2. Expt. No.: Date:
Experiment
Water Purification: Hardness Estimation by EDTA method and its
Removal using Ion-exchange Resin
Problem
definition
Hardness of water is due to the presence of dissolved calcium and
magnesium salts in water. EDTA forms stable complex with
hardness causing salts and is used in the removal of scale and
sludge forming impurities in industrial boilers.
Methodology
EBT indicator-Metal ion complex is weaker compared to EDTA-metal
ion complex. The end point is the color change from wine red (EBT-
Metal ion complex) to steel blue (free EBT indicator).
Solution
Estimation of Calcium hardness (in ppm) in the given unknown
sample. Understanding the water softening using ion-exchange
resins.
Student
learning
outcomes (SLO)
Students will learn to
a) perform complexometric titration
b) understand the efficiency of ion-exchange resins using in water
purifiers
3. 1). Introduction to Hard Water and its Classification:
Hard water contains high levels of dissolved Ca2+and Mg2+ions.
Ground and surface water dissolve the Ca2+/Mg2+ containing
ores/minerals from surrounding soil and rock and the solution gets
enriched with these cations.
Hardness is commonly expressed as mg of CaCO3 eq./L.
Amount of hardness causing chemicals:
60 mg/l = Soft water:
60–120 mg/l = Moderately hard water.
120–180 mg/l = Hard water.
>180 mg/l = Very hard water.
Based on the type of anions (Cl-, SO4
2-, HCO3
-) associated with
Ca2+/Mg2+ ions, the hardness is categorized into temporary
(carbonate, HCO3
-) hardness & permanent (non-carbonate, Cl-,
SO4
2-) hardness.
4. 2). Problems caused by Hard Water:
Hard water can cause costly breakdowns in boilers and plumbing.
When hard water is heated, the hardness causing salts tend to
precipitate out of solution, forming hard scale or soft sludge in
pipes and surfaces, thereby restricting water flow.
Scale formation in boilers prevents efficient heat transfer, resulting
in energy loss and overheating leading to serious accidents.
At the domestic level, hard water lessens the effectiveness of soap
by forming scums/precipitates, which adhere to human skin.
Human consumption of
water containing excess of Ca
and Mg are associated with
increased risks of
osteoporosis, colorectal
cancer, hypertension, stroke,
coronary artery disease,
diarrhea and obesity.
5. 3). Modern Treatment of Hard Water:
Hard water can be softened using ion-
exchange resins (IER).
IERs are very small, porous polymeric
beads with sulphonic/carboxylic
acid/Na+ functional groups attached to
the polymer backbone.
When hard water is passed through
the IER beads, Ca2+/Mg2+ ions in hard
water are trapped by the IER and
hydrogen/Na+ ions are released,
thereby softening the hard water.
Exhausted IER beads (saturated with
Ca2+/Mg2+ ions) are regenerated using
mild acid/brine solution to flush out
Ca2+/Mg2+ ions. The regenerated IER
can be reused for many cycles.
6. 4). Principle:
Ehtylenediaminetetraacetic acid (EDTA) forms complexes with a
large number of cations including Ca2+ and Mg2+ depending upon pH
of solution.
Since EDTA is insoluble in water, the disodium salt of EDTA is used.
The resulting metal-ligand complex, in which EDTA forms a cage-like
structure around the metal ion, is very stable at specific pH.
EDTA
Disodium salt of EDTA Metal-EDTA complex
7. Hardness in water can be estimated by complexometric titration
using sodium salt of EDTA and EBT indicator at pH = 9-10.
EBT forms an wine-red colored weak/unstable complex with
Ca2+/Mg2+ions, which upon titrating with EDTA, results in the
breaking of EBT-Ca2+/Mg2+ unstable bond and formation of
stable/strong EDTA-Ca2+/Mg2+ bond.
The endpoint changes from wine-red (EBT-Ca2+/Mg2+) to steel blue
(free EBT indicator).
8. 5). Reagents and solutions:
Standard hard water (SHW, 1mg/mL of CaCO3 equivalents), 0.01 N
EDTA solution, Eriochrome Black – T (EBT) indicator, hard water
sample, NH3-NH4Cl buffer solution and ion exchange resin.
6). Apparatus:
Burette, pipette, conical flask, standard flask, IER column.
7). Procedure:
(i). Titration-1: Standardization of EDTA
Pipette out 20 mL of SHW containing 1mg/mL of CaCO3 (1000
ppm) into a clean conical flask. Add 1 T.T. full of buffer solution to
maintain the pH around 10.
Add 3 drops of EBT indicator and titrate it against the given EDTA
solution taken in the burette.
End point is change of colour from wine red to steel blue. Repeat
the titration for concordant titer values.
Let ‘V1’ be the volume of EDTA consumed.
9. Calculation:
20 mL of given hard water consumes V1 mL of EDTA
20 mg of CaCO3 requires V1 mL of EDTA for complexation
1 mL of EDTA requires = 20/V1 mg CaCO3 for complexation
This relation will be used in other two titrations
S.
No.
Volume of standard
hard water (mL)
Burette reading (mL) Volume of EDTA
(V1, mL)
Initial Final
1. 20 ml 0 19.8 19.8 ml
2. 20 ml 0 19.8 19.8 ml
3.
Concordant titer value
10. (ii). Titration-2: Estimation of total hardness of hard water sample
Pipette out 20 mL of the given sample of hard water into a clean conical flask.
Add 1 T.T. of buffer solution and 3 drops of EBT indicator.
Titrate this mixture against standardized EDTA solution taken in the burette.
The end point is the change of color from wine red to steel blue. Repeat the
titration for concordant titer value.
Let ‘V2’ be the volume of EDTA consumed.
S.
No.
Volume of sample
hard water (mL)
Burette reading (mL) Volume of EDTA
(V2, mL)
Initial Final
1. 20 ml 0 11.3 11.3 ml
2. 20 ml 0 11.3 11.3 ml
3.
Concordant titer value
Calculation:
20 mL of sample hard water consumes = V2 mL of EDTA.
= V2 x 20/V1 mg of CaCO3 eq.
1000 mL of hard water sample consumes = V2 x 20/V1×1000/20
= V2/V1×1000 ppm
Total hardness of the water sample = “X” ppm
11. (iii). Titration-3: Removal of hardness using ion exchange method
Ion exchange is a reversible process. When hard water is passed through
cation ion-exchange resins (IERs) packed in a narrow column, Ca2+ and
Mg2+ cations in hard water are exchanged with Na+ or H+ ions in IERs.
The exhausted resins are regenerated by passing 10% dil. HCl through the
column.
Applications of IERs: preparation of high-purity water for power
engineering, electronic and nuclear industries, in household water
purifiers.
12. Methodology:
Arrange the IER column to burette stand and pour the hard water sample (around
40 to 50 mL) remaining after the completion of Titration – 2 into IER column.
Collect the water passing through the column in a beaker over a period of 10 min.
Adjust the valve of the column to match the duration of outflow.
From the water collected through the column, pipette out 20 mL into a clean
conical flask and repeat the EDTA titration. Volume of EDTA consumed as ‘V3’.
S.
No.
Volume of sample
hard water (mL)
Burette reading (mL) Volume of EDTA
(V3, mL)
Initial Final
1. 20 ml 0 0.6 0.6 ml
2. 20 ml 0 0.6 0.6 ml
3.
Concordant titer value
Calculation:
20 mL of water sample after passing through IER consumes V3 mL of EDTA.
= V3 x 20/V1 mg of CaCO3 eq.
1000 mL of water sample after softening through IER column consumes = V3 x
20/V1×1000/20
= V3/V1×1000 ppm
Residual hardness of the water sample = “Y” ppm
13. 8). Final Results:
Total hardness of the water sample= …………….(X) ppm
Residual hardness in the water sample= ……………(Y) ppm
Hardness removed through the column = …………….(X–Y) ppm
9). Link for video presentation: https://youtu.be/wkfK98OnrHE
V1 = 19.8 ml
V2 = 11.3 ml
V3 = 0.6 ml
