Ion-association method for the colorimetric determination of neomycin sulphate in pure and dosage forms A.S. Amin a, *, Y.M. Issa b a Chemistry Department, Faculty of Science, Benha University, Benha, Egypt b Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Received 28 January 2002; accepted 20 April 2002 Abstract A simple, fairly rapid, sensitive and accurate method is described for the colorimetric determination of neomycin sulphate (NMS), based on the measurement of the absorbance of the extracted organic soluble ion-association complex formed between neomycin dictation and a bulky counter anion. Different chromotropic acid azo dyes were examined as counter ions. The effect of pH, the counter ion concentration, sequence of addition and solvents for extraction were also illustrated. The most suitable system is based on reagent VIII (pH 7.5) with chloroform as the extraction solvent. The use of other counter ions, in conjunction with their respective solvents, was found to be less sensitive. The neomycin- reagent VIII system exhibits negligible or no interference when used for the determination of up to 58 mg ml 1 of NMS in the presence of several drug excipiences. The method has been used for the determination of up to 58 mg ml 1 with a good recovery (99.89 /1.5%), and the precision is supported by the low relative standard deviation 0/1.35%. The sensitivity is discussed and the results are compared with the official method. The proposed method was applied successfully to the determination of NMS in pure and dosage forms, with a good precision and accuracy compared to the official one. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Neomycin sulphate; Chromotropic acid azo dyes; Colorimetric determination; Pharmaceutical analysis 1. Introduction Neomycin sulphate (NMS) [1405-10-3] is a broad-spectrum antibiotic used in gastrointestinal infections, mainly in child diarrhea caused by suspensions and eye drops mixed with pharma- ceutical agents such as kaolin, pectin and alumi- nium hydroxide. These substances are utilized as antiflatulent, antiacid and antidiarrheal [1,2]. They absorb bacteria and toxins responsible for the onset of diarrhea, vomiting, nausea and cramps in various intestinal infections and coating the in- flamed mucous membrane of the intestinal tract [3]. However, these excipients are capable of absorbing various antibiotics such as neomycin [3,4]. Various procedures [5,6] were suggested to the use of buffers to increase the liberated of adsorbed neomycin. On the other hand Harris [6] * Corresponding author E-mail address: asmin2002@hotmail.com (A.S. Amin). Spectrochimica Acta Part A 59 (2003) 663  /670 www.elsevier.com/locate/saa 1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S1386-1425(02)00216-0