Make diagrams for constant gas pressure:
log PCO2 = constant
Follow these simple steps:
1) Start DATABASE and click on C
(carbon) in the periodic table. Click on CO3 2−
in the list of available components. Both H+ and
CO3 2− will be
listed as selected components. Click on the pull-down menu
File / Save and Exit.
Click on the SPANA button to save the data file and to make a diagram.
Enter a file name, for example CO3 2-.
2) The SPANA main window will
appear. Diagrams made with these chemical components (H+
and CO32−) would have a
total concentration for carbonate (example
with [CO32−]T = 10 mM).
A diagram for constant logPCO2
requires that CO2(g) is one of the chemical components.
It is necessary to exchange
a component with a reaction products.
Click on the pull-down menu Run /
Modify chemical system. The modify window appears.
Click on CO3 2- to select it. Click on the
Exchange button:
A new window will appear listing the available reaction products. Click on
CO2(g) to select it and on the OK-button.
Back on the modify window click on the Save changes and exit
button. Give a name for the new data file, for example CO2(g).
Back on the SPANA main window: click on the pull-down menu
Run / Make a Diagram.
A new window appears to select what type of diagram is wanted.
Click on the Diagram button in the upper-left corner of the window.

The calculations will be done by a console program.
When the calculations are finished the diagram is displayed in a window:
Compare this diagram with that obtained when
CO32−
is a component instead of CO2(g).