CONDUCTIVITY OF SODIUM HYDKOXIDK IN AiJi'Kors SOLUTION 7 . 265 



PART II. DETERMINATION OF THE DENSITY OF AQUEOUS SOLUTIONS OF 



SODIUM HYDROXIDE AT 18 C. 



Determinations of the density of aqueous solutions of sodium hydroxide have been 

 made by some six different investigators, but owing to the difficulty of preparing pure 

 solutions of known concentration, the results obtained have not been characterised by 

 any close approach to accuracy, and the published tables are far less trustworthy 

 than those for a substance like common salt, which can be weighed out in an 

 anhydrous condition, or sulphuric acid, which can be purified by distillation and 

 freezing. In every case the solutions appear to have been standardised by titration, 

 and it is clear that the errors introduced by inaccurate values for the concentration, 

 and by neutral impurities in the solution, must have l>een many times greater than 

 those due to the mere measurements of density. 



In the early tables of RICHTER (1795) and of D ALTON (1810) the density is given 

 to two places of decimals. A more ambitious table was published by TUNNERMANN 

 in 1827 ('Neues Journal der Pharmacie fiir Aertzte/ vol. 18, p. 2), in which the 

 percentages of Na 2 O were given to three places of decimals (1 part in 30,000) and the 

 densities to four ; the additional accuracy of this table is for the most part fictitious, 

 as errors are to be found even in the second place of decimals. TUNNERMANN'S values, 

 in conjunction with those of DALTON, were, however, made use of 30 years later by 

 SCHIFF (' Annalen,' 1858, vol. 107, p. 300), who deduced from them a formula con- 

 necting density with concentration, his object being to obtain by extrapolation a value 

 for the density of fused 100-per cent, sodium hydroxide. SCHIFF'S formula was used 

 by GERLACH in 1867 (' Zeitschrift fiir Analytische Chemie,' vol. 8, p. 279) to 

 calculate the densities for each exact percentage of Na. 2 O and NaOH, and GERLACH'S 

 table was reproduced in all the dictionaries and text-books of chemistry until 

 20 years ago. 



It is remarkable that the crude measurements of density made in the early years 

 of last century should be still in use ; it is even more remarkable that during more 

 than half a century no attempt at revision was made, each successive writer being 

 content to quote from his immediate predecessor the untested figures of a former 

 generation. A revision of the tables of density of sodium hydroxide was attempted 

 by HAQEN, who published in 1883 ('Commentar zur Pharmacopoaa Germanica,' Berlin, 

 1883) a list of densities determined to four places of decimals at no less than 100 

 concentrations between 0'61 per cent, and 49'02 per cent. NaOH. This list is given in 

 LUNGE'S ' Sulphuric Acid and Alkali,' and is quoted thence in the current edition of 

 WATT'S Dictionary, but is rather less accurate than the table of GERLACH, which it 

 was intended to replace. It appears, therefore, that until 10 years ago no table had 

 been published in which the density of aqueous solutions of sodium hydroxide was 

 given with a sufficient approach to accuracy to render even the second place of 



VOL. CCIY. A. 2 M 



