3.1.2. Spectrophotometric detection

3.1.2. Spectrophotometric detection
Thymol blue was chosen (on the basis of the tabulated
pKa values [17] and experiments performed [15]) as a suitable
acid–base indicator that changes its colour during the reaction
of the analyte with the reagent and enables registration of the
FIA peaks. The detection was carried out at a wavelength of
λ = 610 nm, which corresponds to the maximum absorbance
of the alkaline form of thymol blue. The indicator was part
of the titration reagent. At the titration, a decrease in the
absorbance at λ = 610 nm first occurs; following completion
of the reaction of the hydrochlorides with the titration agent,
the absorbance again increases. The FIA peak is registered as
the time-dependence of the absorbance at this wavelength.