The method is based on the reaction of molybdate with
phosphate and silicate ions in an acid environment of pH 1–2. In
the proposed approach, a stream of sample containing both
analytes is merged with a stream of ammonium molybdate to
form molybdophosphoric and molybdosilicic acids (see Fig. 1).
Total absorbance of the compounds is registered in a form of a
steady-state signal (A1, in Fig. 2) and measurements are performed
at wavelength λ¼385 nm, at which radiation is absorbed by both
compounds. Simultaneously, oxalic acid solution is injected into a
carrier stream (H2SO4) and then merged with the stream of
sample containing the formed heteropolyacids. A characteristic
peak is registered as a result of selective decomposition of
molybdophosphoric acid by oxalic acid. The area (or the absorbance
A1 measured at the steady-state signal) and the absorbance
measured at the minimum of the peak (A2 in Fig. 2) were selected
as the measures corresponding to the phosphate and silicate
concentrations, respectively.