Geochemist's Workbench Support Forum

# 3 questions on ACT2

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Hi, I have 3 basic questions about ACT2:

(1) If I want to plot a ACT-ACT diagram on Cu, is that true that for the diagram species I can only input

total activity instead of molality? I see that it only gives me the choice of activity or log activity.

(2) If I input the total activity of 10^-3 for Cu, is the 10^-3 including both the dissolved and solid phases?

(3) How is the boundary between aqueous species and solid phases defined in ACT2? Garrels and

Christ, defined the boundary as the point where the “sum of the activities of the ions in equilibrium

with the solid exceeds some chosen value”. They chose 10^-6 as a default value on the basis that if it is less

than this value, the solid will tend to behave as an immobile constituent in the environment.

My question is whether the boundary is defined following Garrels and Christ (boundary at total dissolved Cu to be 10^-6)

or to be the line where mineral precipitation begins?

Thanks,

Peng

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Hi, I have 3 basic questions about ACT2:

Hi Peng:

1) Indeed, since this is an activity diagram, all species concentrations must be input as activities.

2) If you specify an activity for Cu++, you are constraining the non-ideal concentration (or activity) for the dissolved Cu++ species.

3) Any boundary in Act2 is simply the mass action equation between the species/minerals on either side of the boundary written in terms of the axis species. For example, on a Quartz solubility diagram with log SiO2 activity as the Y-axis and -log H+ activity (pH) as the X-axis, the boundary between dissolved silica and Quartz is expressed by the reaction:

Quartz = SiO2(aq)

Which has the mass action equation:

K = aSiO2(aq)

From the thermodynamic database, the log K value at 25C for this reaction is -3.9993. Substituting for K and aSiO2, and switching to log scale, we come up with the final equation as it's plotted in Act2:

Y = -3.999

Note that since the reaction is only written in terms of the Y-axis (ie. not in terms of H+ activity), there is a Y-intercept, but no line slope- that is, the line plots as a horizontal line on the activity diagram.

I hope this helps.

Regards,

Tom Meuzelaar

RockWare, Inc.

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Hi Tom,

If I set Cu activity in the system as 1e-6 and use "thermo" database to plot,

how are the boundaries of CuOH+ vs Tenorite and CuOH+ vs Cuprite defined?

Does the program assume the CuOH+ to be 1e-6 M when calculating the boundaries?

Or, it will speciate Cu++ and CuOH+ at certain pH value and used the real activity

of CuOH+ (say 1.5e-8 M) to calculate the boundaries?

Peng

Hi Peng:

1) Indeed, since this is an activity diagram, all species concentrations must be input as activities.

2) If you specify an activity for Cu++, you are constraining the non-ideal concentration (or activity) for the dissolved Cu++ species.

3) Any boundary in Act2 is simply the mass action equation between the species/minerals on either side of the boundary written in terms of the axis species. For example, on a Quartz solubility diagram with log SiO2 activity as the Y-axis and -log H+ activity (pH) as the X-axis, the boundary between dissolved silica and Quartz is expressed by the reaction:

Quartz = SiO2(aq)

Which has the mass action equation:

K = aSiO2(aq)

From the thermodynamic database, the log K value at 25C for this reaction is -3.9993. Substituting for K and aSiO2, and switching to log scale, we come up with the final equation as it's plotted in Act2:

Y = -3.999

Note that since the reaction is only written in terms of the Y-axis (ie. not in terms of H+ activity), there is a Y-intercept, but no line slope- that is, the line plots as a horizontal line on the activity diagram.

I hope this helps.

Regards,

Tom Meuzelaar

RockWare, Inc.

##### Share on other sites

Hi Tom,

If I set Cu activity in the system as 1e-6 and use "thermo" database to plot,

how are the boundaries of CuOH+ vs Tenorite and CuOH+ vs Cuprite defined?

Does the program assume the CuOH+ to be 1e-6 M when calculating the boundaries?

Or, it will speciate Cu++ and CuOH+ at certain pH value and used the real activity

of CuOH+ (say 1.5e-8 M) to calculate the boundaries?

Peng

Hi Peng:

You can look at the calculations behind an entire activity diagram by choosing the Run - View - .\Act2_output.txt menu option. I think this will answer all of your questions.

Regards,

Tom

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