Transcript Karst Chemistry I - Illinois State University

Karst Chemistry I
Definitions of concentration units
• Molality m = moles of solute per kilogram of solvent
• Molarity [x]= moles of solute per kilogram of solution
• Molarity =
mg
L
formula weight
• Parts per million (ppm) – weight of solute per million
weight of solution (i.e. mg/L)
• 1% = 1 part per hundred or 10,000 ppm
• Milliequivalent (meq) = mg/L / equivalent weight
• Milligram equivalents per kilogram
(epm) = ppm / equivalent wt.
Basic Karst Chemistry
• Global Equation for weathering of
limestone
• CaCO3+CO2+H2O↔Ca2++2HCO3• This equation comprises three different
attacks on the calcite surface:
• Carbonic Acid
• Water
• Other acids
Dissociation
• In the presence of water Calcite will dissociate:
CaCO3↔Ca2++CO32• This reaction is described by solubility product
aCa aCO
constant
Kc 
2
aCaCO3
2
3
 aCa 2 aCO 2
3
• Where a is the activity of the dissolved species
and is closely related to concentration.
• The solubility product is a function of
temperature.
Dissociation
(cont.)
• The carbonate ions that form by the
dissociation hydrate when in contract with
water:
CO32- + H2O↔ HCO3-+OH1. H2O↔H+ + OH2. CO32- + H+ + OH- ↔ HCO3-+OH-
• This forms a mildly alkaline solution, raising
the pH and decreasing the carbonate
solubility, which is low in water.
Acid Dissolution – Carbonic Acid
•
•
Most carbonate minerals are readily
soluble in acid
The acid most important to karst
processes is carbonic acid (H2CO3),
formed by the dissolution of gaseous
CO2
1. CO2(g)↔ CO2(aqueous)
2. CO2(aqueous)+H2O↔H2CO3
Acid Dissolution – Carbonic Acid (cont.)
• This reaction is described by equilibrium constant:
K CO2 
a H CO
2
3
PCO 2
• Where PCO2 is the carbon dioxide partial pressure
expressed in atmospheres.
• What happens to the concentration of dissolved CO2
as the carbon dioxide pressure changes?
(White, 1988)
•
Neutral carbonic acid dissociates in solution to
form the bicarbonate ion, which in turn
dissociates to form the carbonate ion.
1. H2CO3 ↔HCO3-+H+
2. HCO3- ↔CO32-+H+
•
At the pH and Ionic strength of most
carbonate-bearing waters, which ion species is
dominate?
Bjerrum Plot
• The previous reactions are described by
equilibrium constants:
K1 
aHCO  aH 
3
aH CO
2
K2 
3
aCO 2 aH 
3
aHCO 
3
•
•
The ionization of carbonic acid releases hydrogen ions,
forming a mildly acid solution.
The connection between these reaction and the
hydration of the carbonate ion formed by dissociation of
carbonate minerals is the dissociation of water:
1.
•
•
•
With
H2O↔H+ + OH-
Kw 
aH  aOH 
a H 2O
 aH  aOH 
The activity of the carbonate ion links these reactions to
the solubility of calcite and dolomite.
The activity of carbonic acid ties the system to the
external carbon dioxide pressure.
• The net reaction for dissolution of calcite
by carbonic acid is:
CaCO3+CO2+H2O↔Ca2++2HCO3-
Activity coefficients
• The equilibrium constants for these various reactions are
written in terms of activities of the constituent species.
• Only the H+ activity is determined experimentally by
measuring pH
• Other ions are determined experimentally as concentrations,
since concentration is related to activity by the expression:
ai=gimi
where mi is molal concentration (moles of solute per liter of
solution).
Activity coefficient, gi
• gi connects the activity (a thermodynamically
idealized concentration) with the idealized
concentration.
• The gi can be calculated using the Debye-Hückel
equation
 logg i 
2
i
Az
0
I
1  ai B I
• Parameters A and B are constant for a given
temperature and for a given solvent
Values for A and B for aqueous solutions (Manov et al., 1943)
T(ºC)
0
5
10
15
20
25
30
35
40
A
0.4883
0.4921
0.4960
0.5000
0.5042
0.5085
0.5130
0.5175
0.5221
B
0.3241108
0.3249
0.3258
0.3262
0.3273
0.3281
0.3290
0.3297
0.3305
• zi is the formal charge on the ion and åi is a
parameter specific to each ion that effectively
measures ionic diameter.
Values for åi (Garrels and Christ, 1965)
Cation
åi
Anion
åi
Ca2+
610-8
CO32-
4.510-8
Mg2+
810-8
HCO3-
410-8
Na+
410-8
Cl-
310-8
K+
310-8
SO42-
410-8
H+
910-8
Ionic Strength (I)
• I is a measure of the total concentration of
charged species in solution, whether or not these
species take part in the reactions under
consideration
2
1
2
i i
I
m z
• The equation is valid up to ionic strengths of
about 0.1, it is generally adequate for karst waters
• In most karst waters there
will only be seven
constituents in significant
concentration.
• In most areas Na+, K+, Cl-,
and SO42- can be neglected,
but the should be measured
to be sure.
• Rule of thumb: I for
brackish water ~ 0.1 and for
fresh water ~ 0.01
Cation
Anion
Ca2+
HCO3-
Mg2+
Cl-
Na+
SO42-
K+
Measurements
• Characterization of karst waters requires
certain chemical analyses and
measurements:
– pH
– Temperature
– Conductivity
– Cation & Anion concentrations
– Alkalinity
– If possible CO2 in the gas phase
pH
• The hydrogen ion activity is expressed as
pH (pH=-log aH+)
• Can be measured directly with a pH meter
Temperature
• The temperature of karst waters can be very
stable, a change of 0.1 ºC can reveal a meaningful
fluctuation. Other systems can be highly variable.
16
14
12
Temperature (C)
10
8
6
4
2
0
Oct-00
Nov-00
Jan-01
Feb-01
Apr-01
Jun-01
Jul-01
Time
Sep-01
Nov-01
Dec-01
Feb-02
Apr-02