Dichromatometry: Redox Titrations Using Potassium Dichromate

Dichromatometry is redox titrimetry using standardized potassium dichromate, $\text{K}_2\text{Cr}_2\text{O}_7$, as the oxidizing titrant, following the half-reaction:

so its n-factor (per formula unit, since 6 electrons are gained by the whole dichromate ion, which contains two chromium atoms) is $6$, giving an equivalent weight of $\tfrac{294}{6} = 49\ \text{g equiv}^{-1}$ using the molar mass of $\text{K}_2\text{Cr}_2\text{O}_7$.

Why it is used alongside permanganometry. Potassium dichromate has one major practical advantage over $\text{KMnO}_4$: it can be obtained commercially in a very pure, stable, non-hygroscopic crystalline form and can be weighed out directly as a primary standard, so a solution of exactly known concentration can be prepared in one step, without a separate standardization titration. Its main limitation is that it is a comparatively weaker oxidant than $\text{KMnO}_4$ (it cannot oxidize $\text{Cl}^{-}$ to $\text{Cl}_2$, for instance, which is actually an advantage when titrating iron samples dissolved in hydrochloric acid, since $\text{KMnO}_4$ would otherwise be partly consumed oxidizing the chloride), and its orange-to-green colour change ($\text{Cr}_2\text{O}_7^{2-}$, orange, to $\text{Cr}^{3+}$, green) is far less sharp than permanganate's self-indicating pink end point, so a redox indicator (such as diphenylamine or barium diphenylaminesulfonate) is normally added. …