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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]
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.
Sodium chromate is the inorganic compound with the formula Na 2 CrO 4. It exists as a yellow hygroscopic solid, which can form tetra-, hexa-, and deca hydrates . It is an intermediate in the extraction of chromium from its ores.
Partial predominance diagram for chromate. Molecular chromic acid, H 2 CrO 4, in principle, resembles sulfuric acid, H 2 SO 4. It would ionize accordingly: H 2 CrO 4 ⇌ [HCrO 4] − + H + The pK a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. [4]
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]
Potassium chromate is the inorganic compound with the formula K 2 CrO 4. This yellow solid is the potassium salt of the chromate anion. It is a common laboratory chemical, whereas sodium chromate is important industrially.
Calcium chromate is an inorganic compound with the formula CaCrO 4, i.e. the chromate salt of calcium. It is a bright yellow solid which is normally found in the dihydrate form CaCrO 4 ·2H 2 O. A very rare anhydrous mineral form exists in nature, which is known as chromatite .
It is relevant to chromate conversion coating, which is used to passivate common metal alloys such as aluminium, zinc, cadmium, copper, silver, magnesium, and tin. [2] In conversion coating chromate reacts with these metals to prevent corrosion, retain electrical conductivity, and provide a finish for the appearance of the final alloy products. [3]