Measurement of Hardness of Water

Introduction

Hardness of water is the property of water that prevents the lather formation with soap and is mainly caused by dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts. It is important to measure hardness for domestic, industrial, and agricultural purposes.

Types of Hardness

• Temporary Hardness: Caused by bicarbonates of calcium and magnesium (removable by boiling).
• Permanent Hardness: Caused by chlorides and sulfates of calcium and magnesium (not removed by boiling).

Common Methods to Measure Hardness of Water

1. EDTA Titration (Complexometric Titration):

   This is the most widely used method.

   • Principle: Ethylenediaminetetraacetic acid (EDTA) forms stable complexes with Ca²⁺ and Mg²⁺ ions in water.
   • Indicator: Eriochrome Black T (EBT) indicator changes color at endpoint.
   • Procedure: Water sample is titrated against standard EDTA solution using EBT; the endpoint is indicated by a color change from wine red to blue.
   • Calculation: Total hardness expressed as mg/L (ppm) of CaCO₃.
2. Soap Test:

   Qualitative method.

   • Soap solution forms scum instead of lather in hard water; quantity of soap required to produce lather can indicate hardness.
3. Clark’s Test:

   Used to remove temporary hardness.

   • Boiling converts bicarbonates into insoluble carbonates which precipitate out.
   • Calcium hydroxide (Clark’s reagent) is added to precipitate calcium carbonate.

Stepwise Procedure for EDTA Titration

1. Take a suitable volume of water sample in a conical flask.
2. Add buffer solution to maintain pH around 10.
3. Add few drops of Eriochrome Black T indicator.
4. Titrate against standard EDTA solution until color changes from wine red to pure blue.
5. Calculate hardness based on volume of EDTA used.

Chemical Reaction

EDTA complex formation with calcium and magnesium ions:
Ca²⁺ + EDTA⁴⁻ → [Ca-EDTA]²⁻
Mg²⁺ + EDTA⁴⁻ → [Mg-EDTA]²⁻

Calculation of Hardness

Hardness of water (expressed as equivalent mg/L of CaCO₃) is calculated using the formula:

Hardness (mg/L as CaCO₃) = \(\frac{V_{\text{EDTA}} \times M_{\text{EDTA}} \times 1000 \times 50}{V_{\text{sample}}}\)

Where,

\( V_{\text{EDTA}} \) = Volume of EDTA solution used (mL)

\( M_{\text{EDTA}} \) = Molarity of EDTA solution (mol/L)

\( V_{\text{sample}} \) = Volume of water sample taken (mL)

50 = Equivalent weight of CaCO₃ (g/mol)
    

Units

Hardness is commonly expressed in mg/L or ppm (parts per million) of CaCO₃ equivalent.

Significance of Measuring Hardness

• Helps in determining water suitability for domestic, industrial, and agricultural uses.
• Hard water reduces soap efficiency and forms scalings in boilers and pipes.
• Knowing hardness level assists in water softening treatments.

Precautions

• Use freshly prepared and standardized EDTA solution.
• Maintain proper pH (about 10) with buffer for accurate results.
• Use clean glassware and avoid contamination.
• Perform titration slowly near the endpoint.