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How is seawater salinity measured?

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Seawater salinity, which represents the total weight of dissolved solids in one kilogram of seawater, is measured using both traditional and modern techniques.1

Traditional Methods

Historically, oceanographers determined salinity through titration, a laboratory technique used to measure chlorinity—the concentration of chloride ions. Because the major constituents of seawater exist in constant relative proportions, salinity can be accurately estimated from chlorinity measurements. In this process, silver nitrate is added to a seawater sample to react with the halides, allowing researchers to calculate the degree of chlorinity.1

Modern Methods

Since the late 1950s, shipboard electrical conductivity systems have become the standard for measuring salinity. Modern Conductivity-Temperature-Depth (CTD) systems utilize sensors to measure the electrical conductivity and temperature of seawater. These measurements are interpreted using an algorithm to determine "practical salinity," a scale based on the conductivity ratio. This method is highly precise and more reliable than the traditional chlorinity scale because conductivity is sensitive to changes in any ion, whereas chlorinity is ion-specific.12