A number of minerals commonly referred to as salts give seawater its characteristics. Apart from the laboratory, salinity is commonly measured by aquarium enthusiasts and growers who suspect a possible salt build-up in the soil. While there are several tools that can be used to measure salinity, the exact measurement results depend on your goals. Read aquarium guides or research specific plant information to determine the salinity you need.
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Method 1 of 3: Using a Hand Refractometer
Step 1. Use this tool to accurately measure the salinity in the liquid
A refractometer measures how much light is bent or reflected as it passes through a liquid. The more salt (or other substance) dissolved in the water, the greater the resistance it faces, and the greater the amount of light that is bent.
- A hydrometer is a cheaper option, but with a lower level of accuracy.
- To measure soil salinity, use a conductometer.
Step 2. Use the appropriate refractometer for the type of liquid you are measuring
Different liquids reflect light in different ways, so to accurately measure the salinity (or other solids content) in them, use a refractometer designed specifically for the liquid you are measuring. If the liquid is not specifically stated on the device packaging, it is likely that the refractometer is intended for measuring salt water.
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Notes:
A salt refractometer is used to measure sodium chloride dissolved in water. Seawater refractometers are used to measure salt mixtures that are generally contained in seawater or saltwater aquariums. Inappropriate tools will produce readings with an error rate of about 5%, which may still be acceptable for out-of-laboratory analysis.
- Refractometers are also designed to take into account the expansion of certain materials due to changes in temperature.
Step 3. Open the plate located near the beveled end of the refractometer
Hand refractometers have one rounded end that opens for viewing, and one angled end. Hold the refractometer so that the inclined surface rests on top of the instrument, and look for a small plate near it that slides to one side.
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Notes:
If you have never used a refractometer, it is best to calibrate it first to get a more accurate reading. The calibration process is explained at the end of this section, but you may want to read the following steps first to better understand how to use a refractometer.
Step 4. Pour a few drops of liquid into the opened prism
Use an eyedropper to pick up some of the liquid you want to measure. Pour the liquid into the transparent prism that opens when you slide the refractometer plate. Pour the liquid over the entire surface of the prism.
Step 5. Carefully close the refractometer plate
Close the prism again by returning the plate to its initial position. The components of the refractometer are small and very sensitive. Do not force the prism in if it is slightly stuck, however, wiggle it back and forth with your fingers until it slides smoothly again.
Step 6. Look inside the refractometer to see the salinity reading
Look inside the round end of the refractometer. You should see one or more scale numbers. The salinity scale is generally marked 0/00 which means "parts per thousand", from 0 at the base of the scale to 50 at the end. Find the measure of salinity at the line where the white and blue sections meet.
Step 7. Wipe the prism with a soft, damp cloth
After obtaining the required measurement results, open the refractometer plate again, and use a soft, slightly damp cloth to wipe the prism clean of any remaining liquid droplets. Water remaining in the prism or wetting the refractometer can cause damage.
Moist wipes can be used if you don't have a cloth that is flexible enough to cover the entire surface of the tiny prism
Step 8. Calibrate the refractometer periodically
Calibrate the refractometer between uses periodically to justify readings using pure distilled water. Pour in water like any other liquid, and check if the salinity reading is "0". If not, use a small screw to adjust the calibration bolt, which is usually located under the small cap on the top or bottom, until the salinity reads "0".
- A new, high-quality refractometer may only need to be calibrated every few weeks or every few months. However, cheaper or older refractometers may need to be calibrated before each use.
- Your refractometer may come with a calibration guide stating the specific water temperature. If your refractometer did not come with a guide, use room temperature distilled water.
Method 2 of 3: Using a Hydrometer
Step 1. Use this fairly inexpensive tool to measure water accurately
A hydrometer measures the specific gravity of water, or its density compared to H2Oh pure. Because almost all salt is greater than water, hydrometer readings can provide information on the amount of salt content. This method is accurate enough for most uses, such as measuring salinity in an aquarium, but many hydrometer models are inaccurate or easy to misuse.
- This method cannot be used on solids. If you are measuring soil salinity, use a conductivity measurement.
- To get more accurate measurement results, use a fairly inexpensive evaporation method, or use a faster refractometer.
Step 2. Determine the type of hydrometer you need
Hydrometers, also known as specific gravity gauges, are sold online or at aquarium stores in a variety of shapes. Glass hydrometers floating in water are generally more accurate than the other options, but often don't give precise measurements (with longer decimal places). Plastic swing arm hydrometers may be cheaper and more powerful, but over time their accuracy tends to decrease.
Step 3. Choose a hydrometer that lists the standard temperature
Because different materials expand and contract at different rates as they heat or cool, knowing the calibration temperature of a hydrometer is essential for measuring the salinity of a liquid. Choose a hydrometer that lists the temperature on the appliance or its packaging. It is easiest to measure salinity using a hydrometer calibrated at 15.6 C or 25 C, as they are the most common standards for measuring salinity of brine. You can use a hydrometer with different calibration temperatures, as long as it comes with a guide chart to convert the readings to salinity.
Step 4. Take a water sample
Pour some of the water you want to measure into a clean, clear container. The size of the container must be large enough to accommodate the hydrometer, with sufficient depth to absorb most of it. Be sure to clean the container of dirt, soap, or other ingredients.
Step 5. Measure the temperature of the water sample
Use a thermometer to measure the temperature of the water sample. As long as you know the temperature of the sample, and the standard temperature of the hydrometer, you can calculate the salinity.
To get a slightly more accurate reading, you can warm or cool the sample until it reaches the appropriate temperature for the hydrometer. Be careful not to get the water temperature too high, as evaporating or boiling water can greatly affect its specific gravity
Step 6. Clean the hydrometer if necessary
Scrub the hydrometer to remove any visible dirt or other solids on the surface. Rinse the hydrometer with clean water if it was previously used to measure salt water, as salt may stick to its surface.
Step 7. Dip the hydrometer slowly into the water sample
A glass hydrometer can be partially submerged in water, then released until it floats on its own. Swing arm hydrometers will not float, and usually come with a handle or rod to help you dip them into the water without getting your hands wet.
Do not immerse the entire glass hydrometer, as this may interfere with the reading
Step 8. Gently shake the hydrometer to remove air bubbles
If air bubbles adhere to the surface of the hydrometer, its buoyancy will result in incorrect readings. Gently shake the hydrometer to remove air bubbles, then wait for the water to settle before continuing.
Step 9. Read the measurement results on the swing arm hydrometer
Place the swing arm hydrometer so that it is level, without any part tilting to one side. The size shown is the specific gravity of water.
Step 10. Read the measurement results on the glass hydrometer
On a glass hydrometer, the reading can be seen from the surface of the water that touches it. If the water surface is curved up or down, ignore the curve and read the measurements on the flat side of the water surface.
The curvature of the water's surface is called the "meniscus" and is an event caused by surface tension, not salinity
Step 11. Convert the specific gravity measurement results to salinity if necessary
Many aquarium care guides list a specific gravity, usually between 0.998 and 1,031, so you don't need to convert your measurement to salinity, which is generally between 0 and 40 parts per thousand (per mile). However, if your aquarium care manual only lists salinity, you'll need to convert your specific gravity measurement results to salinity yourself. If your hydrometer doesn't come with a conversion chart, look up a "conversion of specific gravity to salinity" table or calculator online or in an aquarium manual. Be sure to use a table or calculator that matches the standard temperature listed on the hydrometer, or you will get incorrect results.
- This table can be used for hydrometers calibrated at a standard temperature of 15.6 C. Note that the temperature of the water sample is expressed in C.
- This table is used for hydrometers calibrated at 25 C. The temperature of the water sample is expressed in units of C.
- These charts and calculators also vary by type of liquid, but are mostly used for salt water.
Method 3 of 3: Using a Conductometer
Step 1. Use this method to measure the salinity of water or soil
The electrical conductivity meter is the only instrument commonly used to measure soil salinity. It can also be used to measure the salinity of water, but a high-quality electrical conductivity meter may be much more expensive than a refractometer or hydrometer.
To confirm salinity measurement results, some aquarium enthusiasts sometimes use a conductometer and one of the other tools in this article
Step 2. Select the electrical conductor
This tool emits an electric current through a certain material, and measures its conductivity. The more salt contained in water or soil, the higher the conductivity. To get accurate readings of typical water and soil samples, choose a conductometer that can measure up to at least 19.99 mS/cm (19.99 dS/m).
Step 3. Mix the soil with distilled water to measure it
Mix one part soil with five parts distilled water, stir until combined. Let the mixture sit for at least 2 minutes before continuing. Since distilled water doesn't have any salts or electrolytes in it, the measurements you get will reflect the content of both in the soil.
In the laboratory, you may have to let the mixture separate for 30 minutes, or use "saturated earth paste" which can take up to 2 hours for more accurate results. However, this step is rarely performed outside the laboratory, and the method above is still quite accurate
Step 4. Remove the conductometer cap and dip into the sample to the appropriate depth
Remove the protective cover of the thin end of the conductor. Dip the thin end into the water until it hits the mark. Or, if there are no markings on the conductometer, dip it deep enough so that the part is submerged. Most conductometers are not waterproof above a certain point, so do not immerse the instrument in water.
Step 5. Move the conductometer gently up and down
This movement aims to get rid of air bubbles trapped in the tool. Do not move it too hard, because it can actually draw out the water inside.
Step 6. Adjust the temperature according to the conductometer manual
Some conductometers can change the temperature of the liquid (which can affect conductivity) automatically. Wait at least 30 seconds for the conductometer to adjust to the temperature of the liquid, or longer if your liquid sample is really cold or hot. Other conductors have knobs that can be adjusted manually to change the temperature of the liquid.
If your conductometer is not equipped with one of the options above, you may be able to use the chart that came with the device to adjust the reading to the temperature of the liquid
Step 7. Read the results on the screen
Conductometer displays are usually digital and will give you results in mS/cm, dS/m, or mmhos/cm. Fortunately, these three units are the same, so you don't need to convert them.
The length of the above units, respectively, is milliSiemens per centimeter, deciSiemens per meter, or millimho per centimeter. "Mho" (as opposed to ohm) is the old name for the Siemens unit, but is still used in some instruments
Step 8. Determine if the soil salinity is suitable for your plants
In the manner described here, a conductometer reading of 4 or more indicates a hazard. Sensitive plants such as mango or banana may be moderately affected by a conductivity of 2, while vigorous plants such as coconut may still grow at a conductivity of 8-10.
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Notes:
Whenever you're looking for a conductivity range for a particular plant, look for a way to measure it as well. If the soil is diluted with 2 parts water, or with just a little water to form a paste, instead of the 1:5 ratio as in this article, the results may be very different.
Step 9. Calibrate the conductometer periodically
Periodically calibrate the conductometer between uses by measuring the "electrical conductivity calibration solution". If the measurement result does not match the conductivity stated by the solution, use a small screw to adjust the calibration bolt until the correct result is obtained.
