A: Dissolve about 20mg in 2mLof buffer solution,prepared by adjusting 50mLof 1Mmonobasic potassium phosphate with 1Nsodium hydroxide to a pHof 7.0,diluting with water to 100mL,and mixing.Add 0.2mLof ninhydrin solution (1in 200)and boil:a purple color is produced.

B: It meets the requirements of the tests for Sulfate á191ñ.

C: Liquid Chromatographic Identification Test—

Mobile phase— Prepare a mixture of 0.1Mtribasic sodium phosphate and acetonitrile (77:23),and adjust with phosphoric acid to a pHof 3.0.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard solution— Prepare a solution of USP Colistin Sulfate RSin Mobile phasehaving a concentration of about 6mg per mL.Protect this solution from light.

Test solution— Prepare a solution of Colistin Sulfate in Mobile phasehaving a concentration of about 6mg per mL.Protect this solution from light.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 212-nm detector and a 4.6-mm ×25-cm column that contains 5-µm packing L1.The flow rate is about 1mLper minute.

Procedure— Separately inject equal volumes (about 10µL)of the Standard solution and the Test solutioninto the chromatograph,and record the chromatograms.The chromatogram obtained from the Test solutioncorresponds qualitatively with that obtained from the Standard solution,exhibiting a major peak corresponding to colistin Aand minor peaks at relative retention times of about 0.55(colistin B)and 0.8.