Charcoal desorption curves

Figure 3 Adsorption and desorption curves for the sorption of radon on coconut based charcoal. (Jebackumar, 1985).

Bansal, studied the adsorption desorption isotherms of benzene, toluene and o-xylene on sugar charcoal associated with varying amounts of the carbon-oxygen surface groups and observed that the area of the hysteresis loop decreases as the molecular dimensions of the adsorbate increase from benzene to o-xylene (Table 2.7). The point of inception of the hysteresis loop was also found to shift to lower relative vapor pressures as we move up the series of hydrocarbons, which is due to an increase in the molecular size of the adsorbate. The point of inception of hysteresis loop was calculated using Cohn postulates and compared with the values read from the experimental curves (Table 2.8). It is seen that the two values agree closely for all the adsorbate-adsorbent systems. Higute, from thermodynamic considerations, also proposed that the critical radius for the inception of capillary condensation is equal to four times the molecular radius of the adsorbate. 

The desorption efficiency is dependent on the amount of analyte collected on the charcoal. Plot the desorption efficiency versus weight of analyte found. This curve is used in section 10.4 to correct for adsorption losses. 

Procedure for determining desorption efficiency. The reference portion of the charcoal tube is removed. To the remaining portion, amounts representing 0.5X, IX, and 2X (X represents TLV) based on a 201 air sample are injected onto several tubes at each level. Dilutions of acrylonitrile with methanol are made to allow injection of measurable quantities. These tubes are then allowed to equilibrate at least overnight. Following equilibration they are analyzed following the same procedure as the samples A curve of the desorption efficiency... 

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