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Chromium(III) hydroxide is a gelatinous green inorganic compound with the chemical formula Cr(OH) 3. It is a polymer with an undefined structure and low solubility. It is amphoteric, dissolving in both strong alkalis and strong acids. [2] In alkali: Cr(OH) 3 + OH − → CrO − 2 + 2 H 2 O In acid: Cr(OH) 3 (OH 2) 3 + 3 H + → Cr(OH 2) 6 3+
Chromium(III) hydroxide (Cr(OH) 3) is amphoteric, dissolving in acidic solutions to form [Cr(H 2 O) 6] 3+, and in basic solutions to form [Cr(OH) 6] 3−. It is dehydrated by heating to form the green chromium(III) oxide (Cr 2 O 3), a stable oxide with a crystal structure identical to that of corundum. [6]
The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr 2 O 3 in 1838 via a secret process, sold as a pigment. [6] It is derived from the mineral chromite, (Fe,Mg)Cr 2 O 4. The conversion of chromite to chromia proceeds via Na 2 Cr 2 O 7, which is reduced with sulfur at high temperatures: [7] Na 2 Cr 2 O 7 + S → ...
Chromium(III) hydroxide (Cr(OH) 3) is amphoteric, dissolving in acidic solutions to form [Cr(H 2 O) 6] 3+, and in basic solutions to form [Cr(OH) 6] 3−. It is dehydrated by heating to form the green chromium(III) oxide (Cr 2 O 3), a stable oxide with a crystal structure identical to that of corundum. [21]
In acid solution the aquated Cr 3+ ion is produced. Cr 2 O 2− 7 + 14 H + + 6 e − → 2 Cr 3+ + 7 H 2 O ε 0 = 1.33 V. In alkaline solution chromium(III) hydroxide is produced. The redox potential shows that chromates are weaker oxidizing agent in alkaline solution than in acid solution. [6] CrO 2− 4 + 4 H 2 O + 3 e − → Cr(OH) 3 + 5 OH −
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This compound is commonly prepared by reacting chromium(III) oxide and potassium hydroxide at 850 °C under argon: [2]. Cr 2 O 3 + 6 KOH → 2 K 3 CrO 4 + H 2 O + 2 H 2. This compound can be prepared other ways such as replacing chromium oxide with potassium chromate.
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