Chromate-Dichromate Test & Chromyl Chloride Test

Reaction Equations:

$$\mathbf{\text{Chromate-Dichromate Acid-Base Equilibrium:}}$$

$$2\text{CrO}_4^{2-} \text{ (Yellow)} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} \text{ (Orange)} + \text{H}_2\text{O} \quad (\text{Acidic Medium } \text{pH} < 4)$$

$$\text{Cr}_2\text{O}_7^{2-} \text{ (Orange)} + 2\text{OH}^- \rightleftharpoons 2\text{CrO}_4^{2-} \text{ (Yellow)} + \text{H}_2\text{O} \quad (\text{Basic Medium } \text{pH} > 7)$$

$$\mathbf{\text{Chromyl Chloride Qualitative Test (Detection of } \text{Cl}^- \text{):}}$$

$$\text{K}_2\text{Cr}_2\text{O}_7 + 4\text{NaCl} + 6\text{H}_2\text{SO}_4 \xrightarrow{\Delta} 2\text{CrO}_2\text{Cl}_2 \uparrow \text{ (Reddish-Brown Vapors)} + 2\text{KHSO}_4 + 4\text{NaHSO}_4 + 3\text{H}_2\text{O}$$

$$\text{CrO}_2\text{Cl}_2 + 4\text{NaOH} \longrightarrow \text{Na}_2\text{CrO}_4 \text{ (Yellow Solution)} + 2\text{NaCl} + 2\text{H}_2\text{O}$$

$$\text{Na}_2\text{CrO}_4 + (\text{CH}_3\text{COO})_2\text{Pb} \longrightarrow \text{PbCrO}_4 \downarrow \text{ (Yellow Precipitate)} + 2\text{CH}_3\text{COONa}$$

Concept: In the chromyl chloride test, chloride ions react with potassium dichromate and concentrated sulfuric acid to yield volatile reddish-brown vapors of chromyl chloride ($$\text{CrO}_2\text{Cl}_2$$). Passing these vapors into $$\text{NaOH}$$ forms yellow sodium chromate, which precipitates as yellow lead chromate ($$\text{PbCrO}_4$$) upon adding lead acetate. Note: Fluorides, bromides, iodides, and heavy metal chlorides ($$\text{HgCl}_2, \text{AgCl}, \text{PbCl}_2$$) do not give this test.

Notation:

• $$\text{CrO}_2\text{Cl}_2$$ = Chromyl chloride gas (Chromium in $$+6$$ oxidation state)
• $$\text{PbCrO}_4$$ = Lead chromate yellow precipitate