Salinity is the measure of how much salt is present in a sample of water. This measurement is important for many scientific purposes, but in everyday life, salinity is most commonly measured to determine the quality of drinking water and in saltwater aquariums. There are two main tools that are used to measure salinity -- hydrometers and refractometers -- and both are simple to use.
A hydrometer is the simplest method for determining salinity, but also one of the least accurate. A hydrometer works by taking into account the weight of the salt in the water, which is called its specific gravity. A sample of saltwater is collected and weighed, and then a sample of pure (fresh, distilled) water of the same volume is also weighed. The difference is expressed as a ratio (weight of saltwater divided by weight of pure water). Most saltwater tanks should maintain a ratio of between 1.02 and 1.025, depending on the species in the aquarium.
Refractometers are some of the simplest and most common tools for obtaining a reasonably accurate measurement of salinity. They work on the principle that water bends light by slowing it down, and saltwater bends light more than pure water. Most handheld refractometers work by placing a droplet of the water sample on a glass plate and looking through the end to read a measurement of how far the light shining through the droplet is bent.
Salinometers work on the principle that saltwater will conduct electricity while pure water will not. The electrical charge moves through the salt particles. These tools provide measurements in units of microSiemens per centimeter. Drinking water is anywhere from 100 to 1,000 microS/cm, while seawater is about 54,000 microS/cm. The wide range provides the ability to carefully calibrate water salinity for scientific experimentation and especially sensitive aquarium habitats.
In a scientific setting, laboratory titration involves adding chemicals to a seawater sample to determine the exact volume of sodium chloride (salt) present. Equal parts of the water sample and potassium chromate are added to a flask and stirred while silver nitrate is added with a dropper. When the solution turns orange, the nitrate is ceased and the solution stirred until the orange disappears. Then the silver nitrate is added again, one drop at a time. When the solution remains orange permanently, a calculation is performed based on the amount of silver nitrate added to determine the weight of the salt present.