The arrangement of the crucibles in which the determinations were made is represented in [fig. 3]. 1 is a small porcelain crucible, (00) from the exterior and lid of which the glaze had been removed by hydrofluoric acid. The lid was separated from the crucible by hooks made from thick platinum wire, to allow free communication between the contents of the crucible and the external air. This would facilitate the outward diffusion of the oxides of nitrogen when liberated from the nitrate. 2 is an uncovered porcelain crucible (no. II) in which 1 was placed. From the exterior the glaze had been removed to prevent the crucible from adhering to the unglazed porcelain scorifier on which it rested. The exterior was carefully brushed after treatment with hydrofluoric acid to remove all loose particles adhering to its surface. Crucibles 1 and 2 were not separated during a determination.
3 is a nickel crucible about two and a half inches in diameter. The porcelain crucibles were not allowed to touch the nickel at any point. The nickel crucible was covered by a lid of nickel.
The mode of procedure.
A piece of cadmium weighing from two to three grams was cut from the bar of the metal by means of a steel chisel. This was seized with steel forceps and filed with a hard steel file to about one half the original weight. Care was taken to remove the entire exterior portion of the metal which had come in contact with the chisel or had stood exposed to the air. The plug of metal was then carefully brushed and examined with a lens to insure the removal of all loose particles from the surface.
[Crucibles 1 and 2] having been brought to constant weight against their tare, were ready for use. The piece of cadmium was weighed and placed in 1. An excess of pure nitric acid was added and a gentle heat applied until all the metal had dissolved. This required from twenty to forty hours.
A sand-bath was constructed by placing a large porcelain crucible in an iron crucible and filling the intervening space with sand. The pair of crucibles ([1 and 2]) was placed in the porcelain crucible and the contents evaporated to dryness by warming very carefully at first and gradually increasing the temperature. The pair of crucibles was then transferred to a bath constructed as the above where iron filings took the place of sand. This was heated by a single burner until the nitrate was all decomposed when a triple burner was added and finally two for six or eight hours. This was not sufficient to effect complete decomposition. When cold, the pair of crucibles was placed in the nickel crucible as represented in [fig. 3] and sharply heated over a blast-lamp for several hours. This completed the decomposition of the nitrate and the removal of the last traces of oxides of nitrogen.
During the blasting the lid on crucible 3 was raised a little to one side to allow free access of air. The nickel crucible was forced tightly into a hole cut in the center of an asbestos board about ten inches in diameter, to prevent any reducing gases from the lamp entering the crucibles while hot. This was the same arrangement as was used by Partridge[4].
It was found that the final decomposition of the nitrate could not be effected in a muffle furnace as with zinc, since at very high temperatures cadmium oxide attacked the porcelain with great energy and injured the crucibles.
The decomposition of the nitrate was shown to be complete not by constant weight alone, but by testing for oxides of nitrogen with starch paste rendered extremely sensitive with potassium iodide. That the test should be reliable, Morse and Burton have pointed out that all the reagents used must be free from oxidizing agents. The presence of iodate in the iodide is especially to be avoided. This was removed by boiling the solution with zinc amalgam. Air was removed from all the solutions by boiling.
When the starch-potassium-iodide solution had been prepared as sensitive as possible, a portion of it was treated with a little hydrochloric acid, to determine if any iodine was liberated. If no coloration was observed the cadmium oxide was added. It dissolved in the hydrochloric acid and if any oxides of nitrogen were present they would have revealed themselves by the liberation of iodine and a blue coloration of the starch paste.