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I'm wondering what differences you'r making when you choose specificate over x and y, insted og no specificates in an stability pH-eh diagram. And wath the blue broken lines are??


Hi Hansen:


Activity diagrams created in Act2 and Tact2 show the predicted distribution of aqueous species and mineral phases for the main diagram species across the axes ranges you specify. You can also diagram the effect of adding additional chemical components to your system using the add button towards the bottom of the Basis pane:




In the above example, the SO4-- component is added to a system already containing the Fe-O-H components. The original diagram has the Fe component as the main species, Oxygen represented by the y-axis (Eh) and Hydrogen by the x-axis (pH).


If we add the SO4-- component, Act2 assumes by default that SO4-- is the only stable sulfur-containing aqueous species over the entire compositional range represented by the X and Y axis. In other words, SO4-- is stable from pH 0 to 14 and Eh -.75 to 1.25, which results in a diagram that looks as follows:




This is not a realistic representation of a geochemical system, as we expect reduced sulfur aqueous species at low Eh values and oxidized sulfur aqueous species at high Eh values. Similarly, we expect that the SO4-- component will speciate in response to changing pH conditions. Act2 gives a user the option to allow additional system components (SO4--) to speciate across axis components (pH, Eh):




  1. Speciate over X tells Act2 that the additional component speciates over the X-axis
  2. Speciate over Y tells Act2 that the additional component speciates over the Y-axis
  3. Speciate over X-Y tells Act2 that the additional component speciates over both axes


Since SO4-- varies over both pH and Eh, we'll choose the third option so that our setup looks as follows:




Compare the resulting diagram to the original:




The blue lines delineate the speciated SO4-- components - you can view the labels for the various sulfur aqueous species by checking the Plot - View - Mosaic Labels option (in version 6 and older, it looks a bit different- I think you choose the Config - Options - Mosaic Labels checkbox).


When you speciate additional components in an activity diagram, you are essentially overlaying multiple activity diagrams. These are also known as mosaic diagrams.


I hope that helps,


Tom Meuzelaar

RockWare, Inc.

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