CAESIUM TETRAIODIDE BY N O R M A N R A E
I n 1930 a paper was published by Briggs, Greenawald and Leonard’ on the ternary system caesium iodide, iodine and water a t 2 5 O C . They establish the existence of two polyiodides Cs14 and Cs13, and show that the pentaiodide described by Wells and his co-workers* does not exist a t zs0C. They call attention to the fact that all other workers in the same field, including the present writer, have followed Wells and assumed that the higher polyiodide is the pentaiodide. Referring to my papel.3 they say: “Rae studied quantitatively the rate a t which iodine is lost by evaporation from the supposed pentaiodide prepared in accordance with the directions of Wells and Wheeler. He gives curves of weight loss against time and shows that the loss virtually ceased, unless the temperature was raised, when the material attained a weight corresponding to the tri-iodide Cs13. For some unaccountable reason, however, Rae shows no actual experimental data for the first thirty days during which the supposed pentaiodide was losing iodine, nor does he give analyses of the pentaiodide a t the beginning of the experiment. Nevertheless, if his weight curve be extrapolated back to zero time, the original composition does come close to Cs15, but the actual points as given on the curve start out from a stage corresponding almost exactly to a tetraiodide.” The explanation is that the object of the experiment referred to was to show that the break in the volatilisation curve occurs at the point corresponding to the formula Cs13. Wells and Wheeler give three analyses of their compound in which the percentage of caesium varies from 15.20 to 20.96; from this and the description given, the writer concluded that the supposed pentaiodide was a much more unstable substance than it really turns out to be, and that it would not be possible to prepare the pure dry CsIs to give the initial point on the curve. This point was not a necessary one for the object in view and it was a much simpler thing to obtain the composition for the points on the curve from the composition a t the end point nrhen only caesium iodide was left. Iio attempt was therefore made to analyse the moist product prepared by Kells and Wheeler’s method; but, instead, it was placed in the glass weighing bottle in a small desiccator over sulphuric acid to dry for thirty days before transferring to the desiccator containing caustic soda and starting the weighings. The writer was in error in assuming from Wells and Wheeler that the substance was the pentaiodide and that it was more unstable than it really is J. Phys. Chem., 34, 195 (1930). * A m . J. Sci., 44,43 (1892). J. Chem. SOC.,107, 1286 (191j).
J
CAESIUM TETRAIODIDE
1801
and, a t the time, thought that the pentaiodide had lost iodine over the sulphuric acid a t about the same rate as it subsequently did over caustic soda and so the first thirty days were included in the diagram although the curve was not drawn for this period. It now appears that the tetraiodide is not a very unstable substance, for the original data are still available and show that after drying for thirty days over sulphuric acid the composition was
caesium iodide extra iodine
found
calculated for CsI,
40‘ 76 59’24
40‘ 56 59’44
CS‘fs
CSI,
csz,
csz, csr,
0
20
40
60
FIG.I Caesium Tetraiodide
The authors quoted above also say: “Rae’s curves are not very reliable evidence concerning the formulas of the unstable higher polyiodides, since a mixture of iodine and tetraiodide-the supposed pentaiodide-might lose iodine as vapour only very little faster than the tetraiodide alone” ; while Wells and Wheeler, in their paper, (loc. cit.) state: “It (CSIS)loses iodine on exposure about as rapidly as iodine itself volatilises.” To test this statement and also the reliability of the volatilisation method for indicating the existence of a polyhalide, a further experiment has now been made.
I802
NORMAN R.IE
Wells and IVheeler found that the higher polyhalide melts a t 73OC. and also that artificial mixtures of caesium tri-iodide and iodine, representing compositions varying from CsIr to CsL, all melt at a uniform temperature of 73OC. 1,7817 grams of caesium iodide were weighed in a flat form weighing bottle, excess of iodine was added, the stopper placed on and the mixture was heated in an electric oven a t 80°C. till it had all melted: it was then cooled overnight and weighed to get the weight of iodine added : this was 8,4419 grams. The solid mixture therefore had a composition between CsIlo and CsIll. This w-as placed in a desiccator over caustic soda and was weighed a t intervals of two or three days. The caustic soda was renewed three times to ensure that it was not becoming exhausted or coated, thereby affecting the rate of loss. Altogether forty-four weighings were made and these all lie on a very smooth curve which is shown in the diagram. This curve shows a very definite break a t the point corresponding to the tetraiodide CsI4 and also s h o w that the vapour pressure of the polyiodide is much less than that of free iodine. The average loss per day in the first stage nas 0.1j 3 grams and that in the last stage 0.01I grams, that is about fourteen times slower.
Summary The volatilisation method confirms the correctness of the formula CsI, for the higher polyiodide of caesium and shows that its vapour pressure is much less than that of iodine. Unzcersily College, Colombo, Ceylon. January 26, 1931.
