Since CO2, HCO3- and CO3(2-), H+ and H2CO3 are in equilibrium, it is not possible to use chemical methods, because reacting any species to measure its concentration, will change the equilibrium. Only spectroscopic or electrochemical (ion-selective elctrodes) could be used, however, the problem is hard, because a part of H2CO3 is not dissociated, a part of CO2 is physically dissolved, and the equilibirum is affected by the CO2 concentration (partial pressure) above the solutions as well. I cannot see good and simple method to measure them in the solution, however, measurement of pH (it is easy), measurement of CO2 concentration in the ags phase (partial pressure above the solution) will give a chance to calculate teh species distribution in the solution (from Henry law and avaialable distribution data for CO2), and using thermodynamical constants, protonation constants and likes to calcualte the distribution of CO2-origined species (HCO3, CO32-, H2CO3) in the solution. If you know the all CO2 amount, and you know the aoiunt in the gas phase, the diffreence will be in the solvent. BAsed on Henry law, you can calculate the equilibrium CO2 content in the solution dissolved physically, the difference between this and the all solved CO2 will be the CO3, HCO3 and H2CO3 form. THe dissociation constants of H2CO3 are known, you can calculate the distribution among these species at the given pH.

I hope it helps you to solve the problem, and I am satisfied, that you can find distribution data for CO2 (gas and liquid phase) at various chloride concentrations to do these calculations.

Thank you László for your answer. I like your approach in analyzing the thermodynamics of a saline solution saturated with CO2, under let's say 1 bar. Now there is a hypothesis that increasing the chloride concentration restricts CO2 dissociation. It restricts it in a way that the concentrations of H2CO3, HCO3, and CO3 as a result decrease. This decreases their significance with the corrosion behavior in a way the corrosion rates become eventually lower when we increase the chloride concentration!

What I need now is a way that demonstrates that the CO2 hydration decreases with the chloride concentration - by a direct measurement, not with thermodynamic calculations.

Thank you again

Faysal: As László says, you can simply measure the pH. You still have to do SOME thermodynamic calculations though to relate the CO2 pressure and speciation to pH. This is however quite trivial with a software like PHREEQC v2 (free download at USGS). The effect of salts (ionic strength) on the solubility and speciation of CO2 is related to the aqueous activity coefficients of CO2, HCO3- and CO32-. Since molecular CO2 is fixed by the CO2 pressure through the Henrys law (at moderate pressures) and salts make the activity coefficients of molecular CO2 larger than unit, the solubility of CO2 is getting reduced with increasing salt. Depends on the pH, but at, lets say 1 bar CO2 in non-alkaline waters, molecular CO2 will totally dominate and the solubility (TIC) will be determined by the CO2 pressure and salt content.

Carbon Dioxide Transport

Bicarbonate Buffer:

Carbon dioxide produced in the tissue cells diffuses into the blood plasma. The largest fraction of carbon dioxide diffuses into the red blood cells. The carbon dioxide in the red blood cells is transported as: dissolved CO2, combined with hemoglobin, or as bicarbonate,(largest fraction).

The formation of bicarbonate ions, (HCO3- ) takes place by the following reactions:

Hydration of CO2: CO2 + HOH === H2CO3

Dissociation of H2CO3: H2CO3 === H+ + HCO3-

The H2CO3/HCO3- combination acts as the primary buffer of the blood. The hydration of carbon dioxide is a slow process but occurs rapidly in the red blood cells because a high concentration of the enzyme carbonic anhydrase catalyzes the reaction.

Bicarbonate diffuses out of the red blood cells into the plasma in venous blood and visa versa in arterial blood. Chloride ion always diffuses in an opposite direction of bicarbonate ion in order to maintain a charge balance. This is referred to as the "chloride shift".

The changes in concentration of CO2 or HCO3- ion can influence slight pH changes in the blood even though it is buffered. At the same time the concentration of H+ ions will influence the concentrations of CO2 and HCO3- ions.

Dear Faysal,

If I were in your position, first, I would like measure the partial pressure decreasing when the CO2 atmosphere is just contacting with the chloride containing salt solutions at various chloride concentration. It would give the amount of dissolved CO2, you can compare it with pure water. It should first to know.

The main difference in protonation/hydration of CO2 is probably will be derived from different activity of water and CO2 in the solution depending on Cl concentrations as Helge wrote it. pH measurement will give you information about the H+ concentration, and this will be in relationship with HCO3(-) and CO3(2-) concentrations, in first approximation. If you want to measure these concentration in experimentally and directly, I can see only one possibility because the equilibrium and the low concentrations, this is the measurement of concnetrations with ion-selective electrodes.

Good idea to directly measure the CO2 pressure. Just a small correction.. pH measures activity of H+, not concentrations, which is good as the activities depends on the salt/ionic strength.

Great inputs! I am waiting for this discussion to elaborate further and I will update you all.

Yes, Helge is right, we always measure activity - but the concentration is derived from this measured activity value. The more the chloride (salt) concentration the more difference will be observed between the activities and concentration, of course, and the activity coefficients have important role.

The measurements of pH by a standard glass electrode at high ionic strength, I, is a tricky busyness. The electrode is calibrated at I ~ 0.1 M. The liquid junction potential (LJP) changes with I. The reading of a standard pH meter at low and high ionic strength is a sum of pH and LJP. The PHREEQC v2 (free download at USGS) might be helpful but it's not friendly to use. I personally use a free software from www.acadsoft.co.uk/aq_solutions.htm‎

I also think that chloride concentration and partial pressure of CO2 are enough measurements to calculate the species distribution. Calculation might be more sensible because thermodynamic knowledge about these components are well-developed.

The equations needed are activity of CO2 with respect to partial pressure in gas, speciation of CO2 in water and activity of chloride. These should be solved simultaneously since activity coefficients will be affected by each other. For activity coefficients, you can use simple models (like Davies or Debye-Huckel) if it is dilute. It is with respect to total ionic strength. If it is concentrated, you may need more advanced equations for activity coefficients which involves interaction parameters of individual ion pairs (like Extended Uniquac or Pitzer). But these ions are well known so it will not be problem to find the parameters in activity coefficient models for these ions.