The oxidation of isothiocyanato - chromium (III) complexes by bromine in aqueous solution.
Dodge, Carlton N.
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Over the past ten to fifteen years much work has been done on oxidation and reduction reactions of coordination complexes in aqueous solution. Relatively little attention has been devoted, however, to the study of reactions in which ligands of these complexes are oxidized or reduced. In the present work an investigation was carried out on the bromine oxidation of the isothiocyanatopentamiminechromium(III) ion and of tetraisothiocyanatodiamminechromate(III) ion. Werner (16) studied the bromine oxidation of ammonium reineckate, and after extensive treatment of the reaction solution isolated the green salt [Cr(NH3)2(Br)2(H2O)2]Br. In the present study, an attempt was made to establish the stoichiometry of the bromine oxidations of the Reineckate ion and the [Cr(NH3)5NCS]+2 ion. Work was done on the identification of the oxidation products and on correlation of these products with the observed stoichiometry. The oxidation of the Reineckate ion was carried out in both buffered solution, at a pH of 4.5-5, and acidic solution, with a pH of 1-2, in order to note any difference in the measured stoichiometries with the change in acidity. Measurements were also made to determine if dissociation of the complex preceded the initial oxidation step. [Cr(NH3)5NCS](ClO4)2 was prepared from [Cr(NH3)5Cl] by a procedure based on the method of Adamson and Wilkins (3). The [Cr(NH3)5Cl]Cl2 was prepared by the reduction of chromic ion to chromous ion by zinc in the presence of acid. The chromous ion by zinc in the presence of acid. The chromous ion was then oxidized by air in the presence of ammonium chloride in aqueous ammonia. This solution was treated with hot hydrochloric acid to obtain the internediate [Cr(NH3)5Cl]Cl2. The following procedure was used to determine the rate of consumption of bromine and extablish the stoichiometry of the reactant species. Definite volumes of solution containing complex ion and hydrochloric acid or acetate buffer were placed in one flask, and the same volumes of blank solution containing hydrochloric acid or buffer, but no complex, wwere placed in another flask. A certain volume of aqueous bromine was added to each flask, and after a certain selected time, the remaining bromine was quenched with protassium iodide. The liberated iodine was titrated with standard sodium thiosulfate solution using starch as an indicator. The stoichiometries (the moles of bromine consumed per mole of complex ion) were determined using this same method. The chromium content of each solution of complex was determined by destroying the complex in hot alkali, oxidizing the chromite ion solution to chromate ion by hot hydrogen peroxide, and measuring the absorbance of the resulting chromate solution at a wave length of 372 millimicrons. From the measurements made on the bromine oxidation of Reineckate ion, it appears that there is an initial rapid reaction followed by a slower consumption of bromine. From the available data concerning the oxidation of [Cr(NH3)5NCS]+2, there also appears to be an initial rapid consumption of bromine. Measurements did not indicate a slower consumption of bromine. Allowing the buffered oxidation of Reineckate ion by bromine to take place for 10 minutes, at a pH of 4.5-5, the observed stoichiometry was 14.2 moles of bromine per mole of Reineckate ion. In acidic solution, at a pH of 1-2, the apparent stoichiometry of the Reineckate ion oxidation was 14.0 moles of bromine per mole of Reineckate ion. The stoichiometry of the [Cr(NH3)5NCS]+2 ion oxidation, allowing the reaction to take place for 15 minutes, was 3.2 moles of bromine consumed per mole of complex ion. Sulfate ion and nitrate ion were both identified as products of the Reineckate ion oxidation. The cyanide ion and the ammonium ion were not detected and are thus ruled out as possible oxidation products. No suitable test was devised for the identification of carbon dioxide. More work should be done on the study of the slower secondary oxidation by bromine of Reineckate ion. The bromine oxidation of [Cr(NH3)5NCS]+2 should be investigated in more detail and perhaps a comparison of the oxidation rates of it and the Reineckate ion could be made where possible. A reaction apparatus should be designed which would trap any gaseous oxidation products formed and a correlation could be made between the reaction products formed during each reaction interval and the observed stoichiometries.
Thesis (M.A.)--Boston University