Salinity refers to the concentration of dissolved inorganic salts found in water. Sodium chloride is the most abundant salt, but magnesium, calcium, and potassium ions also contribute significantly. Understanding salt content is fundamental for marine biology, as it directly influences water density, buoyancy, and the physiological survival of organisms. Oceanographers monitor salinity changes because it drives global ocean currents, and the slight variations in salt content affect the heat capacity of the ocean, playing a direct role in regulating Earth’s climate system.
The Scientific Basis of Measurement
The most precise and widely adopted method for measuring salinity relies on the water’s ability to conduct an electric current. While pure water is a poor conductor, dissolved salts dissociate into free-moving ions, such as sodium and chloride, which dramatically increase the water’s capacity to carry an electrical charge.
Modern instruments measure this electrical conductance, which is directly proportional to the total concentration of ions. As dissolved salt increases, measured conductivity rises predictably, providing a reliable proxy for salinity. Because temperature significantly affects conductivity, accurate instruments must simultaneously measure water temperature and apply a mathematical correction known as temperature standardization. This process standardizes the reading to a reference temperature, typically 15°C, ensuring a consistent inference of the true salinity value.
Accessible Field Methods
For quick, field-based measurements, the handheld refractometer is the preferred tool due to its portability and simplicity. This device operates on the principle of light refraction, where dissolved salts increase water density, causing light to slow down and bend at an angle proportional to the salt concentration.
The refractometer contains a prism and an internal scale calibrated to translate this angle of light deflection directly into a salinity reading. To use the instrument, a few drops of the water sample are placed onto the prism surface beneath a clear cover plate. Holding the device up to a light source allows the user to look through an eyepiece and read the line where the light and dark fields meet on the internal scale.
This optical method provides immediate results without needing external power or the complex temperature compensation required by conductivity meters. A less precise, but simpler, method involves using a hydrometer, which is a sealed glass tube that measures density by floating at different depths in the sample. Since salinity primarily influences water density, the depth to which the hydrometer sinks can be used to estimate the salt concentration, though with less accuracy than a refractometer.
Laboratory and High Accuracy Techniques
Scientists employ a laboratory-grade instrument called a salinometer, which is an accurate type of conductivity meter. These instruments achieve superior accuracy by maintaining stable measurement conditions, often using temperature-controlled environments for the sample chamber. Sensors are designed to minimize drift and polarization, ensuring a precise and repeatable measurement of electrical conductance that surpasses handheld devices.
Salinometers are routinely used in shore-based laboratories to analyze collected samples. The underlying technology is also integrated into oceanographic instruments known as CTD (Conductivity, Temperature, Depth) sensors. These sensors are deployed on research vessels and fixed buoys to gather real-time data from the deep ocean. Historically, salinity was determined through chemical titration, specifically by measuring the amount of chloride ions present. This method, known as the Knudsen method, was the primary standard for decades and is now used mainly to verify and calibrate modern electronic salinometers.
Interpreting Salinity Units
Measurements are expressed using one of two common units: parts per thousand (ppt) or Practical Salinity Units (PSU). Parts per thousand is the older unit, representing the number of grams of dissolved material found in every one thousand grams of seawater. For example, 35 ppt means there are 35 grams of total dissolved salt per kilogram of water.
The modern scientific standard is the Practical Salinity Unit (PSU), which is a dimensionless quantity. PSU is based on the ratio of the electrical conductivity of the sample water to the conductivity of a standard potassium chloride solution at a specific temperature and pressure. PSU is preferred because it is based on a precisely defined physical property, making it more accurate and internationally standardized for oceanographic data collection.