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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: kriggy on March 17, 2015, 01:33:15 PM

Title: langmuir isotherm
Post by: kriggy on March 17, 2015, 01:33:15 PM

Hi I came across the langmuir equation for isotherm written as:

q= a*c_feed / 1+b*c_feed

I understand that c_feed is the concentration of compound and q is the amount of occupied sites on a surface. But what is the physical meaning of a and b parameters?
I think b is equlibrium constant for the adsorption on the surface but what about the other one?
thanks for help

Title: Re: langmuir isotherm
Post by: mjc123 on March 18, 2015, 06:04:18 AM

The usual form of the Langmuir isotherm (the one I'm familiar with anyway) takes the form
θ = bc/(1+bc)
where θ is the fraction of surface covered and b is the equilibrium constant. This form only requires one parameter, but you would need another if the LHS is a quantity (e.g. amount of compound adsorbed) which does not equal 1 at saturation. For example, you might have
n_ads = n_m*bc/(1+bc)
where n_ads is the number of moles of compound adsorbed and n_m is the number of moles adsorbed to form a complete monolayer.

Title: Re: langmuir isotherm
Post by: kriggy on March 18, 2015, 10:33:11 AM

Thanks,
It was project I was working some time ago and yesteday I opened the files again and realized that I dont know what those parameters mean and it got me curious :D

well we used the form I wrote to create the izotherm and then we fitted our measured data to see how they are following the izotherm using equation:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FbA4siiP.png&hash=ae157ad3b1fc06caeed90a7e0aea53127965bfe3)
we equilibrated the column and by eluing the adsobed cmpd we got m_des, then ε is porosity - the amount of void space in column, V_c is volume of column and cfeed is concentration of compound in mobile phase so basicaly we got the amount of cmpd adsorbed on stationary phase
Then we ploted q/c_feed vs q to get scatchard plot and from the slope and intercept we got the parameters a and b for creating the isotherm.
Then by calculating a/b we got the maximal surface q_max and the fraction of covered surface could be calculated usign θ=q/q_max

I did some googling and the equation we used seems to be called Reddlich-Peterson equation or model rather than Langmuir.