Determination of minocycline by oxidative coupling and diazocoupling reactions in pharmaceutical formulations

Simple and sensitive spectrophotometric methods (M(1)-M(4)) by the application of oxidative coupling and diazocoupling reactions for the assay of minocycline (MC) in pure form and pharmaceutical formulations have been described. Methods M(1) and M(2) involve the oxidative coupling reactions of MC with 3-methyl-2-benzothiozolinone hydrazone (MBTH) (method M(1), lambda(max) 440 nm) or 4-aminophenazone (4-AP) (method M(2), lambda(max) 520 nm) in the presence of periodate. Methods M(3) and M(4) are based on the formation of diazocoupling products of MC with diazotised p-nitroaniline (DPNA) (method M(3), lambda(max) 420 nm) or diazotised sulfanilic acid (DSAC) (method M(4), lambda(max) 420 nm). Regression analysis of Beer's law plot showed good correlation in the concentration range of 8-48, 20-120, 4-20 and 8-40 microg ml(-1) for methods A, B, C and D, respectively. The molar absorptivities fell within the range of 2.23 x 10(3)-1.51 x 10(4) l mol(-1) cm(-1). The recoveries range from 99.02 to 100.61%.