The Chloride Method.
Huntington had determined the ratios of CdBr2to AgBr and also CdBr2to Ag very carefully, obtaining the result 112.24 for the atomic weight of cadmium. Morse and Jones had obtained 112.07 for thisconstant by the oxide method. The object of the work about to be described was to find the cause of this discrepancy if possible. It was thought advisable however to make some determinations of the ratio of CdCl2to AgCl before beginning the bromide method.
Dumas, in 1859, used cadmium chloride to determine the atomic weight of the metal. He did not establish its ratio to silver chloride but to silver by titration. He prepared cadmium chloride by dissolving the metal in hydrochloric acid and melting the resulting product in a platinum capsulefor five or six hours. He made two series of three determinations. The chloride used in the first series was yellow in places and not completely soluble. The result was 112.476. The second series was made with chloride which was perfectly white and soluble and gave 112.007 for the atomic weight of cadmium. It is evidently more reliable than the first series and Dumas himself concluded that the atomic weight is very near 112.01.
Four different specimens of cadmium chloride were used in this work and from thesespecimens portions were taken for analysis. These portions were treated differently in different analyses, therefore it will be necessary to give a brief descriptions of them and mention the number of the determinations, in which each one was used. Chloride of cadmium was prepared in the following manner. A solution of pure hydrochloric acid was prepared by passing a current of hydrochloric acid gas into pure water which was contained in a porcelain crucible until no more was absolved. The water used had been purified by distilling against a platinum dish and thehydrochloric acid gas was obtained by heating ordinary concentrated chemically pure hydrochloric acid in a distilling bulb whose neck had been closed by fusion in order to avoid the use of a cork or rubber stopper. Hydrochloric acid thus prepared will leave no residue on evaporation when air is excluded (Stas, Aronstein’s German translation, p. 111). A piece of platinum foil freed from iron by heating in the vapors of ammonium chloride as recommended by Stas (Aronstein’s translation, p. 112) was introduced and a piece of cadmium laid on it. Solution begins at once, the hydrogen being liberated onthe platinum foil. During the later part of the process, heat was applied. After all of the metal had dissolved, the solution was evaporated, the platinum foil having previously been removed. The crystals of cadmium chloride which separated were not dried but allowed to remain slightly moist with hydrochloric acid. If no platinum foil is used, the solution of the pure metal becomes exceedingly difficult, unless a very large excess of acid is used. No objection can be raised to the use of platinum foil for in making fifty grammes of cadmium chloride it cost less than a tenthof a milligramme and even this could probably have been avoided by using a somewhat larger amount of hydrochloric acid. The foil was always kept submerged in the acid liquid. The moist crystals of cadmium chloride were transferred to a combustion tube passing through an asbestus covered air-bath, and dried in a current of hydrochloric acid gas for several hours at 300°C. The hydrochloric acid gas had been passed through a long calcium chloride tube to dry it, although calcium chloride probably does not do this very thoroughly. The hydrochloric acid gas was then replaced bya current of nitrogen prepared as has already been described under the sulphide method. After the current of nitrogen had been passing for about half an hour, the tube was allowed to cool, and the chloride transferred to another combustion tube, one end of which had been sealed in the flame of a blast lamp. The other end was drawn out and attached to a Sprengel mercury pump. After exhausting, the chloride was sublimed in the vacuum. This takes place at a moderate temperature and the sublimate has a beautiful crystalline structure and is perfectly white.The crystalline mass exposes so much surface that water is taken up very rapidly when exposed to the air. This action is so rapid that the crystals cannot be transferred to a weighing-glass without introducing an appreciable error. The whole sample was accordingly transferred to a stoppered glass bottle which was kept under a bell jar with sticks of caustic potash. Three samples were prepared in this manner, the first being used in determination one, the second in determinations two to seven inclusive, and the third in determinations eight to nineteen inclusive.The samples used in determinations twenty and twenty-one were prepared in the following manner: About three grammes of cadmium were placed in a combustion tube in which three bridges (as in the distillation of pure cadmium) had been made. A section may be represented thus
The metal was placed in cavity A and a stream of chlorine passed through the tube. The chlorine was prepared from potassium bichromate and hydrochloric acid and driedby passing it through a long tube containing calcium chloride. When the air had been displaced, the cadmium was heated. It fused and began to burn to the chloride which partly flowed over the bridge into cavity B and partly distilled over into this cavity. When the reaction had ended, the current of chlorine was replaced by one of dry nitrogen, and the tube was allowed to cool and the chloride taken for analysis XX. The specimen used in analysis twenty-one was prepared in exactly the same way, only the chlorine used was obtained from manganese dioxide, sodium chlorideand sulphuric acid, and was dried with phosphorous pentoxide instead of calcium chloride.
The special treatment of the portions taken for analysis was as follows: Those taken for determinations I, II and from XI to XIX inclusive were placed in a platinum boat and put into combustion tube. A current of hydrochloric acid gas obtained by heating the aqueous acid was passed through the tube. The gas had been dried by calcium chloride. When the air was displaced, the chloride was heated somewhat higher than its fusing-pointi.e. to incipient redness, and maintained there for a length of time varying from a few minutes to more than an hour. The hydrochloric acid was then displaced by a current of nitrogen, and the chloride allowed to cool. The boat with the chloride, while still slightly warm, was placed in a weighing-tube, cooled in a dedicator containing caustic potash and weighed. The chloride thus prepared is transparent and presents only a small surface to the air. It takes water up so slowly that no error is introduced from this source. This was tested in one case byallowing a boat containing some chloride thus prepared to stand in the air for a certain length of time and noting the increase in weight. It was quite slow. In several cases specimens of chloride were tested for hydrochloric acid using tropaeolin as an indicator. It was always found neutral. The portions used for determinations III and VI to X inclusive were prepared in exactly the same manner as the preceding ones except that the hydrochloric acid gas in which they were fused was not dried but used just as it came fromthe aqueous acid. In some cases the platinum boat in which the chloride was fused was weighed before and after the fusion. The weight remained unchanged.
For determinations IV and V, about six grammes of cadmium chloride were placed in a platinum boat, and more than two-thirds of it distilled out in a current of hydrochloric acid gas which had not been dried. Part of the distillate was collected after cooling in nitrogen and used in determination IV while the residue remaining in the boat was used for determination V.The method of preparing the chloride used in determinations XX and XXI has already been described.
Thinking that a Gooch crucible with a platinum sponge on the bottom in place of asbestus would be desirable for this work one was accordingly made and answered the purpose very satisfactorily. All determinations were made by using such filters. C. E. Munroe (Chem. News, Vol 58, p. 101) has described the preparation of these filters.A platinum Gooch crucible was placed on a filter paper and some ammonium platonic chloride which had been thoroughly washed introduced by suspending it in alcohol and then pouring this into it. The precipitate settles to the bottom forming a uniform layer and the alcohol drains, off through the filter paper. The crucible was then dried slowly in an air-bath. After this it was transferred to a porcelain crucible and slowly heated until decomposition was complete. In this manner a layer of platinum felt is obtained which acts as a very efficientfilter. Another layer of double chloride was then decomposed as before so that if there were any imperfections in the first layer they would be covered by the second layer. The surface was smoothed down by means of a glass rod. To prepare a good filter the drying and subsequent heating should be very slow. The heating must not be at too high a temperature, otherwise the felt becomes very compact and is useless for filtering purposes. Pressure produces the same effect. The filters were always treated with strong nitric acid, washed andreheated before being used, but in no case was chlorine detected in the nitric acid after the washing, nor any loss in weight of the crucible. An objection to the use of these crucibles for the purpose named was found in the course of this work, but it will be discussed later. The crucibles were always set in a large weighing-glass, and another weighing-glass containing an equal amount of platinum foil used as a tare, in weighing. This precaution was perhaps unnecessary, but at least it did no harm.
Analytical Process.
The weighed cadmium chloride was dissolved by placing the boat containing it in an Erlenmeyer flask containing water. The boat was then washed, dried and replaced in its weighing-tube. On weighing again, the loss in weight is equal to the weight of cadmium chloride taken. All samples gave a perfectly clear solution except those used for determinations XX and XXI. A drop of nitric acid (1:3) was added to each solution except in determination XIV where the cubic centimetreswere added, and in XVI where ten cubic centimeters were added. A solution of silver nitrate was then added to precipitate the chlorine. This as well as the subsequent washing was done in a dark-room illuminated by a single gas light whose rays had to pass through a strong solution of neutral potassium chromate. The precipitate was contracted by warming on the water-bath. It was then collected in the prepared Gooch crucibles and washed. Before filtering, the flask containing the precipitate and mother-liquor wasallowed to cool. Silver chloride is soluble in water to a considerable extent but is reprecipitated by adding an excess of either silver nitrate or hydrochloric acid. Stas (Ann. de Chem. et Phys. [4], 25, 22; [5], 3, 145; [5], 3, 289.) investigated this very thoroughly. Cooke also did some work on it and used a dilute solution of silver nitrate to wash the chloride thus preventing solution (Proc. Amer. Acad. 17, 7.). In the above work, therefore, a solution containing 0.10 grammes of silver nitrate per liter was first used, followed by one only one-tenth as strong, and finally pure water was used.Only two or three washings could be made with water as the chloride went into solution after this owing to the removal of the silver nitrate. The last silver nitrate solution used is so weak that any error introduced by not washing it out completely is insignificant. After washing, the silver chloride was dried at temperatures varying from 150°C. to 300°C. to constant weight. A glass air-bath was used in order to prevent products from the burning gas from coming in contact with the chloride. It was then weighed.The quantity of silver nitrate used in the determinations was varied very much. The excess over what was required to precipitate the chloride is given in the table of results in those cases in which it is known. The quantity of water used in each determination is also given where it is known. It is given in the number of cubic centimetres used per gramme of cadmium chloride and does not include wash water. All weighings are reduced to the vacuum standard on the basis that Sp. Grs. of CdCl2= 3.94 and AgCl = 5.5. The results are:
In the first five determinations, the analytical operations were conducted as nearly as possible alike, but the preparation of the portions of cadmium chloride taken for analysis was varied very much as will be seen by referring back to this part of this paper. The results do not vary more than ±0.015 from their average. This is verystrong evidence of the purity of the chloride used for, if it contained any impurity, we should have expected to vary the amount in the different portions. After this, attention was paid especially to the analytical process, for it was thought that there probably was some serious error in the method, the result being higher than any that had previously been obtained, if we exclude Dumas’ first series which he himself did not accept. The conditions were varied in many ways to see how much the result could be influenced, but under no conditions wereresults as low as Huntington’s average (112.24) obtained. A number of errors were found in the method during the work, but they seem to neutralize each other to a great extent. The more important ones will now be given. Nearly every filtrate including the corresponding wash water was examined for chlorine after the silver and cadmium had been precipitated by hydrogen sulphide. The excess of hydrogen sulphide was expelled by boiling, after the addition of some nitric acid. In two cases an inverted condenser was used. On adding silvernitrate a precipitate was always obtained showing the presence of chlorine. Care was always taken to filter off sulphur formed by the oxidation of hydrogen sulphide, before adding the silver nitrate. The precipitate was never very heavy, and was not estimated quantitatively. It is evident that cadmium nitrate exerts a solvent action on silver chloride. In some cases a very large excess of silver nitrate was added but it did not change the results markedly. Silver nitrate itself dissolved silver chloride to some extent. The increase in insolubility, if any, on addingan excess of silver nitrate is probably counterbalanced by the increased error due to occlusion of nitrates in the silver chloride. Stas (Aronstein’s Trans. p. 156) says it is impossible to contract silver chloride or bromide in a solution containing salts without there being occlusion and that the precipitate can only be freed from them by dividing up the contracted mass by shaking with pure water. This was not done here owing to the solubility of silver chloride in pure water, and the complications introduced in the analytical part. The occlusionof nitrates by the silver chloride would lower the atomic weight found. The silver chloride obtained always darkened on heating and contained cadmium, as was shown in the following manner: The lump of silver chloride was attached to the negative pole of a cell and electrolyzed in a bath containing dilute sulphuric acid. The resulting metal was then dissolved in nitric acid and the silver precipitated by adding hydrochloric acid. The filtrate was evaporated to expel the nitric acid and the residue taken up with waterand tested for cadmium with hydrogen sulphide. An appreciable quantity was always found. This method of examination does not show the occluded silver nitrate. Another error which tends to lower the atomic weight found is due to the platinum crucibles used for filtering. If a silver nitrate solution is filtered through such a crucible there will be an increase in weight due to silver being deposited. This takes place in acidified solutions as well as in neutral ones. Washing with ammonia does not remove the deposit, butstrong nitric acid does, the washings giving a test for silver. Whether the depositing of silver is due to the action of spongy platinum in contact with the compact metal of the crucible or to some impurity in the platinum sponge was not determined, but the former seems by far the most probable. The increase in weight during the time required for filtering a determination must have been quite small however. The samples of cadmium chloride employed for determinations XX and XXI were prepared by burningcadmium in a current of chlorine. The glass tube used was attached somewhat and the solution of the chloride was very slightly turbid in each case. The turbidity was so slight however, that no very serious error could have resulted from it, particularly as it was probably partly counterbalanced by the formation of some potassium chloride. For more accurate work, it should have been made and redistilled in a porcelain tube. These two samples were tested for free chlorine with potassium iodide and starch paste, but none was found. Someof the specimens of chloride prepared by fusion in a current of hydrochloric acid were found to be neutral, using tropaeolin as an indicator.
As nearly as can be judged, the above errors would probably counterbalance each other to a great extent, and thus give a fairly close approximation to the atomic weight of cadmium when the average of all the determinations is taken. The value 112.383 thus obtained can only be regarded as tentative.