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

2. CHM 101/102
Water Hardness: Determination with EDTA
 Water is said to be “hard” when it contains Ca2+ and Mg2+ ions.
These ions react with soap to form an insoluble substance
called “soap scum”.
 Ca2+ and Mg2+ ions along with other metal ions such as Fe3+ and
Pb2+ can be removed from hard water by the addition of EDTA
(ethylenediaminetetraacetic acid).
 EDTA has a greater affinity for Ca2+ and Mg2+ when it is in the
form of the dihydrogen anion H2EDTA2-. This is the ionic form of
EDTA at pH 10.
H2EDTA2-
dihydrogen ethylenediaminetetraacetate ion
Laboratory Manual

3. CHM 101/102
Water Hardness: Determination with EDTA
 H2EDTA2- binds to a Ca2+ ion by forming four special covalent
bonds called coordinate covalent bonds.
+ + 2 H+
Ca2+
H2EDTA2- Ca2+ ion chelated
by EDTA
 In today’s experiment, you will determine the total concentration
of calcium and magnesium ions in a hard water sample using
EDTA in a solution buffered to a pH of 10.
Laboratory Manual

4. CHM 101/102
Water Hardness: Determination with EDTA
• Using a Volumetric Pipet
 Squeeze the pipet bulb and
place the silicone end over the
top of the pipet. Do not force
the pipet into the bulb.
 Draw liquid up into the pipet until
it is just above the calibration
mark. Slide the bulb off the
pipet and place your index finger
over the top of the pipet.
Laboratory Manual

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

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

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

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

9. CHM 101/102
Water Hardness: Determination with EDTA
• Procedure (Titration of Blank)
 Use a volumetric pipet to dispense 25.00 mL of
deionized water (DI) into a 250 mL flask.
 Add 5 mL of pH 10 buffer, 2 drops of Eriochrome
Black T indicator, and 15 drops of 0.03 M MgCl2.
 Titrate the solution with EDTA from your buret. As
you near the endpoint, the solution will turn purple.
Continue to slowly add EDTA until the solution
turns blue, with no trace of red.
beginning nearing endpoint
color endpoint color
Laboratory Manual

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

11. CHM 101/102
Water Hardness: Determination with EDTA
• Procedure (Titration of Sample)
 Use a volumetric pipet to dispense 50.00 mL of
your hard water sample into a 250 mL flask.
 Add 5 mL of pH 10 buffer, 2 drops of Eriochrome
Black T indicator, and 15 drops of 0.03 M MgCl2.
 Titrate the solution with EDTA from your buret to
the blue endpoint you observed when titrating the
blank.
endpoint
color
Laboratory Manual

12. CHM 101/102
Water Hardness: Determination with EDTA
• Calculations
1 mol CaCO3
(1) mol CaCO3 in sample = LEDTA x MEDTA x
1 mol EDTA
moles CaCO3
(2) MCaCO3 =
50 x 10-3 L sample
100 g CaCO3 1000 mg
(3) mg/L CaCO3 = MCaCO3 x x
1 mol CaCO3 1g
(4) ppm = mg/L
Laboratory Manual