Preparation of arsenic acid
Preparation of arsenic acid from arsenic trioxide and nitric acid
The arsenic acid liquid produced in preparing dinitrogen tetroxide is poured off from the unchanged pieces of white arsenic trioxide, evaporated to dryness in a porcelain dish, and the residue again dissolved by heating with a little water. The solution must be free from arsenious acid (H3AsO3), if not, it must be again evaporated, adding a little sulfuric acid, and dissolved once more. The solution, free from arsenious acid, is evaporated at 120° C (in order to completely remove nitric acid) to a syrup, and left to crystallize at below 15° C in a desiccator over sulfuric acid. If necessary, a small crystal of arsenic acid could be added to initialize crystallization process. If crystallization process is slow a very nice, large, compact glossy crystals are obtained. If the crystallization has occurred too quickly, the mass is dissolved, the crystals are melted at a moderate heat, and then allowed to slowly crystallize, so that larger crystals may be obtained. Arsenic acid prepared by such method contains water forming hemihydrate H3AsO4 · ½H2O or dihydrate H3AsO4 · 2H2O.
Preparation of arsenic acid from arsenic and nitric acid
Alternatively, arsenic acid is prepared by dissolving arsenic in 30% nitric acid. 15 g of finely powdered arsenic are placed in a large glass flask. From a funnel, in small portions 100 g of 30% nitric acid is added. When all arsenic is dissolved, arsenic acid is obtained by the procedure described above.
Laboratory manual of inorganic preparations, by H. T. Vulte, 135-136, 1895