4.2 Important Compounds: Potassium Dichromate and Potassium Permanganate

An old roadside breath-test tube held orange crystals that turned green when a driver who had been drinking blew through them: dichromate, $\mathrm{Cr_{2}O_{7}^{2-}}$, was being reduced to $\mathrm{Cr^{3+}}$ by ethanol. A pinch of purple potassium permanganate turns a bucket of well water pink and pulls dissolved iron out as a brown sludge. In the school laboratory, a burette of permanganate titrates iron(II) or oxalate solutions with no indicator at all: the first drop in excess stays pink. These are everyday uses of two compounds in which chromium ($+6$) and manganese ($+7$) are in their highest oxidation states, the states we met at the top of the "staircase" in Section 4.1.

Such high oxidation states are not found as simple cations. A $\mathrm{Mn^{7+}}$ ion would be impossibly small and highly charged. Instead the metal is bonded to oxygen atoms in an oxide or an oxoanion, where oxygen's ability to form multiple bonds stabilises it. This section first surveys the oxides of the 3d metals and how their acid–base character changes with oxidation state. It then follows potassium dichromate and potassium permanganate from their ores to the titration flask, and shows how to balance every one of their redox reactions by counting electrons.

Oxides and Oxoanions of the 3d Metals

The 3d metals react with oxygen at high temperatures to form oxides. Every metal of the series except scandium forms an ionic oxide $\mathrm{MO}$. Higher oxides are known for most of them, and the highest oxidation state in an oxide equals the group number from $\mathrm{Sc_{2}O_{3}}$ (group 3) to $\mathrm{Mn_{2}O_{7}}$ (group 7). Beyond group 7, no oxide of iron higher than $\mathrm{Fe_{2}O_{3}}$ is known, although iron(VI) occurs in ferrate ions, $\mathrm{FeO_{4}^{2-}}$, formed in alkaline solution and easily decomposed.

Potassium Dichromate, $\mathrm{K_{2}Cr_{2}O_{7}}$

Preparation. Dichromates are made from chromite ore, $\mathrm{FeCr_{2}O_{4}}$ (that is, $\mathrm{FeO\cdot Cr_{2}O_{3}}$). The process has three stages.

1. Oxidative fusion. The powdered ore is fused with sodium carbonate (or sodium hydroxide) in free access of air. Chromium(III) is oxidised to chromium(VI), giving yellow sodium chromate:
   $$\begin{aligned}&\mathrm{4FeCr_{2}O_{4}(s)+8Na_{2}CO_{3}(s)+7O_{2}(g)}\\&\quad\mathrm{\rightarrow8Na_{2}CrO_{4}(s)+2Fe_{2}O_{3}(s)+8CO_{2}(g)}\end{aligned}$$
2. Acidification. The chromate is extracted with water, filtered, and acidified with sulphuric acid; orange sodium dichromate crystallises:
   $$\mathrm{2Na_{2}CrO_{4}(aq)+2H^{+}(aq)\rightarrow Na_{2}Cr_{2}O_{7}(aq)+2Na^{+}(aq)+H_{2}O(l)}$$
3. Conversion to the potassium salt. Sodium dichromate is more soluble than potassium dichromate, so adding potassium chloride to a hot concentrated solution gives orange crystals of $\mathrm{K_{2}Cr_{2}O_{7}}$ on cooling:
   $$\mathrm{Na_{2}Cr_{2}O_{7}(aq)+2KCl(aq)\rightarrow K_{2}Cr_{2}O_{7}(s)+2NaCl(aq)}$$

Structures. The chromate ion is tetrahedral, with chromium at the centre. The dichromate ion consists of two $\mathrm{CrO_{4}}$ tetrahedra sharing one corner oxygen; the $\mathrm{Cr{-}O{-}Cr}$ bond angle is about $126^{\circ}$. The bridging $\mathrm{Cr{-}O}$ bonds are longer than the six terminal ones, which have partial double-bond character.

Potassium Permanganate, $\mathrm{KMnO_{4}}$

Preparation. Manganese(IV) oxide, from the ore pyrolusite, is first oxidised to green potassium manganate, $\mathrm{K_{2}MnO_{4}}$ (Mn $+6$), by fusing it with potassium hydroxide and an oxidising agent such as air or potassium nitrate:

The dark green melt is extracted with water. In neutral or acidic solution the manganate ion disproportionates: some of it is oxidised to purple permanganate and some reduced to manganese(IV) oxide.

Commercially, the alkaline manganate solution is instead oxidised electrolytically at the anode, so that none of the manganese is lost as $\mathrm{MnO_{2}}$:

In the laboratory, a manganese(II) salt can be oxidised directly to permanganate by peroxodisulphate:

Properties and structure. Potassium permanganate forms dark purple, almost black, crystals, isostructural with $\mathrm{KClO_{4}}$. It is only moderately soluble in water ($6.4\ \mathrm{g}$ per $100\ \mathrm{g}$ of water at $293\ \mathrm{K}$). Heated to $513\ \mathrm{K}$ it decomposes and releases oxygen:

Both the manganate and permanganate ions are tetrahedral, the $\pi$ bonding taking place by overlap of oxygen $p$ orbitals with manganese $d$ orbitals. The green manganate ion ($\mathrm{Mn^{6+}}$, $d^{1}$) is paramagnetic with one unpaired electron; the purple permanganate ion ($\mathrm{Mn^{7+}}$, $d^{0}$) is diamagnetic. Permanganate has no $d$ electrons, so its intense colour cannot come from a $d$–$d$ transition. It comes from a charge-transfer transition: light moves an electron from the oxygen atoms to the manganese.

Exercises