Water Analysis: unit of concentration, estimation of hardness by EDTA, and alkalinity

Water Analysis: Hardness by EDTA & Alkalinity

1. Units of concentration (water chemistry)

• ppm (parts per million) = mg of solute per litre ≈ mg/L for dilute water.
• Hardness always expressed as CaCO₃ equivalents so Ca²⁺ and Mg²⁺ add up:

• 1 °Clarke = 1 grain/gallon ≈ 14.3 ppm; 1 °French = 10 ppm. Conversions asked via CaCO₃ equivalents, not direct degrees.

2. Types of hardness

• Temporary (carbonate) hardness: Ca/Mg bicarbonates — removed by boiling ($Ca(HCO_3)_2 \to CaCO_3\downarrow + CO_2 + H_2O$).
• Permanent (non-carbonate) hardness: chlorides, sulphates of Ca/Mg — needs lime-soda/zeolite/ion-exchange.
• Total hardness = temporary + permanent. 1 mole CaCO₃ ≡ 1 mole EDTA ≡ 1 mole Ca²⁺/Mg²⁺ in titration math.

3. EDTA estimation of hardness

EDTA (ethylenediamine tetraacetic acid, disodium salt) complexes Ca²⁺/Mg²⁺ 1:1. Indicator: Eriochrome Black-T (EBT) — wine-red with metal ions, steel-blue when free, at pH ~10 (ammonia buffer).

• Procedure: buffer + EBT + sample → wine red; titrate with standard EDTA → blue end point.
• Steps: (i) total hardness (all Ca+Mg); (ii) boil, filter, titrate filtrate → permanent hardness; temporary = total − permanent.
• Formula:

($V$ in mL, $M$ in mol/L, 100 = mol. wt. CaCO₃).

Numerical: 50 mL sample, 0.01 M EDTA, titre 12.5 mL → hardness $= 12.5 \times 0.01 \times 100 \times 1000/50 = 250$ ppm.

4. Alkalinity

Capacity to neutralize acid — due to $OH^-$, $CO_3^{2-}$, $HCO_3^-$. Titrate with standard HCl using phenolphthalein (P) then methyl orange (M):

• $P$ titre → $OH^- + \frac{1}{2}CO_3^{2-}$; $M$ titre → total alkalinity.

Relation	$OH^-$	$CO_3^{2-}$	$HCO_3^-$
$P = 0$	0	0	$M$
$P = M/2$	0	$2P$	0
$P < M/2$	0	$2P$	$M - 2P$
$P > M/2$	$2P - M$	$2(M - P)$	0
$P = M$	$M$	0	0

Each expressed as CaCO₃ equivalents (1 mL 1 N HCl ≡ 50 mg CaCO₃). High alkalinity → boiler foaming, embrittlement risk.

Exam Points

• EDTA principle, indicator, pH 10 buffer, colour change + hardness formula with one numerical (5 marks favourite — temporary/permanent split included).
• P/M alkalinity table — memorize the five rows (3–5 marks); units ppm as CaCO₃.