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The concept of the amount of oxygen required to decompose organic matter in a water sample is used as a measure of the level of pollution of water samples. |
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Biochemical oxygen demand
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.
- At 273K oxygen solubility is 14.6 mg dm-3.
- At 293K oxygen solubility is 7.6 mg dm-3.
Clearly, the higher the concentration of dissolved oxygen gas, the less environmental pressure is put onto the aquatic animal lifeforms.
The quality of water is often assessed by the concentration of dissolved oxygen. In general the higher the dissolved oxygen content the less polluted the water. This is however, not easy to measure directly. One method relies on a series of indirect redox reactions. This is called the Winkler method.
Parts per million (ppm)
This term can have slighly different meanings depending as to the system to which it is related.
In a gas sample this would mean cm3 per m3 (one million cm3), while in an aqueous solution it means the mass in milligrams per litre (approximately 1000g) of solution.
In NMR spectrometry it means the relative energy needed to change the spin state of a hydrogen atom compared to the energy needed to change the spin states of the hydrogen atoms in tetramethyl silane (TMS).
As a measure of concentration it is useful to describe quantitied that are present in very low concentrations.
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The following table shows permitted atmospheric pollutant levels in Australia
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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.
Experimental procedure - part 1
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
The water sample is placed in a special bottle to prevent oxygen exchange with the atmosphere and equal volumes of manganese(II) chloride solution and alkaline sodium iodide solution are added.
Under these alkaline conditions the managanese(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
Experimental procedure- 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
Summary
An excess of manganese(II) salt, iodide ions and hydroxide ions is added to a water sample causing a white precipitate of manganese(II) hydroxide to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown manganese precipitate. Then sulfuric acid is added to acidify the solution, dissolve the precipitate and allow the manganese(III) ions to oxidise the iodide ions to iodine. These are then titrated against standardised sodium thiosulfate solution.
The amount of dissolved oxygen is directly proportional to the titre of the thiosulfate solution.
