Part 3

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 efficient filter. 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 and reheated 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 centimetres were 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 was allowed 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. , 25, 22; , 3, 145; , 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 CdCl_{2} = 3.94 and AgCl = 5.5. The results are:

╒═════╤════════╤═══════╤════════╤═══════════════════╤═══════╤═════════╕ │ No.│CdCl{2}│ AgCl│ H{2}O│ Excess AgNO{3}│At. Wt.│Melted in│ │ │ │ │per Grm.│ │ │ │ ├─────┼────────┼───────┼────────┼───────────────────┼───────┼─────────┤ │ I│ 3.09183│4.83856│ │ │112.339│ Dry HCl │ │ II│ 2.26100│3.53854│ │ │112.329│ „ „ │ │ III│ 1.35729│2.12431│ │ │112.320│Moist HCl│ │ IV│ 2.05582│3.21727│ │ │112.339│ „ „ │ │ V│ 1.89774│2.97041│ │ │112.306│ „ „ │ │ VI│ 3.50367│5.48473│ │ 8.90│112.283│ „ „ │ │ VII│ 2.70292│4.23087│ 200│ 1.79│112.301│ „ „ │ │ VIII│ 4.24276│ 6.6398│ 300│ 8.10│112.387│ „ „ │ │ IX│ 3.40200│5.32314│ 300│ 18.95│112.368│ „ „ │ │ X│ 4.60659│7.20386│ 300│ 25.62│112.472│ „ „ │ │ XI│ 2.40832│ │ │ │112.434│ Dry HCl │ │ XII│ 2.19114│3.42724│ │ │112.433│ „ „ │ │ XIII│ 2.84628│4.45477│ 300│4.45 + 3cc. HNO{3}│112.319│ „ „ │ │ XIV│ 2.56748│4.01651│ 300│ .10│112.399│ „ „ │ │ XV│ 2.31003│3.61370│ 300│.10 + 10cc. HNO_{3}│112.406│ „ „ │ │ XVI│ 1.25008│1.95652│ 300│ 4.66│112.319│ „ „ │ │ XVII│ 1.96015│3.06541│ 300│ 3.22│112.466│ „ „ │ │XVIII│ 2.29787│3.59391│ 300│ 4.27│112.448│ „ „ │ │ XIX│ 1.94227│3.03811│ 300│ 3.61│112.423│ Dry HCl │ │ XX│ 1.10976│1.73547│ │ │112.471│ „ „ │ │ XXI│ 1.63080│2.55016│ │ │112.476│ „ „ │ │ │ │ │ │ │———————│ │ │ │ │ │ │ Average│112.383│ │ └─────┴────────┴───────┴────────┴───────────────────┴───────┴─────────┘

Discussion of the Results.

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 very strong 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 were results 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 silver nitrate 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 adding an 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 occlusion of 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 water and 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, but strong 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 burning cadmium 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. Some of 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.

The Bromide method.

Huntington (Proc. Amer. Acad. 11.28) working under the direction of J. P. Cooke, determined the ratio of cadmium bromide to silver bromide and using the total quantities for the calculation the result for the atomic weight of cadmium is 112.239. He also determined the ratio of cadmium bromide to silver, obtaining 112.245 for the atomic weight of cadmium.

The work which will now be described was carried out very much like the work described under the chloride method. The ratio of cadmium bromide to silver bromide was investigated.

Preparation of Cadmium Bromide and Hydrobromic Acid.

A large quantity of hydrobromic acid was prepared according to the method described by Dr. Edward R. Squibb (Trans. of Med. Soc. of the State of N. Y.). One part of water was added to seven parts of strong sulphuric acid (Sp. Gr. = 1.83) and the mixture cooled. Then six parts of potassium bromide were dissolved in six parts of hot water and the diluted sulphuric acid added to this hot solution. It was set aside until cold in order to allow the sulphate of potassium to crystallize out. The crystals were drained on a filter-plate and quickly washed with two parts of water. The mother-liquor and washing were then distilled until no more acid was obtained on further heating. The acid thus obtained was distilled three times from potassium bromide, twice from cadmium bromide formed by adding a piece of pure cadmium to it, and twice without the addition of anything. It was tested and found to be free from sulphuric acid. Cadmium bromide was prepared from it, in exactly the same way that the cadmium chloride used in the chloride method was prepared from pure metal and hydrochloric acid. While the crystalline mass of cadmium bromide was still moist, it was transferred to a combination tube and dried at 300°C for several hours in a current of nitrogen. It was then sublimed in a vacuum as the chloride had been. This specimen served for the first three determinations. About nine grammes of it was placed in a platinum boat in a combustion tube, and part of it distilled in a current of nitrogen. The distillate, a portion of which had been tested with tropaeolin and found neutral, was used for determination I. The residue in the boat was used for determination II. Another portion of the main sample was resublimed in a vacuum and used in determination no. III. Cadmium bromide is not hygroscopic or at least only slightly, therefore the sublimed cadmium bromide can be transferred to a weighing-glass without taking up water. This cannot be done in the case of the chloride. It is probable that the hydrobromic acid as above prepared was perfectly free from hydrochloric acid. Chlorine in cadmium bromide would cause the atomic weight to be found lower than it really is. It was thought desirable, however, to prepare an acid which would certainly be free from chlorine. The method described by Stas (Aronstein’s German translation, p. 154.) was employed with the additional precaution that the above purified acid was used to start with and all reagents employed had been especially prepared so as to be free from chlorine. Pure silver was prepared according to Stas’ description (see Aronstein’s translation, page 34, also page 104) by the action of ammonium sulphite on an ammoniacal solution of silver nitrate and copper sulphate. The silver was dissolved in nitric acid free from chlorine, and then slowly added to a dilute solution of the above-described hydrobromic acid, and the precipitated silver bromide thoroughly washed. It was then digested for a long while in a strong solution of potassium bromide, first in the cold, then by heating. The potassium bromide had been made thus: Twice recrystallized potassium hydrogen tartrate was heated in a platinum dish in a muffle furnace until it was converted into carbonate, and the excess of carbon burned off. It was then dissolved in water, filtered and neutralized with some of the hydrobromic acid already described. The carbonate had been tested for both sulphuric acid and chlorine with negative results. After the silver bromide had been digested with the potassium bromide, it was washed very thoroughly, suspended in water, and a current of hydrogen sulphide passed into it. This converts it into sulphide hydrobromic acid being liberated. The acid was drained off on a porcelain plate, and then distilled a number of times. It was finally tested and found to be perfectly free from sulphates and also did not contain free bromine. Having started with an acid which was probably pure and subjected it to these operations with reagents free from chlorine, there can be no doubt as to the purity of the resulting acid. The hydrogen sulphide used was prepared from potassium hydrosulphide as in the sulphide method, and washed first with a solution of the hydrosulphide, then very thoroughly with pure water. From the hydrobromic acid obtained, a specimen of cadmium bromide was prepared as before and sublimed twice in a vacuum. This specimen was used for determinations IV and V.

Method of Analysis.

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 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.