Reading a Water Body by Its Oxygen: From Winkler Titration to BOD5 and COD

Ask an operator whether a river is healthy and you get an answer about colour or smell. Ask a laboratory and you get a number: dissolved oxygen. DO is tied to the partial pressure of oxygen in the air, atmospheric pressure, water temperature and water quality. At 20°C and 100 kPa, pure water holds roughly 9 mg/L. That figure is the reference point against which almost everything else in aerobic treatment is judged.

Where the Oxygen Goes

Aerobic bacteria break organic compounds down and consume dissolved oxygen doing it. Measured as carbon, C+O2=CO2 means every 12 g of carbon burns 32 g of oxygen. Drop the DO value to 5 mg/L and some fish species start struggling to breathe. DO is replenished two ways: oxygen infiltrating from the atmosphere when the water is unsaturated, and oxygen released by aquatic plants through photosynthesis. When organic pollution drives consumption faster than either source can keep up, anaerobic bacteria take over and the water turns black and foul.

Recovery time after a DO sag is a direct read on self-purification capacity. A short recovery means the water body can look after itself, or the pollution is mild. A long or absent recovery means self-purification is weak or gone.

The Winkler Method, Step by Step

Add manganese sulfate and alkaline potassium iodide to the sample. Dissolved oxygen oxidises low-valent manganese to high-valent manganese and a brown tetravalent manganese hydroxide precipitate forms. Acid is added, the precipitate dissolves, and it reacts with iodide ions to liberate free iodine. With starch as indicator, the liberated iodine is titrated against standard sodium thiosulfate, and DO comes out of titrant consumption: DO (O2, mg/L) = M×V×8000/100, where M is the concentration of the standard sodium thiosulfate solution in mol/L and V is the volume consumed in mL.

Reagents That Decide the Result

• Manganese sulfate solution must not turn blue with starch. For the alkaline potassium iodide solution, weigh 480 g of manganese sulfate (MnSO4·4H2O), dissolve in water and dilute to 1000 mL. A separate alkaline iodide preparation weighs 500 g of sodium hydroxide into 300-400 mL of water and 150 g of potassium iodide into 200 mL of water, combines them once the caustic has cooled, and dilutes to 1000 mL.
• (1+5) sulfuric acid: one part concentrated acid to five parts water, mixed well.
• 1% starch solution: 1 g of soluble starch pasted with a little water, diluted to 100 mL with freshly boiled water, then 0.1 g of salicylic acid or 0.4 g of zinc chloride added as preservative once cool.
• 0.02500 mol/L potassium dichromate standard: 1.2258 g of potassium dichromate dried at 105-110°C for 2 h and cooled, dissolved and made up to 1000 mL.
• Sodium thiosulfate solution: 3.2 g of Na2S2O3·5H2O dissolved in boiled and cooled water with 0.2 g of sodium carbonate, made up to 1000 mL, kept in a brown bottle and standardised against the dichromate solution before use. Sulfuric acid at pH=1.84 is used for acidification.

Running the Titration

Fixation happens at the sampling point: pipette below the liquid surface, add 1 mL of manganese sulfate and 2 mL of alkaline potassium iodide, stopper tightly, invert several times and let it stand. Then open the stopper, add 2.0 mL of sulfuric acid below the surface, restopper, invert until the precipitate has fully dissolved, and stand in the dark for 5 min. Pipette 100.00 mL into a 250 mL conical flask, titrate to pale yellow, add 1 mL of starch solution, continue until the blue just disappears, and record the volume used.

The errors are procedural, not chemical. Keep reagents off the air so no atmospheric oxygen rides into the sample. Starch goes in only after the solution has gone from brown to pale yellow, or the endpoint oscillates and becomes hard to judge. Suspended matter adsorbs iodine and biases low - hydrolyse with alum under alkaline conditions and measure the supernatant. Nitrite interferes and is destroyed with sodium azide pre-added to the alkaline iodide solution. Fe2+ at 100-200 mg/L needs 1 mL of 40% potassium fluoride. Oxidising substances such as free chlorine are removed with an equivalent amount of sodium thiosulfate in advance.

The Probe Alternative

A membrane electrode skips the chemistry. The sensor is a gold cathode and a silver anode in potassium chloride or potassium hydroxide electrolyte; oxygen diffuses through the membrane and completes a measuring circuit. Apply a polarisation voltage of 0.6-0.8 V and oxygen reduces at the cathode, O2+2H2O+4e-, while the anode runs Ag+Cl to AgCl+2e-. By Faraday's law the current tracks oxygen partial pressure, and at constant temperature current is linear with concentration. No reagents, and neither colour nor turbidity interferes - but oxygen is consumed at the cathode, so the sample must be kept agitated or the reading drifts low, and temperature has to be logged alongside because it shifts the result more than anything else.

From DO to BOD5

The oxygen microorganisms consume while decomposing organic matter is biochemical oxygen demand, BOD, in milligrams per litre. Degradation runs in two stages: organics to carbon dioxide, ammonia and water, then ammonia to nitrite and nitrate through nitrosating and nitrifying bacteria. BOD normally refers to the first stage only. Rate and extent depend on temperature and time, and 15-30°C is the comfortable band. Complete oxidation theoretically takes infinite time, but 20 days is treated as complete in practice and is called BOD20. Experience says 5 days of culture captures about 70-80% of the total, so the standard method fixes 5 days at 20°C and calls it BOD5. BOD below 1 mg/L indicates clean water; above 3-4 mg/L indicates organic pollution. The test is slow, and on strongly toxic wastewater microbial activity is suppressed and the number becomes unreliable.

Where COD Fits

Chemical Oxygen Demand, COD, is the oxidant consumed during chemical oxidation of oxidisable substances under specified conditions, again reported as milligrams of oxygen per litre of sample. Organic matter ends as carbon dioxide and water. Because different organics oxidise with different ease, COD only describes the oxygen demand of oxidisable substances under the chosen conditions. KMnO4 suits relatively clean water samples; K2Cr2O7 suits heavily polluted water and industrial wastewater. The two give different numbers on the same sample, so the method belongs in the report.

COD is not limited by water quality conditions and is quick, but it cannot separate organics that biodegrade from those that do not, and chemical oxidants also attack reducing inorganic substances. BOD remains the better indicator of organic pollution; COD is the substitute when water quality rules BOD out.

When There Is Too Much

Supersaturation is the mirror image: violent aeration or a spillway drives molecular oxygen into solution until DO exceeds equilibrium. Water conservancy projects do this routinely. Under high dams and large reservoirs, discharge through outlets or flood-discharge holes drags air into the falling water, and dissolved gas content downstream rises enough to harm aquatic organisms, fish especially, over a wide reach.