This was a question asked earlier at RG. But unfortunately, the answer was not to the point.
There are several papers available that have applied DR isotherm model for their data. Surprisingly, the unit for Polanyi potential has not been mentioned anywhere? If the units are derived from the formula RTln(1+1/Ce) then what happens to the unit mg/L corresponding to Ce. This seems to be totally missing in isotherm constant too. So, should the unit for Ce be considered?
1) Should the unit for Ce be considered for Polanyi potential?
2)The unit for the isotherm constant are different as well: in some papers they have been given the unit mol^2/kJ^2, elsewhere it is mol^2/J^2. The addition of "k" becomes very important as the scale is not converted but is a derived unit.
3) Similarly, the unit for E varies as mentioned above.
Clarifications are needed on the same.
Thank you.
Hi Smruthi,
As a ln has no unit, the isotherm must be in RT.
Second, it seems that there is no unit for Ce because it drives initially from P/P0.
Solution may appear in the attached paper:
Tell me if this is th solution, please. Thank you.
Jacques
Dear Prabhu,
I think you should follow the way below:
The linearised form of the Dubinin–Radushkevich equation is given in equation
ln qe= ln Xm-KDR ε
where KDR is related to the free energy of adsorption and Xm is the Dubinin–Radushkevich isotherm constant related to the degree of sorbate adsorption by the adsorbent surface. The initial value of ε can be found using equation below as expressed below and this value was then incorporated into equation above to determine the values of KDR and Xm.
ε= RTln(Co/Ce)
Here you see ε drives initially from Co/Ce. So, there is no unit for Co/Ce but ε.
(Because: Joule/(mol K).K should be equal Joule/mol am I right?)
Confirmation of the isotherm model was obtained by plotting ln qe versus ε2 when the data points yielded a straight regression line.
Reference:
Horsfall, Jr., M.H. and Spiff, A.I. (2005) Acta Chim. Slov. 52, 174.
Thank you !!! Jacques GE Thierie and Olcayto Keskinkan for such a wonderful explanation. It's very clear now.
let us suppose we are considering dye adsorption on activated carbon then, I think Ce should be in "moles of dye /L of solution" and qe in "moles of dye/gm of adsorbent" as the binding energy for dye molecule is "in joules/mole"
It depends on how Ce is defined. If it is a mass concentration, the SI unit may be kg /m3, but g/L or mg/L are also acceptable. Also note whether the volume relates to the solution or only to the solvent.
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