Acid-base Dissociation

1
Acid-base Dissociation

• For any acid, describe its reaction in water
• HxA H2O ? x H A- H2O
• Describe this as an equilibrium expression, K
  (often denotes KA or KB for acids or bases)
• Strength of an acid or base is then related to
  the dissociation constant ? Big K, strong
  acid/base!
• pK -log K ? as before, lower pKstronger
  acid/base!

2
pKx?

• Why were there more than one pK for those acids
  and bases??
• H3PO4 ? H H2PO4- pK1
• H2PO4- ? H HPO42- pK2
• HPO41- ? H PO43- pK3

3
Geochemical Relevance?

• LOTS of reactions are acid-base rxns in the
  environment!!
• HUGE effect on solubility due to this, most other
  processes

4
Dissociation of H2O

• H2O ? H OH-
• Keq HOH-
• log Keq -14 log Kw
• pH - log H
• pOH - log OH-
• pK pOH pH 14
• If pH 3, pOH 11 ? H10-3, OH-10-11

Definition of pH
5
pH

• Commonly represented as a range between 0 and 14,
  and most natural waters are between pH 4 and 9
• Remember that pH - log H
• Can pH be negative?
• Of course! ? pH -3 ? H103 1000 molal?
• But whats gH?? Turns out to be quite small ?
  0.002 or so

6
Henderson-Hasselbach Equation

• When acid or base added to buffered system with a
  pH near pK (remember that when pHpK HA and A-
  are equal), the pH will not change much
• When the pH is further from the pK, additions of
  acid or base will change the pH a lot

7
BUFFERING

• When the pH is held steady because of the
  presence of a conjugate acid/base pair, the
  system is said to be buffered
• In the environment, we must think about more than
  just one conjugate acid/base pairings in solution
• Many different acid/base pairs in solution,
  minerals, gases, can act as buffers

8
Buffering example

• Lets convince ourselves of what buffering can
  do
• Take a base-generating reaction
• Albite 2 H2O 4 OH- Na Al3 3 SiO2(aq)
• What happens to the pH of a solution containing
  100 mM HCO3- which starts at pH 5??
• pK1 for H2CO3 6.35

9

• Think of albite dissolution as titrating OH- into
  solution dissolve 0.05 mol albite 0.2 mol OH-
• 0.2 mol OH- ? pOH 0.7, pH 13.3 ??
• What about the buffer??
• Write the pH changes via the Henderson-Hasselbach
  equation
• 0.1 mol H2CO3(aq), as the pH increases, some of
  this starts turning into HCO3-
• After 12.5 mmoles albite react (50 mmoles OH-)
• pH6.35log (HCO3-/H2CO3) 6.35log(50/50)
• After 20 mmoles albite react (80 mmoles OH-)
• pH6.35log(80/20) 6.35 0.6 6.95

10
Bjerrum Plots

• 2 D plots of species activity (y axis) and pH (x
  axis)
• Useful to look at how conjugate acid-base pairs
  for many different species behave as pH changes
• At pHpK the activity of the conjugate acid and
  base are equal

11
Bjerrum plot showing the activities of reduced
sulfur species as a function of pH for a value of
total reduced sulfur of 10-3 mol L-1.
12
Bjerrum plot showing the activities of inorganic
carbon species as a function of pH for a value of
total inorganic carbon of 10-3 mol L-1.
In most natural waters, bicarbonate is the
dominant carbonate species!
13
Carbonate System Titration

• From low pH to high pH