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 =

• 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.

weightformulaL

mg

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 constant

• Where a is the activity of the dissolved species and is closely related to concentration.

• The solubility product is a function of temperature.

23

2

3

23

2

COCaCaCO

COCac aa

a

aaK

Dissociation (cont.)• The carbonate ions that form by the dissociation

hydrate when in contract with water:CO3

2- + H2O↔ HCO3-+OH-

1. H2O↔H+ + OH-

2. 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:

• 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?

2

32

2

COP

aK COHCO

(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- ↔CO3

2-+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:

32

3

1

COH

HHCO

a

aaK

3

23

2

HCO

HCO

a

aaK

• 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. H2O↔H+ + OH-

• With

• 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.

OHHOH

OHHw aa

a

aaK

2

• 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=imi

where mi is molal concentration (moles of solute per liter of solution).

Activity coefficient, i

• i connects the activity (a thermodynamically idealized concentration) with the idealized concentration.

• The i can be calculated using the Debye-Hückel equation

IBa

IAz

i

ii 0

2

1log

• Parameters A and B are constant for a given temperature and for a given solvent

T(ºC) A B

0 0.4883 0.3241108

5 0.4921 0.3249

10 0.4960 0.3258

15 0.5000 0.3262

20 0.5042 0.3273

25 0.5085 0.3281

30 0.5130 0.3290

35 0.5175 0.3297

40 0.5221 0.3305

Values for A and B for aqueous solutions (Manov et al., 1943)

• zi is the formal charge on the ion and åi is a parameter specific to each ion that effectively measures ionic diameter.

Cation åi Anion åi

Ca2+ 610-8 CO32- 4.510-8

Mg2+ 810-8 HCO3- 410-8

Na+ 410-8 Cl- 310-8

K+ 310-8 SO42- 410-8

H+ 910-8

Values for åi (Garrels and Christ, 1965)

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

• The equation is valid up to ionic strengths of about 0.1, it is generally adequate for karst waters

22

1ii zmI

• In most karst waters there will only be seven constituents in significant concentration.

• In most areas Na+, K+, Cl-, and SO4

2- 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.

0

2

4

6

8

10

12

14

16

Oct-00 Nov-00 Jan-01 Feb-01 Apr-01 Jun-01 Jul-01 Sep-01 Nov-01 Dec-01 Feb-02 Apr-02

Time

Te

mp

era

ture

(C

)