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 highfor 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 caseof 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 solublematter 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.
Fig. 2.
Fig. 2.
Fig. 2.
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 wasbarely 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 wereequal. 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 acidis used. A vertical section of the crucible and weighing-glass is shown in figure 3.
3
3
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 temperaturewhich 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:
1. Could not find any record of its Sp. Gr., 3.0 is assumed.
1. Could not find any record of its Sp. Gr., 3.0 is assumed.
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 acidwe 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 theminimum 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 SO3and not as H2SO4as 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.