Transcript Experiment 4 - Chemistry| |UCC

Experiment 2
Determination of Water Hardness
Water Hardness
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Hard water is due to metal ions (minerals)
that are dissolved in the ground water. These
minerals include Ca2+, Mg2+, Fe3+, SO42-, and
HCO3The 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?
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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
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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
O
HO
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palmitic acid
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
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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
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Permanent hardness is due to the
presence of the ions Ca2+, Mg+2, Fe3+
and SO42-. This type of hardness cannot
be eliminated by boiling
The water with this type of hardness is
said to be permanently hard
Objectives
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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
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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
Ethylenediamine-tetraacetic acid - EDTA
O
O
HO
N
HO
OH
N
OH
O
O
H2X + Ca2+  CaX + 2H+
EDTA
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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
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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
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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
Take approx. 100 mL of the EDTA
solution from the container at the sink
Dissolve the indicator tablet fully before
starting the titration
Water sample in the conical flask
Titrations
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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
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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....
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ppm CaCO3 = 0.02 x [titration vol] x 105
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ppm = mg/L
ppm CaCO3 = 0.02 x [titration vol] x 105
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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
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Temporary hardness = total – permanent
Magnesium hardness = total – calcium
column
1
2
3 (1-2)
4
5 (1-4)
Hardness
type
Total
Permanent
Temporary
Calcium
Magnesium
Aliquot
Volume
Titration
figure
Hardness
(ppm)
Report
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Experimental observations
Balanced chemical equations
All titration results
Calculations
Completed table