Freundlich equation usually fails both at low and high concentrations. Freundlich isotherm is somewhat better for the adsorption from liquid solution and Langmuir equation gives better results for adsorption of gases. Neither Freundlich or Langmuir expressions, however, are adequate for adsorption of vapors on porous solids at high vapor pressures: which is assumed to yield multi-molecular adsorption layers, and the Brauner - Emmett - Teller (B.E.T.) semi - empirical isotherm is applied in such a case, which will not be discussed here. We will prepare our acetic acid-charcoal isotherm by allowing acetic acid solutions of various concentrations to equilibrate with a given mass of charcoal. The amount of acetic acid ( ) not adsorbed to the charcoal will be determined by titration with sodium hydroxide ( ). ( ) ( ) ( ) This will allow us to easily determine the amount of acetic acid that has adsorbed to a given mass of the charcoal.