ANALYTICAL cHEMISTRY - III

MSCM301

Determination of the amount of copper sulphate (CuSO₄) by spectrophotometric titration.

Spectrophotometry is a method to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution.
The basic principle is that each compound absorbs or transmits light over a certain range of wavelength. The measurement of the absorbance or transmittance can be used to measure the amount of a known chemical substance.
Spectrophotometry is one of the most useful methods of quantitative analysis in various fields such as chemistry, physics, biochemistry, material and chemical engineering and clinical applications.

Spectrophotometric titration of copper with EDTA

The spectrophotometric titration is performed at 625 nm and at this wavelength, both the Cu (II)-EDTA chelate and the Cu (II) ions absorb, however, the molar absorptivity of the Cu (II)-EDTA chelate is much higher compared to the copper (II) ion.
Thus, initially, when EDTA titrant is not added to the copper sulphate (CuSO₄) solution, containing Cu (II) ions, the absorptivity of the solution is lower. After each sequential addition of EDTA titrant, due to the formation of Cu (II)-EDTA chelate, absorptivity of the solution increases, till all the Cu (II) ions in the solution have formed Cu (II)-EDTA chelate.
From the spectrophotometric titration, a plot of absorbance v/s volume of the titrant (EDTA) is obtained. The points fall below the extrapolated lines in the end-point region, because the reaction is incomplete near the equivalence point. After the equivalence point, the added excess EDTA titrant forces the reaction to completion. The further addition of titrant leads to dilution; therefore, the absorbance will then decrease slightly.
The pH is critical for this titration, because a large change in pH changes the effective binding constant. An acetate buffer is used to maintain the pH between 2.4 and 2.8 to avoid this problem. This low pH also permits the copper to be titrated in the presence of metal ions that form weaker complexes with EDTA.

Spectrophotometer_1.mp4

Spectrophotometer/Colorimeter, Burette, Pipette, Beakers, Conical flasks, etc.

Given CuSO₄.5H₂O solution of unknown concentration, 0.02 M EDTA solution, Buffer solution (pH 2.2), etc.

Pipette out 10 mL of stock solution of CuSO₄.5H₂O in a measuring flask (250 mL). Add distilled water up to the mark. Prepare 250 mL of 0.02 M EDTA solution by accurately weighing 1.861 gm of EDTA. Prepare buffer solution of CH₃COONa of pH 2.2.
Now prepare 16 sets according to the following table.
Now, fill the cell with distilled water and set on zero absorption and 100% transmittance. Now fill the cell with each set one by one and note down the absorption. Plot the graph of absorption vs. volume of EDTA. Calculate the amount of CuSO₄.5H₂O in the given solution.
From the graph, end point = __________ mL

1_Determination of copper sulphate by Spectrophotometric.mp4

2_Determination of copper sulphate by Spectrophotometry.mp4

3_Determination of copper sulphate by Spectrophotometric.mp4

Amount of CuSO₄.5H₂O in 250 mL diluted solution = _______ g.

G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition
S. Suzanne Nielsen, Food analysis, 4th Edition

What other complexometric titrations can be performed with Cu (II) ions?
What difference would it make if we use EDTA instead of EDTA-disodium salt in the above experiment?
Why we need to use acidic buffer solution in this experiment?
Draw the structure of the Cu (II) - EDTA complex being formed in this spectrophotometric titration.

Dr. Viraj Bhanvadia,

Assistant Professor, Chemistry,

viraj.bhanvadia@gsfcuniversity.ac.in