I have successfully applied non-linear langmuir and freundlich isotherms in origin. However, the fit does not converge in case of Sips model. I am unable to solve the issue behind this. I would be really thankful for help in this regard.
PS: I have attached the data file here.
Dea Noor ul Ain,
unfortunatey, I did not succeed to open your Origin data file, but here are the possible reasons of the failure:
1) a limited amount of the data, e.g., 5-6 experimental points? The Sips isotherm (mean the Langmuir-Freundlich model?) needs three parameters as distinct from two others, and needs more data to be involved. Does your data set even need a Sips isotherm? For example, does the Freundlich model fit the data well? When you linearize the data by using log vs log plot, do you see deviations of the experimental data from the fitted line, exceeding the experimental variability? If not, you have no reason for refitting, using the Sips isotherm, and use of the 3 parameter model will lead most probably to non-realistic values, if at all. The minimization algorithm may become lost.
2) poor initial values of the parameters. Try to figure out the range of the parameters values and change the initial values
Your data opens empty in the Origin workbook. Might be able to help if you can share an Excel sheet instead.
In fact, I had the same problem with the results, but I reformulated the equation well and tried to modify the variables
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