Hardness of Water

Hardness of Water

Water hardness is a measure of the concentration of multivalent cations in water. Primarily, these cations are calcium (Ca²⁺) and magnesium (Mg²⁺), although other metals like iron (Fe²⁺/Fe³⁺), manganese (Mn²⁺), and strontium (Sr²⁺) can also contribute to hardness. The hardness of water affects its suitability for drinking, industrial processes, and as an ingredient in various products.

Types of Hardness

There are two types of water hardness:

  1. Temporary Hardness: This is caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). It can be removed by boiling the water, which precipitates the bicarbonate ions as carbonate salts that can be filtered out.

  2. Permanent Hardness: This is due to the presence of sulfate or chloride compounds of calcium and magnesium. Permanent hardness cannot be removed by boiling and requires a chemical treatment, such as ion exchange, to eliminate.

Measuring Hardness

Water hardness is typically measured in terms of concentration of calcium carbonate (CaCO₃) in milligrams per liter (mg/L) or parts per million (ppm). There are also other units like grains per gallon (gpg) or millimoles per liter (mmol/L).

The hardness of water can be determined using various methods, including titration with a chelating agent such as ethylenediaminetetraacetic acid (EDTA), which forms complexes with the metal ions.

Effects of Hardness

Aspect Effect of Hard Water
Domestic Use Causes scaling in boilers, pipes, and kettles; reduces soap lathering
Industrial Use Affects processes like dyeing, textile processing; can cause scaling in cooling towers
Health Generally not harmful and may contribute to dietary needs of calcium and magnesium

Removal of Hardness

The most common methods for removing hardness from water are:

  1. Ion Exchange: This process uses a resin where sodium ions (Na⁺) are exchanged for hardness ions like Ca²⁺ and Mg²⁺.

  2. Lime Softening: Adding lime (calcium hydroxide) to water precipitates calcium as carbonate and magnesium as hydroxide.

  3. Chelating Agents: Chemicals like EDTA can bind to metal ions, effectively softening the water.

  4. Reverse Osmosis: This process uses a semi-permeable membrane to remove ions from water.

Formulas and Calculations

The total hardness can be calculated using the concentrations of calcium and magnesium ions:

$$ \text{Total Hardness (as CaCO₃)} = \left( \frac{\text{[Ca²⁺]} \times 50}{20.04} \right) + \left( \frac{\text{[Mg²⁺]} \times 50}{12.16} \right) $$

Where:

  • [Ca²⁺] = concentration of calcium ions in mg/L
  • [Mg²⁺] = concentration of magnesium ions in mg/L
  • 50 = molar mass of CaCO₃ in mg/mmol
  • 20.04 = molar mass of Ca²⁺ in mg/mmol
  • 12.16 = molar mass of Mg²⁺ in mg/mmol

Examples

  1. Temporary Hardness Removal: Boiling 1 liter of water containing 100 mg/L of calcium bicarbonate will result in the precipitation of calcium carbonate:

$$ \text{Ca(HCO₃)₂} \rightarrow \text{CaCO₃} + \text{H₂O} + \text{CO₂} $$

  1. Permanent Hardness Removal: Treating water with ion exchange resins can replace calcium and magnesium ions with sodium ions:

$$ \text{2Na⁺-Resin + Ca²⁺} \rightarrow \text{Ca²⁺-Resin + 2Na⁺} $$

  1. Calculating Hardness: If a water sample contains 60 mg/L of Ca²⁺ and 20 mg/L of Mg²⁺, the total hardness as CaCO₃ is:

$$ \text{Total Hardness} = \left( \frac{60 \times 50}{20.04} \right) + \left( \frac{20 \times 50}{12.16} \right) \approx 150 + 82 = 232 \text{ mg/L as CaCO₃} $$

Understanding water hardness is crucial for managing water quality in various applications, from household use to large-scale industrial processes. Proper treatment and monitoring can prevent scaling, improve efficiency, and ensure that water is suitable for its intended use.