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The mineral chromite is an iron chromium oxide with empirical formula FeCr 2 O 4. Structurally, it belongs to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr 2 O 4);. [1] Zincochromite is another example.
Structure of CrO(O 2) 2 (pyridine).Hydrogen atoms bonded to carbon atoms are omitted. Color code: Cr = gray, C= black, H = white, O= red, N = blue. Chromium(VI) oxide peroxide is formed by the addition of acidified hydrogen peroxide solutions to solutions of metal chromates or dichromates, such as sodium chromate or potassium dichromate.
Lead(II) chromate can be produced by treating sodium chromate with lead salts such as lead(II) nitrate or by combining lead(II) oxide with chromic acid.. Related lead sulfochromate pigments are produced by the replacement of some chromate by sulfate, resulting in a mixed lead-chromate-sulfate compositions Pb(CrO 4) 1−x (SO 4) x.
Sodium chromate (Na 2 CrO 4) Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO 5) is formed, which can be stabilized as an ether adduct CrO 5 ·OR 2. [6] Chromic acid has the hypothetical formula H 2 CrO 4. It is a vaguely described chemical ...
The hydrogen chromate ion may be protonated, with the formation of molecular chromic acid, H 2 CrO 4, but the pK a for the equilibrium H 2 CrO 4 ⇌ HCrO − 4 + H + is not well characterized. Reported values vary between about −0.8 and 1.6. [4] The dichromate ion is a somewhat weaker base than the chromate ion: [5]
It may be formed by the salt metathesis reaction of potassium chromate and iron(III) nitrate, which gives potassium nitrate as byproduct. 2 Fe(NO 3) 3 + 3 K 2 CrO 4 → Fe 2 (CrO 4)3 + 6 KNO 3. It also can be formed by the oxidation by air of iron and chromium oxides in a basic environment: 4 Fe 2 O 3 + 6 Cr 2 O 3 + 9 O 2 → 4 Fe 2 (CrO 4) 3
The structure of nickel chromate is the same as for chromium vanadate, CrVO 4. Crystals have an orthorhombic structure with unit cell sizes a = 5.482 Å, b = 8.237 Å, c = 6.147 Å. The cell volume is 277.6 Å 3 with four formula per unit cell. [5] [7] Nickel chromate is dark in colour, unlike most other chromates which are yellow. [3]
The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possible. Both the chromate and dichromate anions are strong oxidizing reagents at low pH ...