ALUMINA CHROME-ALUM. 



35 



The alumina, sesquioxide of chromium, and excess of 

 baryta are, precipitated from the filtrate by carbonate 

 of ammonia mixed with caustic ammonia. After long 

 standing, the precipitate is filtered oft' and thoroughly 

 washed. 



The filtrate is evaporated, the residue heated to 

 expel chloride of ammonium, and the residual chloride 

 of potassium gently ignited in a covered crucible. 



The mixed precipitate is taken and washed off while 

 wet from the filter, dissolved in dilute sulphuric acid, 

 and the sulphate of baryta, filtered and washed. 



An excess of caustic potassa is added to the solution, 

 which is then saturated with chlorine gas, and the 



Fig. 2. 



oxide of chromium forms a yellow solution of chro- 

 mate of potassa. 



kept cool, so that its temperature may not rise above 4(P, until the 

 odor of the gas begins to be perceptible. After some time, octohe- 

 dra of pure chrome-alum are formed, which may be set aside. The 

 mother-liquor is mixed with an equal volume of a solution of com- 

 mon alum, saturated at 40, when the salt in question separate! 

 in yellowish octohedra. 



