Carbonate System

AlkalinityLecture 21

TOTH• TOTH is the total amount of component H+, rather than

the total of the species H+.o Every species containing H+ contributes positively to TOTH while every species

formed by subtracting H+ contributes negatively to TOTH.• For pure water: TOTH = [H+] - [OH–]

o Of course in pure water [H+] = [OH–] so TOTH = 0.• Now we dissolve CaCO3 to our solution and chose Ca2+

and CO32- as components.

o In near neutral pH, almost all the CO32- will react to form HCO3

–:CO3

+ + H2O = HCO3- + OH–

o some Ca2+ (though generally not much) will form Ca(OH)+, so our mole balance equation will be

TOTH = [H+] - [OH–] + [HCO3–] - [Ca(OH)+]

• Since we have not added [H+], TOTH remains 0.

TOTH• Now we dissolve CO2 in our solution:

H2O + CO2 ⇋ H2CO3o In near neutral pH, almost all the H2CO3 will react to form HCO3

–:H2CO3 = HCO3

- + H+

o If we chose CO2 as our component, HCO3

– = CO2 + H2O - H+

TOTH = [H+] - [OH–] - [HCO3–]

• This time HCO3- contributes negatively.

• Every species containing H+ contributes positively to TOTH while every species formed by subtracting H+ contributes negatively to TOTH.

• How we write the TOTH equation depends on how we defined components.

• Since we have not added [H+], TOTH remains 0.

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Conservation Equations• Remember our problem set problem on the dissolution of galena:

PbS + 2H+ ⇋ Pb2+ + H2S• We used mass balance to solve this.• A number of other reactions are possible:

H2S ⇋ H+ + HS–

HS– ⇋ H+ + S2-

Pb2+ + S2– ⇋ PbSaq

• However, we may still write a conservation equation• ΣS = PbSaq + H2S + HS– + S2–

• And if galena is the only source of Pb and S, then ΣPb = ΣS

• Mass balance, or conservation, equations can be useful constraints on our system. However, they often come with caveats, like the above (galena the only source of Pb and S).

Charge Balance• Aqueous solutions are always electrically neutral

(period, no caveats).• Thus the following constraint always holds:

• Often, however, charge and mass conservation equations end up being the same (since we can only add electrically neutral substances to our solution).

The Carbonate System• We now turn our attention to carbonate. • Water at the surface of the Earth inevitably contains dissolved CO2, either as a

result of equilibration with the atmosphere or because of respiration by organisms. CO2 reacts with water to form carbonic acid:

CO2 + H2O ⇄ H2CO3 • Some of the carbonic acid dissociates to form bicarbonate and hydrogen ions:

H2CO3 ⇄ H+ + HCO3–

• Some of the bicarbonate will dissociate to an additional hydrogen ion and a carbonate ion:

HCO3– ⇄ H+ + CO3

2–

• We can write three equilibrium constant expressions for these reactions:

Carbonate Minerals• Another important reaction in the carbonate

system is precipitation of carbonate minerals (mainly calcite) in veins in rocks, in soils (caliche), as shells, on your faucet, etc., and dissolution of carbonate, as in limestone caverns and sinkholes:

CaCO3 ⇄ Ca2+ + CO32-

Carbonate Speciation• Suppose now that we

have a known fixed total carbonate activity, e.g.,o ΣCO2 = H2CO3 + HCO3

– + CO32-

• Combining this with our equilibrium constant expressions, we can solve for the species activities as a function of pH, e.g.:

pH at fixed carbonate concentration

• If we have a solution with fixed ΣCO2 that is closed and contains no other dissolved species, the pH is also fixed.

• We can calculate pH by simultaneously solving charge and mass balance equations together with equilibrium constant expressions:

o (note typo in book -missing exponent 2)• We can guess our solution will be acidic, in which case we can

ignore CO32– and OH–, we means we can drop terms containing K2

and Kw. Therefore:

• This illustrates a key part of solving such equations – knowing when and how to simplify them by neglecting terms.

Equivalence Points• Particularly simple relationships occur when

the activities of two species are equal.• These are determined by equilibrium

constant expressions:• CO2 E.P.:

[H+] = [HCO3–]

• Bicarbonate E.P. • [CO3

2–] = [H2CO3]

• Carbonate E.P.• [OH–] = [HCO3

–]

• Two others [H2CO3] = [HCO3–], [HCO3

–] = [CO3

2-]

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Conservative and Non-Conservative Ions

• Conservative ions are those whose concentrations are not affected by changes in pH, temperature, and pressure, assuming no precipitation or dissolution.o In natural waters, the principal conservative ions are Na+, K+, Ca2+, Mg2+, Cl–, and

. o These ions are conservative because they are fully dissociated from their

conjugate acids and bases over the normal range of pH of natural waters. • Non-conservative ions are those that will undergo

association and dissociation reactions in this pH range. These include the proton, hydroxyl, and carbonate species as well as B(OH)4

–, H3SiO4–, HS–, NH4OH,

phosphate species, and many organic anions. • Virtually all the non-conservative species are anions, the

two principle exceptions being H+ and NH4OH.

Alkalinity• Alkalinity is an important and fairly readily

measured property of natural waters.• Alkalinity is a measure of acid-neutralizing capacity

of a solution and is defined as the sum of the concentration (in equivalents) of bases that are titratable with strong acid.

• Mathematically, we define alkalinity as the negative of TOTH when the components are the principal species of the solution at the CO2 equivalence point (which carbonate species there?) The acidity can be defined as the negative of alkalinity, and hence equal to TOTH.

Carbonate Alkalinity• Consider a solution containing a fixed total dissolved concentration of

CaCO3. At the CO2 equivalence point, H2CO3 is the principal carbonate species, so we choose our components as H+, H2O, CO2, and Ca2+.

H2CO3 = H2O + CO2

HCO3- = H2O + CO2 - H+

CO32- = H2O + CO2 - 2H+

• The proton mole balance equation is then:TOTH = [H+] - [HCO3

- ] - 2[CO32- ] - [OH–]

• The alkalinity is then:Alk = -TOTH = -[H+] + [HCO3

-] + 2[CO32- ] + [OH–]

• The above is also the carbonate alkalinity: the alkalinity due to the presence of carbonate ions.o In many solutions, carbonate alkalinity is very nearly equal total alkalinity. To be clear, we

sometimes call the alkalinity as defined previously Total Alkalinity.

Alkalinity & Non-Conservative Ions

• Charge balance requires:• Σcations - Σ anions = 0• Σcons. cations-Σcons. anions+Σnon-cons. cations-Σnon-cons. anions = 0• Σcons. cations-Σcons. anions= -Σnon-cons. cations+Σnon-cons. anions • The right hand side is equal to the alkalinity:• Alk = Σcons. cations-Σcons. anions

• = -Σnon-cons. cations+Σnon-cons. anions • This equation emphasizes an important point. The difference of the sum of

conservative anions and cations is a conservative property (they cannot be changed except by the addition or removal of components).

• Since alkalinity is equal to this difference, alkalinity is also a conservative quantity (i.e., independent of pH, pressure and temperature). Thus total alkalinity is conservative, even though concentrations of individual species are not.

The Tableau Method of Morel & Hering

• In determining alkalinity, we need to correctly write the TOTH equation.

• To do so, we need to decide on a proper system of components (as usual, we want the minimum number of components) and then decide which species are formed by adding H+ and which are formed by subtracting H+.

• The Morel & Hering method is to produce a matrix-like table (a tableau) with species across the top and components listed vertically.

• Entries in the Tableau are the stoichiometric coefficient of each component needed to form each species.

• Consider a solution containing H3SiO4–, H4SiO4, B(OH)3, B(OH)4

–, H2S, HS–, HPO4

2-, H2CO3, HCO3–, CO3

2–, and, H+, and OH–. We also recall that we always chose H2O as a component.

H+ H2O CO2 H2PO4–

H4SiO4

B(OH)3

H2S

H+ 1OH– -1H2CO3 1 1HCO3

– -1 1 1CO3

2– -2 1 1HPO4

2- -1 1H2PO4

– 1H3SiO4–

-1 1

H4SiO4 1B(OH)4–

-1 1 1

B(OH)3

1

H2S 1HS– -1 1

Alk = -TOTH = -{[H+] - [OH-] - [HCO3–] - 2[CO3

2-] - [HPO42-] -

[H3SiO4-] - [HS-]}

Titration Determination of Alkalinity

• We have learned how to calculate alkalinity, but it can also be measured by titration.

• Titration is the process of progressively adding a strong acid or base to a solution until a specified pH, known as an end-point, is reached.

• In the case of the determination of alkalinity, this end-point is the CO2 equivalence point, as the definition suggests.

• The analytical definition of alkalinity is its acid neutralizing capacity when the end-point of the titration is the CO2 equivalence point.

Titration Example• Consider a solution containing sodium carbonate

(Na2CO3). Because the carbonate ion can act as a proton acceptor, Na2CO3 is a base.o We assume ideal behavior, complete dissociation, and a volume of solution

large enough that the titration results in trivial dilution. • The charge balance equation during the titration is:

[Na+] + [H+] = [Cl–] + [ HCO3–] + 2[CO3

2- ] + [OH–]o Since the Cl– concentration is conservative, it will be equal to the total

amount of HCl added. • Using equilibrium constant expressions to eliminate

some species, e.g.,

• and noting mass balance requires 2[Na]=ΣCO2, we derive the following:

Titrating Alkalinity• This plot shows the how

carbonate species in a 5 mM Na2CO3 solution change with pH as HCl is added during titration.

• Notice how the HCl curve flattens at the equivalence points - pH changes rapidly with small additions of HCl.o This means we don’t have to hit the pH

of the E.P. with great precision to obtain the alkalinity.

o In practice, a solution indicator or pH meter would be used.

• The end-point is reached here at 10 mM HCl.

Calculated and Titrated Alkalinity

• In this example, titration yielded an alkalinity of 10 mM.

• Can we obtain the same result from calculation?TOTH = [H+] - [OH-] - [HCO3

–] - 2[CO32-]

• Charge balance is (before addition of HCl):[Na+] + [H+] = [HCO3

–] + 2[CO32-] + [OH–]

• Solving the two equations, we find:Alk = -TOTH = [Na+] = 10 mM