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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 about −0.8 and 1.6. [4] The dichromate ion is a somewhat weaker base than the chromate ion: [5]
Chromate anions (CrO 2− 4) and dichromate (Cr 2 O 7 2−) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH: 2 [CrO 4] 2− + 2 H + ⇌ [Cr 2 O 7] 2− + H 2 O. Chromium(VI) oxyhalides are known also and include chromyl fluoride (CrO 2 F 2) and chromyl chloride (CrO
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 ...
In atmospheric chemistry, mixing ratio usually refers to the mole ratio r i, which is defined as the amount of a constituent n i divided by the total amount of all other constituents in a mixture: r i = n i n t o t − n i {\displaystyle r_{i}={\frac {n_{i}}{n_{\mathrm {tot} }-n_{i}}}}
Structure of CrO(O 2) 2 (pyridine).Hydrogen atoms bonded to carbon atoms are omitted. Color code: Cr = gray, C= black, H = white, O= red, N = blue. Chromium(VI) oxide peroxide is formed by the addition of acidified hydrogen peroxide solutions to solutions of metal chromates or dichromates, such as sodium chromate or potassium dichromate.
Chromyl chloride can be prepared by the reaction of potassium chromate or potassium dichromate with hydrogen chloride in the presence of sulfuric acid, followed by distillation. [3] [4] K 2 Cr 2 O 7 + 6 HCl → 2 CrO 2 Cl 2 + 2 KCl + 3 H 2 O. The sulfuric acid serves as the dehydration agent.
pyridinium dichromate (PDC) is the pyridium salt of dichromate, [Cr 2 O 7] 2-. pyridinium chlorochromate (PCC) is the pyridinium salt of [CrO 3 Cl] −. These salts are less reactive, more easily handled, and more selective than Collins reagent in oxidations of alcohols.
The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate. (NH 4) 2 Cr 2 O 7 → Cr 2 O 3 + N 2 + 4 H 2 O. The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations. [8]