The document outlines the method for estimating Fe(III) in a complex formed with salicylic acid using colorimetry. It explains the principles of the colorimeter, including absorbance measurement and the creation of a calibration graph to determine unknown concentrations. The experiment involves static titration, where varying volumes of salicylic acid are added to a fixed volume of Fe(III) solution, and the optimal salicylic acid volume for color change is used to calculate the amount of Fe(III).

1. Estimation of the amount of Fe(III) in the complex formed
with salicylic acid by static method.
Dr. P. U. Singare
Associate Professor
Department of Chemistry
N.M. Institute of Science, Bhavan’s College,
Andheri, Mumbai 400 058

2. Understanding the basic concept
• Colorimeter is the instrument which measures the absorbance of colored solution in Visible region
(400nm to 700nm).
• Colorimeter works on the Principal of Beer-Lamberts law according to which absorbance of the
sample solution is directly proportional to the pathlength (b) of the light radiation travelled through
the solution and also directly proportional to the concentration (C) of the light absorbing solution
i.e.
A = εbC ----------- (1)
Here ε is called molar extinction coefficient or molar absorptivity.
• During the experiment the sample solution whose absorbance is to be measured is placed in the
sample holder also called as Cuvette.
• The cuvette used is of uniform thickness (usually 1 cm).
• Hence for all colorimetric measurements pathlength (b) = 1cm which is always constant.
• For constant values of ε and b, A α C -------- (2)
• From eq(2) it is clear that the absorbance of solution will increase linearly with the concentration
of light absorbing species present in the solution.
• Depending on the concentration of the sample solution, the light radiations will be absorbed and
remaining light radiations will be transmitted.

3. Understanding the basic concept (Continued----)
• The light absorbing impurities (if any) if present in the chemicals and solvents used for
sample preparation will interfere during the colorimetric measurement.
• The interference due to light absorbing impurities is overcome by using the Blank solution.
• The Blank solution contain all the chemicals and solvents used for preparing the sample
solution.
• The Blank usually will be a colorless solution having 0 (Zero) absorbance i.e. 100%
Transmittance.
• The absorbance of the sample solution is corrected by subtracting the absorbance of blank
from the absorbance sample solution.
• Usually in the colorimetric experiments, the graph of corrected Absorbance (A) on Y-axis is
plotted against Concentration (C) of the solution on X-axis which will be a straight line
graph passing through the origin.
• This graph is called calibration graph.
• The calibration graph is used to find the concentration of sample solution which is Unknown.

4. Understanding the basic concept (Continued----)
• The source of Visible light radiations used in colorimeter is tungsten filament lamp.
• The Visible light radiations emitted from the lamp will be made to pass through the filter.
• Filter will absorb light radiations of unwanted wavelength and will allow only monochromatic
wavelength of light to pass through it.
• For selecting the particular wavelength of light radiations, different coloured filters are used.
• The monochromatic wavelength of light radiation fall on the sample solution placed in the
sample cell (Cuvette).
• For measuring the absorbance in Visible region, the sample holder (cuvette) is made up of
Glass or Quartz material.
• The detector used in the colorimeter is photomultiplier tube.

5. Colorimeter Vs Spectrophotometer
Colorimeter
• Works only in Visible (400-700nm) region.
• The sample solution is colored.
• The cuvette (sample holder) is made up of
glass or quartz material.
• The tungsten filament lamp is used as a
source of light.
Spectrophotometer
• Works in UV(200-400nm) as well as Visible
(400-700nm) region.
• The sample solution may be colored as well
as colorless.
• For measurement in UV region, quartz
cuvette is used. Since glass strongly
absorbs UV radiations, glass cuvettes can
not be used for measurements in UV
region. For measurement in Visible region,
cuvette made up of glass or quartz material
can be used.
• For measurement in UV region, H2 or
Deuterium lamps are used. While for
measurement in Visible region tungsten
filament lamp is used.

6. Background of the experiment
• The experiment involves estimation of Fe(III) ions present in the Fe(III) Salicylic acid complex by
Static Titration method.
• In acidic medium, the Fe(III) ions will form soluble Violet color complex with Salicylic acid.
• The absorbance of the colored solution is measured on the Colorimeter at 530nm wavelength.
• Here λmax = 530nm.
• λmax is defined as the wavelength at which the solution shows maximum absorbance.
• During the experiment, the fixed volume (fixed concentration) of Fe(III) ions solution are taken in 9
different flasks (9 batches).
• To each flask (each batch) increasing volume of salicylic acid is added.
• As the volume of salicylic acid increases, the concentration of Fe(III)-salicylic acid complex
increases.
• With increase in concentration of Fe(III)-salicylic acid complex in the solution, the violet color
intensity initially increases sharply and further when no more free Fe(III) ions are left in the solution,
the color intensity remains constant.
• As a result, the absorbance of solution also initially increases sharply and latter on remains constant.
• The volume of salicylic acid at which the absorbance remains constant is the end point of titration.
• Since the experiment is performed in 9 batches, it is called static titration method.

7. Fe(III) Salicylic acid complex

8. Requirements
1. Colorimeter
2. 0.002M Fe(III) solution
3. 0.002M Salicylic acid solution
4. 0.002M HCl solution
5. Distilled water
6. Standard measuring flask (100 mL capacity) = 9 Nos.
7. Bulb Pipette (10 mL capacity)
8. Graduated pipette (10 mL capacity)

9. Procedure
Flask No. Volume of
0.002M
Fe(III)
solution
(mL)
Volume of
0.002M
Salicylic
acid
(mL)
Dilution
with
distilled
water
(mL)
1 10 0 100
2 10 1 100
3 10 2 100
4 10 3 100
5 10 4 100
6 10 5 100
7 10 6 100
8 10 7 100
9 10 8 100
• Dilute the given Fe (III) solution (Experimental solution) to
250 mL with 0.002M HCl solution.
• This will give 0.002M Fe(III) solution.
• Prepare 9 different 100 mL solution in 9 flask (100mL
capacity) as shown in the table.
• The solution in flask no. 1 will be colorless (since no salicylic
acid is added) and hence will be used as a Blank.
• Take the blank solution in the cuvette and place the cuvette
in the colorimeter.
• Select the wavelength λmax = 530nm on the colorimeter.
• At the selected value of λmax, adjust the absorbance of
blank solution to 0(zero).
• One after the other measure the absorbance of all the
solutions from flask No. 2 to 9 at selected λmax value.
• Plot the graph of absorbance of solution on Y-axis against
volume of salicylic acid (mL) on X-axis.
• From the graph find the endpoint of titration (Vx).
• The end point represent volume of 0.002M salicylic acid
(Vx) required to react (form complex) with 10mL of 0.002 M
Fe(III) solution.
• From the value of Vx as obtained from the graph, the
amount of Fe(III) ions in the solution can be calculated.

10. Observations & Graphs
Flask
No.
Volume of
0.002M
Fe(III)
solution
(mL)
Volume of
0.002M
Salicylic
acid
(mL)
Dilution
with
distilled
water
(mL)
Absorbance
1 10 0 100 0
2 10 1 100 0.09
3 10 2 100 0.20
4 10 3 100 0.25
5 10 4 100 0.27
6 10 5 100 0.27
7 10 6 100 0.27
8 10 7 100 0.27
9 10 8 100 0.27
0
0.05
0.1
0.15
0.2
0.25
0.3
0 2 4 6 8 10
Absorbance Volume of 0.002M Salicylic acid (mL)
Vx =2.7mL

11. Calculations
10 mL of 0.002M Fe (III) solution = 2.7mL of 0.002M salicylic acid (From graph)
Therefore 250 mL of 0.002M Fe (III) solution =
250 𝑥 2.7
10
= 25 x 2.7 = 67.5 mL of 0.002M salicylic acid
Since 1 mL of 0.002 M salicylic acid solution = 280 µg of Fe(III)
Then 67.5 mL of 0.002M salicylic acid = 67.5 x 280 = 18,900 µg of Fe(III)
Therefore amount of Fe(III) present in 250 mL of diluted solution = 18,900 µg

12. Results
1. Volume of salicylic acid required for complex formation of Fe(III) present in 10 mL diluted
solution = 2.7mL
2. Volume of salicylic acid required for complex formation of Fe(III) present in 250 mL
diluted solution = 67.5 mL
3. Amount of Fe (III) in the given solution = 18,900 µg

13. Experimental method for estimation of Fe3+ using salicylate ion solution
https://www.youtube.com/watch?v=tYILu2igNbY