Discussion
A
solution of the magnesium/EDTA complex is useful for the titration of cations
that form more stable complexes than the magnesium complex but for which no
indicator is available. Magnesium ions in the complex are displaced by a
chemically equivalent quantity of analyte and the liberated magnesium ions are
then titrated with Eriochrome Blake T as the indicator. Note that the concentration
of the magnesium solution is not important; all that is necessary is that the
molar ratio between Mg2+ and EDTA in the reagent be exactly unity.
Procedure
Preparation
of the Magnesium/EDTA Complex, 0.1M (sufficient for 90~100 titrations)
1.
To
3.72g of Na2H2Y2˙2H2O in 50 mL of
distilled water, add an equivalent quantity(2.46 g) MgSO4˙7H2O.
2.
Add
a few drops of phenolphthalein, followed by sufficient 0.1M NaOH to turn the
solution faintly pink.
3.
Dilute
to about 100mL with water. The addition of Eriochrome Blake T to a portion of
this solution buffered to pH 10 should cause a dull violet color to develop.
4.
Moreover,
a single drop of 0.01M Na2H2Y2 solution added
to the violet solution should cause a color change to blue, and an equal
quantity of 0.01M Mg2+ should cause a change to red.
5.
The
composition of the original solution should be adjusted with additional Mg2+
or H2Y2- until these criteria are met.
Titration
1.
Weight
a sample of the unknown( to the nearest 0.1mg) into a 500 mL beaker[1].
2.
Cover
with a watch glass and carefully add 5 to 10 mL of 6M HCl. After the sample has
dissolved, remove CO2 by adding about 50 mL of deionized water and
neutralize with 6M NaOH until the red color is discharged.
3.
Quantitatively
transfer the solution to a 500 mL volumetric flask, and dilute to the mark.
4.
Take
50.00 mL aliquots of the diluted solution for titration, treating each as
follows: Add about 2 mL of pH 10 buffer,1 mLof Mg/EDTA solution, and 3 to 4
drops of Eriochrome Blake T or Calmagite indicator. Titrate[2] with standard
0.01M Na2H2Y to a color change from red to blue.
5.
Report
the number of milligrams of CaO in the sample.
Notes
[1]
The sample taken should contain 150 to 160 mg of Ca2+.
[2]
Interferences with this titration are substantially the same as those
encountered in the direct titration of Mg2+ and are eliminated in
the same way.
Experimental
Record
Concentration
of Na2H2Y(aq)
|
0.010M
|
Weight
of MgSO4
|
0.120g
|
=1.00x10-3
mol
|
|
Mole
of MgSO4 was added
|
1x10-5
mol
|
Weight
of CaO sample
|
0.150g
|
=2.68x10-3
mol
|
|
Mole
of CaO sample was added (assumed as 100%)
|
2.68x10-4
mol
|
Consumption
of 0.010M EDTA solution
|
20.10
mL
|
21.11
mL
|
|
21.63
mL
|
|
Average
|
21.28
mL
|
Theoretical
Consumption of 0.010M EDTA solution
|
26.80
mL
|
CaO
in sample
|
119.2
mg
|
CaO
% in sample
|
79.46
%
|
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