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The Chemistry of Osmium

AN important addition to our knowledge of the chemical nature of this interesting element is contributed by Prof. Moraht and Dr. Wisch in, of Munich, to the current number of the Zeitschri/t/ur Aworgaw- ische Chemie. Two years have scarcely elapsed since the position of osmium in the periodic system was finally decided by the painstaking redetermination of its atomic weight by Prof. Seubert. Previous determinations of the atomic weight of osmium had been made with material which Seubert subsequently showed to be impure, and in consequence the erroneous value 198'6 had been ascribed to it. Indeed previous to the year 1878 the order of precedence as regards atomic weight of the four metals of the platinum group—gold 196 2, iridium 196'7, platinum 196'7, and osmium 198'6—was entirely at variance with the order demanded by their chemical and physical properties, and a standing contradiction of the periodic law of Newlands and Mendeleef. In that year, however, Seubert attacked the case of iridium, and as the result of a series of determinations, made with the laborious care which has characterized all his work, the atomic weight of this metal, when obtained in a pure state, was shown to be 192'5, a number very different to that previously assigned to it, and which was afterward remarkably confirmed, even to the decimal place, by an independent investigation by Joly. Three years later Seubert made his celebrated redetermination of the atomic weight of platinum, which resulted in the number 194'3 being finally derived for the true atomic weight of the perfectly pure metal. This value was likewise subsequently confirmed by Halberstadt. In the year 1887 the position of gold was decided by simultaneous independent redeterminations of its atomic weight by Thorpe and Laurie in this country and Kruss in Germany, the two values being practically identical, 196'7. Lastly, in 1891, Seubert completed his work by redetermining the atomic weight of osmium with a specimen of the metal of practically perfect purity, with the result that the old number, 198'6, was found to be entirely erroneous, due to considerable quantities of impurities being present in the samples previously employed, and that the real value of this constant was 190'3, thus removing osmium from its former situation at the end of the series and placing it in its proper position at the head of it. The order of precedence of the metals of the platinum group is therefore as follows: Osmium 190'3, iridium 192 5, platinum 194'8, and gold 196-7. This order is in full accordance with the relative chemical and physical properties of these metals, and the last outstanding exception to the periodic generalization has disappeared. Although the properties of pure metallic osmium, and particularly its atomic weight, are now known with certainty, the nature of its compounds is yet very little understood. Moreover, it is evident from the result of the investigation of Prof. Seubert that previous workers have been dealing with an impure metal of atomic weight 198'6. It was therefore desirable that not only should the chemistry of this element be extended to compounds hitherto uninvestigated, but that the composition and properties of the compounds already known should be subjected to a re-examination. Prof. Moraht and Dr. Wischin have therefore taken up the study of the compounds of osmium with oxygen, sulphur, halogens, employing material of a very high degree of purity, and the results of their investigation are both novel and interesting. Work with osmium compounds is endowed with peculiar personal danger to the chemist, owing to the great facility exhibited under the most various conditions for the formation of the tetroxide OsO4. a substance which boils at 100° C., and is very volatile at the ordinary temperature, and which attacks the skin, the lungs, and particularly the eyes with most serious consequences. The material started with was a comparatively pure sample of the best known salt containing osmium, potassium osmate, K.OsO. ^2H20. This salt was further purified by distillation with nitric acid or aqua regia and absorption of the liberated tetroxide vapors in a solution of caustic potash. The dark brown solution of potassium perosmate thus formed was largely diluted with water, and reduced to osmate by the addition of alcohol. After the expiration of about 24 hours almost the whole of the osmium had separated in the form of beautiful little crimson octahedrons of the salt K2OsO.r 2H.0, which, after washing with dilute alcohol, proved to be quite free from impurity, showing no trace of iridium. Previous observers have noticed that an aqueous solution of potassium osmate, K2OsO(, is most remarkably affected by sunlight, a rapid decomposition being brought about with deposition of a black precipitate to which tire composition OsO..2H.0 has been ascribed. The specimens experimented with, however, undoubtedly contained iridium, and it was therefore of interest to investigate the action of upon ' solutions of the pure salt just described. When the crimson octahedrons of pure K.OsO.^H.O were dissolved in cold water, and the clear reddish violet-colored solution was exposed to direct sunshine, no evidence of change was apparent for several days, but the moment the vessel containing the solution was immersed in a bath of boiling water, while in bright sunshine, decomposition commenced, and a black precipitate rapidly accumulated, until after the expiration of two or three hours the whole of the osmium present was deposited. As there is a marked tendency for the production of the noxious fumes of osmium tetroxide during this decomposition of the hot osmate solution by the waves of light, it is best to take the precaution of reducing their amount to a minimum by the addition of a little alcohol, which acts as a strong reducing agent under these circumstances, and by passing a stream of hydrogen through the solution during the. whole operation. The precipitate is usually so finely divided that considerable difficulty is experienced in separating it from the solution. The filtration succeeds best when the filter is previously moistened with dilute acetic acid, when a clear colorless filtrate is usually at once obtained. The precipitate cannot be dried in a warm air bath, as it is largely converted thereby into' the volatile osmium tetroxide. It may safely, however, be dried over phosphoric anhydride in the vacuum of an air pump. The accurate analysis of an insoluble substance of the nature of this precipitate, and containing a metal such as osmium, which so readily oxidizes to the volatile tetroxide, is a task of exceptional difficulty. The usual method of reduction to metal in a stream of hydrogen is insufficient, for more or less of the tetrox- ide is always formed during the process, necessitating the use of an absorption apparatus containing a solution of caustic potash, placed in front of the tube containing calcium chloride to absorb the water formed. The difficulty is, then, how to estimate the small quantity of osmium thus dissolved in the large excess of alkali. ' It was eventually found that the weak electric current from three Daniell's cells precipitates the whole of the osmium from such a solution, contained in a nickel dish which forms the negative electrode, in the form of pure osmium dioxide, OsOa, which may conveniently be dried iw vacuo over phosphoric anhydride and weighed as such. By this mode of analysis the fact was eventually elicited that the black insoluble substance formed by the action of light upon a hot solution of potassium osmate is not, as was previously. supposed, a hydrate of osmium dioxide of the composition OsOa” 2H20, but is no other than free osmie acid itself, the hydrate of osmium trioxide, OsO3 •H.O or HaOsO4. Osmic acid is thus formed by the direct action of water, under the influence of sunlight and slight rise of temperature, upon the potassium salt. This remarkable change is expressed by the simple equation: K,OsO4 + 2H.0 = H. OsO. + 2KOH. The liquid, as soon as the change commences, is observed to exhibit a strong alkaline reaction, becoming, as indicated in the equation, a solution of caustic potash. It is singular that the presence of alcohol and the passage of a current of hydrogen during the reaction 'do not cause any reduction, serving only to hinder the further oxidation to the state of tetroxide. Indeed, if the crimson octahedral crystals of potassium osmate are covered in sunshine with warm alcohol and a current of hydrogen is allowed to bubble through the liquid, no trace of blackening is observed upon the faces of the crystals. The moment water is added, however, decomposition is immediately brought about. Osmie acid, H2OsO4, is a soot-black powder, which fumes strongly in moist air, owing to its rapid conversion into the volatile osmium tetroxide, OsO., but which is quite permanent at the ordinary temperature when preserved under water containing alcohol. It dissolves readily in nitric acid, with formation of the hydrate of osmium tetroxide, the so-called per-osmic acid. Cold hydrochloric acid attacks it but very slightly. Upon warming, however, it is entirely soluble, forming an olive green liquid, which will be subsequently considered, with liberation of a small quantity of chlorine' Sulphuric acid does not attack it. Osmic acid reacts in a most energetic and interesting manner with sul- phureted hydrogen gas. Even in the dry state at the ordinary temperature the reaction proceeds with considerable violence. If the experiment is conducted in a piece of combustion tubing, upon which a bulb has been blown for the reception of the osmic acid, the moment that the gas enters the tube the whole of the black powder immediately becomes incandescent, and drops of water and a large quantity of free sulphur are deposited in the portion of the tube not heated by the reacting substances. The residual product of the reaction is a brown powder, which has been found to be a hydrated oxysulphide of osmium of the composition 20sS0-H.0. The reaction occurs in accordance with the equation— 2H.OsO. + 4H.S = 20sS0.H.O + 5H.0 + 2S. This oxysulphide of osmium is soluble in acids with decomposition, even sulphuric acid it with evolution of sulphureted hydrogen. It possesses acid properties, for it liberates carbon dioxide from carbonate of soda and sulphureted hydrogen when fused with sulphide of potassium. It would, moreover, appear to contain SH groups, for it yields mercaptan upon treatment with soda and ethyl iodide, the osmium being reduced to the dioxide OsOa. Its probable constitution is therefore represented by the graphic formula: /SH Os=Q 0s=0 \SH When this oxysulphide is warmed in dry sulphureted hydrogen another violent reaction occurs, the whole mass again becomes incandescent, and the whole of the oxygen is eliminated in the form of water. The product of this second reaction with sulphureted hydrogen is pure osmium disulphide OsS 2. Os.O3(SH). + 2H. S = 20sS. + 3H.O. Of the halogen compounds of osmium only the chlorides have been at all investigated, chiefly by Claus, whose observations may be summarized in a few words. When finely powdered metallic osmium is heated in a stream of dry chlorine, sublimates are formed. The first chlorine compound formed its chromous green in color, but is only produced to a very slight extent. There is next deposited a dense black sublimate, and finally a smaller quantity of a sublimate of the color of red lead. None of these three chlorine compounds are crystalline. Claus subsequently stated that the lowest chloride OsOl, is a bluish-black solid when isolated, and forms a dark bluish violet solution; the sesqui- chloride Os3Cl3 is reddish brown in the solid state, and gives with water a rose-red colored solution, and the dichloride OsCla is the compound which exhibits the color of red lead, and yields a lemon yellow solution. These observations of Claus are completely confirmed by the experiments of Prof. Moraht and Dr. Wischin, who, however, have extended them, and have been able to isolate other and higher chlorides of osmium. They commenced by warming a large quantity of the free osmic acid above described for two days upon a water bath with concentrated hydrochloric acid, the flask in which the reaction was conducted being connected with an upright condenser. A little alcohol was added in order to prevent the formation of osmium tetroxide. The osmic acid eventually entirely dissolved, with formation of the dark olive green colored solution previously incidentally mentioned, a little chlorine being evolved at the commencement of the operation. It was found impossible to evaporate the solution upon the water bath without decomposition, but evaporation iw vacwo over sulphuric acid and solid caustic potash, the latter to absorb the hydrochloric acid, succeeded admirably. The solid left after complete evaporation consisted of well formed crystals which assumed the habit of six-sided pyramids. These crystals were dark olive green in color when moist, but when the last traces of superfluous water were removed, exhibited a bright vermilion color. They were readily soluble in water and alcohol, the solutions being colored dark green, and the salt may be recrystallized from these solvents. Upon analysis they were found to consist of the chloride Os.Cl, crystallized with seven molecules of water. This chloride of osmium, os2c1.7hso, would appear to be a molecular compound of the trichloride, OsCl3, and the tetrachloride, OsCl4. For when potassium chloride solution is added to the solution of the crystals in alcohol, a precipitate of brilliant red octahedrons and cubes of potassium osmichloride, Ka0sCl«, is obtained, showing the presence of osmium tertrachloride, OsCl4. Moreover, when the precipitate is separated by filiation, and the filtrate concentrated by evaporation iw Bacwo, dark green crystals of thetriohloride, OsCl3, are deposited containing three molecules of water of crystallization.' During the reduction of these crystals of the trichloride in a current of hydrogen for the purposes of analysis, a small quantity of a white sublimate was obtained, which probably consisted of the octo-chloride, OsCl, corresponding to the tetroxide OsO4. Bromine does not react with osmium with anything like the energy of chlorine. The free elements do not appear to combine at all, even at moderately high temperatures. Only a small quantity of a sublimate of a dark brown color is obtained by passing bromine vapor over osmic acid. This sublimate dissolves to a brown solution in water, which, however, rapidly decomposes with deposition of a black precipitate. When osmic acid, HOsO4, is treated with hydrobro- mic acid in the manner just described in the case of hydrochloric acid, a similar reaction occurs with formation of a clear reddish brown solution which yields, upon evaporation iw vacuo over sulphuric acid and solid caustic potash;' small crystals of a molecular compound of the tribromide, OsBr3, and the hexabromide, OsBr, together with six molecules of water of crystallization. These crystals of Os.Br '6H20 are dark reddish brown in color and exhibit a beautifUl metallic luster. They are quite stable when preserved in a dry atmosphere, but rapidly deliquesce in moist air. Iodine appears to possess even less affinity for osmium than bromine. When, however, osmic acid is treated with hydriodic acid, a deep greenish-brownsolution is obtained which deposits in vacuo dark violet rhombohedrons, exhibiting a brilliant metallic luster, consisting of the anhydrous tetra-iodide of osmium, OsI4. This iodide, the only one containing osmium yet prepared, is permanent in a dry atmosphere at the ordinary temperature, but rapidly deliquesces like the bromide when exposed to moist air. In relative stability the chloride, bromide and iodide of osmium above described exhibit a gradation.such as would be expected from the relations between the halogen elements themselves. The iodide is readily dissociated by slightly raising the temperature, and upon the.addition of water is decomposed with the deposition of a black precipitate containing the metal. A similar decomposition occurs, although much more slowly, in case of the bromide. The chloride, however, is well nigh permanent under these conditions, only' exhibiting traces of decomposition after the lapse of a considerable time. A. E. TUTTON.

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