The Oxide Method.
As the chloride and bromide methods and the synthesis of cadmium sulphate all lead to approximately the same high result, it seemed probable that the oxide method which had given a much lower result (Morse & Jones 112.07) must be affected by some error. Accordingly it was examined in the manner about to be described. A set of crucibles was prepared as described by Morse and Jones in their work on this method, and in the present paper under the oxalate method. After they had been heated in a nickel crucible overa blast lamp and weighed, a weighed piece of cadmium was introduced into the smaller inside crucible, and dissolved in nitric acid with the aid of heat. An equal quantity of nitric acid was added to the tare. The acid was then evaporated off, and the resulting nitrate converted into oxide exactly as has already been described under the oxalate. The first experiment was made in this way and the second one exactly like it, only the porcelain crucible used was the one which had been employed in the first determination. The glaze had been removed by thecadmium oxide of the first determination, and before using for the second one the crucible was boiled out with nitric acid, and heated to constant weight over a blast lamp as before. Determinations III, IV and V were made in the same way except that the small inner crucible was platinum instead of porcelain. All weighings were reduced to the vacuum standard on the basis of 8.54 for the Sp. Gr. of cadmium and 8.15 for the Sp. Gr of cadmium oxide and 8.4 for the brass and 21 for the platinum weights.
The results are as follows:
The oxides resulting from these determinations were always tested for oxides of nitrogen, sometimes by using meta phenylene diamine and at other times by sulphanilic acid and naphthylamine sulphate, but no traces were ever found. The average of the determinations made inporcelain crucibles is 112.08. Morse and Jones obtained the same figure or, if their results are reduced to the vacuum standard, 112.06, by the same method under the same conditions. The results of the determinations made in platinum crucibles are equally constant, but their average is 111.88 being .20 of a unit lower. Therefore, more oxide is obtained when platinum crucibles are used instead of porcelain ones. In two cases the platinum crucibles were weighed at the end of the determinations after the cadmium oxide had been removed. Their weight remained unchanged. The most probable explanationof these facts seems to be that something is retained in the oxide in both cases, but that the amount is greater in the determination made in platinum crucibles than in those in which porcelain ones were employed. We should expect this, because in porcelain crucibles some of the oxide is absorbed forming a silicate, and any volatile impurity must be expelled from this part of the oxide. Not finding oxides of nitrogen, it was thought that gases probably nitrogen and oxygen might be occluded although Richards and Rogers (Amer. Chem. Jour. 15, 567.) had examined cadmium oxide prepared from the nitrate and foundonly a trace of gas. Accordingly two specimens of cadmium oxide obtained in the above determinations were powdered in an agate mortar and boiled with water for some time in order to remove any adhering air. They were then dissolved in dilute hydrochloric acid from which the air had been removed by boiling. A small amount of gas was found in each case but not nearly enough to account for the difference of .31 unit in the atomic weight of cadmium between 112.38 and the oxide method. In fact not more than about one sixth of the amount required was found. It may be that the powderingof the oxide and then boiling up in water may have been to severe a treatment, and that the greater part of the occluded gas escaped during these processes. It seems that there is at least some error due to occluded gases in methods involving the decomposition of cadmium nitrate to oxide, but no satisfactory idea of its magnitude could be obtained from these two experiments as carried out.
The following experiments were then made and they seem to give definite evidence not only of the existence of an error but also of its magnitude. Carbonate of cadmium wasmade by dissolving pure cadmium in nitric acid, adding an excess of ammonia and a small quantity of ammonium carbonate. After standing for some time the cadmium carbonate was filtered off and rejected. The filtrate was treated with an excess of ammonium carbonate and the precipitated cadmium carbonate allowed to digest in it for some time. After washing by decantation several times the carbonate was transferred to a funnel containing a porcelain filter-plate, covered with a piece of ashless filter paper of slightly larger diameter, and washed thoroughly.with water. It was then transferred to a platinum dish, care being taken to avoid contamination with filter paper and heated gently to convert it into oxide. The resulting oxide was powdered in an agate mortar, returned to the platinum dish and heated to incipient whiteness for seven hours in a muffle furnace. The temperature must not be too high, otherwise the oxide will distill readily leaving no residue. The oxide is slightly volatile at good red heat as was observed in trying to make a determinant at this temperature by the oxide method.A weighed portion of the oxide which had been prepared from the carbonate in the manner described was dissolved in a weighed porcelain crucible and the resulting nitrate converted into the oxide again by heat just as in the oxide method. This constitutes experiment I. Experiments two and three were made in exactly the same way except that a platinum crucible was used instead of a porcelain one. The results are:
As we started with cadmium oxide, and, after passing to the nitrate, converted it back into the oxide, the weight should remain unchanged if the method is correct. However, this is not the case, but a large increase in weight takes place. The increase is larger in a platinum crucible than in a porcelain one, which accounts for the fact that a lower value for the atomic weight is found by the oxide method when they are used. The use of a porcelain crucible therefore diminishes the error, but does not eliminate it. The explanation of this hasalready been given. The oxides obtained in these three experiments were tested for occluded gases in the manner already described, but only small amounts were found. Both of those made in platinum crucibles were tested for nitrate of cadmium with brucine and sulphuric acid with negative results. To show that the impurity was not converted into an ammonium salt when the oxide was dissolved in hydrochloric acid, a slight excess of caustic potash was added to the solution, the precipitate allowed to subside and the clean, supernatant liquid tested forammonia with Nessler’s reagent. No ammonia was found. In order to make these experiments as severe a test as possible, a somewhat higher temperature was employed than had in the five experiments described under the oxide method. This was accomplished by boring out the stopcocks of the blast lamp so that a larger supply of gas was furnished. The two oxides in the platinum crucibles seemed to be constant in weight, but that in the porcelain crucible seemed to lose in weight slowly. The weight given was taken after four hours blasting, which islonger and at a higher temperature than was used in any of the five determinations made by the oxide method. If the cadmium oxide prepared from the carbonate retained any carbon dioxide, it would lose weight in being dissolved and reconverted into oxide. The above experiments therefore seem to furnish very strong evidence that there is an error of at least −.24 unit in the oxide method when porcelain crucibles are used and −.39 of a unit when platinum ones are employed. if .24 of a unit is added to 112.07 the result obtained when porcelain crucibles are used we get 112.31and adding .39 to 111.87 gives 112.26. Considering the small number of experiments made, the fact that they were made in such a way as to give a low value (numerically) for the error rather than a high one, and also that the error is probably variable to some extent, especially when porcelain crucibles are used, the corrected results agree as closely with 112.38, the average of the chloride, bromide and sulphate (synthetical) methods as could be expected. It must also be borne in mind that 112.38 is only to be regarded as an approximation to the atomic weight of cadmium. The increase inweight observed in converting the nitrate back into oxide might also be explained by assuming that the cadmium oxide used in the beginning of the experiments was richer in metal than the formula CdO indicated and that the increase in weight is due to this excess of metal being changed to oxide. The method of preparation of the oxide from the carbonate and the known properties of cadmium oxide render this view highly improbable, and the following two observations render it untenable:
1st. If this were the cause of the increase, the amount of increase would necessarily bethe same in both platinum and porcelain crucibles, which is not the case.
2nd. Three grammes of cadmium oxide made from the carbonate were dissolved in dilute hydrochloric acid from which the air had been expelled by boiling. The oxide, which is very compact, was placed in a glass bulb which had been blown at the end of a tube. After displacing the air by filling the entire apparatus with recently boiled water, the exit of the tube was placed under boiling dilute hydrochloric acid, and the bulb heated until the water boiled. It was then turned over so that the steam displaced nearly all the water. On removing the flame the dilute hydrochloric acid at once filled the bulb. The exit tube was then quickly placed under a narrow tube filled with mercury and inverted over mercury in a dish. The bulb was then heated until the oxide had dissolved. By this method the gas would be boiled out of the solution and collected in the top of the narrow tube. As only a very small amount of steam and dilute hydrochloric acid go over at the same time, there is no danger of the gasformed being absorbed to any considerable extent. It is well to put the oxide into the bulb before the tube is bend. If the hydrochloric acid is too strong, it must be cooled before entering the bulb as otherwise the reaction is too violent, and the experiment may be lost. This experiment shows that there is no excess of cadmium present in the oxide employed for no gas was found. If three grammes of the oxide contained enough metal to take up .00126 grms. of oxygen, .00016 grms of hydrogen should have been set free, and its volume under ordinary conditions of temperature and pressure would have been about 1.9cubic centimetres. This experiment would also have shown the presence of carbon dioxide if any had been present.
After having done the work which has just been described, we are in a position to turn to the oxalate method, which is the first method described in this paper. It involves the decomposition of cadmium nitrate, and is therefore affected by an error from this source, only it is not as large as in case of the oxide method. If 2.95650 grammes ofcadmium oxide prepared in a porcelain crucible contain .00081 grammes of impurity, an error of −.24 of a unit would be introduced in the atomic weight as determined by the oxide method or +.10 in case the oxalate method were employed. That is the oxalate should give about 112.48 for the atomic weight of cadmium, but it really gives a very much lower result. Morse and Jones obtained 112.04 ± .035 by it, while Partridge obtained 111.81 ± .035 by it. If we take 112.38 for the atomic weight of cadmium, there appears to be a second error of .44 of a unit in the method as used by Morse and Jones, whilePartridge’s result indicates an error of .57 of a unit. Partridge only moistened the oxide obtained from the oxalate with a few drops of nitric acid before making the final heating, and it seems probable therefore that he made no appreciable error on account of the final oxide retaining products of decomposition from cadmium nitrate. The most probable cause of this large error seems probably to be incomplete dehydration of the oxalate, or reduction to metal during the decomposition of the oxalate, and subsequent volatilization of some of it, or a combination of both ofthese. The nine determinations given in the earlier part of this paper of course vary so much that they are of no value whatever in determining the atomic weight. The reason that the first four are low is probably due in part to sublimation of cadmium, for on dissolving the resulting oxide in nitric acid a considerable quantity of metal was noticed in each case. In the others, the temperature was kept lower, and the decomposition took a longer time. No metal was observed on taking up in nitric acid. To be certain of what the cause of error is wouldrequire some very carefully conducted experiments, but as there are a number of much more reliable methods for determining the atomic weight of cadmium, it does not seem desirable to spend the time required in making them. It should be mentioned that Lenssen, in 1860, first employed this method. He made three determinations. 1.5697 grms of cadmium oxalate giving 1.0047 grammes of oxide, which gives a value of 112.043 for the atomic weight of cadmium . The difference between the highest and lowest determination was .391 of a unit.