Thompson Rivers University
General Chemistry Laboratory
Instructor Notes to Accompany Analysis of Iron
by Atomic Absorption Spectrophotometry Lab
The purpose of this experiment is to use a spectroscopic technique to analyze an Fe
solution and relate it back to the amount of iron in the original multivitamin. Students gain
experience in the preparation of a real sample for quantitative analysis and in the
construction and use of calibration curves in analytical chemistry. These Instructor
guidelines are written for a laboratory class preparing their samples for analysis and
submitting them to the BC-ILN.
Pre-lab Talk - be sure to mention
Grab student interest by asking if they believe what is written on the labels of foods and
medicines. Today’s lab shows one way this information can be verified.
Part A – Sample prep

Acid is used to break down the multivitamin tablet and release the iron.

The tablet will sit in acid at room temperature while they do Part B and then be
heated gently on a hot plate in a fumehood

The hot plates must be preheated (set the taller, older models at 4 and the newer, flat
models at 23). The beakers should not be allowed to get too hot to handle safely with
bare hands.

During both these times the beaker must be swirled periodically

A small amount of residue may remain after heating but it will be filtered out in the
next step

Demonstrate how to flute a filter paper for maximum surface area during the
gravity filtration
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General Chemistry Laboratory

Demonstrate arrangement of apparatus for gravity filtration into 100mL volumetric
flask

Demonstrate quantitative transfer of solution to volumetric flask and proper filling
and mixing of the flask

Dispose of the used filter papers and residue in the supplied waste beaker
Part B – Preparation of Calibration Standards

Demonstrate how to correctly read and use a Mohr pipet; remember all volumes
must have 2 decimal places; record an initial and a final volume in the data book for
each sample measured out

Review dilution calculation (Coriginal x Voriginal = Cdilute x Vdilute)
Part C – Dilution of Unknown

Make sure students have completed all 5 steps in Part A before moving on to Part C

The 40 mL mentioned in the lab manual is enough to rinse the pipet and measure
out the required aliquot
Part D – Measuring Absorbance

Check your connections and communication software per Instructor Getting
Connected tutorial prior to beginning the experiment

Give the students a brief summary of the theory behind AAS: certain elements absorb
light at a particular wavelength (for Fe, it is 372.0 nm), the absorbance is directly
proportional to the concentration, a set of solutions of known concentration of the
element can be prepared and their absorbances measured to produce a calibration
curve. Then an unknown solutions absorbance may be measured and the
concentration found from the calibration curve.
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Suite 300, San Francisco, California 94105, USA.
Thompson Rivers University

General Chemistry Laboratory
The function of the 4 major parts of the AAS (see lab manual)
Sample Calculations
Remind the students that the first data point is their blank solution. Once the multivitamins
concentration is known, some students have trouble working the calculations back to the
total amount of iron in the original tablet; this should be reviewed, if necessary.
For example:
Concentration from calibration curve is 9.40 mg Fe/L
Therefore, in the original solution the concentration is:
C original = C d x (V d /V o ) = 9.40 mg/L x (100.0 mL/25.00 mL)
= 37.6 mg/L
The multivitamin tablet was initially dissolved in 100.0 mL. Therefore, the Fe in the
tablet was 37.6 mg/L x 0.1000 L = 3.76 mg (note this is not real data).
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licence, visit http://creativecommons.org/licenses/by-sa/2.5/ca/ or send a letter to Creative Commons, 171 Second Street,
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Thompson Rivers University
General Chemistry Laboratory
Sample Marking Scheme
Report Mark =11
Complete and neat
Significant figures
Sample calculation of a conc. of a standard solution (with units)
1 mark
1 mark
1 mark
Calibration curve (good technique and scaling; illustrating det. Of
conc.; title; axes labelled; linear data points; “bestfit” line drawn;
blank included as a data point).
The graph should be landscaped so the x-axis is along the 11” side
of the page.
3 marks
Concentration of diluted multivitamin unknown read correctly with
proper number of sig figs
1 mark
Calculation of conc. of original multivitamin solution
1 mark

dilution calculation
Calculation of mass of iron in tablet

0.5 mark
conc x volume
Calculation of per cent difference from label info
0.5 mark
Possible sources of error
1 mark
Question
1 mark
init ial and mark st udent data /1
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licence, visit http://creativecommons.org/licenses/by-sa/2.5/ca/ or send a letter to Creative Commons, 171 Second Street,
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Thompson Rivers University
General Chemistry Laboratory
10
Questions (from Lab Manual)
1.
When an atom absorbs electromagnetic energy electrons are moving to higher energy
levels. The atom is excited. An exact amount of energy is required for each energy
level change. This means that only certain specific wavelengths of light will be
absorbed.
2.
Absorbance is directly proportional to analyte concentration.
3.
Atomic absorption refers to the absorption of radiation, which causes an electron to
be excited, while atomic emission refers to the emission of light from a substance
when electrons drop from the excited state to the ground state.
4.
A Mohr pipet precisely measures variable volumes and is not drained when
delivering a sample. A volumetric transfer pipet precisely measures one fixed
volume (termed an aliquot).
5.
a)
The acid is added to help break down the solid sample and dissolve the iron.
b)
The solid sample and acid are heated to speed the dissolution.
c)
The sample is filtered to remove any solid residue that remains after the
digestion.
d)
The instrument is calibrated to determine the exact relationship
between concentration and absorbance.
6.
If the iron was not completely released, the final result would be lower than it
should be.
7.
If the instrument is not properly calibrated for quantitative analysis, the values
determined for any unknowns would be incorrect.
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licence, visit http://creativecommons.org/licenses/by-sa/2.5/ca/ or send a letter to Creative Commons, 171 Second Street,
Suite 300, San Francisco, California 94105, USA.
Thompson Rivers University
General Chemistry Laboratory
8.
Possible reasons for experimental error could include: not completely digesting
the tablet; not rinsing out the out the beaker used for the digestion; improperly
filling the volumetric flasks; not completely mixing the solutions to make them
homogeneous; not properly measuring the stock solution with the Mohr pipet
when preparing the standard solutions.
9.
You would use a lamp that produced a wavelength of light specific to Ca.
10.
A.
lamp
B.
burner
C.
monochromator
D.
detector
This work is licensed under the Creative Commons Attribution-Share Alike 2.5 Canada License. To view a copy of this
licence, visit http://creativecommons.org/licenses/by-sa/2.5/ca/ or send a letter to Creative Commons, 171 Second Street,
Suite 300, San Francisco, California 94105, USA.