Winkler Titration for Dissolved Oxygen in Water
Dissolved oxygen (DO) shows whether water can support aquatic life and how oxidation processes are progressing. A Winkler titration is an iodometric method: alkaline manganese fixes oxygen, iodide releases iodine in acid, and thiosulfate measures it. It remains useful when a quick result is needed without an online probe.
For Australian users, the method supports source-water assessment, wastewater process control, reservoir studies and checks on water supplied to producers of still and sparkling bottled water. The Australian Drinking Water Guidelines set national expectations, while state and territory water legislation and regulator requirements govern particular supplies and recycled-water schemes.
Chemistry behind the measurement
Manganese(II) is precipitated in strong alkali and oxidised by dissolved oxygen to higher-valent manganese solids. After acidification, these solids oxidise iodide to iodine. Because one mole of oxygen corresponds to four moles of thiosulfate, the titration has a definite stoichiometric basis rather than relying on colour preference.
Collect the sample without adding or losing oxygen. Immediately record its temperature, avoid splashing, and reject bottles with leaks or bubbles. Measure only what is needed and begin fixation at the sampling point; a stable bottle seal cannot correct oxygen gained or lost earlier.
Reagents and glassware
Use a calibrated volumetric container, suitable volumetric pipettes, a stoppered Winkler or BOD bottle, a burette and a validated reagent set. Typical reagents are concentrated manganese(II) sulfate, alkaline potassium iodide containing sodium hydroxide, sulfuric acid, starch indicator and standardised sodium thiosulfate.
Use the exact strengths and volumes specified by the adopted laboratory method because reagent formulation affects capacity and titre. If sodium hydroxide concentration is uncertain, calibrating sodium hydroxide before preparing alkaline iodide prevents a systematic error. Prepare a reagent blank with the same quantities used for the sample.
Sampling and oxygen fixation
For a common 300 mL procedure, add about 2 mL of manganese solution first and the same volume of alkaline iodide beneath the surface. Stopper immediately and invert repeatedly until the precipitate is uniformly dispersed; a white-to-brown change indicates oxidation. If the bottle was filled to the brim, some reagent overflows, which is preferable to leaving headspace for atmospheric exchange.
A measured 250–300 mL aliquot is often titrated, but its original volume—not the diluted volume—must be recorded. Run separate bottles for replicates. In Sydney and Melbourne, distribution water and recycled-water schemes may have different temperature and residual-management histories, so sample at the point relevant to the investigation rather than at a convenient laboratory tap.
| Method | Measurement principle | Suitable use | Main caution |
|---|---|---|---|
| Conventional Winkler | Manganese fixes oxygen, then liberated iodine is titrated | Clean or low-interference samples | Nitrite and oxidising substances can bias results upward |
| Azide-modified Winkler | Azide suppresses nitrite before iodine is released | Suitable samples containing measurable nitrite | Hazardous reagent that does not correct every interference |
| Membrane electrode | A probe measures oxygen directly | Rapid field or continuous process checks | Calibration, fouling and temperature response affect results |
Acidification and iodine release
Once fixation is complete, add the method-specified amount of sulfuric acid slowly and swirl in a closed system. Strong acidity dissolves the manganese solids and releases iodine, producing a yellow-brown colour. Use an injection port or minimise opening because iodine is volatile and atmospheric exchange can bias the recovered amount.
Follow the validated settling time if one is specified, then confirm that the precipitate has dissolved before titration. Do not acidify before iodine is fixed, and do not acidify alkaline azide-modified reagent outside the approved procedure because sodium azide and acidic mixtures require stringent chemical-safety controls.
Titration and endpoint control
Titrate the iodine with standardised sodium thiosulfate while stirring. Reserve starch until the mixture is pale straw-yellow; adding it too early can cause slow iodine release and a sluggish endpoint. Continue dropwise through the blue colour until it disappears and only the faint colour of dissolved manganese remains.
Run a reagent blank in parallel. For a sample titre (V), blank titre (V_b), thiosulfate normality (N), and original sample volume (V_s), calculate DO as ((V-V_b) \times N \times 8 \times 1000/V_s) in mg/L when all titres are in mL and (N) is in equivalents per litre.
Replicate bottles should agree within the adopted method’s acceptance limit. A drifting result may indicate leakage, incomplete manganese fixation, delayed analysis, endpoint judgement or excessive biological activity. Analyse promptly and retain uncorrected readings so calculations remain traceable.
Interferences and quality assurance
Nitrite and ferric iron can generate extra iodine and bias results upward; sulfite and other reducing substances can consume iodine and bias them downward. Suspended material may also interfere. Use an approved azide-modified Winkler procedure for suitable nitrite-containing samples, while recognising that azide does not correct every oxidant or reductant.
Water from warm regions such as Brisbane or Darwin generally has a lower oxygen-saturation concentration than cool water at the same measured DO. Report temperature with every field result and calculate percent saturation using an appropriate local saturation value. Commercial laboratories may offer this test within a NATA ISO/IEC 17025-accredited scope, but the specific DO method should be verified as included.
Reporting Australian water-quality results
Record the result in mg/L and, where useful, as percent saturation, together with sampling time, location, depth, bottle condition, reagent variant, thiosulfate concentration, sample and blank titres, and any accepted correction. Chlorinated drinking water can be compliant even when its DO is modest, while low DO in receiving water may indicate organic loading; the value needs operational context.
The result to remember is simple: exclude atmospheric exchange, fix oxygen quantitatively in the manganese–iodide matrix, and report a blank-corrected titration with its sampling temperature and time.