The Influence of Dissolved Organic Matter to the Seawater Refractive Index Using Optical Refractometer

Abstract Optic refractive index is a good proxy for in situ salinity measurement of seawater according to TEOS-10. Currently, salinity of seawater is estimated by practical salinity (SP), which is calculated by measuring electric conductivity, temperature and pressure using Conductivity-Temperature-Depth (CTD) sensors. However, the electric conductivity method only takes into account the conductive compositions to seawater salinity and ignores the contribution of non-ionic components. However, non-ionic components in seawater affect seawater salinity according to the definition of salinity of seawater, but they could not be measured by conductivity sensor. Optical refractive index is sensitive to all dissolved matters of seawater including ionic and non-ionic components. Here, we take the dissolved organic matter (DOM) extracted from Aoshan Bay seawater as the representative sample to experimentally study the contribution of the non-ionic component to the refractive index of the standard seawater. Seawater refractive indices with different DOM concentrations were measured by a home-made V-shaped groove refractometer. The experimental results show that refractive index of seawater increases linearly with increasing DOM concentration at rate of 1.19 × 10-4/1(g/kg) DOM.