Determination Of The Hardness Of Water Water Page 2
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1
2
3
4H
C
CH
2
2
CH
H
C
2
2
N
N
CH
H
C
2
2
Ca
O
O
C
C
O
O
C
C
O
O
O
O
In a titration to determine the concentration of a metal ion, the added EDTA combines
quantitatively with the cation to form the complex. The endpoint occurs when essentially all of
the cation has reacted.
In this experiment a solution of EDTA will be standardized by titration against a known
solution made from calcium carbonate, CaCO
. The EDTA solution can then be used to
3
2+
determine the hardness of an unknown water sample. Since both EDTA and Ca
are colorless,
it is necessary to use a special indicator to detect the end point of the titration. The indicator
most often used is called Eriochrome Black T, which forms a very stable wine-red complex,
–
MgIn
, with the magnesium ion. A tiny amount of this complex will be present in the solution
2+
2+
during the titration. As EDTA is added, it will complex free Ca
and Mg
ions, leaving the
–
MgIn
complex alone until essentially all of the calcium and magnesium have been converted to
2+
chelates. At this point EDTA concentration will increase sufficiently to displace Mg
from the
indicator complex; the indicator reverts to its uncombined form, which is sky blue, establishing
the end point of the titration.
+
The titration is carried out at a pH of 10, in a NH
/NH
buffer, which keeps the EDTA (H
Y)
3
4
4
3–
mainly in the form HY
, where it complexes the Group 2 ions very well but does not tend to
3+
react as readily with other cations such as Fe
that might be present as impurities in the water.
Taking H
Y and H
In as the formulas for EDTA and Eriochrome Black T, respectively, the
4
3
equations for the reactions which occur during the titration are:
3–
2+
2–
+
2+
Titration reaction:
HY
(aq) + Ca
(aq)
CaY
(aq) + H
(aq) (also for Mg
)
3–
–
2–
2–
End point reaction:
HY
(aq) + MgIn
(aq)
MgY
(aq) + HIn
(aq)
wine red
sky blue
2+
Since the indicator requires a trace of Mg
to operate properly, a little magnesium ion will be
added to the buffer solution.
Experimental Procedure
1.
Fill a sample vial with calcium carbonate and weigh the vial on the analytical balance.
Carefully pour between 0.16 to 0.18 g of the carbonate to a 250-mL beaker and weigh
the vial again. Determine the mass of the CaCO
sample to 0.1 mg by difference.
3
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