Adsorption Isotherm Using a A. J. Dandy
Colorimetric Method
Makerere University College Kampala, Uganda, East Africa
I
A general chemistry experiment
W e have found the following modification of the conventional Freundlich adsorption isotherm experiment to be attractive to beginning chemistry students. It combines several techniques, illustrates graphical evaluation of constants, and provides an admirable basis for evaluation of errors. Copper sulfate is adsorbed from solution on charcoal; relative concentrations are measured colorimetrically.
Figure 1.
Experiment01 arrangement.
The adsorbent column is set up as shown in Figure 1, 3 g of finely-ground charcoal adsorbent being introduced as a slurry in water into the glass tube (2 cm diameter X 40 cm) after which a vacuum is applied to remove water. An approximately 0.25 M solution of copper sulfate is prepared. Six ml of this solution are pipetted into a 200-ml graduated flask and made up to the mark with water. This is then poured down the column. A vacuum is applied and the emergent solution is collected in a clean, dry filter flask. Ten ml of ammonia solution (sp gr 0.880) are added to the contents of the flask, and the intensity reading for the resulting solution is obtained using a colorimeter. The procedure is repeated for 100 ml portions of solutions containing 4, 6, and 8 ml of 0.25 M CuSOk
solution per 100 ml of solution. Five ml only of ammonia solution should be added to the emergent solutions in these cases. The same column is used throughout, the adsorbate layer being built up each time to an equilibrium value. Two hundred ml of the first solution are needed to establish this equilib~ium;thereafter, only 100 ml are needed. The amount of solute adsorbed a t each stage may be obtained from the differences before and after passing through the charcoal column. I n order to determine the concentrations of the emergent solutions, a calibration graph of log (intensity) against known concentrations must be prepared. Suitable solutions for this purpose contain 2, 3, 4, 5, and 6 ml portions of 0.25 M CuSOn solution per 100 ml, to which 5 ml of ammonia is added in each case. A graph of log x (total amount adsorbed) against log c (equilibrium concentration) is plotted to test the Freundlich relationship, x/m = kc". Since the quantities involved in the plot are logarithms, any consistent set of units for x and c may be used; colorimeter values may be translated simply into ml of
Figure 2.
Freundlich irotherm pld.
0.25 M CuSOp solution, g/l CuSOn or mol/l Cu++. It is instructive for the student to handle the data in differentways to recognize the consequences on the numerical values of k and n thus obtained. Values of n = 0.43 and k = 3.3 X were obtained from the graph of log x versus log c (Fig. 2). Concentrations here are expressed as g CuSOdl00 ml of solution. The adsorption could be reversed only with difficulty. Thus very weak solutions (or pure water) were required in order to obtain measurable desorption, and the rather large volumes required introduced correspondingly large errors in measurement. The value of n is likely to vary with the particular specimen of carbon chosen, and must be determined by the instructor. Volume 4 1 , Number 1, lonuory 1 9 6 4
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