Determination of silicate in natural waters based on the formation of the yellow α-silicomolybdenum using a step flow analysis

This study focuses on the reaction kinetics of the yellow α-silicomolybdenum complex for the determination of silicate in natural waters. The less sensitive yellow α-silicomolybdenum reaction has several advantages: the α-silicomolybdenum color is stable; it has less salt effect and phosphate interference. However, it is still unfavorable to be employed on automated analysis due to the slow reaction rate. To solve this problem the pH of the reaction is adjusted to 4.2-4.4 by a buffer system so as to enhance the reaction completeness to less than 4 minutes under room temperature, and can be further shortened to less than 2 min by moderate heating. Since the adding of bleaching reagent is avoided, the final absorbance may contain some signals attributed to phosphate: 〖Abs〗_(Si+P)^λ ={[P]×ε_p^λ +[Si]×ε_Si^λ }×b×Q_s/Q_f ×10-6+RB Where ε_Si^λ and ε_p^λ are molar extinction coefficients of silicomolybdic acid and phosphomolybdic acid(M-1cm-1), respectively, at a wavelength of 400(nm), b the path length(cm), [Si] and [P] the concentrations for silicate and phosphate(M),Qs and Qf represent the sample and final volume ratios and RB the reagent blank. The silicate concentration should be corrected for the phosphate interference for which the phosphate concentration has been identified by a separate channel: [Si](μM)=(〖Abs〗_(Si+P)^λ-[P]×ε_p^λ×b×Q_s/Q_f )/(ε_Si^λ×b×Q_s/Q_f )×106 The molar extinction coefficient for silicate(ε_Si^λ) was formed to be functions of temperature and salinity, but that for phosphate(ε_p^λ) is not. At the wavelength of 400 nm: ε_Si^400(T,S)(M-1cm-1)= 1166+T×(4.4-0.1532×S+0.00154×S2) ε_p^400(T,S) (M-1cm-1) = 390 20 oC < T <40oC, 0< S <35 A new automated analysis system for the determination of silicate has been proposed and it has been compared with the traditional flow injection analysis. The system uptakes samples and add reagents in a quick flow manner to fill a thermostat cuvette, then the flow is stopped so that the sample is allowed to stay in the cuvette for complete reaction. Since the determination is static and the color is fully developed, it has a higher sensitivity and better precision than do conventional methods. The minimum resolution (Abs=0.001) is 1 μM Si with 1 cm cuvette and can be lowered to 0.2 μM Si if a 5 cm long cuvette is used. The linear range is up to 200 μM Si. The precision at 100 μM Si is less than 0.5%.