Experiment 33
Colorimetric Determination of Iron
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In Lab
• Work in groups of 4
• 2 people do part A; preparing the standard
solutions
• 2 people do part B; preparing the 3 trials of
the one unknown sample
• Everyone makes their own calibration curve
and determine the concentration of %Fe in
the unknown sample
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Part A
•
•
•
•
Prepare all solutions at the same time
Prepare a blank solution (no Fe)
Allow only 15 minutes for color development
Use disposable test tubes instead of cuvettes
for use in the spectrophotometer
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Part B
•
•
•
•
•
Prepare all solutions at the same time
Follow some of Part A procedure (no Fe)
Share the blank solution (no Fe) from Part A
Allow only 15 minutes for color development
Use disposable test tubes instead of cuvettes
for use in the spectrophotometer
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Spectrophotometer
(Parts A and B)
An instrument that will indicate the amount of
light (of one wavelength) absorbed by a
sample
slit
LED
readout
readout
Light
source
monochromator
sample
detector
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Making the Calibration Curve
• You will acquire an absorbance reading for
each standard solution prepared (share data)
• Use the data from Part A to construct the
calibration curve
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Calibration Curve
Absorbance
Concentration, mg Fe / mL (x10-3)
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Using the Calibration Curve to Find
Concentration of the Unknown Solution
Absorbance
Show this on your graph
Unknown solution
absorbance reading
Concentration of
unknown solution
Concentration, mg Fe / mL (x10-3)
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For All Students
• Typing error on x axis of graph paper:
mg Fe / mL (x 10 -3)
• Typing error on y axis; should be Absorbance
• Calculating % Fe in unknown sample:
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mg/mL of original sample
You will know the grams of original sample from
the balance readout, and you will know the
dilutions made during sample preparation:
1. grams converted to mg
2. mg divided by size of first volumetric flask, in mL
3. 1 mL diluted by size of second volumetric flask, in mL
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