BCHE 395-3 2013 pH Buffers Titrations

Segway into buers a buer is a soluyon of a weak acid

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Unformatted text preview: & Bases •  Common range of pKa values for biochemical acids, weak electrolytes –  from 2 to about 13 or 14 –  The smaller the value of pKa, the stronger the acid (think of the pH scale and a low pH) –  Opposite of Ka where a large value indicates a strong acid •  Extent of dissociaYon is determined by the acid dissociaYon constant Ka Conjugate Weak Acid- Base Pairs •  pKa = –log Ka (strong acid → large Ka → small pKa) Weak Acids & Bases Ka = [H+][A- [HA] •  To calculate pH, solve for [H+] [H+] = Ka [HA] [A- •  Take the log + log [H ] = log Ka + [HA] log [A–] www.purplemath.com/modules/logrules.htm The Henderson- Hasselbalch EquaYon MulYply by (- 1): + –log [H ] = Use definiYon of pH and pKa: –log Ka – [HA] log[A–] Soren Sorenson strikes again! pH = pK – log pH = pK + log ‘Have to know equaYon’ [HA] – [A ] [A-] Conj base [HA] Conj acid Henderson- Hasselbalch EquaYon [A-] pH = pK + log [HA] •  pKas are constants •  Allows you to calculate the concentraGon of each species of molecules in a dissociaGon equaGon at any given pH or concentraGons of some species •  Really good for test quesYons •  But for what? (segway into…) Buffers •  A buffer is a soluYon of a weak acid and its conjugate base that resists changes in pH upon addiGon of a strong acid or base –  Resistance is in both direcYons - either up or down •  Two important values define any buffer: •  1.) The pKa(s) of the weak acid •  2.) The total concentraYon...
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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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