Reef Chemistry Question of the Day #22 Calcium Carbonate Solubility

Randy Holmes-Farley · Oct 4, 2014

Reef Chemistry Question of the Day #22

Which form of calcium carbonate dissolves first in seawater as the pH is steadily lowered from 8.2?

A. Calcite
B. Aragonite
C. Struvite
D. Gypsum

Good luck!

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revhtree · Oct 4, 2014

All I can do is follow along and take a guess! I choose A. Calcite!

skinz78 · Oct 4, 2014

B for me!

Randy Holmes-Farley · Oct 5, 2014

And the answer is......................................B. Aragonite

Struvite is magnesium ammonium phosphate.
Gypsum is calcium sulfate.

Both calcite and aragonite are different crystal forms of calcium carbonate. It turns out that aragonite is a little easier to dissolve than is calcite.

This has more detail:

Calcium Carbonate in Seawater
One very important aspect of calcium is that, in seawater, it is actually supersaturated. What supersaturation means in this context is that given the right circumstances, it will precipitate as solid calcium carbonate. Of course, under many other circumstances, it doesn't do so, and the question of why is very important. First, some definitions.

The equilibrium constant expression for the dissolution of calcium carbonate is shown below:

(1) K = [Ca⁺⁺][CO₃^--]

where [Ca⁺⁺] is the concentration of calcium and [CO₃^--] is the concentration of carbonate. When K = Ksp* (the solubility product constant in seawater at any given temperature, pressure, and salinity), then the solution is said to be exactly saturated (equation 2)

(2) Ksp* = [Ca⁺⁺][CO₃^--] (saturation)

When the product of the concentration of calcium and carbonate exceeds the Ksp*, the solution is said to be supersaturated, and there is "too much" calcium and carbonate in solution (equation 3)

(3) Ksp* < [Ca⁺⁺][CO₃^--] (supersaturation)

When the product of the concentration of calcium and carbonate is less than the Ksp*, the solution is said to be undersaturated, and calcium carbonate can dissolve if put into the solution (equation 4)

(4) Ksp* > [Ca⁺⁺][CO₃^--] (undersaturation)

In normal seawater, equation 3 holds (supersaturation). The product of calcium and carbonate is about 3 times the Ksp* of aragonite and 5 times that of calcite (aragonite and calcite are different crystal forms of calcium carbonate; these differences will be discussed more in future articles). Consequently, calcium carbonate is poised to precipitate from seawater, given the opportunity.

Solubility of Calcium Carbonate in Marine Aquaria
There are several interesting results of the supersaturation of calcium carbonate in our tanks. One that many people quickly encounter in maintaining a reef tank is that heaters and other warm objects (e.g., pump impellers) seem to become coated with solids over time. Why is this?

This solid is principally calcium carbonate, though it likely has other ions in the crystal as well (magnesium and other metals, phosphate and other anions, etc.). The two reasons that this happens are readily understood, but the primary one is not at all obvious.

One contribution to the reason that calcium carbonate precipitates on heaters is simply that calcium carbonate in seawater is slightly less soluble as the temperature rises. Since calcium carbonate is already supersaturated, the effect is that when the water is warmed, the supersaturation of calcium carbonate rises, making precipitation more likely.

The supersaturation (omega) for calcium carbonate in seawater is given by:

(5) omega = [Ca⁺⁺][CO₃^--]/Ksp*

When omega = 1, the solution is saturated, and when omega >1, the solution is supersaturated. The higher omega is, the more likely precipitation is to take place. At S=35 and 1 atmosphere pressure, the Ksp* decreases slightly as the temperature rises. Millero ("Chemical Oceanography", 1996) provides a series of long equations for calculating Ksp* for both aragonite and calcite. For aragonite, the log Ksp* drops from -6.19 at 25 Â°C, to -6.23 at 40 Â°C to -6.44 at 80 Â°C. In relative terms, the Ksp* has gone from 1 to 0.91 to 0.55 over this temperature range. Likewise for calcite, the relative Ksp* has changed from 1 to 0.96 to 0.73 over this range.

Consequently, if a tank has a supersaturation of about 3 for aragonite and 5 for calcite at 25 Â°C (typical for seawater), then at 40 Â°C the supersaturation has increased to about 3.3 and 5.2, respectively. At 80 Â°C this supersaturation has increased to 5.4 and 6.8, respectively. Since the supersaturation has increased, the likelihood of precipitation has increased, and this increase is part of the explanation of why precipitation takes place on heaters.

pH Effects

The pH effect in the question arises via the carbonate concentration. As pH is lowered, the concentration of carbonate decreases as some of it is converted into bicarbonate:

CO₃^-- + H⁺ --> HCO₃^- as pH lowers

Since the Ksp* for aragonite is larger than the Ksp for calcite, aragonite begins to dissolve at higher carbonate concentration than calcite. That is, it becomes undersaturated and dissolves first as the pH is lowered.