Determination of pka of the given drug sample. The importance of pKa is that one can easily conclude how much drug is ionized and unionized. Unionized drug is better absorbed

MATERIALS & METHODS:

Materials:

Apparatus: 10 ml volumetric flasks, beaker, pipette (10 ml),spatula.

Chemicals: 0.2 M Sodium hydroxide, 0.2 boric acid, 0.2 M potassium hydrogen phthalate, 0.2 M hydrochloric acid, 0.2 M potassium chloride, distilled water, test drug.

      Instruments: Weighing Balance, JASCO UV-Visible Spectrophotometer, pH meter.

Preparation of buffers:

Following buffers were prepared as per IP 2007.

(1) pH 2 : 5 ml of 0.2 M potassium chloride + 1.3 ml 0.2 M hydrochloric acid

(2) pH 3 : 5 ml of 0.2 M potassium hydrogen phthalate + 2.23 ml of 0.2 M hydrochloric          

                   acid

(3) pH 4 : 5 ml of 0.2 M potassium hydrogen phthalate + 0.01 ml of 0.2 M hydrochloric          

                   acid

(4) pH 5 : 5 ml of 0.2 M potassium hydrogen phthalate + 2.26 ml of 0.2 M Sodium    

                hydroxide

(5) pH 6 : 5 ml of 0.2 M potassium hydrogen phthalate + 0.56 ml of 0.2 M Sodium    

                hydroxide

(6) pH 7 : 5 ml of 0.2 M potassium hydrogen phthalate + 2.9 ml of 0.2 M Sodium    

                hydroxide

(7) pH 8 : 5ml of 0.2 M boric acid +  5 ml of 0.2 M potassium chloride + 0.39 ml of

                 0.2 M Sodium hydroxide

(8) pH 9 : 5ml of 0.2 M boric acid +  5 ml of 0.2 M potassium chloride + 2.08 ml of

                 0.2 M Sodium hydroxide

(9) pH 10 : 5ml of 0.2 M boric acid +  5 ml of 0.2 M potassium chloride + 4.37 ml of

                 0.2 M Sodium hydroxide

Preparation of primary stock: 10 mg of drug was accurately weighed and dissolved in 10 ml of 7:3 = methanol: water mixture as solvent to get concentration of 1 mg/ml.

Preparation of secondary stock:        

2 ml of the primary stock was taken and diluted to 10 ml with 7:3 = methanol: water mixture as solvent to get concentration of  200 mcg/ml.

Procedure for the determination of  pka of given drug: 

1. 1 mL ( 200 mcg/ml) of secondary stock solution was taken in each of the 9 tarson tubes.
2. They were diluted to 10 mL with each of the pH solutions to get a concentration of 20 mcg/ml.
3. All the solutions were immediately scanned in UV range.
4. All the spectrums were overlayed and the suitable wavelength for determination of pka was identified.
5. The absorbance values at that wavelength were noted and pka was determined from the graph between absorbance and pH.
6. Derivative graph was plotted by taking (∆Abs/∆pH) vs. pH.

RESULTS AND DISCUSSION:

The following table gives the absorbance values at 296 nm, 300 nm and 303 nm at different pH values.

Plot of absorbance vs pH and its derivative graph (∆Abs/∆pH vs pH) were ploted at selected wavelengths as shown below.

296 nm:

         

300 nm:

     

303 nm :

     

The peak in the derivative graph corresponds to the pKa of the drug since it is the point at which there is maximum change in absobance with a small change in pH.    

The average of three pka was calculated as:

pka = (6 +6+6)/3

pka = 6

CONCLUSION :

        The pKa of the given drug was found to be 6.