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[OLD] Calcite equilibrium constants for T not 25°

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From: Laura Wasylenki

Subject: calcite equilibrium constants for T not 25°

My goal is to prepare solutions that are supersaturated in CaCO3 to known degrees for experimental growth of calcite crystals at temperatures ranging from 5-35°C. In the literature I have found equilibrium constants for the various reactions involved that are apparently quite different from the ones GWB uses, and I'm wondering if it is possible to put log K values for those reactions into the GWB database in order to calculate, for a given mixture of H2O, NaCl, CaCl2, and NaHCO3, what the degree of supersaturation is at 5 degree increments within the temperature range of interest, based on the equilibrium constants I've found in the literature. I know I can enter constants for temperatures of 0, 25, 60, etc., with the alter command, but how do I do this for temperatures of 5, 10, 15, etc? The most elegant way to accomplish the task at hand would be for me to set in GWB the desired equilibrium constants, pH, (activity Ca++/activity CO3--), and log Q/K, and for the program to tell me what concentrations of NaCl, CaCl2, and NaHCO3 to put in the solution. Does anyone know how I might do this, either the elegant way or by guessing reagent concentrations and then iteratively adjusting them until the a Ca/a CO3 ratio and degree of supersaturation are as desired? How do I make sure each part of the program involved is using data at the temperature of interest?


From: Craig Bethke

Subject: Re: calcite equilibrium constants for T not 25°

To set in the thermo data log K's at your temperatures of interest, try this procedure:

(1) Fit your log K vs. T(°C) data to a polynomial of the form log K = a + bT + cT2 + dT3 + eT4. You can do this easily in Excel.

(2) Use your result to figure log Ks at the principal temperatures of the thermo dataset (0, 25, 60, ... °C).

(3) Either edit these log Ks into a private copy of the thermo dataset, or use the alter command to enforce them in your runs.

(4) To verify that you've ended up with the desired log Ks, read your new thermo dataset into Rxn (or use the alter command).

Then, set temperature to a desired value (e.g., 5°C) and display the reaction of interest. As for an elegant way to solve your second problem, there is in fact no unique combination of these solutes corresponding to a given degree of supersaturation, even if you constrain pH somehow. So I don't think there's a one-step answer. You can iterate to a solution to the problem, knowing that it is no special significance. A quick way is to invoke the grep command to display the saturation index of a mineral after each run, adjusting fluid composition between runs.


From: Laura Wasylenki

Subject: Re: calcite equilibrium constants for T not 25°

I altered CO3-- and Calcite in the database according to the work of Plummer and Busenberg, and now when I iterate to find a solution exactly saturated with calcite, I get back P&B's solubilities to two decimal places. I think the solutions for which I'm solving are unique, because I am fixing pH at 8.5 and the ratio of (a Ca/a CO3) at 1.00. In ten seconds, can you think of (1) where I may have introduced any internal inconsistencies in the database by doing this.

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