Experiment 33: Colorimetric
determination
of iron
PURPOSE
To become aquainted with the
principles of colorimetric analysis.
THEORY
• When mixed with phenanthroline , Fe2+ ions reacts to form an
orange red complex
• The color of the solution is directly proportional to the
concentration of Fe3+ ions present. The concentration of these ions
can be determined by measuring the absorbance of a unknown Fe2+
complex solution and comparing it with the absorbance of a
solution of known concentration.
•
Colorimeters measure the amount of light that is transmitted or
absorbed by a solution. A description of how they work can be
found in your text book
• Fe3+(aq) needs to be reduced to Fe2+ by adding hydroxlyamine
SAFETY PROCEDURES
• Follow all instructions for using the equipment
in this activity.
• 2. Wear safety glasses and a laboratory coat
for this experiment.
• 3. Do not throw any waste down the sink.
Part 1. Preparation of calibration
curve
• Pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of
standard Fe solution into a 50 ml volumetric
flasks 1 – 5 respectively
• Add to each flask
– 1ml of 1M ammonium acetate
– 1ml of 10% hydroxylamine
– 10 ml of .30% phenanthroline
– Dilute the rest to 50 ml with water
Part 1. Preparation of calibration
curve
• Mix well
• Let sit for 40 minutes for the color to develop
•
•
•
•
Clean six cuvettes
Add water to one cuvette ½ way and calibrate spectrophotometer to 510 nm
Fill the other 5 cuvettes ½ way with a solution of increasing concentration
Record absorbance
Part 1. calibration curve
• graph concentration of Fe+2 complex (mg/ml)
vs absorbance
• Absorbance on y axis and concentration on x
as seen on page 425
concentration
Absorbance
of Fe+2 complex
(mg/ml)
1
2
.001
.15
calibration curve
Absorbance
• Prepare calibration curve by plotting
absorbance vs concentration
y = 59.524x
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Y-Values
Linear (Y-Values)
0
0.005
0.01
Fe +2complex mg/ml x 10 -3
Part B: Determination of Fe+3 complex
Create Fe +2 ion
• Weigh 0.1 g to 4 sig figs of Fe unknown into a 50 ml
volumetric flasks
• Add
– 5 drop of 6 M sulfuric acid
– Dilute the rest to 50 ml with water
– Transfer to a 125 erlenmeyer flask
Part B: Determination of Fe+3 complex
Convert Fe +2 ion to Fe+3 complex
• Pipet 1ml of Fe +2 ion from the erlenmeyer flask
into three 50 ml volumetric flasks
• Add to each flask
–
–
–
–
1ml of 1M ammonium acetate
1ml of 10% hydroxylamine
10 ml of .30% phenanthroline
Dilute the rest to 50 ml with water
Part B: Determination of Fe+3 complex
Mix well
Let sit for 40 minutes for the color to develop
•
•
•
•
Clean 3 cuvettes
Fill the 3 cuvettes ½ way with each Fe+3 complex solution
Record absorbance at 510 nm
Using the absorbance determine concentration of Fe+2 complex
(mg/ml) by extrapolating from your graph