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Adding thermodynmic data to DB using Analytic expression for logK vs temperture
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Topic: Adding thermodynmic data to DB using Analytic expression for logK vs temperture (Read 498 times)
EMH121
Frequent Contributor
Posts: 24
Adding thermodynmic data to DB using Analytic expression for logK vs temperture
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January 20, 2017, 05:07:44 AM »
My goal is to add T dependent thermodynamic reaction data to the PHREEQC DB.
Using the "SOLUTION_SPECIES" keyword block, the PHREEQC Help manual says, for a given reaction defined by logK, that LogK vs T data can be added to the DB in the form logK = A1 + A2*T + A3/T + A4*log(T) +A5*(T^2) +A6/(T^2), where A1-A6 are fit constants.
I have a bunch of logK vs T Sulfur species data that I would like to add but don't know how to fit it to the above equation. Does anyone know of a math program that can do the fit. Of course one or more of the fit constants can be zero, reducing the equation to a quadratic, for example, but I am hoping for a more certain answer.
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Pak
Top Contributor
Posts: 64
Re: Adding thermodynmic data to DB using Analytic expression for logK vs temperture
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Reply #1 on:
January 23, 2017, 12:24:04 PM »
I guess you have already think about it but you can also modified directly the database if you wish.
I do not know how you have stored the data. If it is excel you can directly use a fit function that it has, also in Matlab you can do it. In originlab I am pretty sure it is also possible. In general almost all the programs that deal with data will have a polinomial function fitting.
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EMH121
Frequent Contributor
Posts: 24
Re: Adding thermodynmic data to DB using Analytic expression for logK vs temperture
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Reply #2 on:
January 27, 2017, 10:45:23 PM »
Thank you for your reply Pak. I tried Excel but it didn't have enough math options because the math format required by PHREEQC is pretty strict and my data could not always be fitted by a 2 term polynomial. I did not try Matlab, but found that "CurveExpert Professional" worked perfectly (could fit the entire 6 term equation automatically in <1 second) and was <$100. I am one step closer, and have been working on updating my data from 25 years ago as well as comparing the various DBs available in PHREEQC.
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Adding thermodynmic data to DB using Analytic expression for logK vs temperture