Water Characteristics, Hardness and Softening

By the end you should be able to: Distinguish temporary from permanent hardness, state its disadvantages, calculate degree of hardness in ppm CaCO3, and describe softening by ion exchange and reverse osmosis.

Hardness

Hard water is water that does not readily lather with soap, caused by dissolved calcium and magnesium salts.

Temporary (carbonate) hardness — from the bicarbonates Ca(HCO₃)₂ and Mg(HCO₃)₂. Removed by boiling, which decomposes the bicarbonate:

\text{Ca(HCO}_3)_2 \xrightarrow{\ \Delta\ } \text{CaCO}_3\downarrow + \text{H}_2\text{O} + \text{CO}_2\uparrow

Also removable by Clark's process, adding calculated lime.

Permanent (non-carbonate) hardness — from chlorides and sulphates, CaCl₂, MgCl₂, CaSO₄, MgSO₄. Not removed by boiling; requires chemical treatment or ion exchange.

\text{Total hardness} = \text{temporary} + \text{permanent}

Why hardness matters

Soap wastage. Soap is precipitated as insoluble scum before any lather forms: $2\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \rightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca}\downarrow + 2\text{Na}^{+}$ The scum deposits on fabric and skin. Note that detergents do not behave this way — their calcium salts are soluble — which is why detergents displaced soap for laundry in hard-water areas.
Boiler scale. The most expensive consequence. CaCO₃ and CaSO₄ deposit on heating surfaces, and scale conducts heat poorly. The metal beneath overheats, wasting fuel and risking local overheating and boiler failure. A few millimetres of scale can raise fuel consumption measurably.
Blocked pipes and heating elements, reduced flow.
Domestic effects — spotting on glassware, poor cooking of pulses, harsh feel to washed fabric.
Hardness is not a health hazard; hard water is safe to drink and contributes dietary calcium and magnesium.

Degree of hardness

Different salts contribute differently by mass, so all hardness is expressed as an equivalent quantity of CaCO₃.

Why CaCO₃. Its molar mass is 100 g mol⁻¹, which makes the arithmetic easy, and it is the compound that actually forms most scale.

\text{CaCO}_3\ \text{equivalent} = \frac{\text{mass of salt}}{\text{molar mass of salt}} \times 100

Units: ppm (mg per litre, the usual one), or degrees Clark, or degrees French.

Classification: soft below 60 ppm; moderately hard 60–120; hard 120–180; very hard above 180.

The EDTA method

The standard determination. EDTA forms a stable 1:1 complex with Ca²⁺ and Mg²⁺ at pH 10, using Eriochrome Black T as indicator — wine red while free metal ions remain, steel blue at the end point.

\text{Hardness (ppm)} = \frac{V_{EDTA} \times M_{EDTA} \times 100 \times 1000}{V_{sample}}

with volumes in mL and molarity in mol L⁻¹.

Ion exchange softening

Principle. Insoluble resins exchange their mobile ions for those in the water.

Cation exchange resin (R–SO₃H or R–SO₃Na) exchanges H⁺ or Na⁺ for Ca²⁺ and Mg²⁺.
Anion exchange resin (R–N(CH₃)₃OH) exchanges OH⁻ for Cl⁻, SO₄²⁻, HCO₃⁻.

Base exchange (zeolite) softening, the simple domestic case:

\text{Na}_2\text{Ze} + \text{Ca}^{2+} \rightarrow \text{CaZe} + 2\text{Na}^{+}

Regeneration, when the resin is exhausted, using brine:

\text{CaZe} + 2\text{NaCl} \rightarrow \text{Na}_2\text{Ze} + \text{CaCl}_2

Deionisation passes water through both resin types in series, producing water of very high purity:

\text{H}^{+} + \text{OH}^{-} \rightarrow \text{H}_2\text{O}

Advantages: removes hardness almost completely — below 1 ppm — regenerable and long lasting, simple to operate.

Limitations: water must be free of turbidity and suspended solids, which foul the resin; regeneration produces a concentrated brine effluent that is itself a disposal problem; and — the point most often missed — sodium-form softening does not remove dissolved solids, it exchanges them. Calcium is replaced by an equivalent amount of sodium, so the total dissolved solids are unchanged and softened water carries extra sodium, which matters for anyone on a restricted-sodium diet.

Reverse osmosis

Osmosis — solvent flows spontaneously through a semipermeable membrane from dilute to concentrated, until the osmotic pressure opposes it.

Reverse osmosis — apply pressure to the concentrated side greater than the osmotic pressure, and the flow reverses: pure water is forced out, leaving dissolved matter behind.

Process. Feed water is pre-filtered, then pumped at 15–80 bar (higher for seawater) against a thin-film composite polyamide membrane wound in a spiral module. Two streams emerge: permeate, the purified water, and reject or brine, carrying the concentrated salts.

Advantages: removes essentially everything — ions, organics, bacteria, viruses, 95–99% of dissolved solids — not merely hardness; no chemicals needed; compact and continuous; and it is the basis of seawater desalination.

Limitations: high pressure means high energy cost; membranes foul and scale and need pre-treatment and periodic cleaning; membranes are expensive and need replacement; and it wastes water — domestic units typically recover only 25–50%, discharging the rest as reject. That waste ratio is worth knowing before installing one in a water-scarce area.

It also removes beneficial minerals, so remineralisation is often added afterwards.