Total Hardness of given water sample by EDTA Titrimetric Method.
The Hardness of Water
In general definition, the hardness of water is described as a
measurement of the capacity of water to precipitate soap. Water hardness level
is a parameter that is very important in water
quality analysis.
Soap is mainly precipitated by Calcium ions and Magnesium ions. Also,
some other polyvalent cations also have the ability to precipitate soap. But
those cations are normally in a complex form with organic compounds. Therefore,
their role in the hardness of water is negligible and very difficult to
measure.
So, the total hardness of water is defined as the total amount
of calcium and magnesium ion concentrations. Both are stated as calcium
carbonate, in milligrams per liter unit. Hardness water samples are determined
by EDTA Titrimetric Method.
Other than the total hardness of water there is “Carbonate hardness”. Carbonate hardness is also known as carbonate and bicarbonate alkalinity. In carbonate hardness, we measure the milligrams of carbonate and bicarbonate ions dissolved in one liter of water.
Numerically, Total hardness is always greater than carbonate
hardness. The excess amount is the total hardness is the non-carbonate
hardness. Sometimes carbonate hardness may equal the total hardness. That means
there is no non-carbonate hardness present.
The total hardness of water can be zero to several hundred
milligrams per liter according to the water sample. Generally, water can be
categorized according to its hardness as follows.
|
Category |
Total hardness (mg/l) |
|
Soft
water |
0
– 60 |
|
Moderately
hard water |
61
– 120 |
|
Hard
water |
121
– 180 |
|
Very
hard |
>180 |
Table 01: Categories of water based on the
hardness
EDTA titrimetric method
of determining the total hardness of water
Ca2+ and Mg2+ ions
in the water sample are titrated with Ethylenediaminetetraacetic
acid (EDTA) at 10.0 ± 0.1 pH. We use the Eriochrome black T as the
indicator here.
First, use the Eriochrome black T indicator will be complexed
with Ca2+ and Mg2+ ions
in the water. So, the sample is wine red
in color. When it is titrated with EDTA, metal ions are complexed with
EDTA, and the solution becomes blue in
color. At pH = 10,
Reagents
I.
Buffer solution
o Prepared
by dissolving 16.9 g of ammonium chloride (NH4Cl) in
143 mL of conc. ammonium hydroxide (NH4OH).
o Then
this solution is added 1.25 g of magnesium salt of EDTA and diluted to 250 ml
with distilled water.
II.
Eriochrome Black T indicator
III.
Standard EDTA titrant
o To
prepare 0.01M of standard EDTA titrant 3.723 g analytical reagent-grade
disodium ethylenediaminetetraacetate dihydrate, also called (ethylenedinitrilo)
tetraacetic acid disodium salt (EDTA) is dissolved in distilled water.
o This
solution is diluted to 1000 mL.
IV.
Standard calcium solution
o Standard
CaCO3 solution is used to standardize the prepared EDTA
solution.
o First,
1000 mg (1g) of anhydrous CaCO3 is
dissolved in 1+1 HCl. In 1+1 HCl there are one volume of concentrated HCl and 1
volume of distilled water.
o After
dissolving CaCO3, it is added 200 mL of distilled water and boiled for a few
minutes. The purpose of boiling is to expel CO2.
o Then
the solution let to be cooled. The cooled solution is added with a few drops of
methyl orange indicator and adjusted to the intermediate orange color by adding
1 + 1 HCl, as required.
o Finally.
The solution is diluted to 1000 mL with distilled water. 1 mL of this solution
has 1.00 mg of CaCO3.
V.
Sodium hydroxide (NaOH), 0.1N
o Reagent-grade
solution of NaOH is used.
Procedure
25 mL of the test solution is taken and it is diluted to 50 mL
by adding 25 mL of distilled water. Then add 1- 2 mL of prepared buffer
solution. About 1 – 2 drops of indicator solution are added to the sample. If
the indicator is used as a dry powder, use about 1mg of the indicator.
When the indicator is added solution becomes a wine-red color. Then the sample is
titrated with the standard EDTA solution. The color at the endpoint is blue.
As the sample is diluted with distilled water, a blank titration
should be taken place. Because sometimes it may have Ca2+ and
Mg2+ ions in distilled water. The endpoint value of the blank
titration should be subtracted from the sample endpoint in the final
calculations.
Calculation
According to the stoichiometry Ca2+ and
Mg2+ ions react with EDTA at a 1:1 ratio. Therefore, mols of
EDTA equals the total mols of Ca2+ and
Mg2+ ions. To get the EDTA volume consumed by ions in the
sample we should subtract the endpoint value of the blank titration from the
endpoint value of the sample titration.
The hardness of water is
expressed as the milligrams of CaCO3 in
1 liter of sample. Therefore,
A volume of 25 mL of the
sample was taken for titration. To calculate the mass present in the 1 L of the
sample,
