Ornine on the Aiectli Iodides. by ~VILLIAMNORMAN RAE

View Article Online / Journal Homepage / Table of Contents for this issue 1286 RAE : THE ACTION OF BROMINE ON THE ALKAT,I IODIDES. CX LIV.-The Action of &-ornine on the AIEctli Iodides. By ~VILLIAMNORMAN RAE. INa paper by Jackson and Derby on ferrous iodide (Amer. Chem. J., 1900, 24, 15), mention is made of the action of bromine vapour on solid ammonium iodide; these authors state that the ammonium iodide first turned black, but as the absorption went on it finally became converted into the scarlet ammonium bronio-iodo-bromide, NH,BrIBr. A curve constructed from the increase in weight of the ammonium iodide and the time of exposure to bromine showed that therel was a marked diminution in the speed of absorption after the first atom of bromine had been added, and they were unable to decide whether th4e black intermediate1 product was another compound, NH,BrI, or only a mixture of ammoniuni bromide and free iodine. The present investigation was under- taken in order to settle this point, and also to determine whether the action of bromine on other solid iodides follows a similar course. Ammonium bromo-iodo-bromide, NH,BrIBr, was prepared by dissolving the calculated quantities of ammonium bromide, iodine, and bromine in a small quantity of water; the solution was a deep ruby-red colour, and when left in a desiccator over phosphoric oxide slowly deposited crystals of the salt. A similar result was obtained starting with ammonium iodide and bromine. A specimen was analysed by adding a weighed quantity to a Published on 01 January 1915. Downloaded by East Carolina University 14/09/2015 20:03:56. potassium iodide solution, making up to a fixed volume, and titrating with standard sodium arsenite solution ; the results are expressed by comparing the equivalent weight found in this way with that calculated from the formula. Tbe crystals gave E=156*7 (calc. E=l52.4). Too high a value for B denotes loss of halogen; the specimen analysed had been kept for some weeks iu a desiccator, and showed slight decomposition. The crystals obtained in the above manner were of a fine, ruby- red colour, but when examined by reflected light showed a distinct green lustre. The crystals consisted of aggregates, often 3 cm. long and 1 cm. broad, and made up of long prisms arranged parallel to one another. If the compound is sealed up in a glass tube it, is perfectly stable, but when exposed to dry air it loses iodine bromide, and if this is absorbed by potassium hydroxide, the loss continues until only white ammonium bromide is left. View Article Online RAE : THE ACTION OF BROMINR ON THE ALKALI IODIDES. 1287 The compound, like all the polyhaloids, is extremely deliquescent, and very readily soluble in water. If an aqueous solution is treated with successive equal volumes of ether, almost all the colour is removed after the second treatment. Action of Bromine Vapour on Bry Ammortium Iodide. Ammonium iodide crystals were first dried, and then were finely powdered; about 5 grams were placed in a weighed weighing bottle, 4 cm. in diameter and 2 em. high; this was then dried for two days in a desiccator containing phosphoric oxide, and weighed. A beaker of dry bromine was then placed in the desiccator, arid the weighing bottle was weighed at first every ten minutes, and later at longer intervals; previous to each weighing, it was placed in a desiccator containing potassium hydroxide, to remove bromine adhering to the glass. At the commencement of the reaction, which appears to take place more slowly the drier the materials, the dark colour of iodiue at first appears on the top of the solid. The first action, therefore, consists in the replacement of iodine by bromine. That the black solid is iodine is shown by the colour of the vapour and by the fact that iodine is removed by shaking with chloroform, leaving a mixture of white iodide and bromide. As the reaction proceeds, the black colour works its way down through the mass, and the top becomes covered with a red powder, into which, eventually, the whole of the solid is converted. The red solid must be either a compound, ammonium bromo-iodo- bromide, NH,BrIBr, or else a mixture of ammonium bromide and iodine bromide; that the former is actually the case is shown by the colour: iodine bromide is almost black with a brown vapour, Published on 01 January 1915. Downloaded by East Carolina University 14/09/2015 20:03:56. whilst this solid has a very low vapour pressure. Furthermore, the absorption of bromine ceases when the composition reaches NH,BrIBr, whilst if iodine monobromide is exposed to bromine, it continues to take up bromine, forming a liquid, which may consist of higher bromides of iodine or a solution of iodine bromide in bromine (Fig. 1). It appears, therefore, that there is no polyhaloid stable at the ordinary temperature having the composition XIBr, corresponding with the well-known series of compounds XICl, [all the polyhaloids seem to be additive compounds of haloids with one or two molecules either of a single halogen or of a compound of two halogens, and the existence of iodine tribromide corresponding with iodine trichloride seems doubtful (Roscoe and Schorlemmer)]. The red powder was analysed in the same way as the crystals, and gave E=151*5 (calc. E=l52.4). Jackson and Derby state that they found a marked diminution in speed of the reaction after the absorption of the first atom of View Article Online 1288 RAE : THE ACTION OF BROMINE ON THE ALKALI IODIDES bromine, but this result was not observed (Fig. 2); in fact, from the observed colours during the absorption, it was obvious that in the lower layers iodine was being set free, whilst at the same time in the upper ones the iodo-bromide was being formed, so that, a definite break iii the absorption curve was Iiartlly t,o bc expected unless the material was in a very thin layer. A large number of experiments have been made on the rates at which polyhaloids in the solid state ar.e formed and decomposed, but definite and conclusive breaks in the weight-time curves are only obtained when two polyhaloids of the same elements exist in which the polyhaloid halogen has very different vapour pressures A. Gram-atoms of bromine absorbed by one gram-atom of iodine. E. Gram-atoms of bromine lost by exposing the residling liquid iiz n ve.we7 coii- taining potassium hydroxide. 0 10 20 30 40 0 12 24 36 48 60 ho&s 7wws (A) 0 2 4 6 8 ho?crs( R) Published on 01 January 1915. Downloaded by East Carolina University 14/09/2015 20:03:56. FIG.1. (A) Absorption of Frc..2. Ab*qorption of hro- bromine by iodine, and mane by ammni~cm (B) loss of bromine by iodide. the resulting liquid. in the two compounds. Examples of this kind are shown in the curves showing the rate of loss of iodine by czsium pentaiodide and the loss of chlorine and iodine chloride by msium iodochloride, CsIC1, (Figs. 3 and 4). In the first case, the compound was prepared by Wells and Wheeler’s method: the crystals were powdered, placed in a desiccator over potassium hydroxide, and weighed at intervals of a few days. The pentaiodide lost iodine at the rate of 0.31 gram (1 atom of iodine) in twenty-eight days, until the composition was that of the tri-iodide, and then continued to lose iodine at the rate of 0.01 gram in forty-five days, that is, fifty times as slowly; the View Article Online RAE : TEE ACTION OF BROMINE ON THE ATXALI IODIDES. 1289 temperature was then raised from the room temperature (28.) to 63O by placing an electric lamp iii the desiccator and jacketing tlie latter with cotton wool; the remaining 2 atoms of iodine were lost in thirty-seven days at the higher temperature. Chiurn iodochloride, CsICl,, was prepared by a, method which can be used to prepare tlie corresponding compounds of any of the alkali metals, namely, by passing a large excess of chlorine into a concentrated solution of the iodide of the metal and evaporating in a desiccator containing phosphoric oxide and filled with chlorine. The golden-yellow, needle-shaped crystals were dried aiid powdered, and the rate of loss was determined as in the previous case. The CSIS CsCI,T renll'. 3%" Temp 28" \ CSCIJ CST : Temp. 116" Temp. 62' I I .i I I CSCl Csl 60 120 0 60 120 days Published on 01 January 1915. Downloaded by East Carolina University 14/09/2015 20:03:56. Days. FIG.3. Rate of decomposi- FIG.4. Rate of decornposi- tion of CSJ,. tion of csci,r. curve shows a very well-marked break at the composition CsICl,, and a sample analysed at this point had the composition 99.3 per cent. CsIC1,. The loss of weight of the compound CsICl, was almost negligible at 28O, and the experiment was completed at 118O, czesium chloride being left. The reactions here are therefore : CsICl, = CsICl, + c1, ; CsICl, = QCl + ICl, the former proceeding much more rapidly than the latter. The curve (Fig. 1) showing the absorption of bromine by iodine show no breaks, although the compound IBr is well known, whereas the loss curve gives a definite break at the composition IBr. View Article Online 1290 RAE : THE ACTION OF BROMINE ON THE ALKALI IODIDES. The curve (Fig. 2) shows no break at the composition NH,IBr, but the absorption of bromine ceases at, NH41Br,.

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