在乙酸介质中，罗红霉素与藻红B反应生成电荷转移络合物，其最大吸收波长为520 nm，采用分光光度法测定罗红霉素胶囊中罗红霉素的含量。优化的试验条件如下：① 4.0×10^-4mol·L^-1的乙酸溶液作为反应介质；②反应温度为25℃；③反应时间为20 min；④ 2.0×10^-4mol·L^-1藻红B溶液的用量为1.2 mL。罗红霉素的质量浓度在0.020~0.500 mg·L^-1内与反应体系的吸光度差值呈线性关系，检出限（3s/k）为0.014 mg·L^-1。方法用于罗红霉素胶囊样品的分析，加标回收率为101%~107%，测定值的相对标准偏差（n=5）为0.84%~1.6%。

A charge transfer complex based on the reaction of roxithromycin and erythrosine B was formed in acetic acid medium, which showed a maximum absorption wavelength of 520 nm. Spectrophotometry was applied to the determination of roxithromycin in roxithromycin capsules. The optimized conditions found were as follows:① 4.0×10^-4mol·L^-1 acetic acid solution was used as reaction medium; ② temperature of reaction was 25℃; ③ time of reaction was 20 min; ④ amount of 2.0×10^-4mol·L^-1 erythrosine B solution was 1.2 mL. Linear relationship between values of absorbance difference of the reaction system and mass concentration of roxithromycin was obtained in the range of 0.020-0.500 mg·L^-1, with detection limit (3s/k) of 0.014 mg·L^-1. The proposed method was applied to the analysis of samples of roxithromycin capsules, giving values of recovery found by standard addition method and RSDs (n=5) of determined values in the ranges of 101%-107% and 0.84%-1.6% respectively.