UV-VIS Spectroscopy of iron tablets
Back to List of Experiments
Determination of iron by UV-VIS Spectophotometry
Iron in pharmaceutical preparations is normally in the form of ferrous
sulphate, gluconate or ferrate respectively.The active ingredients are
normally held in a matrix which must be destroyed before before the
iron can be effectively determined. In this experiment, the matrix is
destroyed by oxidation with concentrated nitric acid. The iron, after
reconversion to the ferrous state, is complexed with
1:10 o-phenanthroline and determined by visible spectrophotometry. (1)
Place one tablet into each of five clean boiling tubes. Add to each
of these tubes, plus one blank,5 mls analytical grade conc. nitric
acid. Allow to stand in a fume hood overnight to predigest. After this
period, heat the tubes on a sand bath or heating block at 120-130°C
to complete the digestion procedure (2 hours). Cool and then dilute the
contents with 10 mls distilled water. Filter through a Watman #1 filter
paper into 250 mls volumetric flasks and make up to the mark with
distilled water washings of the boiling tube and filter paper.
Pipette out 2 ml each of your diluted solutions into 100 mls volumetric
flasks. To each flask add 5ml of 10% (w/v) aqueous hydroxylamine hydrochloride solution and 5 ml of 1M sodium acetate-acetic acid buffer
(pH 5.0), then add 4 ml of 0.25% (w/v) aquoeus o-phenanthroline and make up
to the mark with distilled water. Allow to stand for one hour before
making measurements on these solutions.
Similarly prepare a series of standards (25 mls) containing 0, 1.0,
2.0, 4.0, 6.0, and 8.0 ppm from standard stock solution provided.
Rinse and fill a clean cuvette with some of the standard blank solution
and wipe the external surfaces clean with tissue. Fill another cuvette
with standard. Place the cuvettes in the reference and sample
compartments respectively of the double-beam uv/vis
spectrophotometer.Scan between 480 and 520nm and note the wavelength
range over which maximum absorbance occurs. Repeat the procedure using
a sample / sample-blank pair to confirm the λ max
All subsequent measurements should be made on a single beam instrument.
Set the mid point of the λ max on the single beam
spectrophotometer and measure the absorption values of each standard
and sample against its respective blank.
Plot a calibration curve and determine the linear regression equation
for the iron-phenanthroline complex and determine the concentration of
iron in each solution. Hence calculate the total iron content of each
tablet analyzed, and then the mean iron content per tablet of the batch
± standard deviation by each method.
Assume that the tablets each contain x mg of iron.Do your results
differ significantly from the above amount at (a) the 95% (b) the 99%
Give the rationale for
(a) Preparation of blanks for samples and standards.
(b) Scanning both sample and standard to establish λ
List sources of error that effect the accuracy
and precision of analysis in this experiment.
(1) Vogel's Textbook of Quantitative Inorganic Analysis
edition (1988). Longman's Publ. 742 - 743.