March 28, 2024, 06:09:18 AM
Forum Rules: Read This Before Posting


Topic: langmuir isotherm  (Read 2228 times)

0 Members and 1 Guest are viewing this topic.

Offline kriggy

  • Chemist
  • Sr. Member
  • *
  • Posts: 1519
  • Mole Snacks: +135/-16
langmuir isotherm
« on: March 17, 2015, 01:33:15 PM »
Hi I came across the langmuir equation for isotherm written as:

q= a*cfeed / 1+b*cfeed

I understand that cfeed 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
« Last Edit: March 17, 2015, 02:56:44 PM by kriggy »

Offline mjc123

  • Chemist
  • Sr. Member
  • *
  • Posts: 2049
  • Mole Snacks: +296/-12
Re: langmuir isotherm
« Reply #1 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
nads = nm*bc/(1+bc)
where nads is the number of moles of compound adsorbed and nm is the number of moles adsorbed to form a complete monolayer.

Offline kriggy

  • Chemist
  • Sr. Member
  • *
  • Posts: 1519
  • Mole Snacks: +135/-16
Re: langmuir isotherm
« Reply #2 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:

we equilibrated the column and by eluing the adsobed cmpd we got mdes, then ε is porosity - the amount of void space in column, Vc 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/cfeed 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 qmax and the fraction of covered surface could be calculated usign θ=q/qmax

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

Sponsored Links