119_5_Buffers&Titrations_07F

119_5_Buffers&Titrations_07F - 1 V. Buffers and...

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Unformatted text preview: 1 V. Buffers and Acid/Base Titrations Harris: 167-176; Ex. 9 H, I Chap 11; Ex. 11 A,B 2 Buffers A buffer is a solution which doesnt change pH much when small amounts of acid or base are added, or when the solution is diluted. A buffer is a solution of a weak acid and its salt or a weak base and its salt. A solution on the titration curve of a weak acid with a strong base (or weak base/strong acid) is a buffer. 3 Buffers: Why They Work: Neutral weak acid and salt HOAc + NaOAc (acetic acid/sodium acetate buffer): Add a little HO , : aOAc + bHOAc + HO H 2 O +(a+ 29 OAc +(b- ) HOAc Add a little H 3 O + , : aOAc + bHOAc + H 3 O + H 2 O +(b+ )HOAc +(a- )OAc 4 Buffers: Why They Work Add base [ ][ ] [ ] [ ] [ ] [ ] = =- +- + OAc HOAc K O H HOAc OAc O H K A 3 3 A 3 A a V H O K , before adding alittle base, b V + & & & & & & = & & & & & & & & 3 A A A a a a V H O K K K b b b V +- & & & &- & & & & = = + + & & & & & & & 5 Buffers: Why They Work You can elegantly expand (a- )/(b+ ) about a/b using the standard formula for a Taylors expansion. You can divide the expression and collect the terms in . You can use a TI-89 to get the expansion to three terms: or you can use Excel or Maple for numerical calculations. 3 2 2 b b)x (a b b)x (a b a x,2,0 , x b x a taylor + + +- = +- 2 ... a a a b a b b b b b b b - + + =- + + + 6 Buffers: Why They Work a=1;b=2 % Diff a=1;b=1 % Diff a=2;b=1 % Diff 0.500 1.000 2.000 0.001 0.499 0.1 0.998 0.2 1.997 0.1 0.005 0.496 0.7 0.990 1.0 1.985 0.7 0.01 0.493 1.5 0.980 2.0 1.970 1.5 0.02 0.485 3.0 0.961 3.9 1.941 2.9 0.05 0.463 7.3 0.905 9.5 1.857 7.1 3 A H O a a K b b +- = + 7 Buffers: Why They Work Add acid [ ][ ] [ ] [ ] [ ] [ ] = =- +- + OAc HOAc K O H HOAc OAc O H K A 3 3 A 3 A a V H O K , before adding alittle acid, b V + & & & & & & = & & & & & & & & 3 A A A a a a V H O K K K b b b V + + & & & & + & & & & = =-- & & & & & & & 8 Buffers: Why They Work: neutral weak base and salt NH 3 + NH 4 Cl (ammonia/ammonium chloride): Add a little H 3 O + , , aNH 3 + bNH 4 + + H 3 O + H 2 O + (a- )NH 3 + (b+ )NH 4 + or add a little HO , , aNH 3 + bNH 4 + + HO H 2 O + (a+ )NH 3 + (b- )NH 4 + 9 Buffers: Why They Work NH 4 + /NH 3 [ ] [ ] [ ] [ ] [ ] [ ] 3 4 A 3 4 3 3 A NH NH K O H NH O H NH K + + + + = = 3 A A b b V H O K K , before adding a little acid a a V + & & & & & & & & = = & & & & & & & & & & 3 A A A b b b V H O K K K a a a V + + & & & & + & & & & = =-- & & & & & & & 10 Buffers: Why They Work NH 4 + /NH 3 [ ] [ ] [ ] [ ] [ ] [ ] 3 4 3 4 3 3 NH NH K O H NH O H NH K A A + + + + = = [ ] base, little a adding before , a b K V a V b K O H A A 3 = = + [ ]...
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119_5_Buffers&Titrations_07F - 1 V. Buffers and...

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