Estimation of Total Hardness of Water

Exp 6

What is hard water?

Hard water is generally considered to be that water which requires considerable amount of soap to produce foam or lather and produces scales in boilers.

How is hardness of water of water caused?

Hardness of water is caused due to the presence of dissolved salts of Ca⁺², Mg⁺² and other metal ions with anions such as HCO3^-, SO₄^2-, Cl^-, SiO₃^2-

What is the difference between temporary and permanent hardness?

Temporary Hardness

• Due to unstable bicarbonates of Ca and Mg

• Temporary Hardness can be removed by boiling water during which bicarbonates decompose to carbonates

Permanent Hardness

• Due to more stable Cl^- and SO₄^2- of Ca and Mg

• Can be removed only by chemical treatment methods like soda lime process or ion exhange process.

How do you express total hardness of water

The sum of temporary hardbess and permant hardness is known as the total hardness. it is expressed in ppm equivalent of calcium carbonate of mg/1

What is EDTA?

Ethylenediaminetetraacetic acid is a complexing agent (sparingly soluble), and also its disodium salt is a strong complexing agent (solubility is more), and it reacts with the metal ions to form a soluble, stable one-to-one complex.

What is a buffer solution?

The solution which resists the change in its pH value even after adding small amounts of an acid or base to it is called a buffer solution.

Why is NH4OH-NH4Cl buffer added in the determination of total hardness of water?

The indicator Eriochrome Black T (EBT) shows a color change at pH range 9 to 11, so the pH of the solution is maintained around 10 using the buffer. Otherwise, pH decreases as H+ ions are released due to the substitution of metal ions for H+ ions in EDTA.

Why does the indicator Eriochrome Black T show wine red in the beginning and blue at the end of the titration?

When a few drops of the indicator are added to hard water (with a pH of about 10), it forms a wine-red colored weak and unstable complex with Ca2+ and Mg2+ ions. As the hard water is titrated with EDTA, EDTA reacts with the free metal ions. Near the endpoint, when all free metal ions are exhausted, further addition of EDTA dissociates the MEBT complex, releasing the free indicator, which is blue in color. Thus, the color changes from wine red to blue.

What are the applications of hardness determination in environmental practices?

The data of hardness of water is an important consideration in determining the suitability of water for domestic and industrial uses. Estimation of hardness serves as a basis for routine control of softening methods.

What is the structure of EDTA?

Why is ammonium hydroxide added to the disodium salt of EDTA?

The addition of ammonium hydroxide to EDTA increases the solubility of the salt in water.

Why are EDTA titrations carried out slowly near the endpoint?

The kinetics of forming a strong and stable complex of EDTA with Ca2+ and Mg2+ ions is reasonably slow, and hence titrations need to be carried out slowly near the endpoint to avoid error.