In situ determination of silicic acid in sea water based on FIA and colorimetric dual-wavelength measurements. Original Research Article

A method has been developed to determine silicic acid in sea water with submersible chemical analyzer. It is based on direct FIA, for fast and discrete measurements, and dual wavelength treatment of the signal to correct the refractive index interference, the main factor that affects in situ analysis. Fast analysis rate (60 discrete samples per hour) is achieved because of a very short lag time between sampling and injection of the sample into the carrier stream. This allow high resolution in situ measurements, giving access to small scale transitory processes concerning nutrients cycles in ocean ecosystem. This colorimetric method is based on the formation of beta silicomolybdic acid reduced in intense colored molybdenum blue. Rapid and non temperature dependent reduction is obtained by using tin(II) chloride as reductor. There is little interference from phosphate in the usual conditions at sea. For in situ applications, we gave special attention to the effect of temperature (0–30°C) and salinity (0–34) variations that can greatly influence the silicomolybdate complex formation. This reaction zone has to be thermostated and kept above 20°C. A maximum signal enhancement of 12% is encountered when samples salinity varies from 34 to 0 (PSS78), but no significant deviations are observed between 34 and 28 (PSS78). The precision of the method is 1% for silicate concentrations above 5 μM and the limit of detection is 0.3 μM. The comparison of the measurements obtained by this method and the usual flow sequential analysis system showed good agreement. The method has been implemented on an in situ chemical analyser and simultaneous silicic acid and nitrate concentration measurements were carried out in coastal waters.