Equilibria 3

• What is solubility in the context of drugs?

  A physico-chemical property of molecules
• What factors influence the solubility of drugs?

  Balance of lipophilic and hydrophilic groups
• What does intrinsic solubility refer to?

  Solubility of the unionised form of the drug
• How do weak organic acids achieve partial solubility?

  Through hydrogen bond formation
• What happens to the ionisation of acids when pH increases?

  Ionisation increases, leading to increased solubility

  
• What is the significance of the nitrogen in Indomethacin?

  It is an amide group, classifying it as an acid

  
• What is the ionisation profile of a typical weak acid?

  Increased pH leads to increased ionisation

  
• What is the rule of thumb for acids regarding pH and pKa?

  If pH > pKa by 2 units, mostly ionised
• What happens to the ionisation of bases when pH decreases?

  Ionisation increases, leading to increased solubility

  
• What is the ionisation profile of a typical weak base?

  Increased pH leads to decreased ionisation

  
• What is the rule of thumb for bases regarding pH and pKa?

  If pH < pKa by 2 units, mostly ionised
• What is the Henderson-Hasselbalch equation used for?

  • To calculate % ionised of drugs
  • Relates pH, pKa, and ionised/unionised forms

  
• How is % ionised calculated for a weak acid?

  Using the formula: $$$\frac{100}{1 + 10^{(pKa - pH)}}$

  
• How is % ionised calculated for a weak base?

  Using the formula: $$$\frac{100}{1 + 10^{(pH - pKa)}}$

  
• What happens when a drug's pKa equals the pH of the solution?

  • 50% of the drug is ionised
  • 50% of the drug is unionised

  
• If a drug has the same pKa as the pH of the solution, what is the % ionised for an acid?

  50% ionised

  
• If a drug has the same pKa as the pH of the solution, what is the % ionised for a base?

  50% ionised