Experiment 2

Determination of Water Hardness

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

Why Be Concerned About Hard Water?

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

O

HOpalmitic acid

Temporary Hardness 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

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

N

N

OH

O

O

HO

O

HO O

OH

Ethylenediamine-tetraacetic acid - EDTA

H2X + Ca2+ CaX + 2H+

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

EDTA solution in the burette

Water sample in the conical flask

Take approx. 100 mL of the EDTA solution from the container at the sink

Dissolve the indicator tablet fully before starting the titration

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

column 1 2 3 (1-2) 4 5 (1-4)

Hardness type

TotalPermanen

tTemporary Calcium

Magnesium

Aliquot Volume

Titration figure

Hardness (ppm)

Report

Experimental observations Balanced chemical equations All titration results Calculations Completed table