Dissolved Oxygen - Winkler Technique

Principle

A divalent manganese solution, followed by strong alkali, is added to the sample. Dissolved oxygen rapidly oxidizes an equivalent amount of divalent manganese to basic hydroxides of higher valency states. When the solution is acidified in the presence of iodide, the oxidized manganese again reverts to the divalent state and iodine, equivalent to the original dissolved oxygen content of the water, is liberated. This iodine is titrated with standardized thiosulfate solution.

Sample collection and handling

Water samples should be collected in rinsed 300 ml BOD bottles. Samples should be placed in bottles as soon as they are taken from the water, the bottles should be filled with a minimum of agitation and exposure of the water sample to air, and bottles should be allowed to overflow. Ideally 1 ml of manganous sulfate and 1 ml of alkaline iodide should be added at once to the bottles when full. The bottles may then be stoppered, mixed, and analyzed at leisure, but should be shielded from strong light.

Procedure

After addition of the manganous sulfate and alkaline iodide and mixing, a precipitate will form that should be allowed to settle at least a third of the bottle's length. Then add 1 ml of conc. sulfuric acid, restopper carefully, and shake to dissolve the precipitate.

Within an hour of acidification pipet 50.0 ml of solution into an Erlenmeyer flask fitted with a white background and a stirring bar, add an aliquot of starch solution, and titrate with standardized thiosulfate (0.01 N or other appropriate concentration). The initial color should be a deep blue-black, followed by lighter blue. The endpoint is when the blue color just disappears and does not return for at least 15 sec. A slight blue color in samples that have been allowed to sit for a while after titration is normal. S

ubtract any blank correction (see Calibration) from the titration volume to obtain the corrected titration in V ml thiosulfate. Calculate the oxygen concentration in millimoles oxygen per liter (mm O2/l or mM O2) from the equation:

mm O2/l = 0.1006 x f x V

(f is a standardization value for thiosulfate, see Calibration)

For convenience,

ml O2/liter = 11.20 x mm O2/liter

and

mg O2/liter (ppm) = 16.00 x mm O2/liter

Reagents

Manganous sulfate: 365 g reagent grade MnSO4 monohydrate and make volume to one liter with distilled water.

Alkaline iodide: Dissolve 500 g reagent grade NaOH in 500 ml distilled H2O. Dissolve 300 g reagent grade KI in 450 ml distilled H2O and mix the two solutions.

Standard thiosulfate: Dissolve 2.9 g reagent grade Na2S2O3 and 0.1 g Na2CO3 in one liter of distilled H2O. Store in a dark bottle; reagent is light sensitive.

Starch indicator: Suspend 2 g soluble starch in 3-400 ml distilled H2O. Add 20% NaOH until solution is clear. Add conc. HCl until solution is slightly acidic, then add 2 ml glacial acetic acid and make volume to one liter with distilled H2O.

Calibration

• Blank: Fill a BOD bottle with sea or distilled water, add 1 ml conc. sulfuric acid and 1 ml of alkaline iodide and mix. Then add 1 ml of manganous sulfate and mix again. Take 50.0 ml aliquots for titration. If addition of starch indicator gives any blue color, titrate to end point with thiosulfate and subtract the resulting blank correction factor (in ml thiosulfate) from sample titration values.

  Determination of f: Repeat the above procedure, except add 5.00 ml of 0.01 N potassium iodate (0.3567 g KIO3 in one liter of distilled H2O) to the 50 ml aliquots and allow iodine to be liberated for 2-5 minutes. Titrate this iodine with thiosulfate. If V is mls thiosulfate used in titration, then:

f = 5.00/V

• A mean value of f, taken from at least three replicate determinations, should be used. Potassium iodate solution is stable for long periods, but f should be checked on batches of thiosulfate every few months or when a new batch is made.

Notes

The Winkler method is used to calibrate other measurement techniques, including various electronic DO meters. Properly conducted, its precision and accuracy are excellent. Under some conditions (presence of reducing agents, etc.) slight modifications of the method are necessary (see APHA, 1985, Standard Methods for Wastewater Analysis).

Proper precautions for handling dangerous chemicals should always be observed, including the use of goggles for eye protection.

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