Definition of DO in water and analysis of the principles and methods of DO determination
Dissolved Oxygen (DO) of Water
The amount of gaseous oxygen dissolved in a water sample under oxygen-saturated conditions at a specific temperature is defined as the DO (Dissolved Oxygen) of that water.
At 15°C, the DO value in oxygen-saturated water is 10 mg/L or 10 ppm (ppm = parts per million). At 20°C, the DO value drops to 9.2 ppm because the solubility of gases in liquids decreases as the temperature rises.
Determination of Water DO
The DO value can be instantly measured using a digital DO meter equipped with an oxygen sensor probe or electrode submerged directly into the water. In laboratories, DO is precisely determined via the chemical Winkler Iodometric Method. For standard surface water, the DO value should strictly be maintained at or above 5 mg/L (5 ppm). However, in river estuaries, due to constant wave action and water currents, the DO value typically stays above 6 mg/L.
Principle and Method of DO Determination
Measuring dissolved oxygen is crucial for assessing the overall purity and quality of water. It serves as the baseline for calculating BOD (Biochemical Oxygen Demand), which evaluates wastewater pollution levels. Monitoring DO is indispensable for maintaining the aerobic characteristics of receiving waters, sewage systems, and industrial effluents. In the modified Winkler’s method, DO is measured by titrating the liberated iodine using iodometry.
Steps and Chemical Reactions (Winkler Method):
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Manganese sulfate (MnSO4) is added to the water sample containing alkaline potassium hydroxide (KOH) and potassium iodide (KI). This produces a white precipitate of manganese hydroxide [Mn(OH)2]:
MnSO4 + 2KOH → Mn(OH)2↓ + K2SO4
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The dissolved oxygen present in the water oxidizes Mn(OH)2 to form a brown precipitate of basic manganese oxide or manganese(IV) oxide hydrate:
2Mn(OH)2 + O2 (dissolved) → 2MnO(OH)2↓ [Brown Precipitate]
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Upon adding concentrated sulfuric acid (H2SO4), the brown precipitate dissolves and converts into manganese(IV) sulfate:
MnO(OH)2 + 2H2SO4 → Mn(SO4)2 + 3H2O
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The generated Mn(SO4)2 reacts with potassium iodide (KI) to liberate an equivalent amount of free iodine (I2):
Mn(SO4)2 + 2KI → MnSO4 + K2SO4 + I2
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Finally, using starch as an indicator, the liberated free iodine is titrated against a standard sodium thiosulfate (Na2S2O3) solution:
2Na2S2O3 + I2 → Na2S4O6 (Sodium Tetrathionate) + 2NaI
*Note: The amount of free iodine liberated in this sequence is directly stoichiometric and equivalent to the amount of dissolved oxygen originally present in the sample.
DO Calculation (Stoichiometric Relationship)
According to the balanced step-by-step chemical equations above:
Therefore, the core relation simplifies to:
Hence, 1 mol Na2S2O3 ≡ 0.25 mol O2 (Dissolved DO)
Significance and Impact of Water DO
(1) Survival of Aquatic Life: The decomposition of organic waste in rivers and ponds triggers a surge in bacterial activity, rapidly consuming and depleting the water’s DO. When DO falls below critical levels, aerobic aquatic organisms, especially fish, suffocate and die. Conversely, anaerobic aquatic plants and harmful bacteria multiply rapidly in oxygen-deprived water, leading to further bacterial infections and massive fish die-offs.
(2) Development of Foul Odors and Toxic Gases: Low DO conditions result in the incomplete oxidation (anaerobic decomposition) of biodegradable organic matter. This pathway produces highly toxic and foul-smelling compounds like methane (CH4), hydrogen sulfide (H2S), phosphine (PH3), and various volatile amines. Sewage, domestic wastes, and untreated industrial liquid effluents are the primary drivers behind severe DO depletion.
Net Ecological Outcome: If the DO level of surface water falls below 5 ppm, aerobic aquatic populations decline sharply while anaerobic micro-organisms surge. The ultimate result is a total collapse of the aquatic ecosystem, structural loss of biological equilibrium, and the widespread emission of hazardous, unhygienic gases.
