pH 7.0 Phosphate buffer— Dissolve 4.54 g of monobasic potassium phosphate in water to make 500 mL of solution. Dissolve 4.73 g of anhydrous dibasic sodium phosphate in water to make 500 mL of solution. Mix 38.9 mL of the monobasic potassium phosphate solution with 61.1 mL of the dibasic sodium phosphate solution. Adjust, if necessary, by the dropwise addition of the dibasic sodium phosphate solution to a pH of 7.0.

Standard preparations— Dissolve a suitable quantity of USP Adenine RS, accurately weighed, in hot water, cool, and dilute quantitatively with water to obtain a solution having a known concentration of about 0.19 mg per mL. Pipet 5-mL portions into three 100-mL volumetric flasks, dilute with 0.10 N hydrochloric acid, 0.010 N sodium hydroxide, and pH 7.0 Phosphate buffer, respectively, to volume, and mix.

Test preparations— Proceed as directed for Standard preparations, using the specimen in place of the Reference Standard.

Procedure— Obtain the UV absorption spectra of the Test preparations and the Standard preparations concomitantly, in 1-cm cells, covering the range between 220 nm and 320 nm, using water as the blank. The respective absorptivities, calculated on the dried basis, at the wavelengths of maximum absorbance, for each pair of corresponding solutions do not differ by more than 2.0%.

0.1 N Perchloric acid— Standardize 0.1 N perchloric acid VS as follows. Transfer about 300 mg of potassium biphthalate, accurately weighed, to a 150-mL beaker, and dissolve in 80 mL of a mixture of 100 mL of glacial acetic acid and 300 mL of acetic anhydride, by stirring. Titrate with the perchloric acid solution, determining the endpoint potentiometrically. Each 20.42 mg of potassium biphthalate is equivalent to 1 mL of 0.1 N perchloric acid.

Procedure— Transfer about 200 mg of Adenine, accurately weighed, to a 150-mL beaker, and dissolve in 80 mL of a mixture of 100 mL of glacial acetic acid and 300 mL of acetic anhydride, by stirring. Titrate with 0.1 N Perchloric acid, determining the endpoint potentiometrically. Perform a blank determination, and make any necessary correction. Each mL of 0.1 N Perchloric acid is equivalent to 13.514 mg of C5H5N5.