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: Once in solution, CO 2 (aq) reacts with water to form carbonic acid: CO 2 (aq) + H 2 O(l)  H 2 CO 3 0

THE CO 2 -H 2 O SYSTEM - II In practice, CO 2 (aq) and H 2 CO 3 0 are combined and this combination is denoted as H 2 CO 3 *. It’s formation is dictated by the reaction: CO 2 (g) + H 2 O(l)  H 2 CO 3 * For which the equilibrium constant at 25°C is: Most of the dissolved CO 2 is actually present as CO 2 (aq); only a small amount is actually present as true carbonic acid H 2 CO 3 0.

THE CO 2 -H 2 O SYSTEM - III Carbonic acid (H 2 CO 3 *) is a weak acid that dissociates according to: H 2 CO 3 *  HCO H + For which the dissociation constant at 25°C and 1 bar is: Bicarbonate then dissociates according to: HCO 3 -  CO H +

THE RELATIONSHIP BETWEEN H 2 CO 3 * AND HCO 3 - We can rearrange the expression for K 1 to obtain: This equation shows that, when pH = pK 1, the activities of carbonic acid and bicarbonate are equal. We can also rearrange the expression for K 2 to obtain: This equation shows that, when pH = pK 2, the activities of bicarbonate and carbonate ion are equal.

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 CO 2 -H 2 O system: H 2 CO 3 *, HCO 3 -, CO 3 2-, H +, and OH -. At each pK value, conjugate acid-base pairs have equal concentrations. At pH < pK 1, H 2 CO 3 * is predominant, and accounts for nearly 100% of total carbonate. At pK 1 < pH < pK 2, HCO 3 - is predominant, and accounts for nearly 100% of total carbonate. At pH > pK 2, CO 3 2- is predominant.

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

SPECIATION IN OPEN CO 2 -H 2 O SYSTEMS - I In an open system, the system is in contact with its surroundings and components such as CO 2 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 p CO 2 = atm. We can calculate the concentration of H 2 CO 3 * directly from K CO 2 : Note that M H 2 CO 3 * is independent of pH!

SPECIATION IN OPEN CO 2 - H 2 O SYSTEMS - II The concentration of HCO 3 - as a function of pH is next calculated from K 1 : but we have already calculated M H 2 CO 3 * : so

SPECIATION IN OPEN CO 2 - H 2 O SYSTEMS - III The concentration of CO 3 2- as a function of pH is next calculated from K 2 : but we have already calculated M HCO 3 - so: and

SPECIATION IN OPEN CO 2 - H 2 O SYSTEMS - IV The total concentration of carbonate C T is obtained by summing:

Plot of log concentrations of inorganic carbon species H + and OH -, for open-system conditions with a fixed p CO 2 = atm.

Plot of log concentrations of inorganic carbon species H + and OH -, for open-system conditions with a fixed p CO 2 = atm.