297, 298] Analysis of Limestones 175 



oxalate. On the other hand, should only just sufficient of the 

 ammonium salt be added, the MgCl 2 present has a distinct 

 solvent action on the calcium oxalate. The most simple way 

 of overcoming this difficulty is by dissolving and re-precipitating, 

 though it may be remarked that less magnesia is precipitated 

 in the presence of acetic acid than in the presence of free 

 ammonia. 



To the total filtrates add acetic acid until slightly acid; 

 boil, and add 1 \ gram of pure ammonium oxalate. Allow to 

 settle, and decant through a filter. Wash once by decantation, 

 and dissolve in the smallest possible quantity of dilute HC1. 

 Add ammonia until only slightly acid ; boil, and add ammo- 

 nium acetate solution in excess. Allow to settle, and filter, 

 washing four times by decantation. Weigh as CaC0 3 . 



297. Magnesia. — The filtrates from the two lime nitra- 

 tions will not only consist of a large bulk of liquid, but will 

 contain great quantities of ammonium salts. It is therefore 

 well to evaporate the liquid to smaller bulk. This is readily 

 done by placing in a large wide-mouthed beaker over a Bunsen 

 burner, keeping the liquid at such a temperature that it only 

 just boils, and adding sufficient liquid from time to time to 

 keep it about a quarter full. To the first lot of liquid add 

 5 c.c. of strong nitric acid, and as more liquid is added to replace 

 what is evaporated, add more acid, 5 c.c. at a time, until 20 c.c. 

 has been used. This will decompose the ammonium salts, 

 which will first form ammonium nitrate, then nitrous oxide and 

 water. When all the liquid has been concentrated to about 

 200 c.c, add to the hot solution 2 grams of pure microcosmic 

 salt, stirring until quite dissolved. Allow to cool, and render 

 strongly ammoniacal. Stir, and allow to stand for one and 

 a-half hour. Filter, dry, ignite, and weigh as Mg 2 P 2 7 (using 

 the precautions described in paragraph 40). 



298. Calculation. — It is not usual to estimate the 



