Chlorinity

Chlorinity is a measure of the chlorine content of seawater and other saline solutions, defined as the mass of chlorine (as chloride) in one kilogram of solution. It is typically expressed in grams per kilogram (g/kg), and by convention 1 g/kg is equivalent to 1 part per thousand by mass (ppt). In seawater, chlorinity reflects the concentration of chloride ions (Cl−) and is closely related to the overall salt content of the water.

Historically, chlorinity was determined by argentometric titration, in which a sample is acidified and titrated with silver nitrate (AgNO3) until silver chloride precipitates, indicating the chloride concentration. The result is converted to chlorinity using established standards, sometimes with an indicator or a pH-stable endpoint for accuracy. The value is then used to infer salinity through a conversion factor.

In typical seawater, chlorinity is about 19.345 g/kg, which corresponds to a practical salinity of approximately 35 PSU. The relationship between chlorinity and practical salinity is commonly approximated by S ≈ 1.80655 × chlorinity, with S representing salinity on the practical scale and chlorinity in g/kg. This link allows historical chlorinity measurements to be related to modern salinity values.

Modern oceanography largely relies on conductivity-based salinity measurements, but chlorinity remains important for historical datasets, calibration, and the study of seawater chemistry. It also serves as a reference point in discussions of seawater composition and paleoceanography.

See also: Salinity, Argentometric titration, Silver nitrate titration, Chloride, Seawater chemistry.