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The Photocolorimetric Determination of Iron in Natural Water
The red-purple complex that forms between iron(III) and sulfosalicylic acid is useful
for determining iron in water supplies. The reagent is a weak acid that reacts to form
ammonium salt in alkali media. Complex formation with iron in thus best described by the
equation:
COO-
COOH
O-
OH
Fe+3 +
Fe+3 +
SO3H
3H+
SO3-
Iron(III) is quantitatively complexed in the range pH between 3 and 10. A pH of
about 9 is ordinarily recommended to most stable and intensity coloured complex formation.
The determination can be performed with a photometer or photocolorimeter equipped
with a blue filter.
Preparation of a calibration curve
1. Transfer 0,1, 0,2, 0,4, 0,5, 0,6, 0,7, 0,8, 0,9, 1,0 and 1,1 ml of the standard iron(III)
solution (concentration of Fe+3-ion is 300 µg/ml) to a raw of 50-ml volumetric flasks.
2. To each flask add 5 ml of 10 % solution of sulfosalicylic acid and 10 % ammonium
hydroxide solution to pH 9 (universal indicator strip). Solutions shake.
3. Dilute all mixtures with distilled water to marks and mix well.
4. Prepare the solution for comparison – "blank" solution.
A "blank" solution contains the solvent and all of the reagents in an analysis,
but none of the sample.
5. Determine the absorbance of the standard solutions with respect to the "blank" using
cuvettes with path length 2 cm.
6. Plot a calibration curve. Attempt to encompass an absorbance range between 0,1 ml and
1,0 ml of standard iron(III) solution. Change cuvettes, if it is possible.
%
7. Calculate specific absorbency coefficient ( E11cm
) for each concentration. Calculate
medium value of specific absorbency coefficient.
Determination of iron(III) in water
1. Transfer 10 ml of tape water to 50-ml volumetric flask.
2. Treat in exactly the same way as the standards.
3. Measure the absorbance with respect to the "blank".
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4. Determine iron amount using calibration curve and specific absorbency coefficient.
Compare results.
5. Report the µg of iron(III) per litre of water and in per cents.
Data for calibration curve plotting
Iron(III) amount
volume of standard
µg
solution, ml
0,1
30
0,2
60
0,3
90
0,4
120
0,5
150
0,6
180
0,7
210
0,8
240
0,9
270
1,0
300
1,1
330
Specific
Absorbance
Iron
absorbency
(optical density),
concentration, %
%
coefficient, E11cm
A
1%
Medium value of specific absorbency coefficient ( E1cm )