BCHE 395-3 2013 pH Buffers Titrations

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Unformatted text preview: t have two or more acidic protons •  Protons in polyproYc acids do not dissociate at the same pKa •  Released in sequence at different pKa values •  More complex YtraYon curves Phosphoric acid in three steps PolyproYc Acids •  TitraYon curve for phosphoric acid –  What are the landmarks? •  (1) At midpoint of curve, pH = pKa(1) (1) Henderson- Hasselbalch pH = pK + log [A- [conj. Base] [HA] [conj. Acid] - Henderson- Hasselbalch can trace YtraYon curves exactly. Know how much of each species (conj.acid, conj.base) at every point - At inflecGon (flat part) point of YtraYon curve, [A- = [HA], so log [A- /[HA] = log 1 = 0 and pH = pKa - At steepest point inflecYon, all of H+ in weak acid is “gone” (H2O) Ex. H- H and LacYc Acid •  LacYc acid is present in our muscles a}er strenuous exercise. What is the value of the raGo [A- /[HA] in a soluYon of lacYc acid at a pH of 7.2? •  CH3CHOHCOOH + OH- ⇔ CH3CHOHCOO- + H2O •  pKa = 3.86 •  pH = pKa + log [A- /[HA] a.  22 to 1, Base to Acid b.  218 to 1 •  7.2 = 3.86 + log [A- /[HA] so c.  2188 to 1 - /[HA] = 3.34 •  log [A d.  21888 to 1 •  [A- /[HA] = 10 3.34 •  = 2188/1 •  So at pH 7.2, there are ~2200 more molecules (or moles) of the conjugate base than of the acid So why do we care about understanding YtraYon curves? Buffer Systems •  Buffers are compounds that resist change in pH •  Very important in biological systems becau...
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This note was uploaded on 03/02/2014 for the course BIOCHEMIST 395 taught by Professor Potenza during the Spring '14 term at NMSU.

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