DETERMINATION OF MG BY TITRATION WITH EDTA
DETERMINATION OF
MG BY TITRATION WITH EDTA
INTRODUCTION:
Many metal ions react with
electron pair donors to form coordination compounds or complex ions. The
formation of a particular class of coordination compounds, called chelates, are
especially well suited for quantitative methods. A chelate is formed when a
metal ion coordinates with two (or more) donor groups of a single ligand. Tertiary
amine compounds such as ethylenadiaminetetraacetic acid (EDTA) are widely used
for the formation of chelates.
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Complexometric titrations with
EDTA have been reported for the analysis of nearly all metal ions. Because EDTA
has four acidic protons, the formation of metal-ion/EDTA complexes is dependent
upon the pH. For the titration of Mg2+, one must buffer the solution to a pH of
10 so that complex formation will be quantitative. The reaction of Mg2+ with
EDTA may be expressed as:
Mg2+ + H2Y2- = MgY-2 + 2H+
The structure of EDTA and the
magnesium-EDTA complex (without the hydrogen atoms) is shown below:
The endpoint of the titration is
determined by the addition of Eriochrome Black T, which forms a colored chelate
with Mg2+ and undergoes a color change when the Mg2+ is released to form a chelate
with EDTA. While it is possible to achieve relatively good results by titration
with EDTA prepared directly from the solid, better results should be obtained
when the EDTA is standardized against a solution containing a known amount of
metal ion. You will be provided with a standard solution of Zn2+ which you will
use to standardize your EDTA solution. Truman State University CHEM 222 Lab
Manual Revised 01/04/08
REAGENTS AND
APPARATUS
EDTA (Na2H2Y 2H2O) pH
10 buffer (2000 mL has been prepared by dissolving 140.0 g of NH4Cl in 650
mL of deionized water, adding 1136 mL of conc. ammonia and diluting to 2000 mL)
Eriochrome Black
T (ground
1:10 with NaCl) pHydrion paper
Standard Zn
solution:
(This has been prepared for you.) An accurate mass (~1.3g) of pure zinc has
been dissolved in a small volume (~15 mL) of 6M HCl. The dissolve zinc was
quantitatively transferred to a 2 L volumetric flask and diluted to the mark.
This ~0.01 M (you need to find the exact concentration!!) solution serves to
standardize the EDTA solution. Remember that the results from your magnesium
determination depend on the accurate preparation of this solution.
PROCEDURE
PART A --
PREPARATION OF STANDARD 0.01 F Na2H2EDTA SOLUTION
1.
Weigh
about 3.8 g of the disodium EDTA salt (Na2H2Y 2H2O) (what balance should you
use?) into a 1 liter volumetric flask, dissolve and dilute to the mark with
deionized water and mix well.
PART B –
STANDARDIZATION OF THE EDTA SOLUTION
1.
Pipet
exactly 25 mL of standard Zn solution into each of three Erlenmeyer flasks. Add
20 Ml of pH 10 buffer (in the hood), 15 mL of water, stir, and add a few
crystals of the Eriochrome Black T indicator. It is critical to add only enough
indicator to produce a light wine-red color.
2.
Titrate
with your EDTA solution until the color changes from wine-red to a clear blue.
3.
Use these results to determine the molar
concentration of the EDTA solution for use in the titration of your unknown Mg
solution.
PART C:
DETERMINATION OF UNKNOWN MAGNESIUM
1.
Your
unknown for this experiment is a solution. When you obtain your unknown, you
need to quantitatively transfer it to a 100 mL volumetric flask and dilute it
to the mark, resulting in the "prepared" unknown solution. You are to
report the results of this "prepared" unknown.
2.
Transfer
exactly 10 mL of the prepared unknown solution into 3 or 4 Erlenmeyer flasks.
Add approximately 15 mL of pH 10 buffer in the hood and 20 or 25 mL of
distilled, deionized water to each flask.
3.
Add
a few crystals of Eriochrome Black T indicator -- it is crucial that you only
add enough indicator to produce a light, wine-red color.
4.
Titrate
each solution with your standardized EDTA solution to a clear blue color.
5.
Report
your results as percent magnesium (% w/v) in your “prepared” unknown sample.
INFORMATION,
HINTS AND HELP FOR MG DETERMINATION
Preparation:
• To prepare the
standard EDTA solution used in this experiment, you must first be aware that the
structure of the EDTA listed in your text and lab manual is not the compound
you will be weighing out to make the solution. The salt you will use is the disodium
salt of EDTA. The formula weight can be obtained from the bottle.
• Since you are
going to standardize the EDTA solution with a standard Zn solution, you need not
dry the EDTA.
• After making
your EDTA solution, if you wish to keep some for an additional lab period, store
it in a plastic bottle, not a volumetric flask.
• The Zn solution
used for standardization may be prepared for you in advance. Consult your class
instructor for this information. Be sure to note the actual concentration of
the Zn solution, as it will not be exactly 0.01 M.
Technique:
• Eriochrome Black
T exists as a wine-red complex when Mg+2 is present in solution at pH =10. When
the EDTA has chelated all the Mg+2 present in solution, the indicator (free and
uncomplexed to Mg+2) will be robin’s egg blue. This color change marks the
endpoint. Add only very small quantities of the indicator are needed. Do your
best to keep the intensity of the indicator color relatively weak and
consistent from sample-to-sample.
• It is best to
test the indicator color change before trying the titration. Try it either with
tap water or a dilute Mg+2 solution. Using a sample with a small amount of Mg+2
and performing a low volume sample titration will help you understand the
endpoint changes.
• A trick which
makes the end point much easier to detect for some is to use methyl red indicator
solution in addition to the Eriochrome Black T. This changes the two endpoint colors
to orange initially and green at the endpoint. Be careful when doing this step
though, as the additions must be done in a way as to keep the intensities of
colors roughly the same in all samples.
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Calculations:
• The
concentration of magnesium that you report is that of the unknown you received
after it was diluted to 100 mL in the volumetric flask (the “prepared”
unknown).
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