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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 666 °C. [1] These structures are complex, although the chromate ion adopts the typical tetrahedral geometry. [2]
Potassium dichromate, K 2 Cr 2 O 7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health.
3 O 2− 10, and tetrachromates, Cr 4 O 2− 13. [2] All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO 4 units sharing corners. [3] The hydrogen chromate ion, HCrO 4 −, is a weak acid: HCrO − 4 ⇌ CrO 2− 4 + H +; pK a ≈ 5.9. It is also in equilibrium with the dichromate ion: 2 HCrO − 4 ⇌ Cr 2 O 2− 7 + H 2 O
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(radius 50 pm), and they can replace each other in some compounds, such as in chrome alum and alum. Chromium(III) tends to form octahedral complexes. Commercially available chromium(III) chloride hydrate is the dark green complex [CrCl 2 (H 2 O) 4]Cl. Closely related compounds are the pale green [CrCl(H 2 O) 5]Cl 2 and violet [Cr(H 2 O) 6]Cl 3.
For example, [Ti(H 2 O) 6] 4+ is unknown: the hydrolyzed species [Ti(OH) 2 (H 2 O) n] 2+ is the principal species in dilute solutions. [11] With the higher oxidation states the effective electrical charge on the cation is further reduced by the formation of oxo-complexes.
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