Table 2.
Chromium oxidation states and main forms in aquatic environments.
| Oxidation State | Form | pH Condition | References |
|---|---|---|---|
| Cr (III) | Hexacoordinate complexes with complexing agents (i.e., water, ammonia, sulphate, urea, and organic acid) | 0 < pH < 4 | [2] |
| Cr(H2O)5(OH)2+ abbreviated as [Cr(OH)]2+ | slightly acidic conditions, 3.8 < pH < 6.3 | [3,68,69] | |
| [Cr(H2O)4(OH)2]+ abbreviated as [Cr(OH)2]+ | 6 < pH < 8 | [68,69] | |
| Cr(OH)3 (aq) * | slightly acidic to alkaline conditions | [69] | |
| Cr(OH)3 (s) | 6.4 < pH < 11.5; max at pH ≈ 8 | [1,66,69,70] | |
| [Cr(OH)4]− | pH > 11.5 | [3] | |
| Cr (VI) | H2CrO4 | pH < 1 | [66] |
| [HCrO4]− | 1 < pH < 6.4 | [1,26,66,69] | |
| [CrO4]2− | pH ≥ 6.4 | [1,26,66,69] | |
| [Cr2O7]2− | pH < 3 | [66] |
* sparingly soluble form, which tends to precipitate quickly.