What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

The plot between the
extent of adsorption
against the pressure of gas (*P)* at constant temperature (T) is
called the adsorption isotherm.

**Freundlich
adsorption isotherm:**

Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.

From the given plot it
is clear that at pressure *P*_{S},
reaches the maximum valve. *Ps* is called the saturation
pressure. Three cases arise from the graph now.

**Case I- At low
pressure:**

The plot is straight and sloping, indicating that the pressure in directly proportional to i.e.,

**Case II- At high
pressure:**

When pressure exceeds the saturated pressure, becomes independent of P values.

**Case III- At
intermediate pressure:**

At intermediate
pressure,
depends on *P* raised to the powers between 0 and 1. This
relationship is known as the Freundlich adsorption isotherm.

On plotting the graph
between log
and
log *P*, a straight line is obtained with the slope equal to
and
the intercept equal to log k.

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