 



VOL. OCIV. A. 2 L 



