2. Drugs in Solution Chemistry
- Created by: RachelOkine
- Created on: 19-02-24 17:00
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- 2. Drugs in solution
- 2.1- Drugs in solution
- 2.1a) acid-base equilibria
- acidic drug (ibuprofen)
- pH= 1/2pKa - 1/2log(Co)
- basic drug (benzocaine)
- pH= 1/2pKw + 1/2pKa + 1/2log(Co)
- acidic drug (ibuprofen)
- 2.1b) acidic and basic drugs and their salts
- b salts: salt of weak acidic drug and strong base
- 2.1c) ionisation of drugs
- % ionised acids = 100/ 1+ 10^(pKa-pH)
- % ionised bases = 100/ 1+10^-(pKa-pH) = 100/ 1+10^(pH-pKa)
- General Rules
- weak acid
- if pHpKa = ionised
- weak base
- if pHpKa = un-ionised (free base)
- weak acid
- amphoteric drugs
- ordinary vs zwitterionic
- 2.1a) acid-base equilibria
- 2.2- Drugs partition
- 2.2 b) pH Partition Hypothesis & Absorption
- explains the extent of drug absorption from the GI tract
- drugs are absorbed by transcellular passive diffusion
- onlt lipid soluble drugs (acidic or basic drugs in un-ionised form are absorbed from GI)
- explains the extent of drug absorption from the GI tract
- lipophilicity - influences bioavailability following oral administration
- 2.2a) partition coefficient- P or log P measures the way a drug distributes itself between two immiscible solvents
- greater value of log P , higher the lipophilicity of drug
- immiscible solvents: oil (organic phase) + water (aqueous phase)
- partition of non-ionisable drugs
- at equ: drug concs remain constant
- log P ideal range: 2.0-4.5 = good absorption and distribution | aqueous and lipid solubility
- partition of ionisable drugs
- pH of aq phase must be adjusted such that drug is unionised
- 2.2 c) distribution coefficient (D or log D)
- ratio between the Udrug in lipid phase/ Idrug + Udrug concs in aqu phase
- 2.2 b) pH Partition Hypothesis & Absorption
- 2.3- Buffers
- aqu sol consisting of mixture of weak acid + its conj base OR weak base + its conj acid
- contains both A and B in equ
- How it works:
- addition of acid: if HCl added, weak base reacts with H3O+ from strong acid to form weak acid and water
- addition of base: if NaOH added, weak acid gives up proton to transform base into water and conj base
- addition of acid: if HCl added, weak base reacts with H3O+ from strong acid to form weak acid and water
- H-H equation
- pH= pKa + log[B]/[A]
- buffer capacity = ability to resist changes in pH
- depends on its molarity they inc together) and pH of sol
- max buffer capacity is when pH of sol = pKa of buffer
- depends on its molarity they inc together) and pH of sol
- 2.1- Drugs in solution
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