Salinity is a measurement that has different meanings in different industries. Overall it is a measurement of salts in water. Salts can be measured by electrolytic conductivity (EC), refractometry, titration of chloride, or specifically for sodium chloride using a sodium Ion-Selective Electrode (ISE).
Salinity measurements can be broken down into two major forms. Those that are for oceanographic studies for seawater salinity and those for food or brine solutions in which there is a weight/weight or weight/volume relationship.
There are meters that offer seawater salinity scales. The seawater salinity scales are Practical Salinity Scale (PSU); 1978, Percent Scale (%); and Natural Seawater Scale(g/L); 1966.
Practical Salinity Units (PSU) - This is a practical scale based on the precise measurement of the electrical conductivity of a solution with a known salinity range. The relationship derived from the scale relates salinity, conductivity, temperature, and pressure and use a solution with a salinity of 35 ‰ as datum point. This is taken to have a conductivity of 42.914 mS/cm of 15 °C at standard atmospheric pressure.
NaCl % Percent Scale – The NaCl % scale is an older salinity scale used for seawater salinity. In this scale 100% salinity is equivalent to roughly 10% solids.
Natural Sea Water Scale (UNESCO 1966) - Salinity is based upon a conductivity ratio of sample to “standard seawater” at 15 oC and 42.914mS/cm being the corresponding conductivity of KCl solution containing a mass of 32.4356 g KCl / 1 Kg solution.
Salinity other than Seawater
Brine solutions and salinity as sodium chloride use a different scale than seawater. Salinity is expressed in a weight/weight (i.e. 10g/100g) or weight/volume (i.e. 10g/100ml). This is not the same as the NaCl % scale found on some conductivity meters. With that scale 100% NaCl is equivalent to Atlantic Seawater that is approximately 35 ppt (3.5 g/100 ml). Titrators and meters are available to measure salinity as w/w or w/v.
Hanna Instruments offers a variety of technologies to measure Salinity. Potentiometric measurements with portable and benchtop pH/ISE meters are available to use with the sodium ISE for direct ion concentration measurements. Potentiometric titrations systems are available for either direct measurement using the ISE mode or for performing a salt titration. Refractometers, testers, and chemical test kits are also available.
Below are the most common products used for salinity measurement.
Titrators are available to perform a titration using the silver/sulfide ISE. With this titration the electrode is used as an indicator to follow the progress and detect the endpoint of an argentometric titration of chloride with silver nitrate titrant. During the titration the silver from silver nitrate reacts with the chloride in the sample to form insoluble silver chloride. When chloride is no longer available then silver ions will be in the solution. The ISE follows the increasing silver ion concentration as silver nitrate titrant is added. The stoichiometric end point, a large change in mV occurs. Titration systems are available with one or two analog board inputs and options for one or two dosing pumps.
Benchtop meters include versions that measure salinity by conductivity along with a version that can use an ISE to measure sodium or chloride. A two channel option allows one channel to be used with a pH/ISE probe and the other channel for use with a conductivity sensor to measure salinity. The benchtop meters have the ion charge and formula weight pre-programmed for use with an ISE. The benchtop meters also have incremental methods pre-programmed including the known addition, known subtraction, analyte addition and analyte subtraction methods.
Portable Salinity Meters
Portable salinity meters include those that measure salinity by conductivity, ISE, or refractometery. Which scale is available is based on the technology used with conductivity using the seawater scales, ISE then weight/weight or weight/volume, or refractometry that has algorithms for both.
Salinity Measurement by Conductivity
The portable meters that use conductivity as a measurement technology are programmed with the seawater salinity scales for oceanographic and environmental studies. This category includes multiparameter meters that measure other key water parameters including pH, ORP, dissolved oxygen, turbidity, temperature, and pressure. Some of the multiparameter meters use nitrate, chloride, and ammonium ISEs and offer a GPS option for automatic tagging of data.
Salinity Measurements by Sodium ISE
This category includes portable meters that measure salinity as sodium chloride or sodium using a sodium ISE. A dedicated salinity meter allows the measurement of sodium chloride up to 300 g/L.
Salinity Measurement by Refractometry
This category includes refractometers for salinity measurement. The salinity scale used is dependent on the application. A seawater version allows results to be displayed as PSU, ppt, or specific gravity (S.G.). A food version is available that can display results as g/100 g, g/100 ml, S.G., and °Baumé.
In this category is a salinity tester that uses a conductivity sensor for the measurement of artificial seawater used in saltwater and reef aquariums. This version allows results to be displayed as PSU, ppt, or specific gravity (S.G.).
Electrode & Probes
Salinity probe include the replacement probes for the portable and benchtop meters. These include conductivity sensors, sodium ISE and the silver/sulfide ISE for salt titrations.
Solutions for salinity measurement include salinity and sodium chloride standards used for calibration. Also in this category are the maintenance solutions for the ISEs. These solutions include condition, storage, and refill solutions.
Chemical Test Kits
A salinity Chemical Test Kit (CTK) is available as a single parameter or with other key parameters for marine studies. The salinity CTK is a simple titration method. For educational use there is the Hanna Backpack Lab for Marine Science and it includes a salinity test kit along with a teacher’s guide that discusses the importance of the parameter.
Replacement reagents used with the Chemical Test Kit. These reagents are used with both the single parameter test kit and the multiparameter versions.
This category has the titrants and reagents necessary to perform an agentometric salt titration.
Accessories include polishing strips for the salinity ISE and magnetic stirrers to ensure adequate mixing when using the ISE.