Determination of Iron in Water
ENVS4010
Theory
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Fe2+ (ferrous) complexed with 1,10 phenanthroline
Orange color
Spectrophotometer measures light absorbed
Selective for Fe2+
Part A
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The test for the thiocyanate ion is to
add iron (III) nitrate to a solution. If the
solution turns blood-red upon the
addition of iron(III) nitrate this
indicates the presence of the
thiocyanate ion
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Fe3+(aq) + 6 SCN-(aq) → [Fe(SCN)6]3-(aq)
LHS: Fe2+
RHS: Fe3+
Chemistry
1.
Reduction of iron:
2Fe3+(aq) + 2NH2OH(aq) + 2OH-(aq) → 2Fe2+(aq) + N2(g) + 4H2O(l)
2.
Complexation by 1,10-phen
Fe2+(aq) + 3(C12H2N2)3(aq) → [(C12H2N2)3Fe]2+(ag)
N
N
N
Fe
N
N
N
[(C12H2N2)3Fe]2+
Questions
1.
What is a chromophore?
A chemical group capable of selective light absorption
resulting in the coloration of certain organic compounds.
2.
Why is it necessary to wait 10 minutes before
reading the spectrophotometer?
To allow time for the formation of he complex
3.
What is a chelating agent?
A chemical compound in the form of a heterocyclic ring,
containing a metal ion attached by coordinate bonds to at least
two nonmetal ions.
Method
1.
Preparation of Standard Solutions Prepare four iron (II) standards having the following
concentrations: 0.5, 1.0, 2.0, and 5.0 ppm. Pipet 0.5, 1.0, 2.0, and 5.0 mL of 100 ppm stock
solution into 100 mL volumetric flasks and dilute to the mark with DI water.
2.
Obtaining a Beer’s Law Plot Transfer a 5 mL aliquot of the 0.5 ppm iron standard to a 125 mL
Erlenmeyer flask and test the pH with test paper. If greater than 4.5, add enough 0.2 M sulfuric
acid dropwise to bring the pH to about 3.5, counting the number of drops. Again counting drops,
add sodium citrate buffer to bring the pH to about 4.5. Pipet 1 mL of 10 % hydroxylamine
hydrochloride and 3 mL of 1,10-phenanthroline into the sample, mix, and allow 5 minutes for color
development.
DO NOT PUT YOUR PIPETTES INTO THESE SOLUTIONS; pour a small amount into your
beaker and pipette from this. Be sure to add the reagents in the order shown here.
Use the same number of drops of sulfuric acid and sodium citrate for the remaining three
standards and a reagent blank, followed by 3 mL of 0.3 % 1,10-phenanthroline and 1 mL of 10 %
hydroxylamine. Mix well. Using water as the reference, measure the absorbance of each standard
at 512 nm.
3.
Analysis of samples Natural and tap water samples often have less than 0.5 ppm iron. Set the
UV/VIS spectrometer to read at 512 nm. Determine the iron in several environmental water
samples.
Use 5 mL samples and treat them the same way as the standards, adding sulfuric acid initially (if
necessary), followed by citrate buffer, reducing agent and the indicator.
NOTE: All iron solutions should be discarded into a “Heavy Metal” waste container.
Calculations
1. For the calibration curve, select the x-y scatter chart that shows only the data points,
then perform a best-fit line of your data without forcing the line through the origin.
Determine the slope, y-intercept, and the correlation coefficient; comment on the
linearity of the plot.
2. Use the equation of your best fit line on the calibration curve to determine the
concentration of iron (in ppm Fe) in your unknown sample. Use correct sig. figs. That
value (the concentration of iron in the diluted solution, as measured in the
spectrometer is the final value you are expected to report.
3. If duplicate determinations were done, report average values and the individual
values of concentration and discuss the reproducibility.
4. For the instrument you used, what would you estimate to be the lowest concentration
of iron that would be reliable?
0.6
A = εbc
0.5
absorbance
0.4
0.3
0.2
0.1
0.0
0.5
1.0
1.5
2.0
2.5
concentration
3.0
3.5
4.0
4.5
Questions
1. Why use a “reagent blank” and not just distilled water to zero the spectrometer?
2. What would the effect be of waiting 30 minutes instead of 10 minutes on step 6?
3. Today’s unknown samples are simple iron dissolved in water. If our sample were
more complex, it might contain other compounds, some which might be colored,
including some which might absorb light at the same wavelength as the iron
phenanthroline complex. What problem would this cause? How could we modify our
procedure to correct for this problem?
4. State the function of each reagent in this experiment: 1,10-phenanthroline,
hydroxylamine hydrochloride, sodium acetate.
5. The analytical sensitivity of the method depends on the slope of the calibration
curve. Was your method sensitive for the determination of iron in these samples?
Why?
Field Instrumentation
• Palintest - Ferrometer 1000
• 1,10 phenanthroline test
• 0.02 to 5.00 mg/L Fe
References
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E. B. Sandell, Colorimetric Determination of Traces of Metals, 3rd Ed.
Interscience Publishers, Inc., New York, 1959.
D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical
Chemistry: An Introduction, 7th ed., Chapters 21 and 22, pp. 547-592.