Spectrophotometric Determination of Mesalazine

Mesalazine (MESA) is determined by a simple and rapid visible spectrophotometric method. This method is depend on oxidative coupling reaction of mesalazine with histidine (HIS) in alkaline media using N-bromosuccinimide (NBS) as oxidizing agent to form a water soluble and stable product, that it has a maximum absorption at 459 nm. Beers law is followed in a concentration range of 50 to 750 μg /20ml (2.5-37.5 μg /ml) with a molar absorptivity of 3.3682×10 l.mol. cm. The recommended method has been successfully applied to the assay of MESA in pharmaceutical preparations.

The suggested method gives good results for estimation MESA in pure and drugs formulations by oxidation with N-bromosuccinimide then coupling the product with histidine in alkaline medium, the formed colored complex prove to be intense, water-soluble and stable.

EXPERIMENTAL Instruments
The UV Spectrophotometer was used (JascoV-630) and a pair of silica cells were used for all experiments, also the pH of solution was estimated by pH meter type HANA .

Analytical reagents were used in this work
Standard MESA solution, 500 µg.ml -1 . A 0.05g of MESA (Fluka) was dissolved in 10 ml of absolute ethanol and diluted to 100 ml distilled water using a volumetric flask. Histidine(HIS) solution, 0.01M. A 0.1551g of HIS was dissolved in 100 ml distilled water using a volumetric flask. N-bromosuccinimide(NBS) solution, 0.015M. Accurate weight of 0.2669 g of NBS was dissolved in 100 ml distilled water using a volumetric flask. Sodium hydroxide solution, 1N. A concentrated solution (10N, fluka) was diluted to 1000 ml distilled water in a volumetric flask then transported to plastic container. Pharmaceutical preparation. An accurate weight (equivalent to 0.05g MESA) of the powder for ten tablets was dissolved in 10 ml absolute ethanol and the volume completed to 100ml by distilled water in a volumetric flask.

General method and calibration graph
To 20 ml volumetric flasks, 0.1-2 ml of MESA solution (500 µg /ml) were transported, then 1 ml of HIS (0.01 M), 0.5 ml of NBS (0.015M) and 1 ml NaOH (1N) were added. The solutions were left to stand for 15 minutes before completing the volumetric flasks with distilled water. The measured absorbance's against the reagent blank were done at 459 nm and Beer ' s law was applied from 50-750 µg MESA / 20ml Fig. (1). From the eqution of straight line, the molar absorbtivity was 3.368×10 3 l.mol -1 .cm -1 .

Choosing of Oxidizing Agent
The best one of oxidizing agents which give the highest intensity was selected after studying different types of available oxidizing agents (Table 1)

The medium of Present Reaction
The primarily experiment has shown that reaction of MESA with HIS in presence of NBS needs alkaline medium, therefor various types of bases were studied (Table 2 ). Results in (Table 2) show that a certain alkaline medium was needed and NaOH gave the best results with volume equal to 1 ml (Table 3).

Effect of HIS Reagent Concentration
The effect of HIS amount on the color intensity of the dye has been studied. From the results , it can be observed that 1 ml of 0.01M HIS is the most suitable amount which gave the highest intensity of color and highest value of correlation coefficient (Table 4).

Effect of NBS Amount on Absorbance
The effect of various volumes of NBS solution (0.015M) on the color intensity has been studied. A 0.5 ml of NBS was the optimum amount which gave the highest intensity of color and highest value of correlation coefficient (Table 5).

Effect of Surfactant
The results in Table (7) showed that no effect of surfactant on the intensity (Table 7).

The Best Order of Addition
The optimum order of reagent addition be followed as given under the general procedure because it gives highest color intensity, otherwise a loss in color intensity occurred (Table 8).

The stability period
The experimental results (Table 9) showed that the absorbance remained constant at least for 4 hours.

Final absorption spectrum
When MESA was treated according to the suggested work, the absorption spectrum, showed a maximum absorption at 459 nm versus the blank solution Fig. (2).  The Nature of the Reaction Product Job ' s of the continuous variation (Delvie, 1997). Fig. (3) indicate that a colored product has a structure of 1:2 MESA to HIS reagent at 459 nm.

Fig. 3: Job' plot for MESA-HIS colored product
Therefore, the probable colored product have the below structure:

Application of the Method
To test the applicability of the present method, it has been applied to estimate MESA in drug formulation (tablet). On applying proposed procedure, a good recovery, accuracy and precision are obtained as shown in (Table 10).

Comparison of method
Table (11) shows the comparison between the various analytical parameters found in suggested work with other spectrophotometric methods. The proposed method is a simple, rapid, sensitive, more stable and can be used to determine MESA in drugs formulations. CONCLUSION A simple, sensitive and rapid spectrophotometric method for estimating MESA in aqueous solution has been carried out by the reaction of MESA with HIS in presence of NBS in alkaline medium. The suggested work has been successfully applied to determine MESA in pharmaceutical preparation (Tablets).