CADMIUM SULPHATE AND THE ATOMIC WEIGHT O F CADMIUM BY W. I,.
t
PERDUE AND G. A. HULETT
The most generally used analytical method of determining the equivalent weight of a metal is to determine the ratio of the metallic haloid to the silver haloid which is obtained from it. This method is not particularly direct but has been found to be the most satisfactory from an experimental standpoint and this is due, in a large measure, to the refinements in method introduced by Richards and his collaborators. The estimation of a metal by electrolysis is one of the simplest and most direct methods for determining the ratio of a metal to its acid radical, haloid or to oxygen, but comparatively few such determinations have been made and not much weight has been given to those which have been made in this way. It would seem that the trouble has been due to the difficulties encountered in removing the metal completely from the solution in a sufficiently pure state and in obtaining it in a weighable condition without oxidation, loss or inclusions. In the preceding paper we have shown that it is possible to avoid these difficulties and the results obtained with cadmium sulphate were so concordant that it seemed worth while to make a more extensive study of this salt and the electrolytic method of determining its percent of cadmium.
Cadmium Sulphate CdS0,.8/3H20 is readily obtained in large clear crystals and it does not seem to be isomorphous with any other salts,' so that recrystallization offers an exceptional opportunity for bringing this substance to a high state of purity. Ferrous and cupric sulphates seem to be isodimorphous with the crystallized CdSOp.8/3H20,' but only very small Rammelsberg: Pogg. Ann., 115, 579; Kopp: Ber. chem. Ges. Berlin, Zeit. phys. Chem., 16,590. Retgers: LOC.cit.
12, 911;Retgers:
156
W . L. Perdue and G. A . Hulett
amounts of these substances are takefi up by the CdSO, crystals even when they are present in considerable concentration, but the method of purification we have used eliminated these metals. Kahlbaum’s cadmium sulphate was dissolved t o about a I O percent solution and H,S was added until the precipitate was clear yellow. The solution was digested on the water bath with this precipitate until all the other heavy metals had been displaced and the precipitate was in a good condition t o filter. The clear solution was now treated with H,S until most of the cadmium was precipitated. The cadmium sulphide obtained in this way was thoroughly washed and treated with redistilled nitric acid until the sulphide was changed t o sulphate and nitrate. A slight excess of sulphuric acid was now added and the liquid evaporated and heated until fumes of sulphuric acid appeared. The whole was now cooled and the cadmium sulphate drained t o free it from the excess of sulphuric acid and then it was heated in a covered platinum dish in a specially constructed furnace until all the excess of sulphuric acid had been driven off. The cadmium sulphate obtained in this way was dissolved, filtered and repeatedly recrystallized. A saturated solution of cadmium sulphate has a density of I .61 and is very viscous so that it was found best t o start with a rather shallow solution I cm deep in the crystallizing dish and then the crystals formed on the bottom of the dish in a very satisfactory manner. The dishes were covered with filter paper and then placed in a constant temperature room so that the evaporation would be slow and uniform. These conditions gave us the maximum number of clear crystals but in every crop there were some cloudy ones which showed inclusions of mother liquor. The difference between the clear and cloudy crystals was very striking, the clear crystals were water” clear in all parts and the microscope did not reveal a suggestion of inclusion. Only these clear crystals were taken for further recrystallizations and finally conductivity water and Jena glass or platinum dishes were used. The crystals were re‘ I
Cadmium Sulphate and the Atomic Weight of Cadmium
I 57
moved from the dish with ivory forceps and wiped with sheet rubber (the rubber dam used by dentists). This was done because it was found that filter paper left the crystals slightly etched. This we think is due to the phenomenon of absorption. The filter paper coming into contact with the mother liquor on the face of the crystal, absorbs some of the dissolved cadmium sulphate and forms an undersaturated solution which attacks the crystals. With sheet rubber this action is a minimum and the faces of the crystals were left in a perfect condition. These crystals were perfectly stable in the dry air of our special balance room; a test extending over 14 days showed no change in weight. Cadmium sulphate was described by Stromeyerl in 1822, who gave it the formula CdS0,.4H20, and it has been the subject of extensive investigations since that time; but only recently' has the composition of these crystals been definitely settled. The trouble has been in determining the percent of water by glowing the crystals. Not until a loss of sulphate was detected, when the salt crystals were heated above a red heat, were the proper precautions taken and then more concordant results were obtained. We find that CdSO4.8/3H,O loses its water at a red heat and some SO, and then the CdO volatilizes. This difficulty was obviated by heating the crystals in an atmosphere which contained a little SO, and then we found that all the water could be driven off and the CdSO, remained perfectly stable up to at least 700'. Under these conditions the determination of the percentage of water in these crystals gave concordant results. The determination of the percent of water of crystallization was carried out as follows : 5 to 7 grams of the crystals were placed in a tall narrow platinum crucible (IS mm by 65 mm). This crucible was furnished with a tight-fitting cover but a Schweiggers' Jour.,
22, 368.
* Follenius: Wied. Ann., 65, 348; Worobieff: Zeit.
phys. Chem., 23, 557; Mylius and Funk: Ber. .chem. Ges. Berlin, 30, 825; Kohnstamm and Cohen: Wied. Ann., 65, 348.
158
sW. L. Perdue and G. A . Hulett
special perforated cover was used when the crucible was heated and through this perforation extended the small porcelain tube which delivered the air with a little SO,. The crucible was heated in an electric furnace and was inside a quartz test tube which was also covered. The temperature was under excellent control and was measured with a thermocouple. The arrangement is sufficiently indicated in Fig. I . The air which was passed over the crystals, while they
Fig. I
were being heated, was filtered and dried and then passed through fuming sulphuric acid which gave the necessary SO, concentration so that there was no loss of SO, or CdO from the crystals. A series of experiments gave the temperature and time needed to remove the water of crystallization so that there was no change in weight when heated to a higher temperature. The range used was from 600° to 700'. After these conditions were worked out we undertook a determination of the water of crystallization in the purest crystals with the following results :
'
Cadmium Sul+hate and the Atomic Weight of Cadmium
I 59
TABLEI Percentage of H,O in CdSO,. 8/3H,O __
No.
IMass CdS0,.8/3H2(
Mass CdSO,
Mass H,O
5.14303 5'46507 5 58677 4,59205 5 53568 5,97274 6,29717 6.40718 5,37196
I . 1856 I . 25986 I . 2886
Percent H,O
I
I
1
2
I
6.32863 6.72493 6 87537 5.65027 6.81125 7.34977 7 74837 7 8843 6.6100 '
' '
'
'
,05822 1.27557 1,37703 1.3572 1.47713 1,2480
I
18'734 18.734 18.742 18,729 18.727 18.736 18.727 18.734 18.730
In the above table the percent of water amounts t o 18.733, which is the arithmetical mean of nine determinations, the probable error of the mean is fo.001, while the probable error of a single experiment is f0.0045. The results seem to be entirely satisfactory, since we found that after the water had been expelled at say 670°, no further loss of weight was observed on heating to 700'. The vacuum corrections used in the above table were calculated from the density of CdS0,.8/3H20 and the density of the anhydrous CdSO,. The determination of the density of CdS0,.8/3H20 was carried out as follows: The crystals were weighed in a small platinum crucible suspended by platinum wires ( 0 . I mm) first in air and then in toluene. The toluene was dried and distilled and its density determined at 25 ' and 23.7 ' with a Sprengle-Ostwald pycnometer. Two determinations gave 3.0900 and 3.0902 as the density (M/V at 24'). The density of the dehydrated crystals was obtained b y heating the crystalline salt to constant weight, obtaining the weight of the dehydrated salt, and then placing it in a vacuum desiccator over calcium chloride where it remained in a vacuum of less than I .o mm for several hours. The vacuum desiccator was provided with a separatory funnel
I 60
W . L. Perdue and G. A . Hulett
so arranged that it was possible to allow the toluene t o drop slowly onto the salt while it was still in vacuo; thus assuring a complete penetration by the toluene into all the interstices of the salt and eliminating any error due to included air. The crucible and contents were now removed and weighed in toluene. The density of the dehydrated salt was found to be 4.691 a t 24'. The salt maintained the general contour of the crystals and did not fall into an amorphous powder but was quite hard. This greatly facilitates the ease of handling. All weighings were made in a special balance room where the temperature was very constant, and where the moisture content of the air was under control. The weight of one cc of air in this room was I . 18 mg and the variations from day t o day were negligible. The weighings were made by the Borda method of substitution: the set of weights used being very carefully calibrated, using a I oo-gram certified standard whose density was 8.391; standard certified I and IO gram weights were used as checks on the calibration at the proper places. The specific volumes of both salts and weights are as follows : CdSO,. 8/3H,O CdSO, (anhyd) Weights
D,, D,, D
3.0901 4.691 = 8.39
= =
SP. V O ~ .=
sp. vol. sp. vol.
=
0.3236 0.2132
= 0.1190
Thus each gram of the crystals displaced 0.2064 cc more than the weights. The correction was 0.2046 X I . 18 = $0.242 mg per gram, For the anhydrous salt the corresponding correction was 0 .T 23. Determinations were next made of the metallic content of both crystalline and anhydrous cadmium sulphate. The percentage of cadmium in both of these substances was determined by the method and apparatus described in the previous article. A slight modification was made in that the crucible and mercury were put into a glass cup which was provided with a split cover as indicated in Fig. 2. The object of this cup was to make sure that there was no loss of electrolyte. After each determination the cup was rinsed
+
Cadmium Sulphate and the Atomic Weight of Cadmium 161 out and the washings tested for cadmium but none was ever found. We took particular pains to convince ourselves that all the cadmium was deposited in the mercury. t
I
I
I
Fig.
2
The weighed crystals of CdS0,.8/3HZO were placed directly on the mercury in the platinum crucible, then conductivity water was carefully added so as to cause as little solution of the sulphate as possible. A drop or two of sulphuric acid was added to aid in the conduction of thecurrent and then the current was regulated so that there was no generation of gas at the cathode. The cadmium was thus deposited as fast as solution took place and the crystals melted away without cadmium appearing in the upper part of the electrolyte. When the cadmium was practically all deposited the current density was increased and the generation of the gas a t both anode and cathode thoroughly stirred the electrolyte, rinsed down the sides of the crucible and a t the same time the last trace of the cadmium was rapidly deposited. The electrolyte was removed as described in the preceding article (p. 152) and it was of course examined for cadmium by the ferrocyanide method, but no trace of the metal
W . L.Perdue a.nd G. A . Hulett
I 62
was discovered in the exhausted electrolyte OL any of the washings. TABLEI1 Percent of cadmium in CdSO, .8/3H,O No.
1
MassCdS0,.8/3H,O
1
Mass Cd
~
7.90902 9.07468 7'32787 6.48847 5 . I 1684 8.02954 5,08743
3 46335 3,97434 3.20936 2.84186 .2,24'57 3.51755 2.22827 '
-Percent Cd
43 790 43.796 43.796 43 799 43.808 43 807 43.799 '
,
'
'
43.799
The determination of the amount of cadmium in the anhydrous sulphate was cArried out in the manner alluded t o under CdS0,.8/3H20. The precaution was taken of thoroughly washing out the crucible in which the salt had been glowed and adding the washings t o the electrolyte. The remainder of the process was carried out exactly as that of the crystallized sulphate. TABLEI11 Percent of Cadmium in CdSO,
-
No.
1
Mass CdSOl
I 2
3 5 7 8 9
1
Mass Cd
I
!
5 . I4303 5,46507 5'58677 5 53568 6,29717 6.40718 5 .37 196 '
~
Percent Cd
I
94566 3.01076 2.98276 3,39295 3.45255 2.89457 2
53.897 53 898 53.891 53,883 53.880 53.887 53 883 '
'
'
1
I I I
'
53.888
The vacuum correction for cadmium dissolved in mercury was readily obtained from the density of cadmium amal-
Cadmium Sulphate and the Atomic Weight of Cadmium I 63
'
gams. Hulett and DeLury' found that the density of these amalgams was a linear function of the percent of cadmium, and at 24' could be expressed as follows: D,, = 13.53640.0609 x p. From this it was easily found that a gram of cadmium dissolved in mercury displaced 0 . 1 0 7 cc of air and as a gram of brass weights displace 0 . I 19 cc, the vacuum correction for each gram of cadmium dissolved in mercury was -0. o I 4. In the crystals we found 43.799 percent of cadmium as the arithmetical mean of seven determinations, and the probable eiror of this mean was fo.0016. The percent of water by difference was 18.733 (see above). The 1911 atomic weights give cadmium I I 2.40, sulphur 32.07, hydrogen I .oo8. This gives the molecular weight of CdS0,.8/3H20 as 256.51 and its percentage composition as follows : Cd
=
43 819
8/3H,O
=
18 7 2 9
so,
-
These percentages are decidedly different from those found above. If, however, we take the atomic weight of cadmium as I I 2.30 the calculated percent is: Cd
so,
=
-
43'797
8/3H,O = 18.736
which is in excellent agreement with our results. If now we take I 12.30 as the atomic weight of cadmium then the anhydrous CdSO, would contain 53.895 of cadmium, while we found in the above table 53.888 & 0.0013. This difference is much greater than our experimental error. If the water had not all been expelled by heating the crystals the anhydride still contained water and the percent of cadmium would therefore have been low. Now the loss in weight of the crystals calculated as water was 18.733, the theoretical calculation being 18.736, which indicated a slight retention ' J o u r . Am. Chem. SOC.,30, 1805.
I64
W . L. Perdue and G. A . Hulett
of water by the anhydride but not sufficient t o measurably affect its percent of cadmium. If the crystals had lost a slight amount of SO,.CdO in spite of the presence of SO, in the air, then the observed loss would not all be water, but some of it would be SO,.CdO and a correspondingly greater amount of water would then be left in the anhydride. The data which we had obtained gave us the possibility of testing this point by a simple calculation: In the experiments in Table I11 the crystals were weighed, glowed, and the percent of cadmium in the residual anhydride was determined. From this data we can also calculate the percent of cadmium in the original crystals and it gives 4 3 . 7 9 3 f o .OOI instead of the 4 3 . 7 9 9 percent found in the unglowed crystals (Table 11). This shows a loss of 0.006 percent of cadmium or 0 . 0 1I percent of CdSO, lost by glowing the crystals so that the total loss, 1 8 . 7 3 3 percent, was therefore composed of 18.722 percent of water and o . O I I percent of CdSO,, consequently IOO grams of the crystals lost 18.722 grams of water and 0 . 0 1 1grams of CdSO,, while the remaining 8 1 . 2 6 7 grams of ahhydride retained 0 . 0 1 4 gram of water (18.736 1 8 . 7 2 2 ) or 0 . 0 1 7 2 percent of water. Now CdSO, which contains 0.0172 percent of water would show 53.888 percent of.cadmium if the atomic weight of cadmium is taken as I I 2 . 3 0 We actually obtained in our results (Table 111) 5 3 . 8 8 8 percent of cadmium in the anhydride. In our endeavor to get pure anhydrous CdSO, we attempted to heat this substance t o its melting point but as the SO,, used to prevent decomposition, is measurably dissociated, long before the melting point of the salt is reached, it is therefore no longer effective in preventing measurable decomposition of the CdSO,. From the constancy of the results and agreement with the calculated value (Table 11) it seemed quite certain that all of the water had been expelled by heating t o 700' but it is evidently a case of small compensating errors and shows how constant such errors may be. The CdS0,.8/3H20 crystals used gave evidence of being
Cadmium Sulphate and the Atomic Weight of Cadmium 165
very pure and free from all inclusions. The analysis of this substance points to I 12.30 as the atomic weight of cadmium, a value so much lower than the accepted I 12.40 that it seems desirable to check this result by determining the percent of cadmium in simpler substances such 'as the oxide and chloride. Princeton Ulziversity, November, I 9 I 0
