Construction of New Coated Carbon Electrodes for Determination of Sildenafil Citrate Drug

In this research, new coated carbon electrodes were constructed and used for the determination of Sildenafil citrate (SILC) drug. The electrodes were constructed by preparing ion-pair for (SILC) with phosphotungstic acid (PTA) or phosphomolybdic (PMA) acid using dibutyl phthalate (DBP) as a plasticizer. These electrodes showed good sensitivity towards SILC with linear range of (1.010 -7 -1.010 -2 ) M, correlation coefficient, (0.9990), the life time (120) days, optimum temperature range (23-65) o C and optimum pH range (2-5) for both electrodes, limit of detection (5.05510 -8 and 5.11610 -8 ) M, slope (57.29 and 59.10) mV/decadefor SILC-PTA and SILC-PMA electrodes respectively. These electrodes were successfully applied for determination of SILC in pure and pharmaceutical preparation form(tablets) with recovery of not less than 98%.


Introduction:
Viagra (Sildenafil citrate SILC) is a white to off-white crystalline powder of molecular weight of 666.7 g/mole and its formula is C 28 H 38 N 6 O 11 S. Has chemical structure showed in Fig. 1. SILC is the first drug which approved for the treatment of erectile dysfunction, due to organic causes SILC is an effective and best drug for erectile dysfunction (ED) [1][2][3]. The studies show that Viagra successfully treats erectile dysfunction in men with co morbidities such as hypertension [4], diabetes [5], and depression [6].The side effects of SILC have been reported such as headache, congestion, flushing and dyspepsia [7][8][9].
Different methods have been reported for the determination of SILC drug in products and biological samples such as high-performance liquid chromatography (HPLC) [10,11], spectrophotometry [12][13], fluorometry [15], voltammetry [16] atomic absorption [17] and LC-ESI-MS [18,19]. The present work describes preparation, characterization and application of coated carbon electrodes for determination of Viagra in pure and pharmaceutical preparations. Performance characteristics of (SILC-PMA) and (SILC-PTA) coated carbon electrodes reveal low detection limit, high sensitivity, good selectivity, fast response time, very long life time and application with accurate determination of Viagra in pharmaceutical preparations.

Sample Preparation:
Four tablets of Jeansiagra 50 mg/tablet (the total tablets weight 0.9746 g) and four tablets of Vegamax 100 mg/tablet (the total tablets weight 2.1404 g) were finely powdered and mixed homogeneously, (0.2437) g of Jeansiagra and (0.5351)g of Vegamax powder was dissolved in (5) ml acetic acid (1.0 M) then adding amount of distilled water in a 100 mL beaker. The resulting solution was then filtered through Whatman filter paper (No. 42) and the volume was completed to the mark with water in a 100-mL volumetric flask. Solutions of concentration range of (1.010 -7 -1.510 -3 and 1.010 -7 -7.510 -4 ) M for Vegamax and Jeansiagra respectively were prepared by appropriate dilution with distilled water.

Preparation of Ion-Pairs:
The ion-pairs were prepared by mixing 50 mL of equimolar of (1.0 10 -2 ) M SILC to 50 mL of either PTA or PMA, a light orange and yellow precipitate of SILC-PTA and SILC-PMA were formed respectively. The precipitate was filtered through Whitman filter paper (No. 42), and washed several times with distilled water. The precipitate was left for 2 days to dry at room temperature.

Preparation of Coated Carbon Sensor:
The two sensors were prepared by using a pure carbon

Calibration Plot of the Fabricated Electrodes and Limit of Detection:
The

Effect of pH:
The effect of pH on the performance of the two electrodes was investigated using concentration of (1.0×10 -3 )M of SILC drug at different pH values (1-9). The pH value was adjusted by addition of small volumes of HCl and/or NaOH solution (0.1-1 M of each). The potential at each pH value was recorded. It is obvious Fig. 4 that the best pH range is from (2-5) for both electrodes where the potential is independent on pH. However the potential decreases gradually at pH values higher than 5 for (SILC-PMA) and (SILC-PTA). It is worth noting that at more than pH 7 a white precipitate of drug is formed and that may be the cause of potential decrease. At pH values lower than 2 the potential readings decrease which can be related to interference of hydronium ion [22,23].

Effect of Temperature:
The change of potential was measured by changing the temperature of the drug solution from (23-65) °C for concentration of (1.010 -2 ) M SILC. The relationship between the temperature and the measured potential was plotted. The results in Fig. 5 showed that the appropriate working temperature is (23-65) °C for both electrodes.

Selectivity of the Studied Sensors:
The selectivity coefficients (log K) of the studied sensors were determined applying separate solution method (SSM) [24,25].

Life time and response time:
For the determination of the storage stability for (SILC-PTA) and (SILC-PMA) electrodes, the potentiometric measurements were carried out at optimum conditions of (pH and Temperature) for several times every week. The performance of the electrodes have been

Precision and Accuracy:
To evaluate the accuracy and precision of the proposed method, pure and pharmaceutical drug solutions at two different levels (within the working limits) was analyzed, each solution was repeated for (5) times. Precision and accuracy were based on the calculated percent relative standard deviation (RSD%) and percent relative error (RE %). The value of RE% is not more than (-0.78) and value or RSD% is not more than (0.426). The results Table 3 show that these methods have reasonable precision and accuracy.

Analytical Applications:
Viagra was determined using a direct calibration method for both pure and  Table 3.

4.1Content Uniformity Assay Test:
To study the content uniformity assay test for pharmaceutical preparations of SILC drug, four individual tablets of Vegamax (100 mg) were placed in separate 100-mL beakers and dissolved in 100 mL distilled water to obtain (1.5 10 -3 ) M the potential of each solution was recorded using the new coated carbon electrodes (SILC-PMA and SILC-PTA). The mean potential was used to evaluate the content uniformity applying straight line of calibration graph. The results are shown in Table 4.

Conclusion:
The proposed method introduced an ion selective electrodes for the determination of SILC based on PVC matrix and DBP as a plasticizer and using PTA or PMA as active materials. These electrodes showed a successful application with low limit of detection and good recovery. The electrodes also showed long life time, fast response time, good selectivity and reasonable working concentration ranges.