Determining Water Hardness: The EDTA Titration Method

Determination of Water Hardness by EDTA Method

EDTA is the abbreviation for Ethylene Diamine Tetra Acetic acid.

Pure EDTA dissolves in water with great difficulty and in very small quantities. Conversely, its di-sodium salt dissolves quickly and completely. Hence, for common experimental purposes, the di-sodium derivative of EDTA is used.

EDTA is a hexadentate ligand. It binds metal ions present in water, such as $Ca^{+2}$ or $Mg^{+2}$, to form a highly stable chelate complex. These metal ions are bonded via oxygen or nitrogen atoms from the EDTA molecule. Therefore, this method is called complexometric titration.

Principle of the EDTA Method

The di-sodium salt of EDTA forms complexes with $Ca^{+2}$ and $Mg^{+2}$ as well as with many other metal cations in aqueous solution.

The total hardness in a hard water sample can be determined by titrating the $Ca^{+2}$ and $Mg^{+2}$ ions present in an aliquot of the sample with $Na_2$-EDTA solution. This process requires an $NH_4Cl$-$NH_4OH$ buffer solution (pH=10) and Eriochrome Black T (EBT) as a metal indicator.

• At pH 10, the EBT indicator forms an unstable wine-red colored complex with the $Ca^{+2}/Mg^{+2}$ ions in the hard water.
• During titration, the EDTA solution breaks this unstable complex, forming a more stable complex with the metal ions and simultaneously releasing the free EBT indicator.
• The release of the free EBT indicator causes the color change from wine-red to blue.

Reaction

The reaction involves the formation of the stable EDTA-metal complex.

Procedure for EDTA Titration

The steps involved in the determination of water hardness are summarized below:

1. Preparation of Reagents
2. Titration

Preparation of Reagents

Preparation of 0.01 M EDTA Solution

0.01 M EDTA is prepared by dissolving 3.7 g of its di-sodium salt (Molar mass = 372.28 g/mol) in distilled water and diluting the solution to 1 liter.

Preparation of Eriochrome Black T (EBT) Indicator

Dissolve 0.5 g of Eriochrome Black T in 100 ml of alcohol.

Preparation of pH 10 Buffer Solution

Add 67.5 g of $NH_4Cl$ to 570 ml of concentrated ammonia solution and then dilute with distilled water to 1 liter.

Titration Procedure

The following steps outline the direct titration procedure:

1. Take 25 ml of the hard water sample.
2. Add 1–2 ml of the pH 10 buffer solution and 2–3 drops of EBT indicator. The color of the solution turns wine-red.
3. Titrate the solution against the EDTA solution (from the burette) until the color changes sharply from wine-red to blue.
4. Record the volume of EDTA solution used.

Recording Titration Volumes

Let $X$ mL be the volume of EDTA solution used in the first titration run.

Titrate the unknown hard water sample similarly.

Let $Y$ mL be the volume of EDTA solution used for the unknown sample.

Water Hardness Classification (PPM)

• 0 to 50 ppm: Soft water
• 51 to 100 ppm: Moderately soft water
• 101 to 150 ppm: Slightly hard water

Advantages of the EDTA Method

• Highly accurate
• Highly convenient
• Highly rapid

Disadvantages of Hard Water

Hard water negatively impacts domestic and industrial applications:

• Washing: It causes wastage of soaps and forms sticky precipitates (calcium and magnesium soaps) which adhere to fabric or cloth.
• Bathing: It causes soap wastage and the production of sticky scum on the body, buckets, or bathtubs.
• Drinking: It can cause harmful effects on the digestive and urinary tract systems.