**
CHAPTER 7 – Pressure
Effect on Equilibrium Reactions**

**
Pressure Acting on Homogeneous Equilibria**

**Pressure Acting on
Heterogeneous Equilibria**

**Pressure Acting on Homogeneous Equilibria**

The
effect of pressure on equilibrium constants is of paramount
importance; sinking down into
the depths of the oceans,
pressure increases by 1 atm with
every 10 meters. As the
intermolecular distances between water molecules decrease slightly,
the density of liquid water increases accordingly. Therefore,**
**interionic
interaction and equilibrium constants become progressively altered in
relation to the**
**pressure itself. The**
**effects become
noticeable when pressure reaches hundreds of bars;
pH and solubility of calcium carbonate alter
to such an extent that
aragonite oversaturation,
and calcite at greater
depths, disappear and,
if formed, these salt readily re-dissolve.

As discussed in Section 4.4 on water density, we recall that water pressure is measured in 'bars' (1 atm = 1.01325 bar; 1 bar = 0.1 Mpa), and that the surface pressure of the sea is assumed to be zero.

The effect of pressure on
equilibrium constants can be calculated (Millero 1995) according to a
second order polynomial expression of the natural logarithm of the
ratio between Ki,P (the
value of i-esimal constant**
**at pressure P) and Ki,0 (the value of i-esimal constant
at reference zero pressure P)

The
constant value **R1**
is given by **R1**
= 83.131
cm^{3}
bar mol^{-1}
K^{-1},
whereas **ΔV**_{i}
is the molal volume change, and
Δki_{ }the
compressibility change. They are in turn deconvoluted in terms
of a second order polynomial
which, strictly speaking,
is only valid for
Salinity = 35.

**ΔV**_{i
}= a0 + a1·Tc
+ a2·Tc^{2}

**Δk**_{i}**
**= b0 + b1·Tc +
b2·Tc^{2}

The values for the
a and b parameters are taken from Millero
1979 and Millero 1995 and
are reported here
for each of the reactions, where Tc indicates the temperature in °C,
P the pressure in bar, and
T the __absolute__
temperature (T = Tc + 273.15) and finally **R1**
= 83.131.
In the formulas, * LnKi,0*
indicates the natural logarithm of

The effect of pressure alters the values for every equilibrium constant, and must therefore be properly accounted for in the program flow. Greater detail on this will be given in Chapter 8

In the appendix, code 002.bas is the general routine for the complete and simultaneous set of calculations. It can be simply downloaded from my web-site (www.oceanchemistry.info) and is ready to run. It requires a text file input (SeaWaterCalc.txt) from which all starting parameters are read, but the reader is strongly recommended to read the lines and the comments related to the pressure effects.

**
Pressure Acting on
Heterogeneous Equilibria**

Generally speaking, in heterogeneous
reactions, reactants are in different phases, like
solids, liquid solutions
or gaseous mixtures. One of the most relevant of such reactions is
the formation or dissolution of calcium carbonate (solid/solution),
according to its oversaturation
value, indicated by **Ω
**and discussed in Section
6.5.

Once formed biologically by
calcifying organisms
or by inorganic route, and
with a density greater
than 1, it eventually sinks into the
dark abyss. Due to increasingly high pressure, solid CaCO3 begins to
dissolve below a certain
depth**,**
referred to as the saturation horizon where **Ω**
is exactly equal
to 1**. **Dissolution
of the solid is not instantaneous,
and the downward flux continues to a depth where the solid particles
of calcium carbonate are completely dissolved. This depth is called
the carbonate compensation depth. If
the sea bottom does not reach such a depth, it becomes
undissolved carbonate sediment.
The two crystallographic forms of CaCO3, calcite and aragonite, have
different solubility products, the former being less soluble.
Therefore**,**
the saturation horizon and the compensation depth for aragonite
are at a
higher level compared to
calcite. Most calcifying organisms
(e.g. Coccolithophores) produce calcite, whilst coral reefs
are made of aragonite.

Solving the equilibria involved in CaCO3 formation with the algorithm described in the preceding sections of this book, (whose usage will be comprehensively described in Chapter 8) the oversaturation profiles at different depths can be calculated. Some of the results can be seen in the graph in Fig. 7.1. The curious reader, intent on modifying input parameters and looking for new results (in a “see what happens” procedure) is directed to Chapter 8.

Fig.
1 Oversaturation for calcite at different ppmCO2
versus pressure

In Fig. 1 oversaturation **Ω**,
is plotted against
pressure being temperature fixed at
4°C, which is the overall temperature for the
ocean's depths
below the thermocline (about 300 meters). The four colored curves
correspond to four concentrations of CO2
values (red = 280 pre-industrial value; cyan = 345; blue = 410 present day
value; green = 475 ppm). The
full line indicates calcite and the
dashed line,
aragonite. Below **Ω**=1
(light blue area) carbonates begin their dissolution process.

One topic frequently debated
today is the potential hazard for coralline reefs
of the rising concentrations of CO2,
through the reduction of ocean pH and carbonate ion concentration.
The effect of this, evident from Fig.2. is however compensated for
by an increase in oversaturation in warmer areas of oceans, where
calcifying organisms
and coral reefs
prosper. Global warming, estimated at
about 1°C from the beginning of the**
**twentieth century to the
present day,
also favours oversaturation and
thereby counteracts the effects of increasing CO2
content by
anthropogenic emissions. Therefore the two figures... should be
considered together to gain a complete picture.

Fig.
7.2 Oversaturation for calcite at 1 atm. versus temperature of
seawater