L10 - Electrolytes and Buffers

• Strong Acids/Bases

  Fully dissociated in aqueous solution
• Electrolytes

  Compounds that are ionised in solution and conduct an electric current

  
• Strong Electrolytes

  Completely ionised in solution.Strong BL acids, bases and their salts
• Weak Electrolytes

  partially ionised in solutionweak BL acids, bases
• Why is it important to control pH of a solution?

  to minimise drug degradation, improve pt comfort and compliance, improve efficacy of delivery
• Non-electrolytes


  Substances that don't ionise in water so don't conduct electric current(eg sucrose, glycerin, naphtalene, urea, steroids). Can dissolve in water, but don't ionise.
• Buffers


  Mixtures of weak acids or bases and their salts, resistant to pH changes.(the weak acid/base holds a common ion with its salt - eg CH3COONa and CH3COOH both have CH3COO- ion when dissociated.)
• Salts as strong electrolytes - eg

  - ephedrine HCl = salt of WEAK base (ephedrine) and STRONG acid (HCl)- sodium salicylate = salt of WEAK acid (salicylic) and STRONG base (NaOH)- NaCl = salt of STRONG acid and STRONG base, so gives pH 7 (neutral).

  
• Buffer Capacity


  Resistance of a buffer to pH changes.Good buffers have a high buffer capacity - resist pH changes well when small increments of acid/base are added.
• Buffer capacity equation


  buffer capacity = ∆m / ∆pHwhere ∆m = no. of moles of acid/base needed to change the pH of 1L of solution by an amount,
• Le Chatelier's Principle: concentration


  equilibrium shifts to counteract the change, eg if reactant concentrations are increased, equilibrium shifts right to increase the product concentrations and therefore reduce reactants.
• Buffer Equation (example with CH3COONa + CH3COOH)

  Ka = [H3O+] [CH3COO-] / [CH3COOH]Follows Le Chatelier's principle(Ka is momentarily disturbed as addition of salt increases ion concentration. H3O+ conc is then decreased to counteract this, with a corresponding increase in CH3COOH conc.)
• Common ion effect

  A decrease in the solubility of an ionic compound caused by the addition of a common ion.Ionisation of weak acid/base is repressed when common ion is added.
• Henderson-Hasselbalch equation for buffers: weak ACID and salt


  pH = pKa + log [salt] / [acid][salt] / [acid] = 10^(pH-pKa)
• Henderson-Hasselbalch equation for buffers: weak BASE and salt


  pH = pKw-pKb + log [base] / [salt]
• pH-dissociation Profiles


  Percentage ionization of weak acids and bases as a function of pH.pH = pKa at 50% ionisation

  
• Ionisation of a weakly acidic drug

  acidic drugs are completely UNionised up to 2pH units below pKa, and completely ionised at greater than 2pH units above pKa

  
• Ionisation of a weakly basic drug

  A basic drug is completely ionised at 2pH units below pKa, and completely unionised at 2pH units above pKa
• Calculating Buffer Compositions

  1. Find pKa of weak acid or base (may need to change from pKb)2. Calculate required molar ration of con. base and conj. acid to give the required pH3. Calculate fraction of con, base and conj. acid needed.4. Calculate moles of each compound to give the buffer concentration required5. Using Mr, calculate weight of each component required to formulate buffer