WHITE PIGMENTS. 25 



alumina precipitation, heat to boiling, add slowly 25 cc of a boiling solution 

 of ammonium oxalate (the solution should be saturated at room temperature) ; 

 let stand on the steam bath for one to two hours, filter, wash free from 

 chlorids (cold water is better for washing than hot water), and determine the 

 calcium by igniting to constant weight in a platinum crucible. A blast lamp 

 is generally recommended for this purpose, but if a platinum triangle is used 

 to support the crucible, from fifteen to thirty minutes' ignition with a good Bun- 

 sen burner will convert the oxalate entirely to oxid with less danger of loss than 

 with a blast. If great accuracy is required, it is well to dissolve the calcium 

 oxalate in hydrochloric acid and reprecipitate. 



Instead of weighing the calcium as CaO, it may be determined volumet- 

 rically by titrating with potassium permanganate as follows: Precipitate as 

 described, and wash free from ammonium oxalate (when free the filtrate will 

 not react for chlorids). Return the moist precipitate to the beaker by washing 

 with a stream of water, remove the small quantity remaining on the paper 

 with hot dilute sulphuric acid, add 20 cc of sulphuric acid (1:1), dilute to 

 300 cc, heat to about 80 C., and titrate with tenth-normal potassium perman- 

 ganate (1 cc=0.0028 gram of calcium oxid). Evaporate the filtrate from the 

 calcium oxalate to about 100 cc, make neutral by the addition of hydrochloric 

 acid, if necessary cool in ice water, add an excess of a solution of sodium 

 ammonium phosphate, stir, allow to stand for fifteen minutes, then add 20 cc of 

 strong ammonium hydroxid, stir well, allow to stand for two hours, filter in a 

 gooch, wash with 2 per cent ammonium hydroxid, ignite gently at first and 

 finally to a bright red, and weigh as magnesium pyrophosphate (MgzPzOi). 



4. Barytes. 



Barytes, being one of the cheapest white pigments, is seldom adulterated. 

 It should be white and well ground. A niiscroscopic examination should be 

 made to determine the evenness of grinding, the size and angularity of the par- 

 ticles, and whether they are amorphous or crystalline. 



(a) MOISTURE. 



Dry 2 grams of the sample at 105 C. for two hours. The loss in weight rep- 

 resents the moisture. 



(b) INSOLUBLE. 



Boil 1 gram in hydrochloric acid (1:3), evaporate to dryness, moisten 

 with hydrochloric acid, dilute with water, boil, filter, and wash. If a pre- 

 vious qualitative examination has determined the absence of lead and other 

 easily reduced metals, ignite in platinum, weigh, and calculate the total insol- 

 uble; this may generally be safely calculated to barium sulphate (BaSOO, 

 but to determine any silica that may be present add hydrofluoric acid and sul- 

 phuric acid, evaporate the acids, ignite, and weigh; calculate the loss as 

 silicon dioxid. 



(c) ALUMINA AND OXIDS OF IRON. 



Add ammonium hydroxid to the filtrate from the total insoluble, boil, filter, 

 wash, ignite, and weigh as oxid of iron and alumina. 



in<;!>-( nun. io<) 12 4 



