Liquid-waveguide spectrophotometric measurement of low silicate in natural waters

Silicon is an essential nutrient required for the growth of some aquatic organisms in both freshwater and marine environment. Such aquatic organisms, most notably diatoms and radiolaria, use silicon as building blocks for their skeletons. In oceanic subtropical gyres, it has been observed that the silicate concentration in surface seawaters is seasonally or chronically depleted to 0.1–0.6 μM, particularly after spring blooms. The low concentration of silicate in ocean surface waters can limit the productivity of diatoms, which can affect the carbon export in these waters. To study the nutrient dynamics, including silicon, in these waters, development of a sensitive spectrophotometric method that can perform onsite measurements of low silicate in surface seawaters is urgently needed.

A robust, sensitive method for measurement of low silicate in natural water is developed. The method is based on the reaction of silicate with ammonium molybdate to form a yellow silicomolybdate complex, which is then reduced to silicomolybdenum blue by ascorbic acid. This method shows no refractive index effect and a small salinity effect that can be corrected for seawater samples. It was found that the use of poly-vinyl alcohol can prevent the precipitation formation in the ammonium molybdate solution and improve the stability of the silicomolybdenum blue complex. The sensitivity of this method is substantially enhanced by using a liquid-waveguide capillary cell. The detection limit is 0.1 μM, and
working range is 0.1–10 μM for using 2-m liquid-waveguide capillary cell (LWCC). The method can be used for both freshwater and seawater samples and has been used to study the distribution of silicate in surface seawater of Gulf Stream in Florida straight. 


Liquid-waveguide analyzer for silicate determination: LWCC, liquid-waveguide capillary cell;

MC, mixing coil; AsAc, ascorbic acid; IV, injection valve; DIW, deionized water; R1, molybdate solution;

R2, oxalic acid solution; R3, ascorbic acid solution; R4, 10%HCl (v/v).



Amornthammarong, N.; Zhang, J.-Z. Talanta2009, 79, 621-626.