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.

Basic Karst Chemistry Global Equation for weathering of limestone CaCO 3 +CO 2 +H 2 O↔Ca 2+ +2HCO 3 - This equation comprises three different attacks on the calcite surface: Carbonic Acid Water Other acids

Dissociation In the presence of water Calcite will dissociate: CaCO 3 ↔Ca 2+ +CO 3 2- 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.

Dissociation (cont.) The carbonate ions that form by the dissociation hydrate when in contract with water: CO H 2 O↔ HCO 3 - +OH - 1.H 2 O↔H + + OH - 2.CO H + + OH - ↔ HCO 3 - +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 (H 2 CO 3 ), formed by the dissolution of gaseous CO 2 1.CO 2 (g)↔ CO 2 (aqueous) 2.CO 2 (aqueous)+H 2 O↔H 2 CO 3

Acid Dissolution – Carbonic Acid (cont.) This reaction is described by equilibrium constant: Where P CO 2 is the carbon dioxide partial pressure expressed in atmospheres. What happens to the concentration of dissolved CO 2 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.H 2 CO 3 ↔HCO 3 - +H + 2.HCO 3 - ↔CO 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:

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.H 2 O↔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.

The net reaction for dissolution of calcite by carbonic acid is: CaCO 3 +CO 2 +H 2 O↔Ca 2+ +2HCO 3 -

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: a i =  i m i where m i 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

Parameters A and B are constant for a given temperature and for a given solvent T(ºC)AB  Values for A and B for aqueous solutions (Manov et al., 1943)

z i is the formal charge on the ion and å i is a parameter specific to each ion that effectively measures ionic diameter. Cationåiåi Anionåiåi Ca 2+ 6  CO  Mg 2+ 8  HCO  Na + 4  Cl - 3  K+K+ SO  H+H+ 9  Values for å i (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

In most karst waters there will only be seven constituents in significant concentration. In most areas Na+, K +, Cl -, and SO 4 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 CationAnion Ca 2+ HCO 3 - Mg 2+ Cl - Na + SO 4 2- K+K+

Measurements Characterization of karst waters requires certain chemical analyses and measurements: –pH –Temperature –Conductivity –Cation & Anion concentrations –Alkalinity –If possible CO 2 in the gas phase

pH The hydrogen ion activity is expressed as pH (pH=-log a H+ ) 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.