U.S. PHARMACOPEIA

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Xanthine, C5H4N4O2—152.11—White, crystalline powder. Decomposes on heating. Slightly soluble in water and in alcohol; soluble in sodium hydroxide TS; sparingly soluble in diluted hydrochloric acid. When subjected to the murexide reaction, a purple color is produced with the ammonia, but on the subsequent addition of fixed alkali hydroxides, the color is not discharged but is changed to violet.
Residue on ignition (Reagent test): negligible, from 100 mg.
Loss on drying 731— Dry it at 105 for 2 hours: it loses not more than 1% of its weight.

Xanthydrol, C13H10O2—198.22—Pale yellow, crystalline powder. Insoluble in water; soluble in alcohol, in chloroform, and in ether. Soluble in glacial acetic acid, forming a practically colorless solution; but when the powder is treated with diluted hydrochloric acid, a lemon-yellow color is produced.
Melting range 741 : between 121 and 123.
Residue on ignition— Ignite 500 mg with 0.5 mL of sulfuric acid: the residue weighs not more than 10 mg (2.0%).

Xylene, C8H10—106.17—Use ACS reagent grade.

m-Xylene, C6H4(CH3)2—106.17[108-38-3]—Clear, colorless, flammable liquid. Insoluble in water; miscible with alcohol and with ether. Use a suitable grade.

o-Xylene, C8H10—106.17—Clear, colorless, mobile, flammable liquid. Insoluble in water; miscible with alcohol and with ether.
Assay— When examined by gas-liquid chromatography, it shows a purity of not less than 95%. The following conditions have been found suitable for assaying the substance: A 3-mm × 1.8-m stainless steel column packed with 1.75% hydrated aluminum silicate plus 5.0% diisodecyl phthalate on support S1. Helium, having a flow rate of about 27.5 mL per minute is the carrier gas, the detector temperature is about 280, the injection port temperature is about 180, and the column temperature is 80. A flame-ionization detector is employed.
Refractive index 831 : between 1.5040 and 1.5060, at 20.

p-Xylene, C8H10—106.17—Colorless liquid.
Assay— Inject an appropriate volume into a gas chromatograph (see Chromatography 621) equipped with a flame-ionization detector, helium being used as the carrier gas. The following conditions have been found suitable: a 0.25-mm × 30-m capillary column coated with a 1-µm layer of phase G14; the injection port temperature is maintained at 130; the detector temperature is maintained at 300; and the column temperature is maintained at 50 and programmed to rise 10 per minute to 100. The area of the C8H10 peak is not less than 99% of the total peak area.
Refractive index 831: between 1.493 and 1.497 at 20.

Xylene Cyanole FF, C25H27N2NaO6S2—538.61—Gray-blue to dark blue powder. Soluble in water.
Assay— Transfer about 50 mg, accurately weighed, to a 100-mL volumetric flask, dissolve in water, dilute with water to volume, and mix. Transfer 2.0 mL of the solution to a 50-mL volumetric flask, dilute with pH 7.0 phosphate buffer to volume (See Solutions, in this section), and mix. Using a suitable spectrophotometer, 1-cm cells, and water as the blank, record the absorbance of the solution at the wavelength of maximum absorbance at about 614 nm. From the observed absorbance, calculate the absorptivity (see Spectrophotometry and Light-scattering 851): the absorptivity is not less than 55.9, corresponding to about 83% of C25H27N2NaO6S2.
Loss on drying 731— Dry it at 110 to constant weight: it loses not more than 6.0% of its weight.

Xylose, C5H10O5—150.13—Use a suitable grade.