How to measure the salinity of water

How to measure the salinity of water

How to measure the salinity of water: useful information onunit of measurement of salinitycome onhow to measurethe amount ofsaltpresent in the water.

Salinity unit of measure: ppt, ppm and psu

Theresalinityindicates theconcentrationof salt dissolved in a solution. In the past, theunit of measurement of salinityit was therechlorinity, that is a quantity that expresses the quantity of chlorides, bromides and iodides (anions) present in water. Today, theunit of measurement of salinityfocuses onelectrical conductivityand therefore takes into account any salt present in a solution. Its official unit of measurement is thepsu.

Theresalinityit is traditionally referred to as a fraction expressed inparts per thousand(ppt, from Englishparts per thousand). Considering roughly thedensityas 1000 grams / liter, it can be said that numerically thesalinityexpressed inpptIt corresponds todensity. This unit of measurement (ppt) is mainly used in oceanography.

In chemistry, theppm which corresponds to the acronymparts per million. Similarly, it is aunit of measurewhich indicates a relationship between quantities, but in this case we refer to chemical elements contained in extremely low concentrations.

By convention, today, thesalinitysee howofficial unit of measurethe psu, introduced with the new onePractical Salinity Scale.PSU is the acronym for Practical Salinity Units and corresponds to the ratio between the conductivity of a seawater sample and that of a standard solution of KCl formed by 32.4356 grams of salt dissolved in 1 kg of solution at a temperature of 15 ° C. Also in this case the relationships are dimensionless and35 psuequal to35 gr of saltdissolved in a solution of 1 kg.

How to measure the salt in water

Regardless of the unit of measurement used, in all cases, thesalinityrepresents the sum of the concentrations of the different chemical elements dissolved in a given volume.

Why do I repeat this? Because it is different to wantmeasure the salinity of the waterfrom wantingmeasure the quantityof a givensaltdissolved in water. The salinity is used to measure all the salts which, when solubilizing, dissociate and influence the electrical conductivity of the solution. To measure the concentration of a specific substance, for example, table salt (NaCl), more in-depth knowledge is required: the concentration of a certain substance present in a mixture is measured with the concept ofMolarity or molar concentration.

Without further detaching ourselves from the main focus of this article, let's see immediately in practical termshow the salinity of water is measured.

How to measure the salinity of water

Measuring salinity can be useful for ensuring the well-being of fish and corals in a marine aquarium or for making the water values ​​in a swimming pool more appropriate. Not only that, in agriculture the need to measure the salinity of the humid fraction of the soil can be useful.

Who needsmeasure the salinity of the waterin a summary way, it can exploit a hydrometer, it is not a very precise economic instrument.

Who wantsmeasure the salinity in the soil, can employ a so-called toolconductivity meter.

Still, whoever wantsmeasure the salinity of the waterwith some precision and accuracy can buy arefractometer.

The refractometer measures the refractivity (or "curvature") that a beam of light assumes when passing through a sample of water. Based oncurvedetected, the instrument will return a numeric digit corresponding tosalinity.

Therefractometerit is used to measure the salinity of water as well as of wines. At the time of purchase, choose arefractometerable tomeasure the salinity of the liquidthat you need to monitor. The most popular and also the cheapest refractometers are those used formeasure the salinity of a marine aquarium.

It is not possible to use a refractometer for marine aquariums to measure thesalinityof the water of a swimming pool! The reason? Seawater refractometers are designed to measure a mixture of salts found only in salt water and, if used in other contexts, could cause errors of 5%.

How much does a refractometer cost?

It depends on many factors. If you need a good refractometer for marine aquariums and you are linked to important brands, know that the one from Red Sea has given calibration problems and, like me, you would find yourself spending a lot of money on an instrument that gives you very inaccurate measurements.

A good refractometer, inexpensive and fairly accurate, can be bought on Amazon at a price of 22.99 euros, you can find more information "on this page".

salinity can undergo slight variations also based on the temperature of the water, there are refractometers that take into account and make automatic corrections and other instruments that, on the other hand, require measurement at a standard temperature. If you want to measure the salinity of the water in your tropical marine aquarium, take a water sample, and let it cool slightly, in this way you will have a more accurate value.

How do you use a refractometer to measure salinity?

The refractometer is composed of two prisms, between these prisms the solution to be analyzed is placed. In practice you will have to put a few drops of water on the refractometer lens and observe from the lens by placing yourself in a very bright place.

Looking from the lens, you will notice a horizon line that will divide the dial into two colors. The dividing line between the two colors (light and dark) corresponds to the value ofsalinity.

The refractometer typically returns asalinityexpressed inppm, that is, parts by the thousands. Therefractometer to measure the salinity of the marine aquariumuses a specific graduated scale which generally includes only the reference range that affects sea water. The scale goes from 1,000 to 1,070 or from 1,015 to 1,030 per thousand. The ideal salinity changes according to the liquid examined, for example, in the case of a marine aquarium, the ideal salinity amounts to 1,025 or 35%. As anticipated at the beginning of the article, salinity on a percentage scale also represents thedensity, therefore with the refractometer it will be possible to measure both density and salinity because from the lens it will be possible to access two different graduated scales.

Table for conversion of salinity, conductivity, density and specific weight

Here is a reference table.

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Video: How to use a hand held refractometer to measure salinity. (December 2021).