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Preformulation Parameters pKa and Solubility

Preformulation Parameters pKa and Solubility. PREPARED BY VAISHNANI BHAVIN M.PHARMA-I (Q.A.). Ionization Constant (pKa).

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Preformulation Parameters pKa and Solubility

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  1. Preformulation ParameterspKa and Solubility PREPARED BY VAISHNANI BHAVIN M.PHARMA-I (Q.A.)

  2. Ionization Constant (pKa) • The Ka value is a value used to describe the tendency of compounds or ions to dissociate and is also called the dissociation constant, the ionisation constant and the acid constant. • The pKa value is a value related to the Ka value in a log manner

  3. Definition of pKa and Ka • Ka is : [H +] [B-]/[HB], where B is the conjugate base of the acid HB • pKa = -Log10 (Ka)

  4. GI absorbtion of weakly acidic or basic is dependent on many factors like • Unionized form • pH at the site of absorption, • pKa, • lipid solubility of the unionized species.

  5. Henderson-Hesselbalch equations: • For acidic comp. pH = pKa + log [ionized drug] [un-ionized drug] • For basic comp. pH = pKa - log [ionized drug] [un-ionized drug]

  6. DETERMINATION OF pKa • Henderson-Hesselbalch equations • UV-VIS. Spectra • Potentiometric titration • Solubility measurements • HPLC techniques • Capillary zone electrophoresis • Foaming activity

  7. Typical titration curves ;

  8. Solubility • Defination: mass of solute that dissolves in a specific mass or volume of solvent at a given temperature. eg. 1 gm. Of NaCl dissolves in 2.786ml of water at 25°C

  9. Solubility-pH profile • plot of the solubility of the drug at various physiologic pH values. • Solubility may be improved with the addition of an acidic or basic excipient. eg. Solubilization of aspirin may be increased by addition of an alkaline buffer.

  10. General methods of increasing the solubility are: • addition of co-solvent. eg. Water, alcohol, glycerin • addition of complexing agent eg. Caffeine, EDTA • Chemical modification of the drug • pH adjustment • addition of surfactant. eg. Span, tween, SLS

  11. Referance (1) Pharmaceutical Preformulation And Formulation A Practical Guide from Candidate Drug Selection to Commercial Dosage Form Editor Mark Gibson Page no. 28,29 (2) Bentley’s Textbook of Pharmaceutics By E. A. Rawlins Page No. 19,20 (3) The science of Dosage Form Design, 2nd edition, Edited by M. E. Aulton, Page No. 115,116,117,118

  12. THANK YOU

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