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Two crystalline forms are known, both being very similar to the corresponding potassium sulfate. Orthorhombic β-K 2 CrO 4 is the common form, but it converts to an α-form above 66 °C. [1] These structures are complex, although the chromate ion adopts the typical tetrahedral geometry. [2]
When heated strongly, it decomposes with the evolution of oxygen. 4 K 2 Cr 2 O 7 → 4 K 2 CrO 4 + 2 Cr 2 O 3 + 3 O 2 When an alkali is added to an orange-red solution containing dichromate ions, a yellow solution is obtained due to the formation of chromate ions ( CrO 2− 4 ).
Predominance diagram for chromate. In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.. 2 CrO 2− 4 + 2 H + ⇌ Cr 2 O 2− 7 + H 2 O. The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium.
The following chart shows the solubility of various ionic compounds in water at 1 atm pressure and room temperature (approx. 25 °C, 298.15 K). "Soluble" means the ionic compound doesn't precipitate, while "slightly soluble" and "insoluble" mean that a solid will precipitate; "slightly soluble" compounds like calcium sulfate may require heat to precipitate.
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The generally yellow chromates or orange dichromates turn to dark blue as "chromium(VI) oxide peroxide" forms: CrO 2− 4 + 2 H 2 O 2 + H + → [CrO(O 2) 2 OH] − + 3 H 2 O. The structure of the pyridine complex has been determined crystallographically. [2] Adducts with other N-heterocycles have also been characterized similarly. [3]
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