⚗️ Dissolved oxygen in water — the iodometric Winkler method (manganese(II) hydroxide, iodide, titration with thiosulfate)

Physicochemistry PN-EN 25813

Overview

ISO 5813:1983 (in Poland PN-EN 25813:1997, which implements EN 25813) specifies the iodometric method of determining dissolved oxygen in water, the so-called Winkler procedure, modified to take account of certain interferences. The iodometric method is the reference method for determining dissolved oxygen; it is used for all types of water with an oxygen concentration above 0.2 mg/l, up to double saturation (about 20 mg/l), free of interfering substances. Interferences come from readily oxidisable organic substances (tannins, humic acids, lignins), oxidisable sulfur compounds (sulfides, thiourea) and actively respiring systems that consume oxygen — in those cases the method with an electrochemical probe (ISO 5814) is better. Nitrites up to 15 mg/l do not interfere, because they are destroyed by the sodium azide that is added. In the presence of oxidising or reducing substances the modifications of clause 9 are used (determination on a second sample with a correction; with reducing agents — oxidation with an excess of sodium hypochlorite), and where suspended solids bind or consume iodine, flocculation with potassium aluminium alum and ammonia according to the annex is used (or a probe). Precision: in air-saturated water (8.5–9 mg/l) the standard deviations within a series are 0.03–0.05 mg/l.

In the PWr (Wrocław University of Science and Technology) textbook "Chemia wody" [Water chemistry] the Winkler method (based on PN-ISO 5813:1997) is described for oxygen contents above 0.5 mg/dm³: into a 125 cm³ oxygen bottle filled to the middle of its neck, 1 cm³ of manganese(II) sulfate (480 g/l) and 2 cm³ of potassium iodide in potassium hydroxide (150 g of KI + 700 g of KOH per 1000 cm³) are added; once the precipitate has settled (brown — a lot of oxygen, white — none), 1 cm³ of concentrated H₂SO₄ is added, and 100 cm³ of the solution is titrated with sodium thiosulfate 0.025 N to a pale straw colour, with 1 cm³ of starch added and titration continued until the colour disappears; X = 0.2·a·1000/V mg O₂/dm³; samples are preserved with 1 cm³ of 5 % HgCl₂ per 1 dm³. Manufacturers describe the Winkler titration as the traditional method with the addition of manganese, iodide and hydroxide, useful among other things for checking optical sensors.

Method principle

Reaction of the dissolved oxygen in the sample with freshly precipitated manganese(II) hydroxide (formed after sodium or potassium hydroxide has been added to manganese(II) sulfate); acidification and oxidation of iodide by the manganese compound of higher oxidation state that has formed, liberating an equivalent amount of iodine; determination of the iodine by titration with sodium thiosulfate. Reagents: sulfuric acid (1+1; 500 ml of the concentrated acid into 500 ml of water; where iron(III) is suspected — phosphoric acid, 1.70 g/ml), sulfuric acid 2 mol/l (½ H₂SO₄), alkaline iodide-azide reagent (35 g of NaOH or 50 g of KOH + 30 g of KI or 27 g of NaI in about 50 ml of water, separately 1 g of NaN₃ in a few ml of water, combined and made up to 100 ml, in an amber bottle; the azide may be omitted when nitrites are absent; after dilution and acidification it should give no colour with starch), anhydrous manganese(II) sulfate 340 g/l (or the monohydrate 380 g/l, the chloride 450 g/l; filter if turbid), standard potassium iodate 10 mmol/l (⅙ KIO₃; 3.567 g dried at 180 °C made up to 1000 ml, then 100 ml → 1000 ml), sodium thiosulfate 10 mmol/l (2.5 g of Na₂S₂O₃·5H₂O in freshly boiled water + 0.4 g of NaOH made up to 1000 ml, in a dark bottle; standardised daily against 20.00 ml of the iodate with about 0.5 g of KI and 5 ml of H₂SO₄ 2 mol/l: c = 6·20·1.66/V), starch 10 g/l freshly prepared, phenolphthalein 1 g/l, iodine about 0.005 mol/l (4–5 g of KI + about 130 mg of I₂ made up to 100 ml), KI or NaI. Apparatus: narrow-necked flasks (Winkler bottles) of 130–350 ml, calibrated to 1 ml, with stoppers bearing the same number (the volume can be established by weighing). Sampling: directly into the bottle in which the determination is carried out — surface waters: fill until it overflows without changing the oxygen content (in shallow waters a probe is better); from a mains supply: a tube of inert material to the bottom, flushed with about 10 volumes; from a given depth: a special sampler filled by displacing the air without turbulence; remove any bubbles and fix the oxygen at once. Fixing: 1 ml of manganese(II) sulfate and 2 ml of the alkaline reagent below the surface with pipettes having a narrow tip, close the stopper without bubbles, invert several times, let the precipitate settle for at least 5 min, then mix again; a sample fixed in this way can be transported and kept for up to 24 h in the dark. Liberation of the iodine: with the precipitate in the lower third of the bottle, slowly add 1.5 ml of sulfuric acid (1+1) (or the corresponding amount of phosphoric acid), close and shake until the precipitate has dissolved and the iodine is evenly distributed (if the titration is carried out in the bottle — draw off a portion of the clear solution without disturbing the precipitate). Titration: transfer the whole content or a portion V₁ to a conical flask and titrate with thiosulfate, adding the starch towards the end (or another indicator). Result: ρ(O₂) = M_r·V₂·c·f₁/(4·V₁) [mg/l], where M_r = 32, V₂ — the volume of thiosulfate, c — its concentration (mmol/l), f₁ = V₀/(V₀ − V′) (V₀ — the volume of the bottle, V′ — the 3 ml of reagents added); report to one decimal place. Before the determination the presence of oxidising/reducing agents is checked (50 ml of the sample neutralised against phenolphthalein + 0.5 ml of H₂SO₄ 2 mol/l + about 0.5 g of KI + starch: a blue colour = oxidising agents; no colour after adding 0.2 ml of iodine for 30 s = reducing agents) and the procedures of clause 9 are applied.

Applications

• The reference method for dissolved oxygen — calibration and checking of electrochemical and optical probes
• Surface waters, intakes and water supply networks, waters at various depths (special samplers)
• Determination of oxygen before and after incubation in the BOD test (chemical method) in laboratories without probes
• Clean and saturated waters (up to about 20 mg/l, double saturation), free of interfering substances
• Waters with suspended solids (after flocculation with potassium aluminium alum and ammonia according to the annex) or with oxidising/reducing agents (procedures of clause 9)

Key parameters

Standard

Standard number

PN-EN 25813:1997 (EN 25813:1992; ISO 5813:1983)

Title (PL)

Jakość wody — Oznaczanie tlenu rozpuszczonego — Metoda jodometryczna

Title (EN)

Water quality — Determination of dissolved oxygen — Iodometric method

Step-by-step procedure

1. Test for oxidising/reducing agents

   Where they may interfere: neutralise 50 ml of the water against 2 drops of phenolphthalein, add 0.5 ml of H₂SO₄ 2 mol/l, about 0.5 g of KI/NaI and a few drops of starch — a blue colour = oxidising agents (proceed according to 9.1); if there is no colour, add 0.2 ml of iodine 0.005 mol/l, shake and wait 30 s — no blue colour = reducing agents (according to 9.2: oxidation with 1.00 ml of hypochlorite of about 4 g Cl/l in two samples, with a correction).

2. Sampling

   Fill the Winkler bottle until it overflows without changing the oxygen content (surface waters); from a mains supply — an inert tube to the bottom, flushed with about 10 volumes; from a given depth — a sampler filled by displacing the air, without turbulence. Remove any bubbles adhering to the glass.

3. Fixing the oxygen

   At once (preferably on site) add below the surface 1 ml of manganese(II) sulfate solution and 2 ml of the alkaline reagent with pipettes having a narrow tip; close without bubbles, invert several times, let the precipitate settle for at least 5 min and mix again. A sample fixed in this way can be taken to the laboratory and kept for up to 24 h in the dark.

4. Liberation of the iodine

   Once the precipitate is in the lower third of the bottle, slowly add 1.5 ml of sulfuric acid (1+1) (or phosphoric acid), close and shake until the precipitate has completely dissolved and the iodine is evenly distributed.

5. Titration

   Transfer the whole content of the bottle or a portion V₁ to a conical flask (if titrating in the bottle, draw off a portion of the clear solution without disturbing the precipitate) and titrate with sodium thiosulfate 10 mmol/l, adding the starch towards the end, until the colour disappears (textbook: to a pale straw colour, then 1 cm³ of starch, then to the disappearance of the colour; any colour returning later is disregarded).

6. Calculation

   ρ(O₂) = 32·V₂·c·f₁/(4·V₁) [mg/l], f₁ = V₀/(V₀ − 3 ml); for the special procedures use the formulae of 9.1.3/9.2.4, and for flocculation (the annex: 20 ml of 10 % alum + 4 ml of ammonia 13 mol/l per 1000 ml, sedimentation, drawing off the clear liquid into two bottles) — the factor V₆/(V₆ − 24 ml). Report the result to one decimal place, together with the ambient temperature and the atmospheric pressure.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sodium azide is a very strong poison — do not acidify the concentrated reagent and do not pour it into drains containing metals (explosive azides); the reagent may be omitted when nitrites are absent
• Concentrated sulfuric acid (1+1) — add the acid to the water, goggles, gloves; where Fe(III) is present use phosphoric acid
• Sodium hypochlorite, mercury(II) chloride (preservation according to the textbook) — toxic; dispense and dispose of them carefully
• Sampling at depth and from the bank — protection against drowning

Frequently asked questions