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[OLD] Activity-activity plots using nonbasis species


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From: James Laurinat

Subject: Activity-activity plots using nonbasis species

I have attempted to use act2 to plot activity-activity diagrams using nonbasis species, Al(OH)4-/H+ instead of Al+++ or Al+++/H+ versus HSiO4--- instead of SiO2(aq). Activities for both the stability field and superimposed scatter data appear to be off by many orders of magnitude. Data that matched up well with the stability field on a Al+++/H+ versus SiO2(aq) plot were off by about 45 orders of magnitude on the aluminum axis and 30 orders of magnitude on the silicon axis. In both cases, the scatter data exhibited higher activities than the corresponding locations on the stability field. The thermo.dat data set was used to plot these diagrams. Prior to making the diagrams, the activities of HSiO4--- were added to the aqueous species list of this data set. Scatter data were entered in terms of the basis species H2O, H+, SiO2(aq), and Al+++. Activities of SiO2(aq) and Al+++ were calculated from the known silicon and aluminum concentrations for the scatter data and equilibrium distribution equations. Is there any way to generate correct activity-activity diagrams using nonbasis species? Does this require an addition of the species Al(OH)4- and HSiO4--- to the basis species list? If so, what should be entered for ion size? How would this impact the use of the related basis species Al+++ and SiO2(aq)?

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