Water hardness edta

1. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
•PurposePurpose
To determine the “hardness” of a water sample To determine the “hardness” of a water sample
using an EDTA titration.using an EDTA titration.
To learn and practice quantitative techniques for To learn and practice quantitative techniques for
determining the concentrations of solutions.determining the concentrations of solutions.
•Safety ConsiderationsSafety Considerations
Keep the pH 10 buffer in the hood. Avoid Keep the pH 10 buffer in the hood. Avoid
breathing ammonia vapors from the buffer.breathing ammonia vapors from the buffer.
Eriochrome Black T will stain skin and clothes.Eriochrome Black T will stain skin and clothes.
All waste materials can be safely rinsed down the All waste materials can be safely rinsed down the
sink.sink.

2. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
Water is said to be “hard” when it contains CaWater is said to be “hard” when it contains Ca
2+2+
and Mg and Mg
2+2+
ions. ions.
These ions react with soap to form an insoluble substance These ions react with soap to form an insoluble substance
called “soap scum”.called “soap scum”.
dihydrogen ethylenediaminetetraacetate iondihydrogen ethylenediaminetetraacetate ion
HH
22EDTAEDTA
2-2-
EDTA has a greater affinity for CaEDTA has a greater affinity for Ca
2+2+
and Mg and Mg
2+2+
when it is in the when it is in the
form of the dihydrogen anion Hform of the dihydrogen anion H
22EDTAEDTA
2-2-
. This is the ionic form of . This is the ionic form of
EDTA at pH 10.EDTA at pH 10.
CaCa
2+2+
and Mg and Mg
2+2+
ions along with other metal ions such as Fe ions along with other metal ions such as Fe
3+3+
and and
PbPb
2+2+
can be removed from hard water by the addition of EDTA can be removed from hard water by the addition of EDTA
((eethylenethyleneddiamineiaminettetraacetic etraacetic aacid). cid).

3. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
HH
22EDTAEDTA
2-2-
CaCa
2+2+
+ +2 H2 H
++
CaCa
2+2+
ion ion chelatedchelated
by EDTAby EDTA
HH
22EDTAEDTA
2-2-
binds to a Ca binds to a Ca
2+2+
ion by forming four special covalent ion by forming four special covalent
bonds called coordinate covalent bonds.bonds called coordinate covalent bonds.
In today’s experiment, you will determine the total concentration In today’s experiment, you will determine the total concentration
of calcium and magnesium ions in a hard water sample using of calcium and magnesium ions in a hard water sample using
EDTA in a solution buffered to a pH of 10.EDTA in a solution buffered to a pH of 10.

4. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
•Using a Volumetric PipetUsing a Volumetric Pipet
Squeeze the pipet bulb and Squeeze the pipet bulb and
place the silicone end over the place the silicone end over the
top of the pipet. Do top of the pipet. Do notnot force force
the pipet into the bulb.the pipet into the bulb.
Draw liquid up into the pipet until Draw liquid up into the pipet until
it is just above the calibration it is just above the calibration
mark. Slide the bulb off the mark. Slide the bulb off the
pipet and place your index finger pipet and place your index finger
over the top of the pipet. over the top of the pipet.

5. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
Carefully release liquid from the Carefully release liquid from the
pipet until the bottom of the pipet until the bottom of the
meniscus is on the calibration meniscus is on the calibration
line. line.
Release your finger and allow Release your finger and allow
the liquid in the pipet to drain the liquid in the pipet to drain
into a beaker or flask. Touch into a beaker or flask. Touch
the tip of the pipet to the side of the tip of the pipet to the side of
the beaker or flask to completely the beaker or flask to completely
drain the pipet.drain the pipet.

6. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
•Using a BuretUsing a Buret
Clean the buret with a buret brush, water, and a small Clean the buret with a buret brush, water, and a small
amount of detergent. Rinse it twice with deionized amount of detergent. Rinse it twice with deionized
water. Be sure to drain deionized water through the water. Be sure to drain deionized water through the
tip. tip.
Rinse the buret again with two 10 mL portions of the Rinse the buret again with two 10 mL portions of the
titrant (EDTA in this experiment). titrant (EDTA in this experiment).
Fill the buret with titrant and drain a Fill the buret with titrant and drain a
small amount from the buret to small amount from the buret to
dispel any air bubbles that might be dispel any air bubbles that might be
in the tip. in the tip.
remove remove
air bubbleair bubble

7. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
Use a marker pen to create a Use a marker pen to create a
black area on a white note card. black area on a white note card.
This will be used to help locate the This will be used to help locate the
bottom of the meniscus when bottom of the meniscus when
reading the buret. reading the buret.
Place the notecard with the black Place the notecard with the black
mark behind the buret and just mark behind the buret and just
below the meniscus. This will below the meniscus. This will
make the bottom of the meniscus make the bottom of the meniscus
much easier to see. much easier to see. Record the Record the
volume reading to the nearest volume reading to the nearest
0.01 mL.0.01 mL.

8. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
If you need to dispense less than a full If you need to dispense less than a full
drop of titrant, open the stopcock slightly drop of titrant, open the stopcock slightly
to allow a small amount of titrant to to allow a small amount of titrant to
accumulate on the tip of the buret. accumulate on the tip of the buret.
Rinse the titrant on the tip into the flask Rinse the titrant on the tip into the flask
using deionized water from your wash using deionized water from your wash
bottle.bottle.

9. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
•Procedure (Titration of Blank)Procedure (Titration of Blank)
Use a volumetric pipet to dispense 25.00 mL of Use a volumetric pipet to dispense 25.00 mL of
deionized water (DI) into a 250 mL flask. deionized water (DI) into a 250 mL flask.
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Add 5 mL of pH 10 buffer, 2 drops of Eriochrome
Black T indicator, and 15 drops of 0.03 M MgClBlack T indicator, and 15 drops of 0.03 M MgCl
22. .
Titrate the solution with EDTA from your buret. As Titrate the solution with EDTA from your buret. As
you near the endpoint, the solution will turn purple. you near the endpoint, the solution will turn purple.
Continue to Continue to slowlyslowly add EDTA until the solution add EDTA until the solution
turns blue, with no trace of red. turns blue, with no trace of red.
beginningbeginning
colorcolor
nearingnearing
endpointendpoint
endpointendpoint
colorcolor

10. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
•Procedure (Titrating Blank and Sample)Procedure (Titrating Blank and Sample)
The following video clip demonstrates the proper The following video clip demonstrates the proper
technique for titrating both the sample and the technique for titrating both the sample and the
blank.blank.
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA

11. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
•Procedure (Titration of Sample)Procedure (Titration of Sample)
Use a volumetric pipet to dispense 50.00 mL of Use a volumetric pipet to dispense 50.00 mL of
your hard water sample into a 250 mL flask. your hard water sample into a 250 mL flask.
Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Add 5 mL of pH 10 buffer, 2 drops of Eriochrome
Black T indicator, and 15 drops of 0.03 M MgClBlack T indicator, and 15 drops of 0.03 M MgCl
22. .
Titrate the solution with EDTA from your buret to Titrate the solution with EDTA from your buret to
the blue endpoint you observed when titrating the the blue endpoint you observed when titrating the
blank.blank.
endpointendpoint
colorcolor

12. CHM 101/102CHM 101/102
Laboratory ManualLaboratory ManualLaboratory Manual
•CalculationsCalculations
Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA
mol CaCOmol CaCO
33 in sample = in sample = LL
EDTAEDTA x M x M
EDTAEDTA x x
1 mol CaCO1 mol CaCO
33
1 mol EDTA 1 mol EDTA
(1) (1)

MM
CaCO3CaCO3 = =
moles CaCOmoles CaCO
33
50 x 1050 x 10
-3-3
L sample L sample
(2) (2)

mg/L CaCOmg/L CaCO
33 = =

MM
CaCO3CaCO3 x x
100 g CaCO100 g CaCO
33
1 mol CaCO1 mol CaCO
33
xx
1000 mg1000 mg
1 g 1 g
(3) (3)

(4) (4)

ppm = mg/Lppm = mg/L