Article written by: Jason Danaher Ph.D.
Nearly 70% of the earth’s surface is covered by water and over 95% of that water is classified as saline. Saline water has a high concentration of dissolved salts or ions in solution compared to freshwater. The dissolved salts are composed of positively charged cations and negatively charged anions. While there are numerous dissolved ions in saltwater, we will use sodium chloride (NaCl) as an example. The solid compound NaCl contains the positively charged sodium (Na+)ion and negatively charged chloride (Cl- )ion held together by an electrostatic force. When added to water the ionic bond of NaCl is overcome by the electrostatic properties of the water creating Na+ and Cl-.
NaCls + H2Ol → Na+aq+ Cl-aq
When salts disassociate into cations and anions in the water they can not only bend light, but have the characteristics to conduct electricity in the water. This property allows an aquatic systems operator to quantify the concentration of salts using a refractometer (Figure 1) or a hand-held meter with electrode (Figure 2) to measure the specific conductance of the water and its ability to carry an electrical charge.
The cumulative mass of dissolved salt concentration (Na+, Ca2+, K+, etc.) is often expressed in one of three ways: Salinity as parts per thousand (ppt); Total Dissolved Solids (TDS) as milligrams per liter (mg/L) or parts per million (ppm); or Conductivity as microsiemens per centimeter (µS/cm). Often a refractometer (Figure 1) is used for aquaculture to quantify the dissolved salt concentration and characterize the sampled water. A digital salinity refractometer works by shining a light source through the water sample placed on a prism. The salt concentration bends or refracts the light at a specific angle. An internal sensor measures the angle and provides the user with a salinity reading displayed on the screen. The refracted light is directly proportional to the concentration of dissolved salts present in the water as well as the temperature of the solution.
A hand-held meter with a probe is often used to measure TDS and Conductivity in aquaponics (fish + plants) or other integrated systems (Figure 2). A portable conductivity meter measures electric current by applying a voltage between electrodes in the probe. The meter detects how easily current flows through ions in the water, then converts this to TDS or Conductivity using a built-in formula. The specific conductance is directly proportional to the concentration of dissolved salts present in the water as well as the temperature of the solution. Mathematical formulas allow an operator to manually convert from one unit to another, but many meters will automatically convert results between salinity, conductivity, and/or TDS if or when conversion is required. The manager and operator of the aquatic system should choose the meter accordingly.
Figure 1. Portable, digital refractometer with automatic temperature compensation to quantify salinity (ppt).
Figure 2. Portable, multi-parameter meter with automatic temperature compensation to quantify conductivity (µS/cm) or total dissolved solids (mg/L).
But meters are only accurate when they are stored and maintained properly. Store the meter in a clean and dry area after each use. After each use the refractometer’s well or the hand-held meter’s probe and electrode should be washed with distilled water to make sure dried salts do not accumulate while being stored. In addition, the devices should be calibrated regularly using fresh standards. The meter’s maintenance should be up-to-date, which includes checking for any signs of wear or damage to the meter’s cable. Besides mineral build up on the electrode, temperature changes of the solution or multiple solutions (i.e. aquatic systems) can result in measurement errors. Mathematical formulas also exist to manually compensate for differences in temperature, but most modern refractometers and hand-held meters are equipped with an automatic temperature compensation (ATC) probe and adjust for temperature differences.
Dissolved salt concentration is an important parameter to monitor as it is vital for osmoregulation and helping fish or plants maintain balanced amounts of ions and water in their body. Refractometers and hand-held meters are relatively simple to use and can quickly and accurately provide reliable and quantifiable results for system operation and maintenance. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com for additional information on measuring dissolved salt concentration in your aquatic system.