Mandatory Experiment 9.3
Estimation of the total hardness of a water sample using edta
Student Material
Theory
Ethylenediaminetetraacetic acid (edta) is a reagent that forms edta-metal complexes with
many metal ions (but not with alkali metal ions such as Na+ and K+). In alkaline
conditions (pH›9) it forms stable complexes with the alkaline earth metal ions Ca2+ and
Mg2+. The edta reagent can be used to measure the total quantity of dissolved Ca2+ and
Mg2+ ions in a water sample. Thus the total hardness of a water sample can be estimated
by titration with a standard solution of edta.
Suitable conditions for the titration are achieved by the addition of a buffer solution of
pH 10. The buffer solution stabilises the pH at 10. There are H+ ions produced as the
reaction proceeds, and without the buffer solution the pH would decrease.
The edta reagent cannot under these conditions distinguish between the hardness caused
by Ca2+ and Mg2+, or (directly) between temporary and permanent hardness. Therefore
the results of this experiment are usually expressed in terms of the quantity of insoluble
CaCO3 that would have to be converted into soluble salts to give the same total number
of moles of dissolved Ca2+ and Mg2+ ions. This enables the total hardness of water from
different sources to be compared easily.
Because it is a primary standard, and is also more soluble in water, the disodium salt of
edta is more commonly used as the reagent rather than edta itself. If Na2H2Y represents
this salt, it ionises in aqueous solution to H2Y2-, which complexes in a 1:1 ratio with
either Ca2+ or Mg2+ ions (which are represented as M2+). The reaction can be represented
as follows:
H2Y2- + M2+ → MY2- + 2H+
The indicator Eriochrome Black T is used to detect the end point. This is an indicator
that has a different colour when complexed to metal ions than when it is a free indicator.
The reaction between the red indicator-metal complex and the edta reagent at the end
point can be represented as follows:
MIn- + H2Y2- → HIn2- + MY2- + H+
Wine
red
blue
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Chemicals and Apparatus
Hard water sample
0.01 M edta solution
n
Buffer solution, pH 10
Eriochrome Black T indicator (“diluted” with sodium chloride)
Deionised (or distilled) water
i
Pipette (25 cm3)
Pipette filler
Burette (50 cm3)
Conical flask (250 cm3)
Filter funnel
Beakers (250 cm3)
Filter paper or white tile
White card
Wash bottle
Graduated cylinder (10 cm3)
Spatula for solid indicator
Retort stand
Boss head
Clamp
Safety glasses
Procedure
NB: Wear your safety glasses.
1. Wash the pipette, burette and conical flask with deionised water. Rinse the burette
with the edta solution and the pipette with the hard water.
2. Using the funnel, fill the burette with the edta solution. Open the tap briefly to fill the
part below the tap. Remove the funnel. Adjust the level of the solution to the zero
mark. Make sure that the burette is vertical.
3. Use the pipette to transfer 50 cm3 of the hard water sample to the conical flask
Add 2-3 cm3 of the buffer (pH 10) solution (measured out using the graduated
cylinder).
4. Add 0.03 g of the solid indicator to the contents of the flask in the following manner:
Add gradually to the flask, swirling after each addition. A deep wine red colour is
obtained.
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Fig. 1
5. Carry out one 'rough' titration to find the approximate end point, followed by a
number of accurate titrations until two titres agree to within 0.1 cm3. At the end
point, the colour should be dark blue, with no tinge of wine-red colour.
6. From the data, calculate the total hardness of the water sample.
Table of Results
Volume of hard water sample
Molarity of edta solution
Rough titre
Second titre
Third titre
Average of accurate titres
=
=
=
=
=
=
cm3
M
cm3
cm3
cm3
cm3
3
Total hardness
Total hardness
Total hardness
=
=
=
mol/l Ca2+
g/l CaCO3
p.p.m. CaCO3
Questions relating to the experiment
1. Why is it important that the reaction between the edta and the metal ions in
solution (i) is rapid and (ii) goes to completion?
2. The water sample could contain metal ions other than Ca2+ and Mg2+. How would
the reliability of the result be affected if this were the case? Suggest two other
metal ions that could be present in the water.
3. This reagent cannot distinguish between temporary and permanent hardness. List
the compounds of calcium and magnesium that cause hardness, and indicate those
which cause temporary hardness.
4. Suggest a method of establishing the amount of permanent hardness in a water
sample.
5. What is the function of the buffer solution?
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Teacher Material

Bottled waters such as Ballygowan are particularly useful hard water sources for
use in this experiment.

The hardness of water samples can vary considerably and it might be worth
carrying out a trial run with the hard water sample to be used by the students.

It may prove necessary to use a larger volume of hard water to avoid very small
and inaccurate titres.

At the endpoint, no metal ions are available to the indicator, as they have formed
a more stable complex with the edta, and so the blue colour of the free indicator
appears.

The complexes formed between edta and the alkaline earth metal ions are only
stable at a pH above 9. The buffer solution of pH 10 satisfies this requirement.

A clear colour change of the indicator at the endpoint will only be obtained
within the pH range of 5.5 - 11. The buffer solution of pH 10 satisfies this
requirement also.
 Addition of too much (or too little) indicator can make it very difficult to detect
the endpoint.

The water samples used in this experiment must contain Mg2+ ions if accurate
results are to be obtained. No sharp endpoint can be obtained if calcium ions
alone are present.

A fuller description of titration procedure is to be found in the Student Material
relating to Mandatory Experiment 4.2.
Extension Work
1. The students could compare the total hardness of a number of brand name mineral
waters, e.g. Perrier, Ballygowan, Tipperary etc, with the local tap water and with water
from a local spring.
2. The students could estimate the temporary and permanent hardness of the water sample
by boiling the sample and filtering to remove the precipitated carbonates. An edta
titration with the boiled and filtered sample gives an estimate of the permanent hardness
of the sample. The temporary hardness can then be found by subtracting the permanent
hardness from the total hardness value.
3. Model building to show the structure of the edta molecule (Fig. 2) is useful.
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Fig. 2
The students will easily appreciate the relationship between the disodium salt of the edta
and the tetrabasic acid itself. Also a model of the edta – metal complex can be made. The
edta is hexadentate i.e., it can bind to the metal ion using the two amino groups and the
four carboxylate groups forming three five-membered rings. The geometry about the
metal atom is octahedral. This model will help the students appreciate the 1: 1
stoichiometry of the reaction between the edta and the Ca2+ or Mg2+. (Note however, in
some edta-metal complexes only four or five bonds to the metal may form.)
Industrial, Environmental & Social Links
Industrial
 The hardness of a water supply in industry is critical whenever water is to be
heated. This is because heating water with temporary hardness in the water will
result in carbonate precipitates forming on the insides of pipes and boilers. This
considerably reduces their efficiency. Hence industries, particularly chemical
industries, often locate where soft water is available, e.g. Cork Harbour.
Environmental
 Some areas of Ireland have geological formations that are largely limestone, e.g.
the Burren, an area that is also of outstanding environmental and ecological
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interest. The water in this area would be expected to have high temporary
hardness levels.
Social
 There has been a huge upsurge in the popularity of drinking mineral waters
because of their taste and calcium content.

Some of the new bathroom cleaners contain edta to help remove scale.
Preparation of Reagents
1. Standard solution of edta
The usual reagent is the disodium salt, which is a dihydrate, Mr = 372.24; it is
available in sufficiently pure form to be used as a primary standard for most purposes. If
necessary, it may be dried at 80 0C for four days to remove a small percentage of water
that may have been absorbed. The solution should be stored in plastic containers rather
than glass bottles. The solution is stable but if stored in glass bottles (particularly if the
glass is new), the concentration of a dilute solution will decrease appreciably with time as
metal ions are extracted out of the glass. To prepare 2 l of the reagent, dissolve 7.44 g of
the salt in enough deionised water to make exactly 2 l of solution.
2. Buffer solution (pH 10)
Add 17.5 g of ammonium chloride to 142 cm3 of 0.88 ammonia solution and make up to
250 cm3 with deionised water.
3. Eriochrome Black T indicator
This indicator can be purchased as a powder. The solid is hygroscopic, and so must be
stored in a tightly stoppered container. The powdered indicator should be 'diluted' with
Analar grade sodium chloride and used as a solid that has a long shelf life. Grind 0.1 g of
the indicator and dilute with 10 g of NaCl (Analar). Store in tightly stoppered bottle.
Sugar may also be used as the dilutant.
Quantities needed per working group
200 cm3 edta solution
250 cm3 hard water sample
10 cm3 buffer solution (pH 10)
2 l deionised water
Access to container of indicator
Safety considerations


The usual precautions when handling glassware for a titration should be observed.
Pipette fillers should be used.
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
Safety glasses and gloves must be worn. .
Chemical hazard notes
The disodium salt of edta is harmful. Contact with the skin and eyes should be
avoided.
n
The buffer solution of pH 10 is corrosive. Avoid breathing vapours. Use in fume
hood.
Eriochrome Black T is an irritant and should not be allowed to come into contact
with the skin.
i
Disposal of wastes
Dilute with excess water and flush to foul water drain.
Specimen Results
Rough titre
Second titre
Third titre
Average of accurate titres
Volume of hard water sample
Molarity of edta solution
= 22.4 cm3
= 22.2 cm3
= 22.1 cm3
= 22.15 cm3
= 50 cm3
= 0.01 M
Specimen Calculations
(a) First principles method
Volume of edta solution used
Moles of edta used
Ratio of metal ions to edta
Moles of metal ions
Volume of water used
Moles/cm3 of Ca2+ and Mg2+
Moles/litre of Ca2+ and Mg2+
Concentration of Ca2+ and Mg2+
= 22.15 cm3
= 22.15 x 0.01 / 1000 moles
= 0.0002215 moles
= 1: 1
= 0.0002215 moles
= 50.0 cm3
= 0.000215/50.0
= 0.00000443
= 0.00443
= 0.00443 M
= 0.00443 x 100 g/l CaCO3
= 0.443 g/l CaCO3
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= 0.443 x 1000 p.p.m. CaCO3
= 443 p.p.m. CaCO3
= total hardness of water sample.
(b) Formula method
VA x MA x nB = VB x MB x nA
50.0 x MA x 1 = 22.15 x 0.01 x 1
MA = 22.15 x 0.01 x 1 / (50.0 x 1)
= 0.00443 moles/litre of Ca2+ and Mg2+
= 0.00443 x 100 g/l CaCO3
= 0.443 g/l CaCO3
= 0.443 x 1000 p.p.m. CaCO3
= 443 p.p.m. CaCO3
= total hardness of water sample.
Suggested Solutions to Student Questions
1. Why is it important that the reaction between the edta and the metal ions in
solution is (i) rapid and (ii) go to completion?
These are general requirements of any titrimetic reaction. If the reaction is not
almost instantaneous the colour change of the indicator will lag behind the end
point and too large a titre would be recorded. If the reagents do not react
completely, no conclusion about the concentration of one of the solutions can be
obtained from the volume of it that reacts with a known concentration and volume
of the other.
2. The water sample could contain metal ions other than Ca2+ and Mg2+. How would
the reliability of the result be affected if this were the case? Suggest two other
metal ions that could be present in the water.
Since alkali metal ions such as sodium or potassium ions do not complex with
edta reagent, the results would be unaffected by their presence in the water
sample. If, however, there were, for example, iron or aluminium ions present, the
value recorded for total hardness by this method would be expected to be too
high.
3. This reagent cannot distinguish between temporary and permanent hardness, List
the compounds of calcium and magnesium that cause hardness, and indicate those
which cause temporary hardness.
MgSO4, MgCl2, Mg(HCO3)2, CaSO4, CaCl2 and Ca(HCO3)2 are the water-soluble
compounds of magnesium and calcium that cause hardness.
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Mg(HCO3)2 and Ca(HCO3)2 cause temporary hardness.
4. Suggest a method of establishing the amount of permanent hardness in a water
sample.
A known volume of hard water is boiled to precipitate the temporary hardnesscausing hydrogencarbonate compounds as carbonates. These are removed by
filtration. The filter paper is washed with deionised water. The filtrate is made up
to an exact volume with deionised water and the edta titration carried out again,
The result is used to calculate the permanent hardness of the water sample.
5. What is the function of the buffer solution?
The buffer solution keeps the pH at about 10 thus ensuring that the necessary
conditions for the effective operation of the indicator are maintained.
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