Spectrophotometric and liquid chromatographic methods are usually employed to determine saponins. The vanillin-sulfuric acid assay is the most selected spectrophotometric method for saponin quantification since it is simple, fast and inexpensive to operate. The basic principle of this method is the reaction of sulphuric acid-oxidized triterpene saponins with vanillin, which gives a distinctive red-purple color measured at wavelengths ranging from 475 to 560 nm using a spectrophotometer.
Please refer to the review article attached below in which the quantification methods of saponins namely, spectrophotometric and chromatographic, are summarized and discussed.