# PhreeqcUsers Discussion Forum

## Kinetics and Equilibrium => Equilibrium => Topic started by: Hydroman on April 23, 2018, 03:11:57 PM

Title: Percentage of HCO3- of total dissolved carbonate
Post by: Hydroman on April 23, 2018, 03:11:57 PM
Hello,

to improve my unterstanding of the carbonic acid system i like to model the percentage of HCO3- of total dissolved carbonate as function of pH (cp. Figure 5.5 in Geochemistry, Groundwater and Pollution).

At the beginnig of the investigation I have a water with pH 14, temp = 25 and a concentration of CO2 of maybe 10 moles.
Then, I put in HCl to lower the pH-value. Therefore I use the Keyword Reaction.

My problem is:

1) If i plot the mol of a species versus the ph-value (appendix 1) the diagram looks nearly like the diagram in the book (figuere 5.5), but the point where HCO3- and CO3-2 have the same concentration is not locacted at the ph-value of 10.3 (or 6.3 for HCO3- and H2CO3*). My model shows that the point where HCO3- and CO3-2 have the same concentration is something about 9.8.

2) In the second step I plottet the activities of the C-species versus the pH-value (appendix 2). Then the pH-values a right (6.3 and 10.3) but therefore the percentage is totally wrong.

3) In the output I see an other species: (CO2)2. Do I have to invoice that into my calculation.

4) Is this the right approch to use HCl or do I have to increase the CO2-concentration in the reaction-part?

So can anybody help me? It would be great.

Hydroman

TITLE pH

SOLUTION 1
ph 14
C(+4) 10 as CO2

REACTION 1
HCl        1
2 moles in 10000 steps

USER_GRAPH 1
-axis_titles            "pH-value [-]" "percentage of C-Spezies [-]"
-axis_scale x_axis      0 14 auto auto
-axis_scale y_axis      0 1 auto auto
-start
10 plot_xy -la("H+"), MOL("HCO3-")/totmole("C(4)"), color = Red, symbol_size = 0, line_width=3, y-axis = 1
20 Plot_xy -la("H+"), MOL("CO3-2")/totmole("C(4)"), color = green, symbol_size = 0, line_width=3, y-axis = 1
30 Plot_xy -la("H+"), MOL("CO2")/totmole("C(4)"), color = blue, symbol_size = 0, line_width=3, y-axis = 1
-end
-active                 true
END

Title: Re: Percentage of HCO3- of total dissolved carbonate
Post by: dlparkhurst on April 23, 2018, 06:06:47 PM
At 10.3 the activity of CO3-2 and the activity of HCO3- are equal. Similarly at 6.3 the activities of HCO3- and CO2* (referred to as H2CO3 or CO2 in different databases) are equal.

If you plot molalities of CO2, HCO3-, and CO3-2, they will differ from the activities because of activity coefficients, ion pairing (including the species (CO2)2, which is added simply to fit solubility data better), and activity of water.  In a H2O-C-Na-Cl system, and low C concentrations, the ion pairing is relatively small. Likewise, when ionic strength is low (say starting pH 12), the activity coefficients are closer to 1; activity of water is nearly 1; and activity will be closer to molality for a carbonate species.

In your system the effects are due to the large concentration of CO2, and the large intitial concentration of OH-. But, as you observe, the expected activities are equal at pH 10.3 and 6.3. The difference with molalities is due to high concentrations that increase the non-ideality of the solutions.
Title: Re: Percentage of HCO3- of total dissolved carbonate
Post by: Hydroman on April 24, 2018, 10:05:04 AM
Thank you for your help. I reduced the initial concentration, started at pH=12 and reduced the OH. You can see the result in Appendix 3. Now it looks good and I can learn more about the system. Tanks.