Determine the composition of the Fe3+-salicylic acid complex solution by Job’s method.

1. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
SAFETY: Refer to MSDS of ammonium ferric sulfate, sodium salt of salicylic acid
-Dr. Mithil S. FalDesai
Aim: Determine the composition of the Fe3+-salicylic acid complex solution by Job’s method.
Chemicals: Ammonium ferric sulfate, sodiumsalt of salicylic acid
Apparatus/Instrument: Standard volumetric flask, beakers, funnel, dropper, glass rod and colorimeter.
Theory: In Job's method, the variation in concentration of the reactants is performed which can reveal the empirical
formula of a complex. The method is employed to find the formula of the compound formed by reacting two or
more chemical species. The absorption is recorded against different wavelengths and wavelength having maximum
absorption is selected. The intensity of solutions with different stoichiometric ratios of the reactants is measured.
The highest observed intensity reveals the maximum amount of compound formed. In this experiment, the Fe3+ and
salicylic acid are reacted and the wavelength at which the complex absorbs strongly is selected. The absorbance of
the different stoichiometric ratios of Fe3+ and salicylic acid at a selected wavelength is determined. The maximum
absorbance of the solution of the stoichiometric ratio reveals the empirical formula of the complex as the maximum
amount of colored complex is formed.
Procedure: Calibrate the colorimeter as per the instructions on the instrument manual. (For blank use distilled water)
A) Preparation of 0.0025 M Fe3+ solution.
B) Preparation of 0.0025 M sodium salicylate in (0.2 M HCl).
C) Determination of the composition of the Fe3+-salicylic acid complex
1) Take 11 nos. of clean 20 mL test tubes and label themA to K.
2) Add Fe3+ solution and sodiumsalicylate solution as shown in Table 2.
3) Using any representative colored solution of Fe3+-salicylic acid complex select the wavelength which shows
maximum absorption (Table 1).
4) Record the absorption of all the solutions at the selected wavelength and determine the formula of the complex.
(Table 2)
5) Plot a graph of absorbance v/s mole fraction of Fe.
Observation
Table 1. Selection of optical filter for maximum absorbance.
Filter 430 nm 470 nm 525 nm 568 nm 601 nm 625 nm 660 nm
Absorbance Fe3+-salicylic acid complex
Table 2.
Test tube A B C D E F G H I J K
Volume of Fe3+ solution (mL) 10 9 8 7 6 5 4 3 2 1 0
Volume of sodiumsalicilate solution (mL) 0 1 2 3 4 5 6 7 8 9 10
Mole fraction of Fe
Absorbance
Results: I) The maximum of absorbance is observed at __ mole fraction of Fe which shows that the complexof iron
and salicylate is formed at a _:_ ratio.
Further reading
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria, and Sons.
Questions
1. What is the IUPAC name of Fe3+-salicilic acid complex?
2. What is the chemical formula of the complex?
3. Why do we need to acidify Fe3+ salt in an aqueous medium?