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Chromium compounds are compounds containing the element chromium (Cr). Chromium is a member of group 6 of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist. [3] [4]
Chromate and dichromate have equal concentrations. Setting [CrO 2− 4] equal to [Cr 2 O 2− 7] in Eq. 3 gives [CrO 2− 4] = 1 / β 2 [H +] 2 . The predominance diagram is interpreted as follows. The chromate ion is the predominant species in the region to the right of the green and blue lines. Above pH ~6.75 it is always the ...
The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. 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 ...
Loss of the second proton occurs in the pH range 4–8, making the ion [HCrO 4] − a weak acid. [citation needed] Molecular chromic acid could in principle be made by adding chromium trioxide to water (cf. manufacture of sulfuric acid). CrO 3 + H 2 O ⇌ H 2 CrO 4. In practice, the reverse reaction occurs: molecular chromic acid dehydrates ...
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 and sodium chloride (they can be used as standard as well as titrant for each other) as potassium chromate turns red in the presence of excess of silver ions.
Basic chromium sulfate is produced from chromate salts by reduction with sulfur dioxide, although other methods exist. [4] [5] The reduction could formally be written: Na 2 Cr 2 O 7 + 3 SO 2 + H 2 O → Cr 2 (SO 4) 3 + 2 NaOH. Since 33% of the anion charges are due to hydroxy ions the basicity is 33% (but in tanning jargon it is known as 33%
The phosphite ion, PO 3− 3, is a strong base, and so always carries at least one proton. In this case the proton is attached directly to the phosphorus atom with the structure HPO 2− 3. In forming this ion, the phosphite ion is behaving as a Lewis base and donating a pair of electrons to the Lewis acid, H +. Predominance diagram for chromate
The reactivity of the chromate anion with silver is lower than with halides (e.g. chlorides) so that in a mixture of both ions, only silver chloride precipitate will form: [9] AgNO 3(aq) + Cl − (aq) + CrO 2− 4(aq) → AgCl (s) + CrO 2− 4(aq) + NO − 3(aq) Only when no chloride (or any halogen) is left will silver chromate form and ...