(1) Absolute freedom of the reagents used from arsenic, antimony,[760] and other impurities.
[760] With regard to purity of reagents, Sonnenschein states that he has once found chlorate of potash contaminated with arsenic.—Sonnenschein, Gericht. Chemie, p. 139.
(2) The sulphuric acid should be diluted with five times its weight of water, and if freshly prepared should be cooled before use. Strong acid must not be employed.[761]
[761] M. A. Gautier uses sulphuric acid diluted with five times its weight of water; when the hydrogen has displaced the air, he adds to the arsenical matter 45 grms. of this acid and 5 grms. of pure sulphuric acid.—Bull. de la Société Chim. de Paris, 1875, t. xxiv.
(3) The fluid to be tested should be poured in little by little.