Adsorbed moisture

When sublimation is complete, further heat is appHed and the pressure further reduced in a controUed manner to drive-off residual adsorbed moisture. A final moisture content of <2% is normally specified. 

Steam explosion rapid vaporization of water within molten metal, molten salts or hot oil or through them eontaeting surfaee or adsorbed moisture (refer to page 47). 

Adsorption - The ability of certain substances to adsorb moisture makes possible the application of this process to drying gases, and to some extent also liquids. The process of adsorption is described in greater detail later on. 

A glass container exploded after shelf storage for several months, (probably from internal gas pressure arising from slow hydrolysis by atmospheric or adsorbed moisture). 

Drying agent. A solid material, such as alumina, with a large surface area per unit weight that readily adsorbs moisture, i.e., moisture will collect on its surface. Used in gas drying towers and other applications that require the removal of moisture for quality control. 

Hygroscopic. Substances capable of adsorbing moisture from the air, such as silica gel and calcium carbide. 

The source of the raw materials can greatly influence hydrolytic reactions. This is exemplified by Gold and Campbell [54] where talc obtained from different sources impacts markedly on the overall stability of the aspirin tablet formulation. This is possibly attributable to the effect of different types and amounts of surface impurities, which are dissolved in the adsorbed moisture layer, where they subsequently react with the API. It could also influence the pH of the micro-environment. 

When solid dosage forms such as tablets adsorb moisture, drug present on the surface will be dissolved (if it is soluble). The drug in solution on the surface of the tablet will be subject to hydrolytic decomposition, and the process will be influenced... 

Stability Residual moisture content—adsorbed moisture on excipient surface protects drug from hydrolytic degradation... 

For extraction of nonpolar analytes, drying agents are mixed with the matrix to adsorb moisture before extraction. Hydromatrix (Celite 566) has been used frequently. Sodium sulfate, and calcium sulfate (Drierite) are also used to dehydrate the matrix. Ratios of sample to drying agent of 1 1 up to 1 5 have been... 

Adsorption of solvent on silica gel. Attempts to use this adsorbent failed since activated silica adsorbs moisture from air more strongly than alcohol and ether vapour. 

Ultramarines adsorb moisture on the external particle surfaces and at the internal surfaces of the zeolite structure. External surface moisture (1-2% according to particle size) is driven off at 100-105 °C, but the additional 1 % of internal moisture needs 235 °C for complete removal. 

Although it is evident that the reaction is almost entirely homogeneous, Stoddart403,404 presented evidence that the rate of reaction is influenced by the history of the reaction vessel it diminishes from experiment to experiment when starting with a new vessel it also diminishes with time during the course of a single experiment. However, the termolecular rate constant does not depend on the surface-to-volume ratio. He concluded that these effects are due to activated adsorption or chemisorption of nitrosyl chloride on the adsorbed moisture of the... 

Moisture analyses are important because samples contain water either as chemically combined hydrates or as occluded surface-adsorbed moisture. Water is an inherent part of most biological substances and constitutes >90% of the fresh weight of some plant materials. To afford reproducible analytical results, samples are usually dried before analysis and the percentage composition of the sample is then calculated on a dry basis. 

Fakes et al. [1.152] evaluated the moisture sorption behavior of mannitol, anhydrous lactose, sucrose, D-(+)-trehalose, dextran 40 and povidine (PVP K24) as bulking agents. Mannitol was found to be crystalline and non-hygroscopic before and after freeze-drying with RM 0.1-0.3% w/w at 25 °C and 10-60% RH. Anhydrous lactose, sucrose and trehalose were crystalline and relatively non-hygroscopic with RM 0.86, 0.15 and 9.2% respectively. After freeze-drying they where amorphous with RM 1.6, 2.5 and 1.2%, respectively, and adsorbed moisture in an increasing RH atmosphere. Lactose adsorbed 10% water and formed its crystalline hydrate at 55% RH. 

The heat-of-adsorption hygrometer uses a column that selectively adsorbs moisture, and the temperature rise due to heat liberation is in proportion to the amount of moisture being adsorbed. 

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