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case the latent heat of vaporization of the liquid under exam. ination is known, this correction is easily made by the application of equation (1), which gives in terms of latent heat, temperature, and volume, the change of the boiling point with the concomitant variation of pressure. But the latent heats of volatilization are known for only a comparatively small number of liquids. In this case, the law” treated of in the foregoing sections is specially applicable. We know from what precedes that near atmospheric pressure
The “constant” varying slightly for different classes of liquids from an average value of 20•7, at least, for normal liquids. If we set for the “constant,” the letter C, neglect the volume of the liquid in comparison with that of the vapor —which will introduce no appreciable error,--and substitute for T its equal", equation (1) becomes transformed into
(B) If now, from the gas equation
(C) we take the value of up (uo = V = a gram-molecule of saturated vapor), and set it in equation (B), we obtain the equality
(D) and if p be the normal pressure of 760mm, we get finally
dT 2T T
By putting for C, that value of the constant found for the class of liquids to which the liquid under examination belongs (see page 359), and for T, the absolute temperature of ebullition, we may obtain with a very considerable degree of accuracy the desired correction, with the restriction, however, that the variation of pressure is but slight, that is, not over 50 millimeters of mercury.
Chicago, January 22d, 1895.
ART. XXXI.-- On the Double Halides of Cæsium, Rubidium,
Sodium and Lithium with Thallium ; by J. H. PRATT.
In previous investigations upon the double halides of trivalent thallium with the alkali metals, the salts of only potassium and ammonium seem to have been carefully studied. The only cæsium and rubidium salts that have been made are Cs, TICI,. 24,0 and Rb, TICI,.2H,0 described by Godfrey, * but in the present investigation the compounds of this type were found to have one instead of two molecules of water of crystallization.
The present research has been carried out very carefully and systematically in order to obtain as complete a series of double salts in each case as possible. The salts that have been made belong to four types, corresponding to those previously made with potassium and ammonium, and are as follows, 3:1 2:1
1:1 Cs, TICI,. H,0 Cs, TICI, Cs, Ti, ci,
--- ---Cs TICI.H.0
Cs, Ti, Br, CsTIBr.
RbTII,. 24,0 Na TICI,.12H Li TICI,: 81,0 For comparison, a list of the previously described double salts with potassium and ammonium is also given.
1:1 K TICI.. 2H,0 K TICI,. 37,0 K,TI,CI.. 17H,O KT1Br. (NH),TICI.2H2O
K TI, Br, . 13,0 KTUI,H,O (NH) TIC
(NH)TIBI,.5H,O (NH)TIBr. 2H 0 '(NZ)TIBr,
(NH) TII. Several points of interest, already noticed in connection with double salts prepared in this laboratory, are well illustrated by the series of new compounds to be described. With cæsium, a more complete series of salts was prepared than with the other alkali metals ; and there is also an increase in ease of formation and in number of salts, from the iodides to the chlorides. The salts, formed from the alkali metal with
* Landenberg's Handwörterbuch.
the lower atomic weight are generally more soluble in water, form in larger crystals and with more water of crystallization thar those with higher atomic weight.
Preparation.—The double salts were prepared in each case by mixing solutions of the thallic halide with the alkali halide in widely varying proportions, evaporating and cooling to crystallization. With the bromides and iodides the conditions for obtaining the double salts were improved by the presence of a little free bromine and iodine.
The crystals, soon after forming, were removed from the solutions, quickly pressed between filter papers to remove the mother-liquor, and, with the exception of the sodium and lithium salts, allowed to stand exposed to the air for some time. The latter on account of their instability, were placed in tightly stoppered weighing-tubes as soon as they were free from the mother-liquor.
Method of analysis.—In determining thallium, the salt was dissolved in warm water and a slight excess of ammonium sulphide added to precipitate the thallium as thallous sulphide. This was filtered and washed with water containing a little ammonium sulphide. The precipitate was then dissolved in hot dilute nitric acid, the solution evaporated with sulphuric acid in a platinum crucible, and then heated to constant weight within a porcelain crucible over a small flame. The filtrate from the thallous sulphide precipitation, was evaporated with sulphuric acid, the ammonium salts driven off, and the residual alkali sulphate ignited in a stream of air containing ammonia. The halogens were determined as silver salts in separate portions, with the precaution of adding sulphurous acid in the case of the iodides to prevent loss of iodine in dissolving, and it was found to be necessary in all cases to use a large excess of nitric acid in order to obtain the silver halide in a pure condition. Water was determined by igniting in a combustion tube, behind a layer of dry sodium carbonate, in a stream of dry air and collecting it in a weighed calcium chloride tube.
3:1 Cesium and Rubidium Thallic Chlorides, Cs, TIC.. H,O and Rb,TICI. H,0.—The cæsium salt is obtained, as a white precipitate, when 0.25 g. of thallic chloride is added to a solution of 50 g. of cæsium chloride. The precipitate dissolves somewhat slowly upon heating the solution and crystallizes out on cooling. The range of conditions is very narrow as 3 g. of thallic chloride to 50 g. of cæsium chloride give the salt, Cs,TICI.. The salt is soluble in hot water, but Čs,T),CI, crystallizes from the solution.
The rubidium salt has a much wider range of formation. It is obtained when 1.5 to 25 g. of thallic chloride are added to a solution of 40 g. of rubidium chloride. It is very soluble in
cold water but gives another salt, Rb, TICI, .H,O upon crystallization. Both salts are white as are all the chlorides with one exception. Two separate crops of each were analyzed with the following results :
47.84 Thallium .... 24.21 24:45 24:37
Calculated for A.
Rb,TICI,H,O. Rubidium ..
36.81 Water ............. 2:51 1.72
2:60 The cæsium salt was obtained in hair-like crystals, too small for measurement. The rubidium salt crystallized in thin plates having a rhombic outline. Under the microscope these showed an extinction parallel to the diagonals and in convergent light a bisectrix at one side of the field, with the plane of the optic axes at right angles to the longer diagonal, indicating monoclinic symmetry.
2:1 Cæsium and Rubidium Thallic Chlorides, Cs, TICI, Cs, TICI,. 1,0 and Rb,TICI.. H,0.—The anhydrous cæsium salt is formed when 5 to 8 g. of thallic chloride are added to a somewhat concentrated solution of 100 g. of cæsium chloride, and the hydrous salt, when 8 to 15 g. of thallic chloride are added to a more dilute solution of 100 g. of cæsium chloride. The rubidium salt was observed when 1.25 to 18 g. of rubidium chloride were added to a rather concentrated solution of 30 g. of thallic chloride. The two hydrous salts are white and the anhydrous compound is pale green. The cæsium salts are readily soluble in hot water but the salt Cs, T),CI, crystallizes from the solution. The rubidium salt recrystallizes unchanged from water. The following analyses were made upon separate crops.
Calculated for 1. 11.
Cs,TICI.. Cæsium ........
41:07 Thallium .. 31:11 31.82 31.62
31:52 Chlorine ...... 27.19 2 7.30 27.20
27:41 Water ....... •81
•81 The small amount of water found in the above analyses, equivalent to about one-fourth of a molecule, was probably held mechanically by the crystals.