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Through Chromate and dichromate, Potassium dichromate and Titration
Structure
Two crystalline forms are known, both being very similar to the corresponding potassium sulfate. Orthorhombic β-K2CrO4 is the common form, but it converts to an α-form above 666 °C (1,231 °F).
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Production and reactions
It is prepared by treating potassium dichromate with potassium hydroxide:
- K2Cr2O7(aq) + 2 KOH → 2 K2CrO4 + H2O
Or with potassium carbonate:
- K2Cr2O7 + K2CO3 → 2 K2CrO4 + CO2
Or, the fusion of potassium hydroxide and chromium trioxide:
- 2 KOH + CrO3 → K2CrO4 + H2O
When treated with lead(II) nitrate, it gives an orange-yellow precipitate, lead(II) chromate.
Applications
Unlike the less expensive sodium salt, the potassium salt is mainly used for laboratory work in situations where an anhydrous salt is required, or as an oxidizing agent in organic synthesis.
It is used in qualitative inorganic analysis, e.g. as a colorimetric test for silver ion. It is also used as an indicator in precipitation titrations with silver nitrate to measure levels of chloride ion (the Mohr method of determining chloride) since red silver chromate is precipitated in the presence of any excess of silver ions when potassium chromate is present. This titration proceeds by the following reactions, where silver nitrate is used as the titrant:
- Ag+ + Cl− → AgCl
- 2 Ag+ + CrO−4 → Ag2CrO4
Safety
As with other Cr(VI) compounds, potassium chromate is carcinogenic. Positive associations with lung cancer at a very high rate, and nasal / sinus cancer at a 100x lower rate than lung cancer have been found using worker exposure data. In general, less soluble chromates are a larger chronic hazard as they can be encapsulated in the lung without being absorbed and excreted, giving more time for reactive intermediates to be produced. Animal data indicates a potential for impaired fertility, heritable genetic damage and harm to unborn children, along with other types of cancer via less common exposure routes.
As a highly soluble hexavalent chromium compound, potassium chromate is also acutely toxic, though it is poorly absorbed from the intestinal tract. The compound is also corrosive and exposure may produce severe eye damage or blindness.