§ 120. GYPSUM. 237 



analyze n solution of the ash obtained in b, in nitric acid. 

 ^f(/2Jhurlc add and chlorine^ however, must always be 

 determined in the nitric-acid solution of the original sub- 

 stance. 



GYPSUM. 



120. a. Water. — Ignite 2 grms. of the finely pulverized 

 gypsum gently. 



b. Insoluble matters.— Digest 2 grms., likewise finely 

 powdered, witli very dilute hydrochloric acid, as long as 

 anything appears to be dissolved, filter out the insoluble 

 sand and clay, wash well Avith hot water, dry, ignite, and 

 weigli the insoluble residue. 



c. Sulphuric acM, ferric oxide, lime, etc.— Divide 

 the acid solution in two equal parts, and in one j^re- 

 cipitate sulphuric acid with baric chloride (§ 59), and 

 in the other, after dilution with water and heating with 

 a little concentrated nitric acid, precipitate ferric oxide 

 and alumina, with ammonia in slightest possible excess 

 (§ 51). Filter the j^recipitate out quickly, so as to 

 avoid the precipitation of calcic sulphate in the alkaline 

 solution, wash it Avell, and then weigh it after ignition. 

 Immediately on filtering out this precii)itate by ammonia, 

 add ammonic oxalate in excess to the filtrate to pre- 

 cipitate the lime (§ 49, a), and estimate magnesia in tlio 

 filtrate from the lime, in the usual way Avith hydric 

 disodic phosphate (§ 50, b) 



d. If there is a considerable precipitation of ferric oxide 

 by ammonia, some calcic sulphate is very liable to be 

 mixed with it. In this case, it is better to boil about 

 1.5 grms. of the finely powdered gypsum an hour with a 

 solution of 6-8 grms. of pure sodic carbonate ; by this 

 operation, if the gypsum was properly pulverized, it is 

 completely converted into calcic carbonate. Filter, wash 

 the contents of the filter well with liot water, and precipi- 



