Occluded material

Occlusions, which are a second type of coprecipitated impurity, occur when physically adsorbed interfering ions become trapped within the growing precipitate. Occlusions form in two ways. The most common mechanism occurs when physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced (see Figure 8.4a). In this case the precipitate s mass is always greater than expected. Occlusions also form when rapid precipitation traps a pocket of solution within the growing precipitate (Figure 8.4b). Since the trapped solution contains dissolved solids, the precipitate s mass normally increases. The mass of the precipitate may be less than expected, however, if the occluded material consists primarily of the analyte in a lower-molecular-weight form from that of the precipitate. 

Occlusions are minimized by maintaining the precipitate in equilibrium with its supernatant solution for an extended time. This process is called digestion and may be carried out at room temperature or at an elevated temperature. During digestion, the dynamic nature of the solubility-precipitation equilibrium, in which the precipitate dissolves and re-forms, ensures that occluded material is eventually exposed to the supernatant solution. Since the rate of dissolution and reprecipitation are slow, the chance of forming new occlusions is minimal. 

Occlusion — A type of - coprecipitation in which a contaminant species is trapped within the growing crystal. The amount of occluded material is greatest in the part of a crystal that forms first [i]. Occlusion of organic additives can occur in - electrodeposition, e.g., incorporation of cumarin in nickel [ii]. 

One advantage of sublimation is that no solvent is used, and therefore none needs to be removed later. Sublimation also removes occluded material, such as molecules of solvent, from the sublimed substance. For instance, caffeine (sublimes at 178°C, melts at 236°C) absorbs water gradually from the atmosphere to form a hydrate. During sublimation, this water is lost, and anhydrous caffeine is obtained. If too much solvent is present in a sample to be sublimed, however, instead of becoming lost, it condenses on the cooled surface and thus interferes with the sublimation. 

Of the numerous techniques referred to in this volume and not discussed so far, special mention must be made of FTIR and NMR. These techniques may be applied in many different ways and for various reasons including study of the pore structure and the properties of adsorbed or occluded material. 

Minor components such as occluded material and thermally unstable end groups can be readily estimated by using a more sensitive absorption band such as that near 1170 cm. ... 

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