Comparison of Salinity Measurement Based on Optical Refractometer and Electric Conductivity: A Case Study of Urea in Seawater

Urea is one of the important components of the dissolved organic nitrogen (DON) in seawater and plays an important role in the marine carbon and nitrogen cycles. However, to our best knowledge, the effect of urea on seawater salinity is still unclear. Currently, seawater salinity is estimated using practical salinity
${S}_{P}$
, which is calculated based on conductivity, temperature and pressure measured by the conductivity-temperature-depth (CTD) sensor. The conductivity sensor is unable to detect non-ionized constituents, leading to potential errors in salinity measurements. In contrast, the optical refractive index of seawater is sensitive to both non-ionized and ionized solutes. In this article, a refractometer based on a V-shaped groove was established. The influence of urea dissolved in seawater on the refractive index and salinity of seawater was investigated, using the optical refractometer and conductivity sensor. The comparative experimental results demonstrate that the refractive index and salinity of seawater measured by the optical refractometer are linearly proportional to the urea concentration. On the contrary, the conductivity and salinity measured by CTD decrease slightly with the increased urea concentration. By comparing these results, the contribution of urea to salinity could be distinguished.