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

A method has been developed to determine silicic acid in sea water with sub mersible chemical analyzer. It is based on direct FIA, for fast and discret e measurements, and dual wavelength treatment of the signal to correct the refractive index interference, the main factor that affects in situ analysi s. 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 th e carrier stream. This allow high resolution in situ measurements, giving a ccess to small scale transitory processes concerning nutrients cycles in oc ean ecosystem. This colorimetric method is based on the formation of beta s ilicomolybdic acid reduced in intense colored molybdenum blue. Rapid and no n temperature dependent reduction is obtained by using tin(II) chloride as reductor. There is little interference from phosphate in the usual conditio ns at sea. For in situ applications, we gave special attention to the effec t of temperature (0-30 degrees C) and salinity (0-34) variations that can g reatly influence the silicomolybdate complex formation. This reaction zone has to be thermostated and kept above 20 degrees C. A maximum signal enhanc ement of 12% is encountered when samples salinity varies from 34 to 0 (PSS7 8), but no significant deviations are observed between 34 and 28 (PSS78). T he precision of the method is 1% for silicate concentrations above 5 mu M a nd the limit of detection is 0.3 mu M. The comparison of the measurements o btained by this method and the usual flow sequential analysis system showed good agreement. The method has been implemented on an in situ chemical ana lyser and simultaneous silicic acid and nitrate concentration measurements were carried out in coastal waters. (C) 1998 Elsevier Science B.V. All righ ts reserved.