Triprotic System Example

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Triprotic System Example

• What is the pH of each 0.1 M solution prepared
  from dissolving each of the forms of histidine in
  water?

pK11.7
H2His
H3His2
pK26.02
pK39.08
HHis
His-
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Polyprotic Acids and Bases

• H3A is treated as if it were monoprotic
• H2A- is treated as an intermediate form
• HA2- is treated as an intermediate form
• A3- is treated as monobasic

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Which is the Principal Species?

• We often want to know which species of an acid,
  base, or intermediate is dominant for a
  particular set of conditions
• For a monoprotic system, the base (A-) will be
  dominant when pHgt pKa. When pHltpKa, the acid
  form (HA) will be dominant
• For a polyprotic system, the reasoning is
  similar, but there are multiple pKas to look at

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Example-Monoprotic

• What is the predominant form of ammonia in a
  solution at pH 7.0? Whats the approximate
  fraction?
• pKa for ammonium (NH4) is 9.24
• so, at pH9.4, NH3NH4
• pH 7.0lt9.2, so NH4 would be dominant
• Fraction in the form of NH4 is gt99 because pH7
  is over 2 orders of magnitude lower than the pKa

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Example-Polyprotic

• Arginine has three acid-base proton sites
• From pH1.82-8.99, H2Arg is dominant. What is
  the 2nd highest species at pH6.0? pH5.0?
• Above pH5.4, wed expect HArg, the conjugate
  base of H2Arg to be next highest. From
  pK1ltpHlt5.4, H3Arg2 would be the 2nd most
  important

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Fractional Composition
We can derive equations that give the
fraction of each species (a) for a given pH.
There are software programs available to draw
these curves out if you have pKas for the
species. Were not going to derive them here,
but you should be aware of what a means.
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Software Example for Arginine
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Arginine Fraction Plot
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Buffer Capacity for Arginine