A new method for nanomolar determination of silicic acid in seawater

A novel method is proposed to determine concentrations of silicic acid in seawater in the nanomolar range of 3–500 nM. It preconcentrates silicic acid through a “Magnesium Induced Co-precipitation” (MAGIC) step before a classical spectrophotometric measurement. The detection limit (3 (plus-minus) 2 nM) is improved by a factor 10 in comparison to the conventional colorimetric methods. The best precision obtained to date is (plus-minus)2 nM for a natural sample of 69 nM Si. No interference of phosphate was observed by contrast to previous methods. This simple method offers a simple, sensitive and accurate tool for silicic acid determination in depleted seawater, where its availability remains unknown.