Carbonate Species vs pH (Open System)

Given is H2O in contact with the atmosphere (open CO2 system). What is the carbonate species distribution in the pH range between 1 and 10?

Titration Calculations

For a fixed CO2 partial pressure (0.00039 atm) and a given temperature (25), the equilibrium state of the open system is uniquely determined by a set of equations that yields pH = 5.61. In order to lower or raise the pH, you should add an acid (HCl) or a base (NaOH).

Addition of HCl. Start with pure water (button H2O) and select “Open CO2 System” with the default parameter pCO2 = 3.408. Then enter the desired pH value, say pH = 3, and set an arbitrary value for chloride (to mimic HCl addition), say 1 mM Cl.

Click on Start and perform charge-balance adjustment with “Cl”. The program promptly outputs the correct amount of Cl (which is equivalent to the molar amount of HCl). The output tables (including table Ions) display the complete speciation. Some data are listed in the table below.

Repeat the calculation for any other pH < 5.61. The table below gives you some idea. One calculation, one row.

Addition of NaOH. For pH > 5.61 repeat the above procedure, but use Na instead of Cl (in order to simulate NaOH addition). Examples are given in the bottom half of the table below.


Table. The table below summarizes the titration calculations at 25. It contains the amount of HCl or NaOH to achieve the target pH, the carbonate speciation, the ionic strength (I in mM), and the alkalinity (Alk in meq/L). Here, CO2 stands for the composite carbonic acid; this value remains constant throughout the whole titration.1

table of carbonate speciation as a function of pH

The pure CO2 system is marked by orange color.

Diagram. The diagram below displays the calculated data: DIC and all carbonate species in logarithmic scale as a function of pH. The dashed H+ curve is a straight line that results from H+ = 10-pH.

carbonate speciation in open CO2 system as a function of pH

Mass Conservation (Mole Balance). In contrast to the closed CO2 system, the DIC is no longer a constant:2

(1) DIC  =  CO2 + HCO3- + CO3-2 + NaHCO3 + NaCO3-  ≠  const

At pH ≈ 5.5 the DIC begins to increase exponentially.3

Footnotes & Remarks

  1. At pH ≤ 1 the CO2 value slightly deviates from 0.0133 mM, which is an artifact of the enhanced ionic strength (I = 0.122 M) that enter the activation corrections. 

  2. The difference between the open and closed CO2 system is also explained in the PowerPoint presentation. For a strict math description we refer to the review (2021) or lecture (2023). 

  3. Due to the small amount of Na and DIC the two aqueous complexes, NaHCO3 and NaCO3-, do not play any role at all. 

[last modified: 2023-10-31]