LXXXI.-Connectio~~ Between the C~~Ystalloyr~Z~Hicul Clzarcicters of Isomolpjhous Salts Ccnd the Atomic Weight of the Metals Contuined
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View Article Online / Journal Homepage / Table of Contents for this issue 846 TUTTON : CONPARhTIVE CItYSThLLOGRhP€I1CAL STUDY Oh' LXXXI.-Connectio~~ between the C~~ystalloyr~z~hicul Clzarcicters of IsomolpJhous Salts ccnd the Atomic Weight of the Metals contuined. A Comjmratitv C~~ystallogrc~~hicclStudy of the Nomzal Selenutes of Potassium, Rubidium, and Casium. By ALFREDEDWIN TUTTON, Assoc. R.C.S. INthis communication are presented the results of a detailed investi- gation of the morphological and physical characters of the normal selenates of potassium, rubidium, and czsium, of an identical, and therefore strictly comparable, nature to that of the analogous sulphates of the same three alkali metals, the results of which were laid before the Society in the year 1894 (Trans., 1894,65, 628). Only the more easily procurable of these selenates, the potassium salt, appears to have been hitherto subjected to a crystallographical study, beyond the cursory inspection which revealed the fact that they were isomorphous with the corresponding sulphates. Potassium selenate was included by Topsoe and Christiansen in their well-known investigation published twenty-three years ago (Ann. Chim.phyls., 1874, [ v], 1, 1). The densities of all three salts were determined by Pettersson in 1872-1876 (A'ova Actcc B, Xoc. Upsaka, [iii], 1873 and 1876), with small quantities of material. The investigation of the selenates has been attended by far greater difficulties than beset that of the sulphates, on account of the fact that they are so much more soluble in water than the latter salts as Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. to be in no small degree hygroscopic. At the very outset of the inves- tigation, the progression from potassium to cmium was unmistakably manifested with regard to this property ; for while the deliquescence of the potassium salt is only suficiently rapid to prove inconvenient, that of the rubidium salt is so much more marked, that it was rarely possible to complete the goniometrical measurement under ordinary conditions, exposed to the atmosphere, with one and the same crystal, the rare occasions being on remarkably dry days; and the rapidity of deliquescence of the c&um salt is 80 extreme that a crystal usually becomes a dropof solution in a few minutes after its exposure to the air on ordinary moist days. The manner in which this difficulty has been overcome mill be described at a later stage. The potassium selenate employed in this investigation was supplied specially for the purpose in a high state of purity by Merck. It was subsequently repeatedly fractionally crystnllised, and the crystals found to bo free from any appreciable traces of impurity. The View Article Online NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CACSIUM. 847 rubidium and cssium salts were prepared by the author from the pure carbonates of the metals and pure selenic acid, all of which were also specially supplied to the author by Jlerck. The carbonates were dissolved in water and the solutions decomposed by the correct quantity of selenic acid. The solutions of the selenates were then allowed to crystallise, and the crystals obtained several times fractionally re- crystallised. The crystals thus obtained proved, as will be subsequently shown, to be pure. Crystallisation of solutions of the selenates is almost impossible in the open air in ordinary moist conditions of the atmosphere ;the potas- sium salt, being least soluble, of course shows the greatest tendency to crystallise, but concentrated solutions of the ccesium salt may remain exposed for weeks and months without depositing crystals. Indeed, saturated solutions of both rubidium and cmium selenates appear to attract moisture from the air. Hence, the crystallisation of the salts was effected over oil of vitriol under reduced pressure. Half a dozen large receivers were independently connected with the same double- barrelled air-pump, so that several clearly labelled crystallising dishes containing solution could be placed over vitriol in each, only crystals of any one salt being grown in the same receiver, to avoid all chance of admixture. The receivers stood on ground glass plates, and con- nection with the air pump and manometer was made by suitable glass connections provided with stopcocks. Thus all use of metal mas avoided, and as the whole arrangement stood on a separate table covered with thick non-conducting baize, all chance of sudden thermal disturbance of the crystallising solutions was avoided. The pressure was only reduced sufficiently to permit of slow crystallisation, in order to Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. obtain the most satisfactory crystals. The author desires to express his thanks to the Research Fund Committee of the Society for their grant to defray the cost ol the large quantities of the expensive chemicals employed in the investi- ga tion, Gyavimeti.ic uncl Xpectyoscopic Ancclyses. Each of the three selenates was gravimetrically and spectroscopically analysed, as has been the author’s custom throughout the whole of these investigations, in order to afford the highest guarantee of their purity. The gravimetric analysis consisted in estimations of the content of selenium. The method found most convenient in the cases of potassium and rubidium selenates was the reduction of tbe selenates to selenites by prolonged boiling with hydrochloric acid, and subsequent precipita- tion of the selenium from the selenites by means of sulphur dioxide gas. The clear characteristic crystals selected from one of the crops 3L2 View Article Online 848 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF used in the goniometrical and optical work were in each case finely powdered in an agate mortar, and the powder rendered perfectly anhy- drous by heating in a current of dried air to 150" for some hours, employing the apparatus, and taking the extreme precautions to prevent contact with the moist air, which will subsequently be more fully described in connection with the density determinations. The weighed quantity of salt was dissolved in distilled water, and boiled with about an equal quantity of pure hydrochloric acid, in a flask fitted with an upright condensing tube, to prevent loss by spirting and retard loss by evaporation, for a couple of hours, when all trace of liberated chlorine had disappeared. The contents of the flask were then trans- ferred with washings to a large three-bulbed U absorption tube, which was connected on one side with a large flask containing a concentrated solution of sodium sulphite into which sulphuric acid could be allowed to fall from a dropping funnel, and on the other with an empty flask fitted with doubly perforated stopper, through whose second hole was inserted a long tube to lead excess of the sulphur dioxide into the flue of the large fume cupboard in which the operation was performed, Sulphur dioxide was then liberated from the generating flask by running in the vitriol and applying heat, and a stream of such rapidity was maintained that a few bubbles escaped absorption every minute. Towards the end of the precipitation, when the red selenium had coagulated to a mass in the lowest bulb on the bend of the U-tube and the liquid began to clear, the absorption apparatus was gradually warmed to about 60", the flame was then altogether removed from the generator, and the reaction was left to complete itself for another couple of hours, the contents of the bulbs being surrounded on each side by a flask filled with the reducing gas. The precipitate Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. was subsequently collected on a weighed filter, thoroughly washed with hot distilled water, dried at SO", and weighed. This method, which proved eminently satisfactory in the cases o€ the potassium and rubidium salts, was found to be inapplicable to cmium selenate. The much greater stability of the caesium salt is strikingly indicated by the fact that, in the first place, boiling with hydrochloric acid appears incapable of reducing it to selenite, there being little or no indication of liberated chlorine, and, further, barely a trace of sele. nium is precipitated by sulphur dioxide. Under these circumstances, a method was tried, and found to answer admirably, which has recently been described by Pierce (Zed. ccnorg. Chem., 1896, 12, 409). It consists in adding to the highly diluted hydrochloric acid solution a considerable excess above the calculated quantity of potassium iodide solution, and boiling for upwards of half an hour until all free iodine is expelled. The precipitated selenium, in the state of black powder, is then transferred to a weighed filter, dried at loo", and weighed. View Article Online NORMAL SELENATES OF POTASSITJM, RUBIDIUM, Ah’D CfiSTUM. 849 The analytical results obtained mere as follows : Potccssium SeZenccte.-l*O453 gram K,SeO, gave 0.3732 gram Se, corresponding to 35.61 per cent. The calculated percentage of Se in K,SeO, is 35.69. Rubidium 8eZencite.-l.4494 gram Rb2St.0, gave 0.3686 gram Se, corresponding to 25.43 per cent. The calculated percentage of Se in Rb,SeO, is 25.16. Cmsiuna SeZenccte.-l-6667 gram Cr;.,SeO, gave 0,3235 gram Se, corresponding to 19.41 per cent. The calculated percentage of Se in Cls2Se0, is 19.30. The above numbers thus indicate that the material of the three selenates which was employed in this investigation mas analytically pure. The rubidium and casium salts were each further spectroscopically tested for traces of the other two salts, in the same manner as in the case of the sulphates (vide that memoir, p. 631). No traces of the ciesium lines mere detected in the spectrum of the rubidium salt, and, likewise, no evidence of rubidium in the casium salt was afforded, although very brilliant spectra were obtained.