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The influence of urea in seawater on the seawater refractive index using optical refractometer

Salinity is one of the most important characteristic parameters of seawater. The absolute salinity 𝑆𝐴 is defined to be the “Total amount of dissolved material in grams in one kilogram of seawater”, and it is directly related to the seawater density. However, the absolute salinity is difficult to be measured in practice. At present, practical salinity 𝑆𝑃 is calculated by using conductivity, temperature and pressure measured by CTD sensors. The conductivity of seawater is dependent on the ions concentrations. However, the non-ionic components cannot be detected, though they may have significant influence on density. The optical refractive index is sensible to all dissolutions of seawater, therefore it is a good proxy of the concepts of salinity/density. In this paper, urea was used to study the influence of urea on the seawater refractive index based on optical refractive index method. The changes of seawater refractive index versus the urea concentration were studied by a Vgroove refractometer. This method provides a new method for measuring urea in seawater.