Determination of the Formulae of Complex Ions by a Colorimetric Method
Introduction
Copper(II) ions form complex ions with ammonia molecules, or molecules of 1,2-diaminoethane
(ethylenediamine), as the ligands. The complex ions are in these cases, highly coloured and
assuming that maximum colour intensity coincides with maximum number of complex ions formed,
one can determine the number of ligand molecules surrounding the central copper(II) ion. The
method of continuos variation is used, in which solutions, containing different proportions of
copper(II) ion to ligand molecule, are mixed and their colour intensity compared, using a
colorimeter.
Apparatus:
6 test tubes to fit a colorimeter and of minimum capacity 15 cm3
3 graduated pipette, a 5.0 cm3 pipette, 4150 cm3 beakers, colorimeter
Chemicals:
1.5M ammonium sulphate solution (50 cm3), 0.10 M copper(II) sulphate solution (60 cm3),
0.10 M ammonia solution, 0.10 M 1,2-diaminoethane (50 cm3)
Procedure
A. Determination of the formula of the Cu(NH3)x2+ complex ion
1. Take six test-tubes, which will contain at least 15 cm3 of solution and fit into the colorimeter,
and label them 1 to 6.
2. Using a graduated pipette, or burette, make up the six mixtures as indicated below shaking the
contents of the tube to make sure they are homogeneous. The 5.0 cm3 of approximately 1.5 M
ammonium sulphate should be added to each tube first, since it is present to prevent
precipitation.
3. After the mixtures have been made, set the pointer on the colorimeter to the zero mark
(minimum absorption of light) with tube 1 in place and no filter.
4. Record the results in table 1. Plot a graph of light absorption (the logarithmic scale) against the
volume of 0.10M CuSO4 solution, reading from left to right and, on the same scale, the
volume of 0.10M NH3 solution, reading from right to left.
Data
Test-tube number
Volume of CuSO4(cm3)
Volume of NH3 (cm3)
Absorption
1
2
3
4
5
6
Questions for discussion
1. How does the ammonium sulphate solution prevent precipitation?
2. What are the volumes of these two solutions which, on mixing would give maximum colour
intensity?
3. What is the value of x in the formula [Cu(NH3)x]2+?
B. Determination of the formula of the Cu(H2NCH2CH2NH2)y2+ complex ion
1. Take six test-tubes and label them as before.
2. Make up mixture of the copper(II) sulphate and 1,2-diaminoethane solutions of the
compositions shown below, and shake until the mixtures are homogeneous.
3. Place tube 1 in the colorimeter and set the pointer on the zero mark, without using a filter.
Record the reading of absorption on the colorimeter for the other tubes relative to this zero
reading.
4. Record the results in table 2. Plot a graph of light absorption (the logarithmic scale) against the
volume of 0.10M CuSO4 solution, reading from left to right and, on the same scale, the
volume of 0.10M 1,2-diaminoethaqne solution, reading from right to left.
Data
Test-tube number
Volume of CuSO4(cm3)
Volume of diene (cm3)
Absorption
1
2
3
4
5
6
Questions for discussion
4. What are the volumes of these two solutions which, on mixing would give maximum colour
intensity?
5. What is the value of y in the formula Cu(H2NCH2CH2NH2)y2+?
6. What are the features of a ligand?
7. What explanation can you give for the relative values of x and y in the above formulae?
E ND