How To Test the Salinity of Water

Salinity is the measurement of salts dissolved in a solution of water. Salinity is measured in percentage (%) or parts per thousand (ppt) and is tested either with a conductivity meter, hydrometer, or refractometer. 

Salinity is an important water parameter to measure in terms of water quality as different crops, livestock, and aquatic organisms require different salinity environments to survive. Small fluctuations in salinity can pose issues such as stress, or can even be fatal to organisms, which has a devastating effect on surrounding ecosystems. 

Different water bodies have different salt content/salinity ranges; freshwater has a salt value of >0.5ppt, saltwater ~34-36ppt, and brackish water is a mix of both so it can vary. The salinity of brackish water fluctuates depending on how much fresh or saltwater flows into the water body. 

How to Test the Salinity of Water

Salinity can be tested in three different ways:  

• Handheld Refractometer
• Hydrometer
• Conductivity meter

The most important factor when testing the salinity of the water is to understand salinity measurements depending on the type of water you will be testing.

Handheld Refractometer

This tool looks like a telescope and is the simplest way to measure the salinity of water accurately. They work by measuring how much light bends or refracts (refractive index) when water is added under a plate, and are most commonly used in aquaculture industries such as aquariums. 

How to use it?

1. Calibrate it first to get an accurate reading.
2. Open the plate to expose the prism.
3. Pipette the solution you wish to test.
4. Add a couple of drops of the solution being tested onto the prism.
5. Close the plate gently.
6. Look through the round end of the meter to read the salinity level. You will see a scale, most likely in parts per thousand (ppt) ranging from 0 to 50.
7. Record the result.
8. Gently wipe the prism with a cloth or tissue.

Hydrometer

This is the most inexpensive tool to test the salinity of water, whilst giving reasonably accurate measurements. Hydrometers measure the specific gravity of water that uses the Archimedes principle. They are great for measuring salinity in aquariums but cannot be used for testing the salinity of water in soils. 

If using a hydrometer, you must note the temperature when it was calibrated to calculate an accurate salinity reading. It is recommended to calibrate a hydrometer to 60 °F (16 °C) or 77 °F (25 °C), as these are the general standards for testing the salinity of water. 

Hydrometers come in 2 types: swing-arm hydrometers and glass hydrometers. 

How to use them?

1. Take a water sample and place it into a container ensuring the container has been cleaned with the water being tested to avoid any errors. You will need enough water so the hydrometer can be submerged deep enough. 
2. Measure the temperature of the water being tested – this is needed later to calculate the salinity level. 
3. Clean the hydrometer in freshwater.
4. Place the hydrometer into the sample. Note that, glass hydrometers will float, this is normal, so never try to fully submerge glass hydrometers as this will give inaccurate readings. 
5. If there are any air bubbles on the hydrometer, gently shake the hydrometer until they are gone as bubbles can affect the reading. 
6. If using a swing-arm hydrometer, whilst keeping it level, read the measurement, this will be pointing to the specific gravity of the water sample. 
7. If using a glass hydrometer, the measurement will be where the surface of the water touches the meter – make sure you measure the flat water surface, not the meniscus. 
8. Convert the relative specific gravity (sg) into salinity if needed. 

Conductivity Meter

Conductivity meters do not provide direct salinity readings, instead, they measure the conductivity of water (mS/cm: S=Siemens, an electrical conductivity unit. mS=milliSiemens), which is why they are called electrical conductivity (EC) sensors. EC sensors are used to test the salinity of water as they record the number of total dissolved solids (TDS) in water. High conductivity readings from EC sensors indicate water with a high salinity, which is why they are most commonly used. 

EC sensors use an electrical current that passes between two metal plates of electrodes, measuring the current that travels between the two plates. Conductivity is the measurement of electrical flow through the water, which is directly related to the ion (salt) concentration of water. 

Conductivity probes usually come in 2 types, a regular conductivity probe and one for industrial industries. The great thing with EC probes is that they do not only measure salt content in water. Most EC probes will also measure nutrients and impurities in the water, covering a wider range of industries like hydroponics, aquaculture, and freshwater systems for example.

How to use them?

1. Calibrate it first to ensure an accurate reading. 
2. If testing salinity in the soil, mix the soil with distilled water (1 part soil, 5 parts water). 
3. Remove the protective cap on the EC sensor.
4. Dip the EC probe in the sample to the required level – most EC probes have a marker on them. Ensure that you only submerge the probe in the sample as EC sensors are not waterproof.
5. Move the probe up and down to remove any air bubbles. 
6. Adjust the temperature as some EC meters automatically adjust which can give you inaccurate readings. If your EC meter doesn’t have this function, you can manually correct this later using a chart. 
7. Read the result from the display. Most EC sensors are measured in mS/cm, so if you require the salinity result to be in ppt, you will need to convert this. 

Why is Testing the Salinity of Water Important? 

It is important to test the salinity of water for many reasons. Salinity meters most commonly measure the salinity of water in 4 main industries:

• Wastewater 
• Surface Water 
• Groundwater 
• Agriculture

Wastewater

In wastewater treatments, salinity must be tested to determine the salt content of the water before it reenters the surrounding environment. 

Salinity enters the wastewater system via agricultural, sewage, and stormwater runoff. Therefore, it is important to measure salinity as any minor changes in levels can cause issues to plants, animals, and the environment. 

Surface Water

Testing salinity is vital in surface waters and aquaculture, as different aquatic organisms have different optimal levels to survive. Organisms living in saltwater have an optimum salinity level of 40 ppt, and freshwater organisms cannot tolerate salinity levels exceeding 1 ppt. 

Organisms that live within brackish conditions must be able to tolerate salinity changes as the inflow and outflow of water can fluctuate depending on how much fresh or saltwater enters. 

Groundwater

Measuring the salinity in groundwater indicates the salt content in surface soil. By testing the salinity levels in soil, you can see how much salt will be extracted via the plant’s roots. Testing salinity levels in the soil are usually done after the irrigation season as this is when salinity is at its highest. 

In addition to regular salinity testing, samples can be sent to a laboratory for further analysis. In lab settings, samples can be screened to look at ion content, TDS levels, total hardness, carbonates, sodium absorption ratio, and sodium analysis. 

Agriculture

The salinity of water is measured in the agricultural industry to check if the water is safe and clean for irrigation and livestock, and in industrial settings to ensure that industrial discharge is free from high levels of dissolved salts. 

Testing the salinity of water is also useful within the health and manufacturing industries to measure the water quality, and is also paramount for basic field testing. 

Salinity Values & Their Uses

Water is an extremely valuable source, covering a wide range of uses in different industries. Depending on the salinity value, depends on what industry the water is used for. 

What Are the Effects of Salinity?

Salinity is found in different bodies of water and is always changing due to factors such as runoff or inflow and outflow of fresh or saltwater. 

When the salinity in water increases it can cause many environmental and economical problems, such as:

• Affecting crop yields: poor health & growth to salt-tolerant crops.
• Poor health or even death of plants, reducing biodiversity, and increasing salt-resistant species in an area.
• Salt can corrode infrastructures such as bridges and roads costing money to repair and requiring regular maintenance.
• Killing aquatic organisms that require low or specific salinity levels, or they will need to learn to adapt to changes, however, not many species can do this.
• Additional water treatment solutions may need to be created.

How to Adjust Salinity?

In most situations, salinity must remain stable. After testing you may find the salinity level is too high or too low, therefore you will need to adjust it.

If the salinity level is too low, you can simply add saltwater in small quantities until the ideal salinity level is reached. For precision, sometimes a titration method is used. 

If the salinity level is too high, you will need to change the balance of the water. To reduce the salt content in water, the easiest way is by adding freshwater, or in agricultural industries, irrigation methods are used. 

Summing Up Testing Water Salinity

Salinity is a vital water parameter to test in different industries as water is such a useful resource. Salinity can be defined as the number of salts dissolved in a solution of water that is measured either in percentage (%) or parts per thousand (ppt). 

Before purchasing any testing equipment like a refractometer, hydrometer, or conductivity meter, make sure you know it will provide you with accurate readings and that you feel comfortable using it. 

If you have any questions regarding testing salinity, the conductivity probes we have to offer, or any other water parameter testing kits, please feel free to reach out to our world-class team at Atlas Scientific, we are always happy to help!

Conductivity Circuit & Probes