Potassium Dichromate ($$\text{K}_2\text{Cr}_2\text{O}_7$$)

Step 1 (Roasting):

$$4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \longrightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2$$

Step 2 (Acidification):

$$2\text{Na}_2\text{CrO}_4 + 2\text{H}^+ \longrightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{Na}^+ + \text{H}_2\text{O} \quad (\text{Yellow} \longrightarrow \text{Orange})$$

pH Equilibrium:

$$2(\text{CrO}_4)^{2-} + 2\text{H}^+ \rightleftharpoons (\text{Cr}_2\text{O}_7)^{2-} + \text{H}_2\text{O} \quad [\text{Chromate stable in basic pH; Dichromate in acidic pH}]$$

Oxidizing Action (Acidic Only):

$$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \longrightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O} \quad (\text{n-factor} = 6)$$