The eluent (eluate) eluted from a column is mixed with a reaction reagent solution (contains pH indicator) in a mixing module. When organic acids are present in the eluate, the pH indicator changes its color. This color change is detected by a visible detector (430 nm). More detailed analysis flow is explained below.
Organic acid samples are analyzed by a column with an acidic eluent such as low-concentration aqueous perchloric acid solution. On the other hand, the reaction reagent solution is adjusted to basic to make the pH indicator in a dissociated state (I- and H+).
This reaction reagent solution becomes acidic when mixed with eluent in the mixing module. This causes the pH indicator to undergoes the reaction "I- + H+ → IH" and changes its color. When organic acids elute from the column, the eluate becomes even more acidic, this accelerates the color change of the pH indicator and makes the visible detector signal stronger.
The post-column method using a pH indicator is more selective than direct organic acid detection method, making this method suitable for samples with complex matrices, such as food products. RSpak KC-811, a column for organic acids analysis is a well fit for the post-column method.

Refer to "Preparation of Reaction Reagent Solution Used for Post-column Organic Acids Analysis Method".

Organic acid analysis by post-column method

pH indicator's spectrum