APPLIED CHEMISTRY DISOCIATION CONSTANT PKa

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APPLIED CHEMISTRY DISOCIATION CONSTANT- PKa
Mark Welliver CRNA MS
Mark Welliver Slide 2006
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Definition

• Drugs which dissociate (ionize) (give up or
  accept an electron (H)) do so in varying amounts
  at varying pHs
• The pH at which equal amounts of ions and ions
  dissociate is the pKa.
• pKa (the dissociation constant) is the pH at
  which 50 of a drug is ionized and 50 is
  unionized.
• Different drugs may have different pKas

Mark Welliver Slide 2006
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Drug x pKa 7
100
50/50

0
14
pH
7
Mark Welliver Slide 2006
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Drug x pKa 9
100
50/50

0
14
pH
7
9
Mark Welliver Slide 2006
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Drug y pKa 7
100
50/50

0
14
pH
7
Mark Welliver Slide 2006
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Why the different curve direction?

• The area under the red curve represents the
  logarithmic function of the H concentration (the
  amount of ionization at different pHs)
• Different drugs have more or less ionization at a
  given pH

Mark Welliver Slide 2006
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Ionization of Basic Drug x
w/pKa 7
100
50

0
14
7
pH
Mark Welliver Slide 2006
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Ionization of Basic Drug x
w/pKa 9
100
50

0
14
7
Mark Welliver Slide 2006
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Ionization of Acidic Drug x
w/pKa 7
100
50

0
14
7
Mark Welliver Slide 2006
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Henderson-Hasselbach Equation

• This equation calculates the degree (amount) of
  ionization of a drug based on its dissociation
  constant. It is a logarithmic function
  (logarithmic by Hasselbachs choice not mine)

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• Henderson Equation H x HCO3- K x
  CO2 x H2O
• Henderson-Hasselbach Equation pH pK log
  ( HCO3- / CO2 )
• Same equation except different measurement units
• H in moles/L vs pH on 0 to 14 scale
• K vs pK (log of K)
• Simple multiplication vs logarithmic
  function

Mark Welliver Slide 2006
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Reality Check

• Do we as anethetists need to know the exact
  amount of ionized or unionized drug?
• What do ionized drugs look like?
• What do unionized drugs look like?
• What is the active form of a drug?
• Does ionized/unionized really matter?

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3 Must Knows for Understanding a Drugs Ionization
Action

• What is the pKa
• Is the drug an acid or base
• What is the pH of the environment

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Putting it all Together

• pKa 50/50 point (pH at which 50 ionized, 50
  un-ionized)
• Drug an Acid or a Base? Not its commercially
  prepared vial pH but the drugs characteristic of
  acidity or alkalinity. Does the drug accept or
  donate H.
• Ie. Weak acid drug- STP, weak base-local
  anesthetics
• Environment Acidic pH or Basic pH
• Ie. pHlt7 acidic,
• Birds of a feather flock together

Mark Welliver Slide 2006