﻿washing of Aluminum. 113 



paper and stopped them up, thus hindering the filtration. To 

 make a successful precipitation and washing of alumina it is 

 quite essential not to have very much ammonia salts present 

 and even with the greatest care it is found that if the filtrates 

 and washings are set away in a warm place, slight precipitates 

 will almost invariably settle out. The above holds good for 

 solutions containing ammonium chloride and nitrate. The fact 

 that alumina passed through the filter only during the washing 

 suggested to us that if we could wash the precipitate with a 

 saline solution which would be completely volatile and do no 

 harm to the precipitate we might avoid the passage of the 

 alumina through the filter and it might also hinder the packing 

 of the precipitate. Ammonium nitrate suggested itself to us 

 as a salt which would be volatile on ignition and do no harm, 

 and our experiments with it have been very satisfactory. The 

 following strength of NH 4 N0 3 has been used in all of our ex- 

 periments— 2 c.c. of pure concentrated HN0 3 neutralized with 

 ammonia and diluted to 100 c.c. with water; this strength has 

 proved so satisfactory that we have tried no other. Using this 

 hot saline wash instead of hot water the precipitation can be 

 made in solutions containing large or small quantities of am- 

 monium salts and no very great care is needed in adding the 

 ammonia. According to our experience, the precipitates from 

 solutions containing a goodly quantity of ammonia salts, re- 

 sulting we will say from 4 to 8 c.c. of pure concentrated hydro- 

 chloric or nitric acid, filter and wash better than precipitates 

 from solutions containing less saline matter. After having 

 made a large number of precipitations we can say that only in 

 one or two cases have we found a trace of alumina in either 

 the filtrate or washings, and that unless the precipitate becomes 

 too dry and packs too firmly upon the sides of the funnel, the 

 washing goes on as well at the end as at the beginning, and 

 there is no difficulty in washing the precipitate free from all 

 traces of chlorine. 



Where large quantities of sodium or other fixed salts are to 

 be removed and a second precipitation of the alumina is required, 

 we recommend that the first precipitate after washing be dis- 

 solved in nitric instead of hydrochloric acid, then very little 

 washing is enough to free the second precipitate from the last 

 traces of chlorine and fixed salts. 



Why ammonium nitrate acts in this way we are not able to 

 state. It may be something on the same principle that sedi- 

 ment deposits much faster in saline than in fresh water. We 

 are convinced that any one who has to handle troublesome 

 alumina precipitates will find the above method of washing of 

 great advantage. 



