牛皮癣洗剂收载于北京市制剂手册(1978年版172页,人民卫生出版社)。处方如下:升汞7g,水杨酸30g,蓖麻油20ml,40%甲醛溶液5ml,95%乙醇加到1000ml。原处方只收载升汞的含量测定方法。考虑到水杨酸在处方中含量达3%,为治疗成分之一,故亦应进行含量测定。水杨酸为有机酸(K_a=1.06×10~(-3)),可在乙醇中,以酚酞为指示剂,用标准氢氧化钠液直接滴定,粉红色为终点。但由于药液中含有Hg~(2+),也能与氢氧化钠液起反应生成黄色氧化汞沉淀。此反应对滴定有干扰,使得终点推迟,同时黄色的氧化汞影响终点。因汞盐与过量碘离子作用,可生成络离子。该络合物具有 Psoriasis lotion contained in Beijing preparation manual (1978 edition 172, People’s Health Publishing House). Prescriptions are as follows: Mercury 7g, salicylic acid 30g, castor oil 20ml, 40% formaldehyde solution 5ml, 95% ethanol added to 1000ml. The original prescription contains only mercury content determination method. Taking into account the content of salicylic acid in the prescription up to 3%, as one of the ingredients, it should also be determined. Salicylic acid is an organic acid (K_a = 1.06 × 10 -3), which can be directly titrated with standard sodium hydroxide solution using phenolphthalein as an indicator in ethanol, with pink as the end point. However, due to the solution containing Hg ~ (2+), but also with sodium hydroxide solution from the reaction of yellow mercury oxide precipitation. This reaction interferes with the titration, delaying the end point, while the yellow mercuric oxide affects the endpoint. Mercury salt due to excessive iodine ions and can generate complex ions. The complex has