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How to use the function to fit the Langmuir model

 
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clwei



Joined: 12 Sep 2016
Posts: 6

PostPosted: Thu Sep 22, 2016 1:43 am    Post subject: How to use the function to fit the Langmuir model Reply with quote

Hi David,
Recently, I do the research about adsorption, and I need fit the data using the Freundlich model and Langmuir model. These two models can be done in the Origin (the results of two models are similar), I finish the Freundlich fit(y=a*x^b) in DG. Now the problem is that the Langmuir formula y=a*b*x/(1+a*x) canít fit. I donít know how to achieve the result. Thanks![/img]
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David
Site Admin


Joined: 25 Nov 2006
Posts: 1938
Location: Chapel Hill, NC

PostPosted: Fri Sep 23, 2016 12:14 am    Post subject: Reply with quote

In the Fit command, select the "Fit function" as "Arbitrary". The detail view will get a few more user interface elements. In the "Function form" field just type in the expression

a*b*x/(1+a*x)

You can use the gear menu to the right of that field to add the function to a menu so you can use a shortcut next time you need this function.

DataGraph will automatically detect a,b as the fitting parameters and use a non-linear least squares method to fit the data. Since the fit is non-linear, it might have multiple minima and the fit can depend on the initial guess. If the fit fails you get a dashed line that shows what the initial guess was. You might need to tweak the initial values. Typically you don't need to change this at all or very much. Since the fit is done in real time, you can use the blue slider button to change the initial guess continuously.

David
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clwei



Joined: 12 Sep 2016
Posts: 6

PostPosted: Fri Sep 23, 2016 8:59 pm    Post subject: Reply with quote

David,
Thanks a lot, I have fit the data as what you said, and it works well,Thanks!
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