Part 5

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.

Amer. Journ. Science XL, 379.

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.

In no one of the ten determinations was the slightest coloration detected.

An equal volume of nitric acid was added to the pair of crucibles used as a tare as to those containing the determination, and they were heated in exactly the same manner and for the same length of time.

The crucibles containing the cadmium oxide were heated over the blast-lamp for an hour, weighed against their tare, reheated, again weighed, and this continued until there was no further change in weight. Usually from two to four hours heating over the blast-lamp was sufficient to completely decompose the nitrate. The test for oxides of nitrogen was then applied.

I found that practically constant weight could be reached short of compete decomposition, at a temperature below that necessary to transform all the nitrate into the oxide. This necessitated the final test for oxides of nitrogen.

The Weighing.

The balance used was a No. 8 long-armed one, made by Becker and Sons. It was supported by iron brackets fastened to one of the foundation walls of the laboratory.

Here it would be subjected to the least jar and was also well protected from air currents. All weighings were made between the hours of one and five in the morning when the surroundings were as quiet as could be desired. A very slight disturbance was detected by the vibrations on the surface of a cup of mercury placed conveniently between the pans.

That the presence of the operator might not produce any change in the balance during the weighing, he closed the room, placed the light above and behind his head and took his position in front of the balance at least an hour before making a weighing. When his presence no longer affected the balance (which was shown by the zero point remaining constant in a series of determinations) the weighing was begun. The method of weighing by vibrations and upon both pans was employed throughout.

Each zero point was taken as the mean of three closely agreeing zero determinations; each one of the three being the mean of seven readings. The zero of the balance empty was determined just before and after each weighing to detect any change in its position. Usually none was observed. The sensibility of the balance was taken at each weighing with the weights used at that weighing. A displacement of the zero point about six divisions of the ivory scale was effected by the addition of one milligram.

The weights had been especially adjusted and were carefully compared with each other before using.

Weighing by tares was adopted as preferable to any other method. By this means all errors resulting from changes in the moisture of the air were avoided and any errors which might have been introduced by heating or manipulating the crucibles would be counteracted by treating the tare in exactly the same manner.

Taring The Crucibles.

A pair of crucibles (1 and 2 in the figure) was selected and treated as described. Another pair about the same size but a little lighter was prepared in exactly the same way. Each pair was placed in the nickel crucible and heated by means of the blast-lamp for half an hour.

After cooling in desiccators, both pairs of crucibles where placed in the closed balance until no longer affected by the moisture of the air, which was also dried by calcium chloride. The tare was brought to within one tenth of a milligram of the weight of the crucibles against which it was being tared, by adding fragments of porcelain obtained from another crucible of the same composition. The difference in weight between the tare and its mate was then accurately ascertained.

Each pair of crucibles was again placed in the nickel crucible and blasted for half an hour. They were then reweighed, to determine if the difference in weight previously found had remained constant. In no case was any change detected, yet this precaution was always taken.

The Results.

The following table contains the results of ten successive determinations.

At. Wt. Cd. At. Wt. Cd. Wt. of Cd. Wt. of CdO. (O = 16) (O = 15.96) I 1.77891 2.03288 112.070 111.790 II 1.82492 2.08544 112.078 111.798 III 1.74688 1.99626 112.078 111.798 IV 1.57000 1.79418 112.053 111.773 V 1.98481 2.26820 112.061 111.781 VI 2.27297 2.59751 112.059 111.779 VII 1.75695 2.00775 112.086 111.806 VIII 1.70028 1.94305 112.059 111.779 IX 1.92237 2.19679 112.083 111.803 X 1.92081 2.19502 112.078 111.798 ------- ------- ------- ------- Mean, 112.0705. 111.7905. Maximum, 112.086. 111.806. Minimum, 112.053. 111.773. Difference, .033. .033.

Calculating the atomic weight of cadmium from the total amount of metal used and oxide found, we have:

At. Wt. of Cd. At. Wt. of Cd. (O = 16) (O = 15.96) 112.0706. 111.7904.