BIOC 2300 Lecture Notes - Lecture 3: Titration Curve, Ph, Conjugate Acid
3 May 2018
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3 – Acid Base Chemistry
January 8th, 2016
Water can ionize: it’s a very weak acid (or base)
• Acid and bases are substances that can donate and accept a proton (respectively)
• Water can partially ionize into H+ ions and OH- ions
o The former exists as hydronium ions (H3O+)
• The ionization can be expressed as an equilibrium
o [H2O] = ~55.5 M
o Keq can be converted to Kw
o Kw = 10-14
o Pure water is neutral: [H+]=[OH-]=10-7 M
o If [H+]>10-7 Acidic
The pH scale:
• [H+] amd [OH-] are expressed as pH and pOH
• pH = -log[H+]
• pH + pOH = 14
• Therefore, as H+ decreases, OH- increases
An acid’s tendency to ionize is defined by its pKa
• Proton dissociation from a weak acid produces its conjugate base *& the other way
around
• pKa = -log(Ka)
• Ka = [H+]*[A-]/[HA]
• The lower the pK value, the stronger the acid
• Some substances have multiple ionizable groups and thus pK values
The pH of an acidic solution can be related to its pK.
• HA H+ + A-
• [H+] = Ka ([HA]/[A-])
• Henderson-Hasselbalch Equation:
pH = pK + log([A-]/[HA])
o The titration curve of an ionizable group is described by the logarithmic nature of
the H-H equation
o Note that when pH=pK, the ratio of [A-]/[HA]=1
o Note that [A-]/[HA] ratio is 10:1 and that 1:10 at pH values one unit above and
below pK value
o Note that the curve is very shallow where pH~pK this is the buffer region
which resists pH change upon an acid or base addition
o **buffers are extremely important in maintaining homeostasis
Buffer Scenario #1 – add 10mM HCl to pure water
• strong acids (e.g. HCl) and bases (e.g. NaOH) completely dissociate in water
• so, adding 10mM to HCl would result in 10mM=.01 M H+ and thus a pH of 2
• Therefore, water is a terrible buffer
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