Mr Wilks, Note on some double fluorides of sodium. 77 



hydrofluoaluminic acid formed is then so stable that hydrofluoric 

 acid does not seem to be split off even on boiling. The solution 

 was in fact boiled in a test tube for half an hour without any 

 apparent action on the glass. 



To diminish the concentration of hydrogen ions in the resulting 

 solution, an acetate must be added. The acetates of nickel or 

 copper can be used for this purpose as neither of these metals give 

 precipitates of double fluorides. Hence to the hydrofluoaluminic 

 acid solution an equal bulk of a cold saturated solution of copper 

 acetate is added. This solution is then boiled and any slight 

 precipitate filtered off. To this solution an equal volume of a 

 50 per cent, solution of alcohol is added. This makes the reaction 

 still more sensitive. Care must be taken however not to add too 

 much alcohol, otherwise a precipitate will be formed which appears 

 to be redissolved only with difficulty on adding more water. If 

 this precipitant has been carefully made the test for sodium can 

 be carried out in test tubes. 



The test is carried out as follows. About 5 c.c. of the precipi- 

 tant is boiled in a test tube and then the sodium solution added. 

 If the latter is strong there will be an immediate precipitate. If 

 it is very weak, however, it will be required to be boiled for some 

 time before the precipitate appears. To make quite certain that 

 the precipitation was not due to action on the glass a blank 

 experiment was carried out at the same time in another tube 

 without adding sodium chloride. In this case there was no pre- 

 cipitate. The test appears to be very delicate, one part of sodium 

 chloride in 20,000 parts of solution being readily detected. 



The precipitate in these cases has not the composition of 

 cryolite. Analysed by heating with strong sulphuric acid and 

 weighing the sodium sulphate and alumina it seems that sodium 

 and aluminium are present in the atomic proportions of 1 . 1 : 1. 

 Further analyses are however being carried out. 



Pure potassium and ammonium salts are not precipitated 

 under the conditions stated above. The metals other than sodium 

 which give precipitates are silver, magnesium, calcium, strontium, 

 barium and lead. 



Attempts are now being made to determine whether sodium 

 can be quantitatively estimated in this way. It has already been 

 proved that precipitation is almost if not quite complete. 



It was considered possible that the elements related to 

 aluminium treated in the same way would give precipitates with 

 sodium. It was found that ferric hydroxide dissolved in hydro- 

 fluoric acid gave with sodium a light brown precipitate which has 

 not yet been examined. 



Chromium and beryllium, however, fail to give a precipitate 

 even with concentrated solutions of sodium chloride. 



