Medium: 0.01 N hydrochloric acid; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes.

Color reagent— Slowly, and with constant mechanical stirring, add 325 mL of sulfuric acid to 173 mL of cold water. Allow the solution to cool, add 2 mL of ferric chloride solution (1 in 40) and 1 g of p-dimethylaminobenzaldehyde, and stir to dissolve. Store in a low actinic flask. [note—Prepare this Color reagent on the day of use.]

Standard solution— Dissolve an accurately weighed quantity of USP Erythromycin RS in Medium to obtain a solution having a known concentration of about 0.44 mg per mL, using sonication if necessary to dissolve. [note—Use this Standard solution within 5.5 hours.]

Test solution— Pass a portion of the solution under test through a filter having a porosity of 0.5 µm or less, discarding the first 5 mL of the filtrate. Use the filtrate.

Procedure— To three separate 50-mL glass-stoppered conical flasks add 2.0 mL of Standard solution, 2.0 mL of Test solution, and 2.0 mL of Medium (to serve as the Blank), respectively. Place the flasks in an ice bath for about 15 minutes. At precise 1-minute intervals add 10.0 mL of Color reagent to the Standard solution, the Test solution, and the Blank, in turn. Immediately after adding the Color reagent, remove each flask from the ice bath, insert the stopper, mix, and allow to stand at room temperature for exactly 30 minutes. Sequentially determine the absorbance at 480 nm of the Standard solution and the Test solution at precise 1-minute intervals, using the Blank to set the spectrophotometer to zero. Calculate the quantity, in mg, of C37H67NO13 equivalent dissolved by the formula: in which C is the concentration, in mg per mL, of USP Erythromycin RS in the Standard solution, P is the designated content, in µg per mg, of erythromycin in the USP Erythromycin RS, and AU and AS are the absorbances of the Test solution and the Standard solution, respectively.

Tolerances— Not less than 75% (Q) of the labeled amount of C37H67NO13 equivalent is dissolved in 45 minutes.

Arsenomolybdate stock solution— Dissolve 100 g of ammonium molybdate in 1.7 liters of water in a 2-liter volumetric flask. Slowly add, with mixing, 84 mL of sulfuric acid. Add 12 g of sodium arsenate dissolved in 100 mL of water. Dilute with water to volume, and mix. Store in an amber bottle for 24 hours before using. [note—This solution should not come in contact with rubber.]

Arsenomolybdate solution— Use a mixture of water and Arsenomolybdate stock solution (2:1). [note—Prepare fresh on the day of use.]

pH 4.8 Acetate buffer— Dissolve 133 g of sodium acetate in about 3.5 liters of water. Adjust with glacial acetic acid to a pH of 4.8 ± 0.1, dilute with water to 4 liters, and mix.

4.5 M Sulfuric acid— [note—This reagent is called out in the Diagram of Dissolution Test Method for Erythromycin Ethylsuccinate Tablets Labeled as Chewable under Automated Methods of Analysis 16.] Add 1.5 liters of water to a 2-liter volumetric flask, and place the flask in an ice bath. Slowly add, with stirring, 300 mL of sulfuric acid. Allow the solution to cool, dilute with water to volume, and mix. At the time of use, add 0.5 g of sodium dodecyl sulfate to each L.

Medium: 0.1 M acetate buffer, pH 5.0; 900 mL.

Apparatus 2: 75 rpm.

Time: 60 minutes.

Standard stock solution— Dissolve an accurately weighed quantity of USP Erythromycin RS in pH 4.8 Acetate buffer to obtain a solution having a known concentration of about 0.5 mg per mL.

Standard solutions— Transfer 8.0-, 4.0-, and 1.0-mL volumes of the Standard stock solution to separate 100-mL volumetric flasks, add 6.0 mL of pH 7.6 phosphate buffer (see Solutions under Reagents, Indicators, and Solutions) and 6.0 mL of Medium to each flask, dilute with pH 4.8 Acetate buffer to volume, and mix to obtain solutions having known concentrations of about 40, 20, and 5 µg per mL of USP Erythromycin RS, respectively.

Procedure— Transfer 6.0 mL of the solution under test to a 50-mL volumetric flask, add 6.0 mL of pH 7.6 phosphate buffer, and heat in boiling water for 30 minutes. Cool to room temperature, and dilute with pH 4.8 Acetate buffer to volume. To determine the absorbance values, use an automated analyzer (illustrated in the Diagram of Dissolution Test Method for Erythromycin Ethylsuccinate Tablets Labeled as Chewable under Automated Methods of Analysis 16) consisting of a liquid sampler; a proportioning pump; a manifold; and a spectrophotometer equipped with matched flow cells, suitable recording devices, and analysis capability at 660 nm. Adjust the system until a steady baseline is achieved. Start the sampler, and conduct determinations at a rate of about 40 to 60 per hour with a 3:1 (sample/wash) ratio. [note—Adjust the analyzer flow rates if necessary to optimize system performance.] Determine the amount of C37H67NO13 dissolved from the absorbance values of the Test solution and the Standard solutions.

Tolerances— Not less than 75% (Q) of the labeled amount of C37H67NO13 is dissolved in 60 minutes.