Part 3

The first three determinations were made exactly like those in the chloride method. The last two were also made in the same manner, only the washing of the precipitate was varied. After the silver bromide had been contracted by warming on a water-bath it was washed by decantation and then agitated violently with cold water to remove occluded nitrates, but it was then so finely divided that it could not be filtered. The artifice used by Stas to contract it a second time was to pass a current of steam into the milky liquid. This was tried here, but for some reason or other did not work very well, and considerable difficulty was had in filtering it. The results of the five determinations are tabulated below. All weighings are reduced to the vacuum standard on the basis of Sp. Gr. of CdBr_{2} = 4.6 and Sp. Gr. AgBr = 6.62.

────────┬────────┬────────┬────────┬────────────┬────────┬──────────── No.│CdBr_{2}│ AgBr│ H_{2}O │Ex. AgNO_{3}│ At. Wt.│Remarks ────────┼────────┼────────┼────────┼────────────┼────────┼──────────── I│ 4.39941│ 6.07204│ │ │ 112.43│Distillate } II│ 3.18030│ 4.38831│ │ │ 112.42│Residue } III│ 3.60336│ 4.97150│ │ │ 112.45│Resublimed. IV│ 4.04240│ 5.58062│ │ │ 112.29│ V│ 3.60505│ 4.97519│ │ │ 112.38│ ────────┼────────┼────────┼────────┼────────────┼────────┼──────────── │ │ │ │ Average │ 112.394│

Discussion of the Results.

The first three specimens were prepared under widely different conditions yet the results agree quite closely. The last two were prepared from the repurified hydrobromic acid. If chlorine had been removed during the second purification we should expect a higher result but the results are lower. There seems to be hardly any doubt that this is due to analytical errors rather than a change in the composition of the bromide. Whether this be true or not, the five determinations all fall within the limits obtained by the chloride method and confirms it as fully as can be expected.

The errors of the method are the same as those of the bromide method, only they are probably less in most cases. One filtrate was examined for bromium, but none was found showing the method to be more perfect in this respect.

Syntheses of Cadmium Sulphate.

It was next thought of examining the method based on the conversion of cadmium sulphate into cadmium sulphide, which has been used by von Hauer whose result is 111.94 for the atomic weight of cadmium, and more recently by Partridge who obtained a much lower result, namely 111.73. They dried cadmium sulphate in porcelain boats, and then reduced it to sulphide by heating in a current of hydrogen sulphide. The reduction begins in the cold and is probably complete or at least nearly complete before the temperature is sufficiently high for cadmium sulphate to decompose into cadmium oxide, for the sulphate is very stable with respect to heat. This being the case, probably no error results from the formation of a silicate of cadmium in this method. The main difficulty in this method would be to prove that the cadmium sulphate used is free from water. Neither von Hauer nor Partridge has done this because drying a substance to a constant weight is not sufficient evidence of its anhydrous character, especially if the drying is done at a constant temperature. This has been shown very clearly in the case of copper sulphate, by Richards (Proc. Amer. Acad. Sci. 26. 263.)

It was therefore decided to attempt the synthesis of cadmium sulphate, hoping to be able to fix a minimum value for the atomic weight of cadmium.

A piece of hard glass tube was closed at one end by fusion and the other end drawn out into a small tube which was then bent twice at right angles. The large part was cut off near the beginning of the smaller tube, and the edges rounded by fusion. It was filled with dilute sulfuric acid and heated for some time to remove soluble matter from the glass. After removing this acid, a weighed piece of cadmium was introduced and an excess of dilute sulphuric acid (1: 3) added. The tube contained a small piece of platinum to aid the solution of the cadmium. During the process of solution, the two parts of the glass tube were held together by a rubber band, and the outlet of the smaller tube dipped under pure water contained in a small tube closed at one end.

A section of the arrangement is shown in figure 2. Solution was aided by the application of heat. These precautions in dissolving the metal were taken to prevent loss by spraying. After the metal had been dissolved, the solution and the water through which the hydrogen had escaped were transferred to a porcelain crucible. An equal amount of sulphuric acid was then added to the tare and both were heated until fumes of sulphuric acid ceased to come off. The crucible containing the dry sulphate was next placed an a porcelain plate in a nickel crucible set in a hole in an asbestos board. This was placed over the flame of a Bunsen burner, so that the bottom of the nickel crucible was barely at a red heat. The temperature on the inside of this bath was considerably lower. After the weight had become nearly constant, the sulphate was tested for sulphuric acid by means of standard alkali using tropaeolin as an indicator. It was found acid, but so slightly that no attempt was made to estimate it. Result = 112.35 is preliminary.

Another synthesis was made as follows: A platinum crucible, lid and perforated cone were place in a large weighing-glass and tared with a similar weighing-glass containing a platinum crucible, platinum foil being added until the weights were equal. After these had been accurately weighed, a weighed piece of cadmium was added to the one containing the cone. The cone was inverted over the piece of metal on the bottom of the platinum crucible. A considerable excess of dilute (1: 3) sulphuric acid was then added, the lid whose edge was bent down placed on the crucible, and the weighing-glass stoppered loosely. This was placed in an air-bath, and gently warmed during the later part of the process of solution. There is no difficulty in getting complete solution if a sufficient excess of acid is used. A vertical section of the crucible and weighing-glass is shown in figure 3.

This arrangement avoids loss from spraying, and the necessity of transferring the solution from a tube to a crucible as in the first experiment.

An equal quantity of sulphuric acid was added to the crucible used as a tare and evaporated. After the metal had been dissolved, the platinum cone was lifted to one side and the excess of acid evaporated off. It was then heated in a glass air-bath for a long time at a temperature which was probably about 400°C. After the weight had become constant, the amount of free sulphuric acid was estimated by titration with a standard alkali using tropaeolin as an indicator. 1.25 milligrammes were found and this weight was subtracted from that found at the balance. Weighing were reduced to the vacuum standard, assuming the Sp. Grs. of cadmium and anhydrous cadmium sulphate[1] to be 8.54 and 3.0 respectively. The results were as follows:

Footnote 1:

Could not find any record of its Sp. Gr., 3.0 is assumed.

Cd CdSO_{4} At. Wt. I. (112.35 can only be regarded as a preliminary experiment) II. 1.15781 2.14776 112.35

Discussion of the Results.

These results agree fairly well with those obtained by the chloride and bromide methods. The second experiment is more trustworthy than the first. In it, we started with pure metal and the manipulations were so simple that no serious error could have been made in them. Hence it will only be necessary to consider the end-product, i.e., the cadmium sulphate. The titration showed that the sulphate was not basic owing to loss of sulphur trioxide, and after deducting the weight of the excess of sulphuric acid we must have left a weight of cadmium sulphate which is equivalent to the metal employed. The question now is, did it contain anything else and what would be its effect? Clearly the effect of water or any other impurity would be to lower the atomic weight found, hence the atomic weight must be at least as high as the experiment indicates. As the cadmium sulphate is deposited, at least the later part of it is from a strong sulphuric acid solution, it probably does not contain any water and in this case would fix a maximum value as well as the minimum value, and thus determine the atomic weight. It might be objected to the second experiment that the sulphuric acid found may have been present as SO_{3} and not as H_{2}SO_{4} as was assumed. This seems highly improbable, and even if it were so the error introduced would be only about .03 of a unit in the atomic weight. As the first determination was found practically neutral, it does not apply to it at all. The most probable conclusion from these experiments is that the atomic weight of cadmium is about 112.35. A more thorough study of this method would have been made if time had permitted it.

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 over a 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 the cadmium 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:

Cd CdO At. Wt. Cd. I. 1.26142 1.44144 112.11 II. .99785 1.14035 112.04 —————— Average 112.08

III. 1.11321 1.27247 111.84 IV. 1.02412 1.17054 111.91 V. 2.80966 3.21152 111.87 —————— Average 111.87

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 in porcelain 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 explanation of 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 found only 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 powdering of 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 was made 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:

Initial Wt. Final Wt. Gain Corresponding Error in At. Wt. I. 2.95469 2.95650 .00081 −.24 II. 2.67717 2.67835 .00117 −.39 III. 3.00295 3.00422 .00127 −.38

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 has already 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 for ammonia 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 is longer 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 112.31 and 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 in weight 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 be the 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 gas formed 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 1.9 cubic centimetres. This experiment would also have shown the presence of carbon dioxide if any had been present.

Discussion of the Oxalate Method.

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 of cadmium 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, while Partridge’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 of these. 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 would require 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.

Other Methods