Hardness in Water

Hardness in Water

• Hardness in water is caused by divalent cations, namely Calcium, Magnesium, Strontium, Ferrous iron, and Manganese.

Characteristics of Hardness-Causing Cations

• These cations react with soap to form precipitates.
• They also react with some anions to form scale.
• Hardness is measured using the amount of Calcium and Magnesium ions in the water.

Cations That Cause Hardness

• Ca2+ reacts with HCO3-.
• Mg2+ reacts with SO42-.
• Sr2+ reacts with Cl-.
• Fe2+ reacts with NO3-.
• Mn2+ reacts with SiO32-.

How Hardness Occurs

• Rainwater is initially soft.
• It picks up hardness as it flows through the ground.
• Soil contains carbon dioxide, which is released by bacteria.
• The carbon dioxide acts as a weak acid, which dissolves limestone.
• Limestone is made up of Calcium Carbonate (CaCO3) and contains impurities like sulfates, chlorides, and silicates.
• As the limestone dissolves, the dissolved minerals make the water hard.

Classifying Hardness

• Soft water has a concentration of 0-75 mg/L.
• Moderately hard water has a concentration of 75-150 mg/L.
• Hard water has a concentration of 150-300 mg/L.
• Very hard water has a concentration of 300+ mg/L.

Determining Hardness Accurately

• Hardness can be accurately determined by titration with EDTA (ethylenediaminetetraacetic acid) or its disodium salts.

Structure of EDTA

• EDTA has the structure: HOOC-H2C-N-CH2-CH2-N-CH2-COOH.

Determining Hardness by Complexometry Titration

• Hardness can be determined using a standard solution of EDTA and complexometric titration.
• Metal ions (M2+) react with EDTA to form a stable complex.
• The reaction can be represented:
• M2+ + In → [M-In] (wine-red).
• [M-EDTA] + 2Na+ + In → [M-EDTA] + In (blue).