One of the most useful titrations involving iodine is that originally developed by Winkler to determine the amount of oxygen in samples of water. The dissolved oxygen content is not only important with respect to the species of aquatic life which can survive in the water, but is also a measure of its ability to oxidise organic impurities in the water. Despite the advent of the oxygen-selective electrode direct titrations on water samples are still used extensively.
In order to avoid loss of oxygen from the water sample it is 'fixed' by its reaction with manganese(II) hydroxide which is converted rapidly and quantitatively to manganese(III) hydroxide:
4Mn(OH)2
+ O2 + 2H2O = 4Mn(OH)3
The brown precipitate obtained dissolves on acidification and oxidises iodide ions to iodine:
Mn(OH)3
+ I- +3H+ = Mn2+ + 0.5I2 + 3H2O
The free iodine may then be determined by titration with sodium thiosulphate.
2S2O32-+I2=
S4O62- + 2I-
This means that 4 moles of thiosulphate correspond to 1 mole of dissolved oxygen.
The main interference in this process is due to the presence of nitrites (especially in waters from sewage treatment). This is overcome by treating the original water sample with sodium azide, which destroys any nitrite when the sample is acidified:
HNO2
+ HN3 = N2 + N2O + H2O
The water sample should be collected by carefully filling a
200-250 mL bottle to the very top and stoppering it while it is below the water
surface. This should eliminate any further dissolution of atmospheric oxygen.
By using a dropping pipette placed below the surface of the water sample, add 1
mL of a 50 per cent manganese(II) solution (50 g of manganese(II) sulphate
pentahydrate in water and making up to 100 mL) and in a similar way add 1 mL
of alkaline iodide-azide solution (40 g of sodium hydroxide, 20 g of
potassium iodide and 0.5 g of sodium azide made up to 100mL with water). Re-stopper
the water sample and shake the mixture well. The manganese (III) hydroxide
forms as a brown precipitate. Allow the precipitate to settle completely for 15
minutes and add 2 mL of concentrated phosphoric(V) acid (85 per cent). Replace
the stopper and turn the bottle upside-down two or three times in order to mix
the contents. The brown precipitate will dissolve and release iodine in the
solution. If the brown precipitate has not completely dissolved then add a
little more (a
few drops) phosphoric(V) acid.
Measure out a 100 mL portion of the solution with a pipette and titrate the iodine with approximately M/80 standard sodium thiosulphate solution adding 2mL of starch solution as indicator as the titration proceeds and after the titration liquid has become pale yellow in colour.
Calculate the dissolved oxygen content and express it as mg L-1; 1 mL of M/80 thiosulphate = 1 mg dissolved oxygen.