05-10_AB_equilibria_4_web

05-10_AB_equilibria_4_web - 1 A buffer is a solution that...

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Unformatted text preview: 1. A buffer is a solution that resists change in pH when a small amount of an acid or base is added, or the solution is diluted. 1. A buffer solution consists of a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid). An acetate buffer. A solution of acetic acid: + +- + → ← O 3 H COO 3 CH a K O 2 H COOH 3 CH ] COOH 3 CH [ ] O 3 H ][ COO 3 CH [ a K +- = Buffers ( and buffering capacity) (A SHORT REVIEW) ] COO 3 CH [ ] COOH 3 CH [ a K ] O 3 H [- = + ] COOH 3 CH [ ] COO 3 CH [ log a pK pH- + = We “nearly” obtained a Henderson - Hasselbach equation (H- H) • equilibrium concentration of the anion can be appr oximated by the analytical concentration of the salt • equilibrium concentration of the undissociated acid (of the CH 3 COOH form) by the analytical concentration of the acid- 2245- COO CH C COO CH 3 ] 3 [ COOH CH C COOH CH 3 ] 3 [ 2245 COOH CH COO CH a pK pH 3 3 log C C- + = The Henderson - Hasselbach equation (H-H) The maximum in buffering capacity is at pH = pK a , that is, when c HA = c A- . + +- + → ← O 3 H COO 3 CH a K O 2 H COOH 3 CH c HA = [HA] + [A- ] FRACTIONAL COMPOSITION DIAGRAMS (Harris, pp. 197-204) α , a fraction of species in a given form is the ratio of the equilibrium concentration “” of such a form to the analytical concentration of the acid or base, c HA or c B . α , fraction of dissociation for (weak) acids For a monoprotic acid , two forms are present, HA, A- . Then , we have 2 fractions of dissociation, α o and α 1 : α = [ ] HA c HA α 1 =- [ ] A c HA and ( the mass balance ): c HA = [HA] + [A- ] K a H A HA = +- [ ][ ] [ ] How does the fraction of dissociation depend on solution pH? The goal: α ( α 1 ) = f ( pH and K a ) or α ( α 1 ) = f ([ H + ] and K a ) ([H + ] for simplicity) For α (looking for [HA]; [A- ] : undesired, it needs to be eliminated; expressed as a function of [HA]): [ ] [ ] [ ] A K a H HA- = + c HA HA K a H HA HA K a H = + + = + +...
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This note was uploaded on 12/28/2010 for the course CHEMISTRY 222 taught by Professor Wieckowski during the Spring '10 term at University of Illinois at Urbana–Champaign.

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05-10_AB_equilibria_4_web - 1 A buffer is a solution that...

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