Spectrophotometric Estimation of Paracetamol in Bulk and Pharmaceutical Formulations

A new, simple and sensitive spectrophotometric method for the determination of paracetamol has been developed. The proposed method is based on the reaction of paracetamol with iron(III) and a subsequent reaction with ferricyanide in an hydrochloric acid medium to yield Prussian bluish green coloured product with a miximum absorption at 715 nm. There were no interferences observed from the common excipients present in the formulations. The method is successfully employed for the determination of paracetamol in various pharmaceutical preparations and the results have been statistically compared with those obtained by the official method.


Introduction
Paracetamol (N-acetyl-p-aminophenol) is widely used as analgesic and antipyretic drug, together with caffeine, ibuprofen and diclofenac sodium.Several spectrophotometric methods have been reported for their determination.The majority of published methods for paracetamol depend on hydrolysis of the compounds leading to the formation of a Schiff base with a substituted benzaldehyde 1,2 , or reaction with o-cresol 3 , sodium nitroprusside 4 , cerium(IV) 5 and oxidative coupling with m-cresol 6 and sodium iodylbenzoate 7 .Other spectrometric methods are based on indophenols blue formation [8][9] , nitrosation and subsequent chelation 10 , ultraviolet absorption 11 and its change with pH 12 .Most of these methods require lengthy treatments and lack the simplicity and sensitivity needed for routine analysis.
Sangavi et al 13 reported a colorimetric method for the hydrolysed product of the paracetamol with 1,2-naphthoquinone-4-sulphonate (NQS) in acid medium to form a Schiff base having an absorption maximum at 480 nm.
The present communication reports on the investigation for the use of iron(III) salts in the presence of ferricyanide as reagent for the spectrophotometric determination of paracetamol.No method has been found for dealing with the use of combinations of these two reagents.This method offers the advantages of simplicity, specificity without the need of extraction or heating, besides having higher sensitivity range than most of the existing spectrophotometric methods.Moreover, this method is totally free from the twin disadvantages of critical acid or reagent concentration and instability of the coloured species.

Experimental
A JASCO model UVI DEC-610 UV-VIS spectrophotometer with 1.0 cm matched cells was used for electronic spectral measurements.Standard solutions of paracetamol (1000 µg/mL was prepared by dissolving 100 mg in distilled water and diluted to the mark in a 100 mL calibrated flask.A working standard solution of paracetamol containing 50 µg/mL was prepared by further dilution.A potassium ferricyanide solution (0.002 M), ferric chloride solution (0.1 M) and hydrochloride acid solution (5 M) were prepared in deionised water.
Aliquots of standard paracetamol solution were transferred into 10 mL calibrated flask, 2 mL of potassium ferricyanide and 0.4 mL of ferric chloride were added and the mixture was set aside for 10 min.Then 1.0 mL of hydrochloric acid was added, the contents were diluted to the mark with water and mixed thoroughly.After 20 min.the absorbance values at 715 nm were measured against a reagent blank and a calibrated graph was constructed.
Five tablets (commercial tablets) were powdered and weighed.An amount equivalent to 50 mg (for syrup and injection forms an appropriate volume of the samples) of paracetamol was taken and dissolved in distilled water and filtered if necessary.A suitable aliquot of this solution in individual paracetamol working range was treated as described in the procedure.

Results and Discussion
This method involves the oxidation of the drug with iron(III) and subsequent chelation with ferricyanide to form a Prussian blue coloured product.The factors affecting the colour development, reproducibility, sensitivity and adherence to Beer's law were investigated with paracetamol and listed in Table 1.

Spectral characteristics
A bluish green complex is formed when paracetamol was allowed to react with iron(III) in the presence of ferricyanide in hydrochloric acid medium with maximum absorption as shown in Figure 1.The colourless reagent blank has practically negligible absorbance at this wavelength.

Optimum conditions for complex formation
It was found that a 0.1 M solution of ferric chloride in the range of 0.3-0.5 mL and a 0.002 M solution of potassium ferricyanide in the range of 1.0-3.0mL were necessary to achieve the maximum intensity of the product.The colour intensity decreases above the upper limit and below the lower limit.Therefore 0.4 mL of ferric chloride and 2.0 mL of ferricyanide were recommended for all measurements.Dilution of the bluish green product with different solvents like water, methanol, ethanol, acetic acid, sulphuric acid and hydrochloric acid were tested.Results showed that hydrochloric acid gives clear blue colour with maximum intensity compared to acetic acid and sulphuric acid.It was found that hydrochloric acid in the range of 1.0-10.0mL was necessary to get clear, stable colour.Therefore, a 1.0 mL volume of hydrochloric acid was recommended for all subsequent measurements.

Interference studies
A detailed study on the interference of different excipients were made, it was found that many of the cations, anions and other concomitant substance such as glucose, lactose, starch, talc etc., do not interfere in analysis and the tolerance limits of these and other ions are listed in Table 2.

Applicability of the method
The applicability of the method to assay pharmaceutical preparations was examined.To results of assay of available paracetamol preparations are summarized in Table 3.We prepared tablets of paracetamol in the laboratory and used for the assay.The results obtained compare favourably with the results obtained by the official method

Conclusion
The method is found to be simple, economical, sensitive and rapid.The Statistical parameters and the recovery study data clearly indicate the reproducibility and accuracy of the method.The recommended procedure is well suited for the assay and evaluation of drugs in pharmaceutical preparations to assure high standard of quality control.

Figure 1 .
Figure 1.Absorption spectra of paracetamol by proposed method

Table 1 .
Parameters for the spectrophotometric determination of paracetamol

Table 3 .
Determination of paracetamol by the proposed method Recovery (%, mean± R.S.D.) b a Trade name, b Average of five determinations, assayed as a percentage of label claim.R.S.D., relative standard deviation (n=5)