Experiment 2
Complexometric Titration of Zinc
A. Purpose
1. To learn the concepts of complexometric titration.
2. To standardize an EDTA solution.
3. To determine the amount of zinc (present as zinc ions Zn2+) in an unknown
solution.
B. Theoretical Background
Metal ions such as Zn2+ may form many complexes in solution, such as
Zn(NH3)42+, for example. Such complexation reactions can serve as a means of
determining ion concentration; however, as several complexing molecules must
be added, one at a time, the equivalence point in such a titration would not in
general be sharp enough to be readily determined. An ingenious way around this
problem is the incorporation of several complexing entities (called “ligands”) into
one molecule. This would ensure that the reaction take place in one step. Ethylene
diamine tetraacetic acid (EDTA) is such a molecule. In buffered solution at pH =
10, it is fully ionized and has the formula (OOCCH2)2NCH2CH2N(CH2COO)2.
Both N atoms and four of the O atoms can act as ligands (EDTA is thus a
hexadentate complexing agent: hexa = 6, dentate = “toothed”- reference to the
fact that it surrounds the metal ion like a mouthful of teeth surrounding a pistachio
nut) and EDTA can therefore form an immensely stable 1:1 complex with Zn2+:
Zn2+ + EDTA4

[Zn (EDTA)]2
Ka = 3.2  1016
The criteria needed to allow a reaction to be used for titrimetric analysis are: 1.
goes to completion, 2. is stoichiometric, 3. proceeds rapidly, and 4. allows for
easy end-point detection. EDTA satisfies the first three of these very well, but an
indicator must also be sought. Another complexing agent, Eriochrome T (or Erio
T), forms metal complexes and changes color when it does so. The form of the
indicator and its color changes are pH dependent, since Erio T is in fact a triprotic
organic acid. At pH = 10, the red complex ZnE exists at Zn++ concentrations
greater than 108 M, whereas at lower concentrations, the blue uncomplexed acid
anions HE2 is the stable from. Thus, as the Zn2+ is titrated by EDTA at pH = 10,
in the presence of Erio T, the free Zn2+ concentration steadily decreases, and at
the equivalence point, the solution color changed from red to blue.
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C. Chemicals and Setup
Zn powder, EDTA solution, Erio T indicator, NH3/NH4Cl buffer (pH 10),
unknown Zn2+ solution.
250 mL volumetric flask, 50 mL buret, 25 mL volumetric pipet, three clean 125
mL (or 250 mL) Erlenmeyer flasks, 50 mL beaker, polyethylene dropping bottle,
buret stand.
D. Procedure
1. THIS SHOULD BE DONE ONE LAB PERIOD IN ADVANCE:
Accurately weigh between 0.25 and 0.40 g of pure Zn and dissolve it in about
5 mL of 6 M HCl in a 50 mL beaker covered with a watch glass. Label and
store in your locker.
2. Transfer the Zn solution (prepared before) quantitatively to a 250 mL
volumetric flask. Dilute to the mark and mix well.
3. Prepare a comparison solution containing 25 ml distilled water, 25 mL
NH4Cl/NH3 buffer solution, 1 mL of EDTA solution, and one drop of Erio T
indicator in a 250 ml Erlenmeyer flask. All end-point determinations will be
made in comparison to this solution.
4. Pipet 10.00 mL of the standard Zn solution into a 250 mL Erlenmeyer flask,
add 15 mL of water, 25 ml of buffer and 1 drop of Erio T indicator. Titrate
with EDTA until the red color is completely gone and the solution is clear
blue, identical (in hue but not necessarily in intensity) to the comparison
solution color.
5. Repeat this titration two additional times.
6. Obtain a sample of unknown Zn solution from the stock room. Pipet 10.00
mL of this unknown solution, into a 250 mL Erlenmeyer flask, then add 15
mL of water and 25 mL buffer, and one drop of Erio T indicator. Titrate with
EDTA to the clear blue end-point. Do three such titrations.
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E. CHEM. 203 Lab Report
NAME : _______________________
EXPERIMENT No. : ______________
TITLE : __________________________________
DATE : ________________
Partner : ________________________
 Preparation of a standard Zn solution
M 0Zn 
m Zn

M Zn  Vsoln
=
 Standardization of the EDTA solution
1
2
3
Upper reading (mL)
Lower reading (mL)
Volume (mL)
Average volume, v EDTA 
M 0EDTA 
M 0Zn  v Zn

v EXTA
=
 Determination of the concentration of an unknown Zn solution
1
2
3
Upper reading (mL)
Lower reading (mL)
Volume (mL)
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Average volume, v EDTA 
M 0unk 
M 0EDTA  v EDTA

v unk
=
Questions
1. Convert the result obtained (in molar units) to mg Zn/100 mL solution.
2. Sketch a schematic representation of the Zn-EDTA complex (ZnY2), showing the
chelation effect.
3. How many electrons are added to Zn2+ by the coordinate-covalent bonds in the
complex ZnY2? In what orbitals will those electrons be accommodated? Write
the electronic configuration of Zn in the complex.
4. Write out the formulas of the following metal-ligand complexes (coordination
compounds): diammine silver (I) ion, tetraammine copper (II) ion, hexaaqua
cobalt (II) ion, tetrachloro platinate (II) ion, tris(ethylenediamine) cobalt (III) ion.
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