Adsorption isotherm Determination of the Adsorption Parameters of Oxalic acid on Charcoal Objectives: To demonstrate the adsorption phenomena and to show students how to determine the adsorption parameters. Theory : Colloidal dispersions have to the property of adsorption solutes at their surfaces. Thermodynamically such adsorption process occurs to reduce the surface free energy of the dispersion and therefore increase its stability. The degree of adsorption of a solute (adsorbate) on the adsorbent depends on the chemical nature of the adsorbent and the adsorbate, the specific surface area of the adsorbent, the temperature and the concentration or the pressure of the adsorbate. At constant temperature the relation between the amount adsorbed and concentration in a limited concentration range may be represented by one of the three adsorption isotherms of Freundlich (Eq. 5), Langmuir (Eq. 7) and BET (Brunauer Emmett and Teller). Freundlicn isotherm y = x/ m = kC1/n
Where x = weight of adsorbate in grams, adsorbed by m grams of the adsorbent. K & n = emperial constants. Ce = equilibrium concentration. Ym = weight of the adsorbate in grams, adsorbed by one gram of the adsorbent to form a monolayer. b= Equilibrium constant of the adsorption process. Freundlich equation can be linearized by taking the logarithmic form ( Eq. 6) of Eq .5. However langmuir equation can be linearized by taking the reciprocal of the equation as shown in equation 7 & 8: Log (x/m) = (1/n) Log C +LogK (6)
Langmuir isotherm In the contrary of frendlich isotherm ; langmuir is based on a theoretical consideration of the process of adsorption. These consideration are: 1. A maximum of monolayer will be adsorbed . 2. The adsorption surface is homogeneous with each site having the same affinity for adsorption . 3. There are a finite number of site to which molecules can be absorbe . 4. There is a competition between solvent and solute molecules for the adsorption site. 5. Adsorption is a function of the number of adsorption site available and therefore the rate of adsorption (or the occupation of these sites) will decrease with time. 1/Y = 1/Ym + ( 1/Ym . b ) (1/Ce)
Ce/Y = 1/Ym . b + (Ce/Y )
Adsorption Of Oxalic Acid On Charcoal In this experiment oxalic acid will be used as the adsorbate and charcoal will be used as the adsorbent . Experimental : Procedure : 1. In each of 6 Erlenmeyer flasks introduce 5 gms of the adsorbent. 2. To each of them add 50 ml a known dilution of a standard solution of oxalic acid provided (e.g . 1N , 0.8N , 0.4N and O.1N) according to Table 7. 3. Shake occasionally for 15 minutes and set aside to achieve equilibrium. 4. Filter , reject the first portion of the filtrate after washing the receiver with it.
5. Titrate, 20ml of the aliquot filtrate in each case with 0.5N sodium hydroxidusing phenophthalin as an indicator (2 drops). 6. Calculate the amount adsorbed in each case and list your result in Table . Data Analysis : 1. Each group of students should cooperate to run experiment . 2. From Table 7, plot x/m vs. Ce, Log x/m vs. Log Ce and Ce l(x/m) vs. Ce. 3. Analysis each graph and write your report as you had done in Experiment No 5.