Question

Assuming a starting condition of dissolved inorganic carbon of 2000 μmol kg-1, total alkalinity of 2400μmol kg-1, temperature is 18C, salinity is 35.3, pressure is 0, and a closed system (no gas exchange with the atmosphere)

1. What are the resulting values of pH, saturation state with respect to aragonite, and bicarbonate's concentration if you add pure carbon dioxide gas to the system until you reach a seawater pCO2 of 820μatm?

2. What are the resulting values of pH, saturation state with respect to aragonite, and bicarbonate's concentration if you add HCl (a strong acid) to the system until you reach a seawater pCO2 of 820μatm?

Answer 1

The precipitation and dissolution of aragonite in seawater can be described by the chemical reaction:

Ca2+ + CO3 2- ↔ CaCO3

The saturation state (known as Ω) of seawater for a mineral is a measure of the thermodynamic potential for the mineral to form or to dissolve.

The saturation state of seawater with respect to aragonite can be defined as the product of the concentrations of dissolved calcium and carbonate ions in seawater divided by their product at equilibrium:
( [Ca2+] × [CO3 2-] ) / [CaCO3] = Ω

The dissolved carbonate species in seawater provide an efficient chemical buffer to various processes that change the properties of seawater.

For instance, the addition of a strong acid such as hydrochloric acid (naturally added to the ocean by volcanism), is strongly buffered by the seawater carbonate system. In distilled water, the addition of HCl leads to an increase of [H+] and [Cl− ] in solution in a ratio 1:1. This is not the case in seawater.