76 Mr Wilhs, Note on some double fluorides of sodium. 



Note on some double fluorides of sodium. By W. A. R. Wilks, 

 B.A., Caius College. (Communicated by Dr Fenton, F.R.S.) 



[Read 8 February 1909.] 



Many of the insoluble compounds by which elements are 

 quantitatively estimated are, as might be expected, found in 

 nature as minerals. Hence it was thought probable that if sodium 

 could be estimated by means of some simple inorganic compound 

 the latter would be found as a mineral. On considering the 

 simple minerals containing sodium, the only one which might be 

 suitable for the purpose appeared to be cryolite. 



This compound has been artificially prepared by Deville {An. 

 Ch. Ph. (8) 59, 82, 1860) and by Baud. The former saturated 

 hydrofluoric acid by sodium carbonate and alumina in the propor- 

 tion of three molecules of carbonate to one of alumina, evaporated 

 and obtained the substance 3NaF . AIF3. The latter precipitated 

 a solution of aluminium fluoride by sodium fluoride and obtained 

 the hydrate of cryolite 6NaF.2AlF3.7HoO. This hydrate is 

 gelatinous and is soluble in water to the extent of '352 gms. per 

 100 c.c. at 16° C. Natural cryolite which is SNaF.AlF^ has the 

 solubility "034 gms. in 100 c.c.'s at 15°. 



In order to find whether sodium could be detected as double 

 fluoride, aluminium was dissolved in dilute hydrofluoric acid in a 

 platinum dish until the acid was exhausted. On adding this 

 solution to a solution of sodium chloride a gelatinous precipitate 

 was obtained whose composition has not yet been determined. If 

 the sodium chloride was dilute, however, the precipitate did not 

 appear. It was further observed that boiling the solution facili- 

 tated the precipitation and that in this case the precipitate was 

 no longer gelatinous but crystalline. It was also found that 

 acids had a marked solvent action on the precipitate. After 

 these preliminary observations another mode of preparation of 

 the precipitant was resorted to. 



Precipitated aluminium hydroxide which has been simply 

 dried by a filter pump was added to a small quantity of strong 

 hydrofluoric acid in a platinum dish. The hydroxide seems to 

 exhaust the acid much better than does aluminium itself, and in 

 the process of solution the moist hydroxide furnishes sufiicient 

 water to form a solution of convenient strength. In order to get 

 the best results the hydrofluoric acid is allowed to stand for at 

 least two days in contact with excess of hydroxide. The complex 



