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THE CO 2 -H 2 O SYSTEM - I

THE CO 2 -H 2 O SYSTEM - I. Carbonic acid is a weak acid of great importance in natural waters. The first step in its formation is the dissolution of CO 2 (g) in water according to: CO 2 (g)  CO 2 (aq) At equilibrium we have:

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THE CO 2 -H 2 O SYSTEM - I

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  1. THE CO2-H2O SYSTEM - I Carbonic acid is a weak acid of great importance in natural waters. The first step in its formation is the dissolution of CO2(g) in water according to: CO2(g)  CO2(aq) At equilibrium we have: Once in solution, CO2(aq) reacts with water to form carbonic acid: CO2(aq) + H2O(l)  H2CO30

  2. THE CO2-H2O SYSTEM - II In practice, CO2(aq) and H2CO30 are combined and this combination is denoted as H2CO3*. It’s formation is dictated by the reaction: CO2(g) + H2O(l)  H2CO3* For which the equilibrium constant at 25°C is: Most of the dissolved CO2 is actually present as CO2(aq); only a small amount is actually present as true carbonic acid H2CO30.

  3. THE CO2-H2O SYSTEM - III Carbonic acid (H2CO3*) is a weak acid that dissociates according to: H2CO3*  HCO3- + H+ For which the dissociation constant at 25°C and 1 bar is: Bicarbonate then dissociates according to: HCO3- CO32- + H+

  4. THE RELATIONSHIP BETWEEN H2CO3* AND HCO3- We can rearrange the expression for K1 to obtain: This equation shows that, when pH = pK1, the activities of carbonic acid and bicarbonate are equal. We can also rearrange the expression for K2 to obtain: This equation shows that, when pH = pK2, the activities of bicarbonate and carbonate ion are equal.

  5. BJERRUM PLOT - CARBONATE • closed systems with a specified total carbonate concentration. They plot the log of the concentrations of various species in the system as a function of pH. • The species in the CO2-H2O system: H2CO3*, HCO3-, CO32-, H+, and OH-. • At each pK value, conjugate acid-base pairs have equal concentrations. • At pH < pK1, H2CO3* is predominant, and accounts for nearly 100% of total carbonate. • At pK1 < pH < pK2, HCO3- is predominant, and accounts for nearly 100% of total carbonate. • At pH > pK2, CO32- is predominant.

  6. Bjerrum plot showing the activities of inorganic carbon species as a function of pH for a value of total inorganic carbon of 10-3 mol L-1. In most natural waters, bicarbonate is the dominant carbonate species!

  7. SPECIATION IN OPEN CO2-H2O SYSTEMS - I • In an open system, the system is in contact with its surroundings and components such as CO2 can migrate in and out of the system. Therefore, the total carbonate concentration will not be constant. • Let us consider a natural water open to the atmosphere, for which pCO2 = 10-3.5 atm. We can calculate the concentration of H2CO3* directly from KCO2: Note that MH2CO3* is independent of pH!

  8. SPECIATION IN OPEN CO2-H2O SYSTEMS - II • The concentration of HCO3- as a function of pH is next calculated from K1: but we have already calculated MH2CO3*: so

  9. SPECIATION IN OPEN CO2-H2O SYSTEMS - III • The concentration of CO32- as a function of pH is next calculated from K2: but we have already calculated MHCO3- so: and

  10. SPECIATION IN OPEN CO2-H2O SYSTEMS - IV • The total concentration of carbonate CT is obtained by summing:

  11. Plot of log concentrations of inorganic carbon species H+ and OH-, for open-system conditions with a fixed pCO2 = 10-3.5 atm.

  12. Plot of log concentrations of inorganic carbon species H+ and OH-, for open-system conditions with a fixed pCO2 = 10-2.0 atm.

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