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Analytical Chemistry Trinidad & 

Tobago Lab Resources

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Revised labscript :

Determination of iron in pharmaceutical preparations
by visible spectrophotometry

1. Introduction:

Iron in pharmaceutical preparations may be provided in the form of ferrous sulphate, gluconate or ferrate salts.The active ingredients are normally held in a matrix which must be destroyed, before the iron can be accurately determined. In this experiment, the matrix is destroyed by oxidation with concentrated nitric acid. The iron, after conversion to the ferrous state, is complexed with 1:10-phenanthroline and determined by visible spectrophotometry (1)

2. Experimental procedure:

(A) Sample Preparation:

A ground tablet sample will be provided for analysis.

(i) Weigh about 0.1g of the ground sample into each of three clean, dry boiling tubes.

(ii) Add to each of these tubes, plus one blank tube, 5 mL analytical grade nitric acid. Allow to stand in a fume hood overnight (10-12h) to predigest.

(iii) Heat the tubes on a sand bath or heating block at 135-140C,for 2-3h, to complete the extraction procedure.

(iv) Cool to room temperature, dilute the contents with 10mL distilled water and mix well. Filter through Whatman #1 filter paper into 250mL volumetric flasks and make up to the mark with distilled water washings of the boiling tubes and filter papers.

(v) Pipette out 2 mL of each sample solution into a 100mL volumetric flask. Add 5mL of 10% (w/v) aqueous hydroxylamine hydrochloride solution and 5mL of 1M sodium acetate-acetic acid buffer (pH 5.0).

Add 4mL of 0.25% (w/v) aqueous 1,10-phenanthroline and make up to the mark with distilled water. Allow to stand for one hour before making measurements on these solutions.

(vi) Similarly prepare a series of Fe standards (25mL each) containing 0, 1.0, 2.0, 4.0, 6.0, and 8.0ug Fe/mL respectively, using the standard Fe solution provided.

(c) Visible Spectrophotometry:

(i) Switch on and allow spectrometer to warm up and stabilize.

(ii) Rinse and fill a clean cuvette with some of the standard blank solution and wipe the external surfaces clean with a soft tissue.

Fill another cuvette with the 1.0ug/mL Fe standard solution.

(iii) Place the cuvettes in the reference and sample compartments respectively of the double-beam uv/vis spectrophotometer. If a diode-array spectrometer is used, the blank sample is scanned first and the spectrum stored, for correction of the absorbances of the sample solutions.

(iv) Scan the solutions over a wide wavelength range (see Demonstrator) and locate the the wavelength at which maximal absorbance occurs, and the absorbance value at this wavelength. All subsequent measurements can be made on a single beam instrument at this wavelength.

(v) Measure the absorbances of all the other standard and sample solutions at the chosen wavelength, corrected for their respective blank values.

(c) Calculations:

Plot a calibration curve of Absorbance vs Fe concentration of the iron-phenanthroline complex and determine the Fe concentrations of the sample solutions.

Alternatively, calculate the linear regression equation and correlation coefficient for the calibration curve and hence determine the Fe concentrations of sample solutions.

Hence calculate the mean iron concentration of the sample provided standard deviation.

The Fe concentration of the sample will be provided (see Demonstrator). Do your results of analyses differ significantly (p<.05) from the reference value provided?

(d) Exercises:

(i) Give the rationale for:
- preparation of blanks for samples and standards;
- scanning both sample and standard to establish λ max

(ii) How may the efficiency of this method be improved, in terms of:
- speed of analysis;
- economy of use of reagents?


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

anal-chem resources
Chem. Dept. UWI. St. Augustine Campus