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# 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

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Introduction

Determination of pka of the given drug sample INTRODUCTION: Dissociation constant of a drug/compound is commonly refered to as pKa. It is a characteristic of drug. It is the estimation of ionized and unionized drug concentration at a particular pH. It is the pH at which 50% of drug is ionized and 50% is unionized. The dissociation constants of both acidic and basic drugs are expressed by pKa values. The importance of pKa is that one can easily conclude how much drug is ionized and unionized. Unionized drug is better absorbed. pKa can be calculated by following equation. pKa = pH +log(di -d/ d- dm) Where, di = absorbance of the ionized species. d = absorbance of solution tested. dm = absorbance of the unionized species. U.V. spectroscopy is widely used in pKa determination as we know lambda max is different for ionized and unionized drug or we can say that there is great difference between absorbance of ionized and unionized drug at a particular lambda. ...read more.

Middle

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. ...read more.

Conclusion

pH 296 nm 300 nm 303 nm Abs ?Abs/?pH Abs ?Abs/?pH Abs ?Abs/?pH 2 0.5229 - 0.5651 - 0.568 - 3 0.0215 0.5014 0.0571 0.508 0.1 0.468 4 0.01 0.0115 0.0158 0.0413 0.0437 0.0563 5 0.0436 0.0336 0.0448 0.029 0.1244 0.0807 6 0.2592 0.2156 0.5629 0.5181 0.5095 0.3851 7 0.436 0.1768 0.4736 0.0893 0.4318 0.0777 8 0.455 0.019 0.4683 0.0053 0.4541 0.0223 9 0.4824 0.0274 0.4909 0.0226 0.4707 0.0166 10 0.4788 0.0036 0.4708 0.0201 0.4399 0.0308 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. ...read more.

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