(H₄N)₂CO₃ + CaSO₄ = (H₄N)₂SO₄ + CaCO₃.
285. Analysis of Gypsum.—For agricultural purposes it will be sufficient to determine the quantity of sulfuric acid, and to calculate therefrom the amount of calcium sulfate in the sample: Or the lime may be determined and the quantity of sulfate calculated therefrom.
(1) Insoluble Matter.—In the conduct of the work the sample of gypsum is rubbed to an impalpable powder in an agate mortar. The washed sample, about one gram, is dissolved in a large excess of dilute hydrochloric acid, the digestion being continued at near the boiling-point, with frequent stirring, for at least two hours. The solution is made alkaline, filtered, and the residue washed and dried to constant weight.
(2) Sulfuric Acid.—The washings and filtrate from the above determination are made up to a definite volume with water and divided into two equal parts. The sulfuric acid is estimated in one part by adding to it sodium carbonate until the acidity is nearly neutralized. The sulfuric acid is then thrown down at near boiling temperature by the gradual addition of barium chlorid solution. The barium sulfate formed is separated, washed, dried, and weighed in the usual manner.
(3) Iron and Alumina.—To the other half of the solution a little nitric acid is added and boiled to convert any ferrous into ferric iron. On the addition of ammonia the iron and alumina are separated as hydroxids, collected on a gooch, washed, dried, ignited, and weighed as oxids.
(4) In the filtrate the lime is thrown out as oxalate, and separated and weighed in the usual way as oxid. One part of CaO is equal to 2.4286 parts of CaSO₄.
(5) Moisture.—Dry about two grams of the sample to constant weight at 80°.
(6) Water of Crystallization.—Heat the residue from the above to 150°, until a constant weight is obtained. The loss represents water of crystallization.
(7) Carbonates.—Determine the quantity of carbon dioxid evolved by the usual process, and calculate the calcium carbonate.
286. Solution in Sodium Carbonate.—Gypsum is also easily decomposed by boiling with a solution of about ten times its weight of sodium carbonate. The calcium, by this operation, is converted into carbonate and can be collected on a gooch, washed, and estimated as usual, but in this case it will contain all the insoluble matters, from which the lime can be separated by solution in hydrochloric acid.