Emmanuel Attih*, Cyril Usifoh, Victor Anah and Timma Uwa


Two titrimetric, one spectrophotometric and one kinetic methods were developed and validated for the estimation of Dihydroartemisinin (DHA) in pharmaceuticals. The methods were based on the oxidative bromination of DHA, by active bromine generated in situ by the action of acid on Bromate-Bromide mixture. The first method was the direct titration of the Bromate-Bromide mixture with DHA in acid medium using methyl orange as indicator. In the second titrimetric method the excess bromine generated after the oxidative bromination of DHA by Bromate- Bromide mixture was determined iodometrically. In the spectrophotometric method a measured excess of Bromate-Bromide mixture was reacted with DHA in acidic medium, the residual Bromine was determined by a fixed amount of methyl orange resulting in a chromogen whose absorbance was measured at 520mm. In the Kinetic method, the time taken for the liberated bromine to bleach the methyl orange when different concentration of DHA was reacted with the Bromate-Bromide mixture at 4-5oC was measured and plotted against the drug concentration. The two titrimetric methods were stoichiometric in the ratio of 1:1 (DHA : kBrO3) and applicable within the range of 5.0-20mg/ml and 5.0-30mg/ml, respectively. In the spectrophotometric method Beer’s law was obeyed, with a linear regression equation of (A = 0.0113C + 0.0009, r = 0.9998, n=10 ) and applicable in the range of 0.5-40mg/ml. the molar absorptivity and Sandell sensitivity of 2.10 x 104L/μ g/cm and 1.35 x 10-2ug/cm2 respectively. The limits of detection and quantification were 0.75μg/ml and 1.72μg/ml respectively. In the kinetic method the time taken for the bleaching of the methyl orange was proportional to the DHA concentration in the range of 5-60mg/ml with the regression equation of T = 0.0188C + 0.006 (r = 0.9979, n=5). The methods were evaluated statistically via relative error (accuracy) and relative standard deviation (precision) the intra and inter day variations were < 3%. The method were compared statistically with a pharmacopoeial method and successfully used to assay DHA in tablets and paediatric powders. The accuracy and applicability were confirmed by recovery studies via standard addition method with the results showing no interference from pharmaceutical excipients.

Keywords: Bromate-Bromide mixture, dihydroartemisinin, kinetic, malaria, spectrophotometric titrimetric.

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