Editing Winkler titration (section)

==Chemical processes==

In the first step, [[manganese(II) sulphate]] (at 48% of the total volume) is added to an environmental water sample.  Next, [[potassium iodide]] (15% in [[potassium hydroxide]] 70%) is added to create a pinkish-brown precipitate.  In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the [[tetravalent]] state.

: 2 Mn<sup>2+</sup>{{abbr|(aq)|aqueous solution}} + O<sub>2</sub>{{abbr|(aq)|aqueous solution}} + 4 OH<sup>−</sup>{{abbr|(aq)|aqueous solution}}  → 2 MnO(OH)<sub>2</sub>{{abbr|(s)|solid}}

Mn has been oxidised to 4+, and MnO(OH)<sub>2</sub> appears as a brown precipitate.  There is some uncertainty about whether the oxidised manganese is tetravalent or [[Electron shell|trivalent]]. Some sources claim that Mn(OH)<sub>3</sub> is the brown precipitate, but hydrated MnO<sub>2</sub> may also give the brown colour.

: 4 Mn(OH)<sub>2</sub>{{abbr|(s)|solid}} + O<sub>2</sub>{{abbr|(aq)|aqueous solution}} + 2 H<sub>2</sub>O → 4 Mn(OH)<sub>3</sub>{{abbr|(s)|solid}}

The second part of the Winkler test reduces (acidifies) the solution.  The precipitate will dissolve back into solution as the H<sup>+</sup> reacts with the O<sup>2−</sup> and OH<sup>−</sup> to form water.

: MnO(OH)<sub>2</sub>{{abbr|(s)|solid}} + 4 H<sup>+</sup>{{abbr|(aq)|aqueous solution}} → Mn<sup>4+</sup>{{abbr|(aq)|aqueous solution}} + 3 H<sub>2</sub>O{{abbr|(l)|liquid}}

The acid facilitates the conversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.

The Mn(SO<sub>4</sub>)<sub>2</sub> formed by the acid converts the iodide ions into iodine, itself being reduced back to manganese(II) ions in an acidic medium.

: Mn(SO<sub>4</sub>)<sub>2</sub> + 2 I<sup>−</sup>{{abbr|(aq)|aqueous solution}} →  Mn<sup>2+</sup>{{abbr|(aq)|aqueous solution}} + I<sub>2</sub>{{abbr|(aq)|aqueous solution}} + 2 {{chem|SO|4|2−}}{{abbr|(aq)|aqueous solution}}

[[Thiosulfate]] is used, with a starch indicator, to [[titrate]] the iodine.

: 2 {{chem|S|2|O|3|2−}}{{abbr|(aq)|aqueous solution}} + I<sub>2</sub> → {{chem|S|4|O|6|2−}}{{abbr|(aq)|aqueous solution}} + 2 I<sup>−</sup>{{abbr|(aq)|aqueous solution}}