1. Definition & Types

Hardness of water is the property caused mainly by dissolved Ca²⁺ and Mg²⁺ ions which prevent soap lathering and cause scale. Hardness is commonly expressed as mg·L⁻¹ as CaCO₃.

• Temporary (carbonate) hardness — due to bicarbonates: Ca(HCO₃)₂, Mg(HCO₃)₂. Removable by boiling or lime treatment.
• Permanent (non-carbonate) hardness — due to chlorides/sulfates: CaCl₂, MgSO₄. Not removed by boiling; chemical or ion-exchange treatment required.
• Total hardness = Temporary hardness + Permanent hardness.

2. Estimation of Hardness — EDTA (Complexometric) Method

Principle

EDTA (ethylenediaminetetraacetic acid) forms 1:1 chelates with Ca²⁺ and Mg²⁺. At pH ? 10 (NH₄OH / NH₄Cl buffer) Eriochrome Black T (EBT) indicator is wine-red when bound to metal ions and turns blue when EDTA complexes all Ca²⁺/Mg²⁺.

Representative reactions

Procedure (summary)

1. Measure aliquot of sample (V_sample, e.g. 50.00 mL).
2. Add ammonia–ammonium buffer (pH ? 10) and a few drops of EBT indicator.
3. Titrate with standard EDTA solution (M_EDTA) until color changes wine-red ? clear blue.
4. Record EDTA volume used (V_EDTA).

Calculation (symbolic)

(Use M_r(CaCO₃) ? 100.09 g·mol⁻¹ and consistent units.)

3. Estimation of Alkalinity (Relation to Hardness)

Principle

Alkalinity is the buffering capacity due to OH⁻, CO₃²⁻ and HCO₃⁻. Determined by titration with standard acid using phenolphthalein and methyl orange end points.

Endpoints & interpretation

• Phenolphthalein end point (pH ? 8.3) — indicates OH⁻ and ½ CO₃²⁻ (P-alkalinity).
• Methyl orange end point (pH ? 4.5) — indicates total alkalinity from HCO₃⁻, CO₃²⁻, OH⁻.

4. Removal of Hardness — Methods

4.1 Soda-lime Process (Chemical Softening)

Uses lime (Ca(OH)₂) and soda ash (Na₂CO₃) to precipitate hardness as carbonates and thus remove both temporary and some permanent hardness.

Precipitates are settled/filtered and residual alkalinity is adjusted before use.

4.2 Zeolite (Permutit) Process

Natural/synthetic sodium zeolite (Na₂Ze) exchanges Na⁺ for Ca²⁺/Mg²⁺ in water. Widely used for domestic softening and boiler pre-treatment.

4.3 Ion-exchange Process (Resin Based)

Synthetic resins exchange undesirable cations/anions for H⁺ and OH⁻ (or Na⁺), enabling softening or full demineralization.

• Cation exchanger (H-form): R–SO₃H exchanges H⁺ for Ca²⁺, Mg²⁺, Na⁺.
• Anion exchanger (OH-form): R–NR₃OH exchanges OH⁻ for Cl⁻, SO₄²⁻, NO₃⁻.

Combined cation + anion exchange yields deionized water (H⁺ + OH⁻ → H₂O). Resins are regenerated with strong acid (HCl/H₂SO₄) and alkali (NaOH) respectively.

5. Applications, Advantages & Monitoring

• Boiler-feed water treatment — prevents scale and improves heat-transfer efficiency.
• Domestic & industrial processes — reduces soap scum, extends equipment life.
• Electronics and laboratories — provides deionized water for sensitive processes.
• Monitoring: periodic EDTA titrations, conductivity monitoring, scheduled resin regeneration, and sludge handling for chemical softening.