POSSIBLE ERRORS OF STAS. 



Anyone with experience will recognize that a much higher 

 apparent atomic weight of sodium was to be calculated by the first 

 procedure than by the second, other conditions being the same, and 

 yet the results were almost identical. The immediate inference is, of 

 course, that some other condition or conditions must have varied in 

 order to compensate for the effect of the altered method of titration . 

 This inconsistency, which appears also in his work on potassium and 

 ammonium, has never been explained, and one of the functions of the 

 present work was to account for it. 



Among the possible sources of error in Stas's work, the most serious 

 seems to have been the practice of dropping solid salt into the solution 

 of argentic nitrate.* Of course this practice caused the greater part 

 of the argentic chloride to be precipitated in the presence of a 

 concentrated solution of salt, immediately around the solid a circum- 

 stance tending greatly to promote the occlusion or inclusion of sodic 

 chloride in the precipitate. Stas, in his early work especially, was 

 fully awake to the danger of the occlusion of argentic nitrate in this 

 precipitate ; f but he seems to have had no fear of occlusion of other 

 salts. In the sequel we shall show that the chloride of silver tends to 

 carry down with it many salts from their concentrated solution, and 

 that the details of treatment determine whether or not these impurities 

 may be removed by washing. 



Another possible cause of the discrepancy is to be found in the 

 methods used for preparing the materials. In Stas's earlier investiga- 

 tion, where the analytical end-point was erroneous, greater care was 

 taken in preparing the sodic chloride than in the later experiments. 

 In general, when purifying this salt, he employed violent treatment, 

 and very rarely recrystallized or in any way fractioned his material 

 afterwards. His favorite method was fusion with ammonic chloride 

 and ammonic chlorplatinate. Presumably the ammonic chloride was 

 used to expel other halogens, and the deposited platinum, Stas believed, 

 carried down silica and alumina with it. Except for these possible 

 advantages, this admixture of foreign materials seems to be a doubtful 

 expedient. In the ten experiments of i86o,| six different preparations 

 were made, from the following sources: 



i. Sodic carbonate neutralized with hydrochloric acid. 



2. Rock salt recrystallized. 



3. Sodic sulphate repeatedly fused with ammonic chloride. 



4. Sodic tartrate recrystallized and fused with ammonic chloride. 



5. Sodic nitrate, similarly treated. 



6. Sodic chlorplatinate recrystallized and fused. 



tOeuvres, 1,337 



*Oeuvres, 1, 759; also 3, 479. 



JOeuvres, 1, 365. 



