| This report is concerned with an investigation of magnesium-potassium dichromate, magnesium oxide-potassium dichromate and magnesium chromate (VI) systems at elevated temperatures using differential thermal analysis (DTA) and thermogravimetry (TG). Other methods used were chemical and X-ray diffraction powder analyses and determinations of the time-to-ignition of the magnesium-potassium dichromate system. The magnesium-potasstmdichromate system gave an exothermic reaction between 400° and 600°. A reaction mechanism is proposed in which magnesium chromate (V) is an intermediate. The Borchardt and Daniels theory of DTA was adapted to determine the activation energy for the reaction between 490° and 560° which was shown to be -87.86+/-0.35 kcal/mole. The results suggested that the initial reaction step involved chemisorption of chromium (VI) oxide onto magnesium oxide to form magnesium chromate (VI). After subtraction of the heat of formation of the chromium-oxygen bond, the true activation was shown to be either +9.14+/-0.05 kcal/mole or +14.14+/-0.05 kcal/mole. The heat of activation for the system determined by time-to-ignition measurements was shown to be 9.105 kcal/mole (0.005 standard deviation) between 450° and 535°. The magnesium-potassium dichromate system gave two endotherms (607°-620°, 620°-630°) on heating and three exotherms (615°-605, 605°-585°, 5850-550°) on cooling probably associated with unit changes in the oxidation state of chromium: Cr(VI)→Cr(V)→Cr(IV)→Cr(III) The exothermic reaction was limited by the formation of a coherent layer of magnesium chromite around each particle of magnesium. The magnesium oxide-potassium dichromate system decomposed between 575° and 630° with the formation of magnesium chromite. Magnesium chromate (VI) decomposed in two endothermic stages (607°-620°, 620°-677°) to form magnesium chromite. |
