UV-VIS Spectroscopy of iron tablets

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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)

Sample Preparation

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.

Visible Spectrophotometry

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 range.

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% confidence level?

Give the rationale for

(a) Preparation of blanks for samples and standards.
(b) Scanning both sample and standard to establish λ _max

List sources of error that effect the accuracy and precision of analysis in this experiment.

Reference:

(1) Vogel's Textbook of Quantitative Inorganic Analysis
4th. edition (1988). Longman's Publ. 742 - 743.


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