A methodforthesimultaneousdeterminationofphosphateandsilicatebasedonspectrophotometric
measurement at385nmofasinglepeakusinga flow injectionsystemwithtwocomponentcalibration
is described.Inthesystem,astreamofsamplecontainingbothanalytesismergedwithastreamof
ammonium molybdatetoform(at1opHo2) molybdophosphoricandmolybdosilicicacids.Total
absorbance ofthecompoundsisregisteredinaformofaconstantsignal.Simultaneously,oxalicacid
solution isinjectedintoacarrierstream(H2SO4) andthenmergedwiththestreamofsamplecontaining
the mixtureofheteropolyacids.Acharacteristicpeakisregisteredasaresultofselectivedecomposition
of molybdophosphoricacidbyoxalicacid.Thearea(ortheabsorbancemeasuredattheconstantsignal)
and theabsorbancemeasuredattheminimumofthepeakcanbeusedasmeasurescorrespondingtothe
phosphate andsilicateconcentrations,respectively.Thetimeofthepeakregistrationisabout3min.
Two-component calibrationwiththeuseoffourstandardsolutionsofthephosphate/silicateconcentra-
tions establishedinaccordancewith22 factorial designwasapplied.Phosphateandsilicatecanbe
determined withintheconcentrationrangesof0.20–15.00and0.20–20.00mgL1, withprecisionless
than 2.7and0.9%(RSD),respectivelyandaccuracybetterthan6.2%(RE).Thedetectionlimitis0.054and
0.092 mgL1 for phosphateandsilicate,respectively.Themethodwasappliedtodeterminationofthe
analytesincertified referencematerialsofgroundwater,wastewater,andriverwatergivingresults
consistent withthecertified values.