experiment 2 determination of water hardness. water hardness hard water is due to metal ions...
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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
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