The post-column method of organic acids uses the reagent, the mixing module and the visible detector of 430nm wave length. The detector detects the color which comes from organic acids. Organic acids in a sample are separated by the column for organic acid analysis (RSpak KC-811) using acidic solution (ex. perchloric acid) as the eluent. The pH indicator in the reagent is adjusted to be dissociated state (I– + H+). When the pH indicator and the eluted solution from the column are mixed in the mixing module, the acidic substances (organic acids) will be free state (AH -> A– + H+) and the pH indicator generate color by the reaction of I– + H+-> IH.
Right figure shows the spectrum of the pH indicator.