*Carbon Dioxide - Carbonic Acid Equilibrium*

Carbonic acid is formed when atmospheric CO_{2} is dissolved in water. The
chemical equilibria are

1) Gas dissolution

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2) Carbonic acid formation

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3) Carbonic acid equilibrium

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In an open system, the partial pressure of CO_{2 }(g) is relatively
constant at *P*(CO_{2}) = 0.000355 Atmosphere.

The equilibrium expression is the Henry's Law equation (@ 25 °C)

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Thus

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Using this one may obtain

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Click here
to learn more about calculating the p*H* of water in equilibrium with air

*Dependent Carbonate Equilibrium*

Using carbonate mass balance and acid equilibrium expressions, the fractional
amounts of all carbonate species can be found as a function of [*H*^{+}]

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The *pH* dependent fractions are

Of interest is the *total carbonate* as a function of p*H*.

Remember that [*H*_{2}*CO*_{3}] is "fixed" by
the atmosphere in an open equilibrium system and

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Total carbonate increases with p*H*.

*Solubility of Calcium Carbonate*

(A Systematic Treatment)

*Chemistry*

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*Charge Balance*

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*Mass Balance*

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*Equilibrium Equations*

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**WOW!**

This is obviously a difficult system of equations to solve. Though it could be done, let's look for the approximations (AKA, the easy way out)

Click here to learn more about systematic treatment of chemical equilibrium

*Molar Solubility of Calcium Carbonate Approximations*

*Open system*; The trick to solving for carbonate equilibria is
to first identify the major species. That is, we should be able to look at all equilibria
and determine which carbonate species are in greatest abundance.

The major equilibrium process here is

By examining the alpha plot, we see that *HCO*_{3}^{-} is a
maximum at p*H*=8.3. This will help. Solving…

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*Closed System*: The real trick here, if there is one, is to
find the p*H*. With the p*H*, we can easily solve any carbonate problem with
the fractional amounts. For example, the calcium carbonate solubility is a function of the
fractional amount of carbonate as carbonate and the total amount of carbonate. In a closed
system at fixed p*H*

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Click here to learn about calcium carbonate solublity as a function of pH.

*This page was last updated Tuesday, August 03, 2004*