Karst Chemistry I

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Karst Chemistry I
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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.

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Basic Karst Chemistry

• Global Equation for weathering of limestone
• CaCO3CO2H2O?Ca22HCO3-
• This equation comprises three different attacks
  on the calcite surface
• Carbonic Acid
• Water
• Other acids

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Dissociation

• In the presence of water Calcite will dissociate
• CaCO3?Ca2CO32-
• 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.

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Dissociation (cont.)

• The carbonate ions that form by the dissociation
  hydrate when in contract with water
• CO32- H2O? HCO3-OH-
• H2O?H OH-
• CO32- H OH- ? HCO3-OH-
• This forms a mildly alkaline solution, raising
  the pH and decreasing the carbonate solubility,
  which is low in water.

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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
• CO2(g)? CO2(aqueous)
• CO2(aqueous)H2O?H2CO3

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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?

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(White, 1988)
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• Neutral carbonic acid dissociates in solution to
  form the bicarbonate ion, which in turn
  dissociates to form the carbonate ion.
• H2CO3 ?HCO3-H
• HCO3- ?CO32-H
• At the pH and Ionic strength of most
  carbonate-bearing waters, which ion species is
  dominate?

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Bjerrum Plot
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• The previous reactions are described by
  equilibrium constants

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• 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
• 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.

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• The net reaction for dissolution of calcite by
  carbonic acid is
• CaCO3CO2H2O?Ca22HCO3-

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

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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

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• 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)
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• 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)
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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

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• 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

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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

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pH

• The hydrogen ion activity is expressed as pH
  (pH-log aH)
• Can be measured directly with a pH meter

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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.

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