Desiccants, calcium sulfate anhydrous

Calcium sulfate, the substance used to absorb water in desiccators, provides an example of this temperature sensitivity. Anhydrous calcium sulfate absorbs water vapor from the atmosphere to give the hydrated salt. The reaction has a negative AS° because water molecules become more constrained when gaseous water molecules move into the solid state. The reaction also has a negative AH ° because of the electrical forces of attraction... 

The calcium sulfate in a desiccator is effective at removing water vapor only as long as some anhydrous salt remains. When all the anhydrous salt has been converted to the dihydrate, the desiccator can no longer maintain a dry atmosphere. Fortunately, the thermodynamics of this reaction makes it possible to regenerate the diying agent. At 100 °C (373 K), A S ° contributes more to A G ° than does A H ° ... 

A G373 f. - (-104.9 kJ) - (373 K) (-290.2 J/r) (10 kJ/7) = +3.3 kJ At this temperature, the reverse reaction is spontaneous. Calcium sulfate dihydrate can be converted to anhydrous calcium sulfate in a drying oven at 100 °C. Then it can be cooled and returned to a desiccator, ready once more to act as a chemical sponge for water. 

A desiccator is a storage container used either to dry samples or, more commonly, to keep samples and crucibles dry and protected from the laboratory environment once they have been dried by other means. An indicating desiccant is a desiccant that changes color when saturated with adsorbed water. Drierite is an example of a desiccant. It is anhydrous calcium sulfate. 

The product is very hygroscopic and should be handled under a moisture-free environment. If the iminium salt is kept dry it has a substantial shelf life. The submitters recommend storing the product in a desiccator over anhydrous calcium sulfate. 

Storage. The plates are stored in a desiccator over anhydrous calcium sulfate and protected from the laboratory fumes. 

Reactions run under a nitrogen atmosphere were arranged with a mercury bubbler so that the system could be alternately evacuated and filled with nitrogen and left under positive pressure. Syringes and reaction flasks were dried at least 12 hours in an oven at 120 °C and cooled in a desiccator over anhydrous calcium sulfate prior to use. Reactions at room temperature (rt) were conducted under ambient laboratory conditions T = 20-27 °C, P = 720-770 mmHg. References to removal of volatile components or concentrated under reduced pressure refer to rotary evaporation of a sample at 25-65 °C under pressure (15-25 mmHg) at room temperature. 

Sample pellets were weighed in the combustion crucible to 0.05 mg. after overnight storage in a desiccator over anhydrous calcium sulfate. To our knowledge none of the compounds was hygroscopic. The sample was ignited using the usual iron wire supplied by Parr. 

Procedure. Mixp-hydroxybenzonitrile, methanol, ether (10 ml), and 5 pellets of molecular sieve in a 50-ml round-bottom flask equipped with a drying tube containing anhydrous calcium sulfate or similar desiccant. Cool the mixture to -20° (ice-salt bath) and bubble HCl gas through an inlet tube reaching nearly to the bottom of the solution. After the solution is saturated with HCl, allow the temperature to warm to 4°. After 30 min collect the orange crystals by filtration and wash them quickly with cold methanol-ether (1 2) followed by cold ether. The product (90% yield, m.p. 171-172°) can be stored at 0° in a vacuum desiccator for at least 6 months without deterioration. 

The surfactant AOT ( purum grade, Fluka) was purified as described by Kotlarchyk 22). The AOT solution was filtered through a 0.2-)im Millipore filter prior to drying in vacuo for eight hours. The AOT was stored in a desiccator over anhydrous calcium sulfate. The molar water-to-AOT ratio (W) was assumed to be 1 in the purified, dried solid (2J ). Water was distilled and filtered through a Millipore Milli-Q system. Ethane, propane ("CP" grade, Linde), and xenon (Research grade, Linde) were used as received. The alkanes had a reported purity of >99% (Aldrich) and were used as received. 

All glassware was flame-dried and cooled in a desiccator charged with anhydrous calcium sulfate. Once completely cooled, the glassware was quickly assembled and purged with nitrogen. 

The product then is dried in a vacuum desiccator over anhydrous calcium sulfate. (The checker finds that drying for 64 hours over CaS04 and 1 hour in vacuo at room temperature produces a 71% yield of product.) The X-ray pattern... 

The anhydrous form of calcium sulfate also has a number of practical applications, the most important of which are in the manufacture of cement and as a filler in the production of paper. A filler adds body to paper, making it firmer, brighter, and easier to write, draw, and print on. Soluble anhydrite is used as a desiccant, which is a material that removes water from other substances. It is usually sold under the trade name of Drierite . 

Anyhydrous compounds have important applications in the chemistry laboratory Calcium chloride forms three hydrates—a monohydrate, a dihydrate, and a hexahydrate. As shown in Figure 10.18, anhydrous calcium chloride is placed in the bottom of tightly sealed containers called desiccators. The calcium chloride absorbs moisture from the air inside the desiccator, creating a dry atmosphere in which other substances can be kept dry. Calcium sulfate is often added to solvents such as ethanol and ethyl ether to keep them free of water. 

Desiccant (Drierite anhydrous calcium sulfate, W.A. Hammond Drierite Co. Xenia, OH, USA). 

The dry methosulfate, dissolved in about 30 ml. of water, is made alkaline with 2-3 ml. of 10% sodium hydroxide, and the solution is then extracted exhaustively with successive 15-ml. portions of chloroform until no more blue substance is remov ed from the aqueous solution (Note 12). The combined chloroform solutions are extracted three times with 20-ml. portions of 5% hydrochloric acid. The combined acid extracts are made alkaline to phenolphthalein with 10% sodium hydroxide and reextracted exhaustively with 25-ml. portions of chloroform until no more blue substance is removed from the aqueous solution (Note 12). The combined chloroform solutions are dried over anhydrous sodium sulfate and decanted, and the chloroform is removed by distillation under reduced pressure. The blue crystalline residue is recrystallized by dissohnng it in the least possible amount of water at 60° and then cooling the solution in an ice bath. The product is filtered on a 5-cm. Buchner funnel and dried in the dark in a v acuum desiccator over calcium chloride. The yield is 1.35 g. (58%) of dark blue needles that melt at 133° (Note 13). 

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