Ammonium Hydroxide → chromium (III) oxide sulfate

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To 500 parts of a solution containing 150 parts chromic sulfate, 150 parts of ammonium hydroxide were added to precipitate a mixture of chromium hydroxide and chromium sulfate, which was filtered, washed with distilled water, dried and heated at about 500° C in air to form a chromium (III) oxide-sulfate starting mixture. Thirty-eight parts of this mixture was combined with 22 parts of chromium trioxide and 25 parts of water. The mixture was placed in a water bath and stirred until an homogenous paste was obtained. The test tube was then placed in a bomb which was fitted with piping to admit oxygen under pressure and with a pressure gauge to indicate the amount of oxygen present. The bomb was sealed and sufficient oxygen was pumped into the bomb to obtain a pressure of 5000 psi. The bomb was then placed in a furnace and the furnace was then heated to 350° C and maintained at that temperature for 4 hours. The pressure gauge on the bomb was monitored to maintain a pressure of 15,000 psi. The bomb was then cooled and the pressure released. Product obtained from the test tube was washed several times with distilled water and dried in an oven at 100°-120° C. The product obtained was found to be fine, acicular chromium oxide of tetragonal crystal structure of the rutile type having a coercivity of 255 Oersteds and a sigma of 73 emu/gm. at room temperature (VSM). The product was found to have a Curie temperature of 76° C which, when compared to chromium oxide similarly prepared without sulfate ion addition, represents a Curie temperature depression of 32° C.

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