Caffeine is a white, odorless powder with a slightly bitter taste. The anhydrous form obtained by crystallization from nonaqueous solvents is a crystalline solid that melts at 235 to 237°C. At atmospheric pressure it begins to sublime without decomposition at 120°C and at 80°C under high vacuum. Its crystals are hexagonal and form parallel plates. When crystallized from water, the monohydrate forms long, silky, white needles. It becomes anhydrous at 80°C. Caffeine is soluble to the extent of 0.6% in water at 0°C, 2.13% at 25°C and 66.7% at 100°C. The pH of dilute solutions is 6.9. In aqueous solution, caffeine forms dimers and higher polymers by base stacking (3). It is fairly stable in dilute bases and acids, forming salts with the latter. Caffeine is considerably more soluble in chlorinated solvents: 8.67% in methylene chloride and 12.20% in chloroform at 25°C. Its ultraviolet absorption spectrum shows a maximum at 274 nm with no variation over the pH range 2 to 14.
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