8.10: Measuring Salinity

As noted in Chapter 10, chemical oceanographers were thrilled when they discovered that by using the Principle of Constant Proportions, they only had to measure one major constituent in the ocean to obtain the concentrations of the rest. But advances in the 20th century gave oceanographers a tool that provided an instantaneous readout of seawater salinity.

In 1729, Stephen Gray (1666–1736), a little-appreciated British chemist, discovered that electricity could flow through a wire, a property called electrical conductivity. By the 19th century, the power of that discovery would be fully harnessed. The Transatlantic Cable, completed in 1858, and the “wireless telegraph,” such as the one used aboard the Titanic, serve as two historic applications of electrical conductivity. Another application was driven by a need to measure the salinity of water in boilers aboard ships. A buildup of salt deposits can damage boilers. To solve this problem, engineers developed the first salinometers, instruments for measuring salinity.

Driven by an interest to use salinometers in the ocean, American physicist Frank Wenner (1873–1954) and oceanographers Rear Admiral Edward “Iceberg” Smith (1889–1961) and Captain Floyd Soule (1901–1968) published in 1930 Apparatus for the Determination aboard Ship of the Salinity of Sea Water by the Electrical Conductivity Method (Wenner et al. 1930). In doing so, they set the stage for one of the most important measurements in the ocean today.

Salinometers, also known as conductivity meters, measure the “strength” of electrical conductivity—that is, how quickly a material conducts electricity. Because seawater contains charged molecules—cations and anions—it acts as a weak conductor of electricity. In a salinometer, electrical conductance is measured as the flow of electricity through a wire connected at each end to pieces of metal called electrodes. Because the electrodes are separated by a gap, no electricity will flow through the wire unless a conductor, such as seawater, is present in the gap. By submersing the electrodes in seawater, a current is generated that is proportional to the concentration of salts in the seawater: the greater the conductance, the greater the salinity.

But simply knowing that a solution conducts electricity doesn’t get you very far. The key to making a salinometer useful lies in calibration, a method used to determine the quantitative relationship between conductivity and salinity. When you place a conductivity probe into a sample of seawater, you want to know that the number you read on the meter matches the real salinity of the sample. To do that, oceanographers calibrate their conductivity meters using solutions with a known salinity. Placing the conductivity meter into a solution with a known salinity permits the instrument to be adjusted so it displays and records an accurate salinity.

The most common solution used for calibration by oceanographers is something called standard seawater. It’s obtained from the North Atlantic Ocean, modified to precise specifications, and sold by a single company. That single source ensures that oceanographers around the world can calibrate their instruments with identical seawater. Standard seawater has an accuracy of ±0.0003, meaning any one bottle of standard seawater may differ from others by ±0.0003.

For decades the salinity of the ocean was reported in parts per thousand. Thus, if a sample of water has a salinity of 10 ppt, it’s made up of 990 parts water and 10 parts salt. Those units remain popular among some oceanographers and in other fields of study that require measurements of salinity. However, in 1978, oceanographers adopted the practical salinity scale, which reports salinity as a ratio of the conductivity of standard seawater to that of a precise concentration of potassium chloride (e.g., Millero et al. 2008). The practical salinity scale, unlike the traditional salinity scale, has no units. Thus, you can choose to report the average salinity of the ocean as 35 ppt, if you’re old school, or 35, if you’re new school.