Abstract
Flow injection analysis was applied to the spectrophotometric determination of silicic acid. A sample solution (80 μl) was introduced into a carrier stream of water via a loop-valve sample injector. The carrier stream met a molybdenum reagent stream {0.05 M Mo(VI), 0.135 M H2SO4}, and flowed together into a reaction tubing (PTFE, 0.5 mm i.d., 5 m). To complete chemical reaction the reaction tubing was heated at 50 °C in a silicone oil-bath. Residence time of the sample in the reaction tubing was about 1 min. The absorbance of heteropoly complex was monitored at 400 nm by a spectrophotometer with a flow-through cell (8 mm light path, 8 μl volume). The standard sample could be measured in a working range of 2 ppm to 100 ppm (SiO2) at a sampling rate of 60 samples h-1. Detection limit (S/N=2) was 0.3 ppm (SiO2). The precisions (C.V.) were 1.7 %, 0.2 % and 0.07 % for monosilicic acid of 2 ppm, 20 ppm and 100 ppm (SiO2), respectively.
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