(last revised: 1/20/06)
Introduction
The adsorption of acetic acid on charcoal is studied using both the Freundlich isotherm and the Langmuir isotherm. This is an example of physical adsorption, where dipole and van der Waals forces are the predominant sources of attraction, and the heat of adsorption is typically less than 50 kJ /mol.
The amount of acetic acid (adsorbate) adsorbed per gram of charcoal (adsorbent) will depend on the surface area of the charcoal, the temperature of the solution, and the adsorbate concentration in solution. The adsorption will be followed by titrating the acetic acid not adsorbed by the charcoal, then determining the amount adsorbed by difference. Isotherms (plots of moles of adsorbate adsorbed per gram of adsorbent versus solution concentration) will be constructed, then compared with two models: (a). the Freundlich isotherm,
N=Kca
where K and a are empirical constants, N is the number of moles adsorbed per gram of charcoal, and c is the solution concentration, and (b). the Langmuir isotherm,
c/N = C/Nm + 1/(k Nm)
where Nm is the number of moles required to cover one gram of charcoal with a monolayer of adsorbate, and k is a function of temperature.
The heat of adsorption may be determined by measuring the concentration necessary to achieve one-half of a monolayer of coverage as a function of temperature:
d ln(1/kNm)/dT = (dln(c)/dT)0.5 = H/RT2.
Procedure
Each group will work at a single temperature. Place approximately 1 g (weighed accurately to the nearest 0.001 g) of charcoal into each of seven dry Erlenmeyer flasks. Prepare solutions of 0.15, 0.12, 0.09, 0.06, 0.03 and 0.015 M acetic acid. Add 100 mL of a different acetic acid solution to each flask, with 100 mL of distilled water to the seventh.. An eighth flask should contain 0.03 M acetic acid, but no charcoal. The flasks should be tightly stoppered, then shaken periodically for thirty minutes, and allowed to equilibrate for several hours (preferably overnight) in a water bath maintained at a constant temperature.
After equilibration, the samples are filtered (discard the first 10 mL of filtrate). Titrate two 25 mL aliquots of each sample with a standardized NaOH solution, using phenolphthalein as an indicator.
Calculations
Determine the final concentration of acetic acid in each sample, then calculate the number of moles of acid adsorbed onto the charcoal by subtracting the final concentration from the initial concentration, then multiplying by the solution volume. Adjust this value, if necessary, if the charcoal is determined to have residual acidity (the seventh flask data should tell you this). Plot isotherms (N vs. c) for each temperature, then determine whether a Freundlich (log N vs. log c) or Langmuir (c/N vs. c) isotherm provides a better fit to the data. Plot ln c (0.5) vs. T to obtain the heat of adsorption for this system.