GEOCHEMISTRY

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GEOCHEMISTRY CLASS 4
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An acid is a proton (H) donor

An acid is monoprotic if it gives off 1
H Example hydrofloric Acid HF ? H F-
The extent to which an acid dissociates is given
by the equilibrium constant for the dissociation
which is known as the acid dissociation constant
designated as Ka.
Ka is often reported as pKa pKa - log Ka
For example for HF pKa 3.18 Hence aH aF-
10-3.18 aHF
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Sample problem What is the pH of a solution in
which 0.2 moles of HF is dissolved in 1 liter of
pure water.
Step 1 Develop the same number of equations as
we have unknowns
aH aF- 10-3.18 aHF
Equation 1 Equilibrium relationship
Equation 2 From mass balance Since all F-1
comes from dissociation of HF and the initial
HF 0.2 then HF 0.2 F-1
Equation 3 From mass balance If we assume that
almost all H comes from dissociation of HF And
that there is no additional sink of H then
H F-
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Note equilibrium relationship is in terms of
activities, mass mass balance equations in terms
of concentrations to make a system of equations
we need a relationship between ais and is.
aH ?HH and so on
Recall from Debye-Huckel Model
Log ?i -Azi2I0.5 1 BaiI0.5
And I ½ S i zi2
Where i concentration of I in moles per liter
Note that in order to calculate ?i we need to
know the concentrations of all of the charged
dissolved species but it is these concentrations
we are trying to calculate in the first place.
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Solution Begin by assuming that the solution
will be so dilute that I 0, then
Log ?i 0 or ?i 1 and ai i
After we calculate all of the i we will go back
and calculate I. If I is not approximately 0 we
will use I to calculate ?is .
We will then calculate new is and when we are
finished calculate a new I. We will compare the
new and the old I. If they are close we will
quit. If not we will use the new I to calculate
new ?is and will redo the calculations.
We then repeat
and repeat
and repeat
and repeat
and repeat
Until I no longer changes significantly.
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What is the pH of a solution in which 0.2 moles
of HF is dissolved in 1 liter of pure water.
aH aF- 10-3.18 aHF
Equation 1 Equilibrium relationship
Equation 2 From mass balance HF 0.2 then
HF 0.2 F-1
Equation 3 From mass balance H F-
And aH H, aF- F-, aHF HF
H2 10-3.18 0.2 H
Plugging these relationships in we get
Rearranging we get the quadratic polynomia
H2 10-3.18 H 0.2 10-3.18 0
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H2 10-3.18 H 0.2 10-3.18 0
Using the quadratic equation X - b /- ( b2
4ac)0.5 2a
We get H -10-3.18 /- ((10-3.18)2
-4-0.210-3.18)0.5 2
H either 0.0111 or -0.01183
Only positive concentrations are possible so
H 0.0111
And pH - log H - log (0.0111) 1.95
I ½ S i zi2 ½ ( 0.0111 12 0.011112)
0.0111
Should probably calculate activity coefficients
and redo the problem, but we wont.
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Acids that give off two protons are known as
diprotic acids
Diprotic acids will have two pKa pka1 for the
first dissociation and pKa2 for the second
dissociation.
Carbonic acid (H2CO3) is one of the geologically
most important examples
H2CO3 ? HCO3- H pKa1 6.35
HCO3- ? CO3-- H pka2 10.33
Acids that give off three protons are triprotic
and have 3 dissociation constants. Most
important geological example is phosphoric acid
H3PO4
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H2CO3 ? HCO3- H pKa1 6.35
Note from this equation aHCO3- 10-6.35/aH
aH2CO3
A similar equation can be derived from the
equation
HCO3- ? CO3-- H pka2 10.33
Thus it is relatively easy to calculate the
relative proportion of Carbonate species as a
function of pH
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An acid is a strong acid if it has a small pKa
and hence undergoes extensive dissociation. Most
important natural strong acid is sulfuric acid
H2SO4 pKa1 -3,
It forms by weathering of sulfides under
oxidizing conditions 4FeS2 8H2O 15O2 ?
2Fe2O3 8H2SO4
Pyrite
Hematite
Or by the dissolution of volcanic gasses in water.
Less important naturally occurring strong acids
are
Nitric acid HNO3 pKa 0
Hydrochloric acid HCl pKa -3
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Weak acids have relatively large pKa and hence
dissociate to a relatively small degree.
Most important naturally occurring weak acids are
1. Carbonic acid
2. Silicic acid
H4SiO4 pKa1 9.83, pKa2 13.17
It forms through the weathering of silicate
minerals MgSiO3 2 H H2O ? Mg
H4SiO4
enstatite
3. organic acids
Most important naturally occurring organic acids
contain the carboxylic group COOH
Most concentrated in waters in contact with
decaying organic material.
In most environments poorly characterized
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Bases General definition a proton acceptor More
restricted definition a substance that produces
OH- when it dissociates in water.
The extent to which an base dissociates is given
by the equilibrium constant for the dissociation
which is known as the base dissociation constant
designated as Kb.
Kb is often reported as pKb pKb - log Kb
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Example of a geologically important base
Amorphous Al (OH)3 pKb1 12.3
Al(OH)3 ? Al(OH)2 OH-
Al(OH)2 OH- 10-12.3
Al (OH)3