Experiment 03LimnologyWater Quality
Limnological Analysis: Dissolved Oxygen (DO) & Biological Oxygen Demand ($text{BOD}_5$) via Azide-Modified Winkler Titration
Duration: 5 Days (Incubation) • Bench Chemistry
🎯 Experimental Aim & Learning Objectives
To quantify Dissolved Oxygen (DO) levels in lotic and lentic water samples using the standard azide modification of the Winkler iodometric titration, set up 5-day dark incubations at 20°C to determine 5-day Biochemical Oxygen Demand ($text{BOD}_5$), and evaluate organic pollution loads using the Streeter-Phelps oxygen sag curve model.
🔬 Theoretical Foundation & Chemical Stoichiometry
Dissolved oxygen is the master limnological variable governing aquatic aerobic metabolism. The azide modification suppresses interference from nitrites ($NO_2^-$), which otherwise catalyze false iodine liberation:
- Oxygen Fixation: Manganese Sulfate ($MnSO_4$) reacts with Sodium Hydroxide ($NaOH$) in the presence of dissolved $O_2$ to form an oxidized brown basic manganic oxide precipitate:
$$2MnSO_4 + 4NaOH + O_2 longrightarrow 2MnO(OH)_2downarrow + 2Na_2SO_4$$
In the absence of oxygen, a white precipitate of pure manganous hydroxide ($Mn(OH)_2$) forms. - Acidification & Iodine Liberation: Adding concentrated $H_2SO_4$ dissolves the precipitate, reducing $Mn^{4+}$ back to $Mn^{2+}$ while quantitatively oxidizing iodide ($I^-$) to free elemental iodine ($I_2$):
$$MnO(OH)_2 + 2KI + 2H_2SO_4 longrightarrow MnSO_4 + I_2 + K_2SO_4 + 3H_2O$$ - Thiosulfate Titration: The liberated iodine is titrated against standard Sodium Thiosulfate ($0.025text{ N } Na_2S_2O_3$) with a starch indicator (turning from intense blue-black to crystal clear):
$$2Na_2S_2O_3 + I_2 longrightarrow Na_2S_4O_6 + 2NaI$$
Because $1text{ mL of } 0.025text{ N } Na_2S_2O_3 equiv 0.2text{ mg } O_2$, titrating a $200text{ mL}$ aliquot gives:
$$text{DO (mg/L)} = frac{V_{text{titrant}} times N_{text{thiosulfate}} times 8 times 1000}{V_{text{sample aliquot}}}$$ - Biochemical Oxygen Demand ($text{BOD}_5$): Reflects the amount of oxygen utilized by heterotrophic microorganisms to aerobically decompose organic matter over 5 days at 20°C in total darkness:
$$text{BOD}_5text{ (mg/L)} = (text{DO}_{text{initial}} – text{DO}_{text{day 5}}) times text{Dilution Factor}$$
Figure 1.3: Azide-modified Winkler titration stoichiometry, Streeter-Phelps stream reaeration/deoxygenation sag model, and 5-day BOD calculation protocols.
🧪 Reagents & Laboratory Equipment
- Reagents: Manganese Sulfate solution ($480text{ g } MnSO_4cdot 4H_2O / text{L}$); Alkali-Iodide-Azide reagent ($500text{ g } NaOH + 150text{ g } KI + 10text{ g } NaN_3 / text{L}$); Concentrated Sulfuric Acid ($H_2SO_4$, sp. gr. 1.84); $0.025text{ N}$ Standard Sodium Thiosulfate ($Na_2S_2O_3cdot 5H_2O$); freshly prepared 1% soluble starch indicator solution.
- Apparatus: 300 mL ground-glass stoppered BOD bottles; automatic burette (50 mL, 0.1 mL precision); volumetric pipettes (1 mL, 2 mL); BOD refrigerated incubator calibrated at $20 pm 1^circtext{C}$.
📋 Step-by-Step Analytical Protocol
- Sample Collection & Initial Fixation: Fill a 300 mL BOD bottle underwater without creating turbulence or bubbles. Immediately insert pipette below liquid surface and add $2text{ mL}$ Manganese Sulfate followed by $2text{ mL}$ Alkali-Iodide-Azide. Stopper securely, avoiding trapped air bubbles, and invert 15 times. Allow brown flocculent precipitate to settle past the upper third.
- Acidification: Add $2text{ mL}$ concentrated $H_2SO_4$ down the neck of the bottle. Re-stopper and invert vigorously until all precipitate completely dissolves into a clear golden-brown iodine solution.
- Titration: Transfer exactly $203text{ mL}$ (accounting for $4text{ mL}$ reagent displacement in a $300text{ mL}$ bottle, giving an effective $200text{ mL}$ sample) into an Erlenmeyer flask. Titrate with $0.025text{ N } Na_2S_2O_3$ until pale straw-yellow. Add $1-2text{ mL}$ starch indicator (turns deep blue-black). Continue dropwise titration to the persistent colorless endpoint. Record $V_1$.
- BOD5 Setup: Prepare a duplicate BOD bottle with the sample (or diluted sample if treating wastewater). Incubate in the dark at $20^circtext{C}$ for exactly 5 days. On Day 5, fix and titrate to determine $text{DO}_{text{day 5}}$.
📊 Representative Data & Interpretation
Unpolluted Lake Sample: Initial DO = $8.2text{ mg/L}$, Day 5 DO = $6.8text{ mg/L}$ (No dilution).
$text{BOD}_5 = 8.2 – 6.8 = 1.4text{ mg/L}$ (Prisinte, oligotrophic / Class A standard).
Municipal Drain Outfall: 1:20 dilution ($15text{ mL}$ effluent in $300text{ mL}$ aerated blank).
Initial DO = $8.0text{ mg/L}$, Day 5 DO = $2.4text{ mg/L}$.
$text{BOD}_5 = (8.0 – 2.4) times 20 = 112text{ mg/L}$ (Heavy organic sewage pollution, requiring wastewater treatment).
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