S3. SPECTROPHOTOMETRIC ANALYSIS OF IRON
PURPOSE
To determine the concentration of an iron (III) solution by reacting it with a colour-forming
reagent and analysing the solution by visible spectrophotometry using a filter photometer, a
non-scanning spectrophotometer and a computer controlled spectrophotometer.
PREWORK
1. Describe how you would prepare a 1000 mg/L solution of Fe3+.
2. Describe how you would prepare a 30 mg/L solution of Fe3+ by using a serial dilution of
the 1000mg/L Fe3+ solution. The first dilution made is termed the intermediate.
PROCEDURE
I. Preparation
1. Clean 9, 100 mL volumetric flasks and label 0, 5, 10, 15, 20, Unknown and Recovery
Check. The remaining flasks are for the intermediate and the 30 mg/L solution
2. Prepare the intermediate Fe3+ solution as described in your Prework adding 5 mL 5M HCl.
Preparation of working Standards
3. Pipette the required amount of the intermediate standard (as per prework 2) into a 100 mL
volumetric flask. This is the 30 mg/L flask. Using a Pasteur pipette, add 0.2% KMnO4
solution dropwise until a pale pink colour persists. NOTE; - If you have added too much
KMnO4 and a considerable amount of colour is present, start again.
4. Make up to the mark with distilled water.
5. For the 5 standard solutions, pipette 0, 5, 10, 15, 20 mL of the 30 mg/L Fe3+ solution into
the labelled 100 mL volumetric flasks. Add 50 mL purified water, 10 mL of 5 M HCl and
treat with permanganate as in step 3. Do not make the solutions up to the mark yet.
6. To each of the standard flasks add 5 mL of 5M NH4SCN and mix well
7. Make the standard flasks up to the mark with purified water.
Preparation of unknown and Recovery Check samples
8. Obtain an unknown solution of Fe, and pipette 10 mL of it into a 100 mL volumetric
flask, add 10 mL of 5 M HCl and approximately 50 mL of water, treat with permanganate
as in step3. Do not make the solutions to the mark yet.
9. To the recovery check flask repeat step 8 also adding 100 µL of 1000 mg/L Fe solution
using a micropipette.
10. To each of the unknown and recovery check solution flasks, add 5 mL of 5M NH4SCN.
Mix well
11. Make each flask up to 100 mL with distilled water.
II. Spectrophotometric Determination of Fe
This practical involves the use of different types of instruments:
• a filter photometer
• a non-scanning spectrophotometer
• a computer-controlled scanning spectrophotometer
12. Record the spectrum on one of your standards, from 400-800 nm, on the computercontrolled scanning spectrophotometer.
13. Record the absorbance of the standard at:
a) 480 nm
b) 475 nm
c) 450 nm
d) 445 nm
14. Zero the instrument with the blank as instructed by the teacher.
15. Rinse the sample cell with each solution to be analysed before recording its absorbance at
480nm. For the filter photometer it may be necessary to use a different wavelength if there
is no 480nm filter.
CALCULATIONS
1. Calculate the concentration of each standard using C1V1 = C2V2
2. Calculate the difference in absorbance between 475 and 480nm and between 445 and
450 nm.
For each instrument:
3. Plot the absorbance vs standard concentration calibration graph.
4. Use the graph to determine the concentration of the diluted unknown Fe solution
5. Using the dilution factor, calculate the concentration of the original solution
6. Repeat calculation for the recovery check sample calculating the % recovery for the
diluted solution.
DISCUSSION
• Comment on differences in results obtained from the three instruments
• Compare the absorbance data recorded at the four different wavelengths, explain why
recording absorbances at the wavelength of maximum absorption is more accurate than
down the side of the peak
• Comment on the % recovery of the iron.
QUESTIONS
1. Why are blanks used in spectrophotometry?
2. Why is NH4SCN used in the experiment?
3. Why are the iron solutions made acidic?
4. Why was KMnO4 added to the iron solutions?
5. The absorbance of a particular solution would have varied from one instrument to the
next. Did this matter in the analysis? Explain your answer
6. What colour was the filter chosen for the analysis using the filter photometer? Explain
why this colour was suitable for the analysis.
S3. RESULTS SHEET
Date of analysis
Identity of instruments
Filter photometer:
Non-scanning:
Scanning:
Unknown Code
Absorbances at varying wavelengths
Wavelength (nm)
480
Absorbance
475
450
445
Absorbances at 480nm (unless indicated)
Volume
30mg/L
Fe
0
Conc
standard
Photometer
mg/L
Filter =
Absorbances
Photometer
Non-scanning
Filter =
Scanning
5
10
15
20
Sample
Recovery
check
Have you?
Completed the instrument log
Completed the sample register
Completed the standard register
Teachers signature
Date
Signature