Water Hardness

Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca²⁺, Mg²⁺, Fe³⁺, SO₄^2-, and HCO₃^-.

The concentration of the Ca²⁺ ions is greater than the concentration of any other metal ion in our water.

Water hardness is usually expressed in ppm CaCO₃

Originally, water hardness was defined as the measure of the capacity of the water to precipitate soap. Hard water does cause soap scum, clogs pipes and clogs boilers as limescale.

Hard water can be softened by boiling

Mineral deposits are formed by ionic reactions resulting in the formation of an insoluble precipitate of calcium carbonate

Ca²⁺ + 2HCO₃^- ® CaCO₃ + H₂O + CO₂

Soaps are long chain fatty acids.

Soap scum is formed when the Ca²⁺ ion binds with the soap. This causes an insoluble compound that precipitates to form the scum you see. Soap actually softens hard water by removing the Ca²⁺ ions from the water

When hard water is heated, CaCO₃ precipitates out, which then clogs pipes and industrial boilers. This leads to malfunction or damage and is expensive to remove

Temporary Hardness is due to the bicarbonate ion, HCO₃^-, being present in the water. This type of hardness can be removed by boiling the water to expel the CO₂, as indicated by the following equation

Ca²⁺ + 2HCO₃^- ® CaCO₃ + H₂O + CO₂

Bicarbonate hardness is classified as temporary hardness.

Permanent hardness is due to the presence of the ions Ca²⁺, Mg⁺², Fe³⁺ and SO₄^2-. This type of hardness cannot be eliminated by boiling.

The water with this type of hardness is said to be permanently hard.

Objectives

To quantitatively determine Total, Permanent, and Calcium hardness in a sample of tap water

To gain some basic analytical knowledge through analysis of water samples

To become familiar with terminology such as ppm and to apply techniques learned from volumetric analysis to basic environmental analysis

Complexometric Titration

Water hardness is usually determined by titrating with a standard solution of ethylenediamminetetraacetic acid, EDTA

EDTA is a complexing, or chelating agent used to capture the metal ions

This causes the water to become softened, but the metal ions are not removed from the water

EDTA simply binds the metal ions to it very tightly

EDTA

EDTA is a versatile chelating agent

A chelating agent is a substance whose molecules can form several bonds to a single metal ion

Chelating agents are multi-dentate ligands. A ligand is a substance that binds with metal ions to form a complex ion

Multidentate ligands are many clawed, holding onto the metal ion to form a very stable complex

EDTA can form four or six bonds with a metal ion

EDTA

It is frequently used in soaps and detergents because it forms complexes with calcium and magnesium ions

Certain enzymes are responsible for food spoilage. EDTA is used to remove metal ions from these enzymes

Used to promote colour retention, and to improve flavour retention in foods

Titrations

Use one tablet of indicator to develop a good colour

Titrate water with EDTA until colour changes from red to blue

Titrations

Titrate for total hardness

Titrate a boiled sample for permanent and hence temporary hardness

Add murexide to a sample at pH 12 to precipitate any Mg²⁺ as Mg(OH)₂. Then titrate to obtain Calcium and hence Magnesium hardness

Treatment of Results

Water hardness is usually expressed as ppm CaCO₃. Since the reaction between calcium or magnesium ions and EDTA has a 1:1 ratio, hardness is given by....

ppm CaCO₃ = 0.02 x [titration vol] x 10⁵

ppm = mg/L

ppm CaCO₃ = 0.02 x [titration vol] x 10⁵

M₁V₁ = M₂V₂ [n₁,n₂=1]

M₁ x 10^-2 = 0.02 x [volume]

M₁ = 0.02 x [volume] x 10²

Mol mass CaCO₃ = 100 g mol^-1

gL^-1 = 0.02 x [volume] x 10²

ppm = 0.02 x [volume] x 10⁵

Temporary hardness = total – permanent

Magnesium hardness = total – calcium

Дата добавления: 2015-10-29