L8 - The Winkler Method

objectives

Understandings

1. The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree of pollution in a water sample.

Applications

1. Application of the Winkler Method to calculate BOD.

BOD and the Winkler Method

The biochemical oxygen demand, or BOD, is used as a measure of water quality. It is defined by the amount of oxygen needed to oxidise the organic components of a water sample over five days at a specific temperature.

BOD is often measured in parts per million (ppm).

Biological activity in water depends on the ability of the life forms to extract dissolved gases such as carbon dioxide and oxygen in order to metabolise. Plants breathe in carbon dioxide and this also applies to aquatic plants, while animals need to respire using dissolved oxygen, which they can extract from the water using structures similar to lungs called gills.

Oxygen is a sparingly soluble gas in water and, in common with most gases, its solubility decreases with increasing temperature.

BOD

The Winkler Method

An effective quantitative method for determining dissolved oxygen was developed by a Hungarian chemist, Lajos Winkler in 1888. The Winkler method uses the dissolved oxygen to convert manganese(II) hydroxide into manganese(III) hydroxide, and then analyzing for the latter by titration.

The following reagents are used for the first part of the procedure:

• Solution 1: Manganese(II) chloride solution 3 mol dm-3.
• Solution 2: A mixture of Sodium iodide solution (4 mol dm-3) and sodium hydroxide solution (4 mol dm-3).
• Solution 3: 50% sulfuric acid

Winkler Method Part 1

Under these alkaline conditions the manganese(II) ions are precipitated as manganese(II) hydroxide:

Mn2+(aq) + 2OH-(aq) → Mn(OH)2(s)

The precipitated manganese(II) hydroxide is then oxidised by dissolved oxygen giving manganese(III) hydroxide:

4Mn(OH)2 + O2 + 2H2O → 4Mn(OH)3

Winkler Method Part 2

Immediately before the analysis the brown precipitate of manganese(III) hydroxide is then dissolved by acidifying (using the same volume of sulfuric acid solution as solution 1) liberating manganese 3+ ions in solution:

Mn(OH)3(s) + 3H+(aq) → Mn3+(aq) + 3H2O

These manganese(III) ions then oxidise the iodide ions that were previously added in solution 2 liberating iodine:

2Mn3+(aq) + 2I-(aq) → I2 + 2Mn2+(aq)

At this point the solution should turn a deep yellow due to the liberated iodine which can then be determined by titration using a standardised solution of sodium thiosulfate.

2S2O32-(aq) + I2 → S4O62-(aq) + 2I-(aq)

Just before the titration end-point (when the yellow colouration of the iodine disappears completely) some starch solution is added to give the blue-black starch iodide complex, which imparts a deeper colour. The titration is continued until the blue-black colour is completely discharged.

Analysing the Data

Working backwards through the equations for the process:

moles of thiosulfate = moles of iodine x 2

moles of iodine x 2 = moles manganese(III)

Hence moles of thiosulfate are equivalent to moles of manganese(III)

moles manganese(III)/4 = moles of oxygen in the sample

Commercial systems use solution volumes and concentrations that give the mass of dissolved oxygen in mg dm-3 as the number of ml of sodium thiosulfate titrant added