S

a Reproduced from N. Nambu, S. Sakurai and T. Nagai, Chem. Pharm. Bull., 23, 1404 (1975).

in the form x

where a and n are constants, the form 1/n being used to emphasise that c is raised to a power less than unity. 1/n is a dimensionless parameter and is related to the intensity of drug adsorption. Equation (6.18) can be written in a linear form by taking logarithms of both sides, giving log(xlm) = log a + (lln)log c

A plot of log(x/m) against log c should be linear, with an intercept of log a and slope of 1/n; it is generally assumed that, for systems that obey this equation, adsorption results in the formation of multilayers rather than a single monolayer. Figure 6.17 shows Freundlich isotherms for the adsorption of local anaesthetics on activated carbon; the method of calculating the constants a and 1/n from these plots is given in Example 6.5.

EXAMPLE 6.5 Use of the Freundlich equation

The following data refer to the adsorption of tetracaine from aqueous solution at 25°C on to a sample of activated charcoal:

Equilibrium conc. 0.155 0.468 1.259 2.510 5.370 (mg dm ~3)

Amount adsorbed 202.8 217.8 232.0 243.2 254.7 (mg g01)

Show that these data can be represented by the Freundlich isotherm and calculate the constants a and 1/n.

Answer

Plot a graph of log(x/m) against log c, noting that the data for the amount adsorbed are given per g of carbon (x/m). This graph is linear, showing that the data can be represented by the Freundlich equation.

At log c = 0, the value of log(x/m) interpolated from this graph is 2.359. Therefore, a = 229 mg g-1

The gradient of this plot = 0.065. Therefore,

Factors affecting adsorption

Solubility of the adsórbate

Solubility is an important factor affecting adsorption. In general, the extent of adsorption of a solute is inversely proportional to its solubility in the solvent from which adsorption

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