Preparation of Pure Cadmium.

The work of preparing pure cadmium was begun more than two years ago by Mr. W. V. Metcalf with Dr. H. N. Morse. I wish to express here my sincere thanks to him for the material with which the following determinations were made. The cadmium used by him was obtained from Schuchart and marked “Met. prss. (galv.) redus.”

The method of purification by fractional distillation in a vacuum, was essentially that employed by Morse and Burton for the purification of metallic zinc.

The distillation was carried out in hard glass tubes of the size of ordinary combustion tubing.

Fig. 1.

Fig. 1.represents such a tube. A hard glass tube, 600-700 mm. in length, was closed at one end and about 130 grams of cadmium introduced. The walls of the tube were heated and indented at the two points a, and b, with a red-hot file, dividing the tube into three sections marked A, B and C. The open end of the tube was drawn out, bent, and attached to a Sprengel air-pump by means of a rubber tube.

The joint was tied tightly with waxed cord and surrounded by mercury. When the manometer indicated that the tube was exhausted, it was gradually heated by the combustion furnace in which it rested. The metal in A melted and distilled slowly into the front portion of the tube. Most of it condensed in B, while a small part, together with any more volatile impurity, collected in C which was kept cooler than the remainder of the tube. When about four-fifths of the metal placed in A had distilled over, the tube was very slowly cooled. When cold, the tube was broken open, the portions in A and C being rejected in every case, while the metal was recovered from B in the form of a bar restingon the bottom of the tube, together with some crystal aggregates, suspended from the top and sides. A few crystal individuals were secured but the measurement of these will be considered later. The metal separated from the glass with a highly lustrous surface and did not attack the glass in the least.

The first distillation was effected in a tube bridged as represented inFig. 1, but drawn out at each end. The original cadmium powder was heated in the tube in a stream of pure hydrogen gas, for the purpose of obtaining the metal in the form of bars, and to reduce any cadmium oxide contained in the powder.

Six distillations were made in a vacuum. In the first, 630 grams of metal were used being distilled in quantities of about 130 grams each.At the end of the sixth distillation, there were about 100 grams of pure cadmium at disposal. In the fifth and sixth distillations, the metal was heated just above the melting point for from twenty to twenty-four hours, before being forced over into the middle portion of the tube. By this means all the remaining traces of the more volatile arsenic were driven into the front part of the tube and separated from the cadmium.

The distillations.

The residue represents the undistilled portion remaining in A. The distillate, the material obtained from B after the distillation was completed. The coating, the substance which condensed in C.

The distillate from the last distillation was examined spectroscopically by Professor Rowland and found to be free from all traces of impurity which would be detected by that method. The chemical test for arsenic was more delicate than the spectroscopic and this failed to reveal a trace.

The method of preparing the pure acid and of preserving and transferring it was the same as adopted by Morse and Burton in their work on the atomic weight of zinc.

Fig. 2.

The simple form of apparatus is represented infig. 2. A large platinum vessel containing fragments of ice was supported on a smaller platinum dish, from which it was separated by hooks of large platinum wire. The acid was distilled from a small flask as represented in the drawing.

The purest nitric acid which could be obtained was diluted with about an equal volume of water. The vessel containing the acid was heated very gently that the distillation might take place without boiling. The dilute acid condensed on the cold surface of the larger dish and collected in the smaller, in which it was preserved until used. This acid gave no residue on evaporation.

Fig. 3.

The arrangement of the crucibles in which the determinations were made is represented infig. 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 facilitatethe 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.

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 2having 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 applieduntil 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 completedecomposition. When cold, the pair of crucibles was placed in the nickel crucible as represented infig. 3and 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 hightemperatures 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 littlehydrochloric 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, againweighed, 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 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 remainingconstant 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.

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 following table contains the results of ten successive determinations.

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

These results agree more closely with those of von Hauer and Lenssen than with those of any other experimenter. The following table gives a comparison of the work of these investigators with that herein described:

A difference of three or four tenths of a unit between the different results of a series leaves considerable doubt as to the accuracy of the method employed and to the value obtained.

The figure selected by Ostwald,[5]as most probable for the atomic weight of cadmium is 112.08. This is the mean of the results on von Hauer and Huntington. My own work leads me to believe that this number is very close to the true value when oxygen is taken as 16.

Marignac[6]offered the objection to this method for determining the atomic weight of zinc that the zinc oxide dissociated when heated in platinum over the blast-lamp. The same objection might be urged against this method for determining the atomic weight of cadmium, had it not been shown that the objection does not hold for zinc[7]. What took place was a reduction of the zinc oxide by the highly heated hydrogen which passed through the hot platinum.

It was shown that zinc oxide can be heated in a platinum vessel in a muffle furnace, to the melting point of steel, without undergoing any dissociation, or in any wise losing in weight. This source of error was avoided by using porcelain vessels, which were not brought into contact with the free flame.

The statement of Marignac that the oxide of zinc derived from the nitrate retains oxides of nitrogen even when heated to the temperature at which it begins to undergo dissociation, was shown by the same authors to be without foundation. The basis of this objection is doubtless to be found in the imperfect method of testing for such oxides.

It might be urged as an objection to this method that the difference in weight between the metal and oxide is not very great, therefore any error in weighing would be multiplied in the result. At first sight this objection may appear valid, but since the substances weighed were so well adapted to that purpose and the weighings could be made with such a high degree of accuracy no appreciable error could have resulted from this source.

A crucible with its contents was repeatedly weighed against its tare and weights to ascertain the difference between successive weighings under the conditions employed. A number of weighings agreed to .00002 gr. and in some instances to half this amount.

1 The great advantage of the method is its extreme simplicity. From the beginning of an experiment until the end the contents of the crucible are not brought into contact with any foreign substance. By this means small errors resulting from incomplete precipitation, and filtration and all other errors incident to ordinary processes of analysis were avoided.2 The nature of the metal and its oxide rendered them well adapted to weighing. The specific gravity of the metal and oxide approached so closely to that of the weights, that it was unnecessary to reduce the weighings to a vacuum standard.3 The advantages derived from weighing by tares have been pointed out.4 The closely agreeing results speak strongly in favor of the accuracy of the method.

2 The nature of the metal and its oxide rendered them well adapted to weighing. The specific gravity of the metal and oxide approached so closely to that of the weights, that it was unnecessary to reduce the weighings to a vacuum standard.

3 The advantages derived from weighing by tares have been pointed out.

4 The closely agreeing results speak strongly in favor of the accuracy of the method.

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