Lab 2: Determination of Water Alkalinity

🎯 Objective

To determine the phenolphthalein alkalinity and total (methyl orange) alkalinity of a given water sample.

Alkalinity of water is due to the presence of carbonates (CO₃²⁻), bicarbonates (HCO₃⁻), and hydroxides (OH⁻). Two indicators are used:

Phenolphthalein (P): Pink → colourless at pH ~8.3 (neutralises OH⁻ and half CO₃²⁻)
Methyl orange (M): Yellow → orange-red at pH ~4.5 (neutralises all remaining alkalinity)

BIS Permissible limit: Total alkalinity ≤ 200 mg/L (max 600 mg/L)

Conical flask, burette, pipette (50 mL), white tile.

N/50 H₂SO₄ standard solution, phenolphthalein indicator, methyl orange indicator, water sample.

Pipette 50 mL of water sample into a 250 mL conical flask.
Add 2 drops of phenolphthalein indicator. If solution turns pink, proceed; if colourless, phenolphthalein alkalinity = 0.
Titrate with N/50 H₂SO₄ until the pink colour just disappears. Volume used = P mL.
Add 2 drops of methyl orange indicator to the same flask.
Continue titrating with N/50 H₂SO₄ until colour changes from yellow to orange-red. Volume used = M mL.
Calculate P alkalinity and Total alkalinity (P + M) in mg/L as CaCO₃.

ℹ️

P alkalinity = (P × N × 50 × 1000) / 50 mg/L as CaCO₃. Total alkalinity = ((P+M) × N × 50 × 1000) / 50 mg/L as CaCO₃.

Observation	Trial 1	Trial 2	Trial 3
Initial burette reading (mL)	______	______	______
Final burette reading (mL)	______	______	______
Volume of titrant used (mL)	______	______	______
Concordant volume (mL)	______

Calculation:
Volume of titrant (V) = ______ mL
Result = ______ (using appropriate formula)

Phenolphthalein alkalinity = ______ mg/L as CaCO₃. Total alkalinity = ______ mg/L as CaCO₃.