Chemical Analysis of Seawater – Formula – Composition – Salinity – Uses

Sea water or salt water obviously has special chemical and physical properties than the regular water. The chemical analysis of seawater is important in order to fully understand the properties, containment, chemical reaction and other qualities of seawater. This article will focus on describing the chemical analysis of seawater, chemical composition of seawater, salinity and other useful facts about seawater.

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Chemical Composition of Seawater

In detail, seawater consists of many different element with various concentration. These elements include hydrogen (H2O), Oxygen (O2), Sodium (NaCl), Chlorine (NaCl), Magnesium (Mg), Sulfur (S), Potassium (K), Calcium (Ca), Bromine (Br), Helium (He), Lithium (Li), Berylium (Be), Carbon (C), Boron (B) and many others. These elements don’t come by itself, but they were attracted to the opposite charged ions. As an example, sulphate will produce sulphur, sodiumm chloride will create sodium and many others.

In general, seawater has the salinity level of 35% or 35 gram per liter. This means that in every kilogram of seawater (or 1 liter volume), there are 35 grams of dissolved salts. Seawater has the average density of 1025 kg per liter. We know that they are more dense than the pure water since seawater contain salt which improves the mass. Seawater generally has the pH of 7.5 to 8.4. Chemical analysis of seawater is the great way to deeply understand the chemical containment in the seawater.

Salinity of Seawater

One of the important chemical analysis of seawater is the salinity level. It’s the amount of the salt dissolved in the seawater. As we mentioned before, the common seawater has salinity level of 35%. We also know this number as the salt concentration in the seawater. Previously, to measure the salinity, we do evaporate the water and measure the weight of the salt remained.

However, this method is not really accurate and difficult. As an replacement, scientists measure the conductivity of seawater to check the seawater’s salinity. The basic principle of this measurement is that electrical conductivity will increase if the salt containment increases. Usually, scientists use the special instrument named Conductivity Temperature Depth or CTDs to measure the salinity level of seawater.

This type of measurement will give the high accuracy rather than the conventional way. Salinity is really important to calculate the seawater density. Knowing the density itself is really useful to understand the water’s vertical circulation. The number of salinity will be high in the tropical water where the temperature is warm. In this temperature, water do evaporate more than precipitate. In contrast, salinity level will be low in the place if there are high numbers of fresh water from the river.

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Salinity Effects

Aside from its important in the chemical analysis of seawater, salinity level also has strong impact to the sea living organisms. We know that there’s a process osmosis where the water will move from the low concentrated water to the high concentrated water. In the sea, the fish who has the cellular salinity of 1.8% will suffer from dehydration.

The seawater fish can drink and excrete water in continuous manner since they have the high level of cellular sanity. For those who can’t live in the high density water, like the river fish, they will excrete without drinking the salty water. Finally they will dehydrate. There’re some sea plants or seaweeds which don’t have control to their osmosis process. This condition makes them really sensitive to the salinity level of seawater.

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Seawater Ions

We know that seawater is considered as the neutral substance. It has the equal amount of positive and negative charges. The positive charge is called as cation, while the negative charge is named as anion. Seawater has the equal numbers of anion and cation. The chemical analysis of seawater includes the chemical reaction of salt in the water. When dissolve in the water, salt will release the cations and anions. Here’re two popular salts reaction in the water

NaCl –> Na(1+) + Cl(1-) (in water)
MgSO4 –> Mg(2+) + SO4(2-)

We can see from the above examples that salt has the same number of anion and cation while dissolving in the water. Sodium chloride will create the anion of chlorine and cation of natrium. Magnesium sulfate will have two anions of sulfate and two cations of magnesium.

In term of ions, there’re only few types of ion which build up almost all dissolved substances in seawater. The major ions in the seawater consist of Natrium (Na+), Chloride (Cl-), Sulfate (SO4-), magnesium (Mg 2+), calcium (Ca 2+) and potassium (K+). Besides them, there’re more ions which has the quite large amount in the seawater. They are bicarbonate (HCO3-), Bromide (Br-), Borate (BO3 3-), Strontium (Sr +2), and Flouride (F-)

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Seawater Ions Properties

Another important part in chemical analysis of seawater is the properties of seawater ions itself. From the experiment, we can see that most of seawater ions is conservative. It means that the containment of ions in seawater always has the same ratio in almost all sea conditions. The composition of seawater is actually always constant. In common, seawater has this default ratio of ions:

Sodium ion of 55%
Chloride ion of 31%
Sulfate ion of 8%
Magnesium ion of 4%
Calcium ion of 1%
Potassium ion of 1%

Above condition will occur in normal sea types, unless the sea has the high amount of fresh water supply from river. As we know that river water has different ions containment than the seawater. There’s more calcium ion in the freshwater from river. This composition has been established for 1.5 billion years. Its containment and ratios has remained same for that period of time. We can understand that the properties of seawater will not change as the time goes.

This steady condition can probably happen because the number of salt which is removed from the sea equals to the number of salt which comes to the ocean. The salt will determine the level of salinity. However, there’re still other substances which are not conservative in the seawater. They are not the salts for sure. Its variation can be caused by different location, geography condition, elevation and containment of other living organisms.

Dissolved Gas in Seawater

Even though that the number of salt ratio always same in any type of ocean, the number of dissolved gas may vary. There are some dissolved gas types in seawater which are not conservative. Basically, the number of dissolved gas in sea will equal with atmosphere but its concentration will vary based on the salinity level and temperature. In seawater with high salinity level, the number of dissolved gas will be decrease. This is due to the presence of salt ion which block the movement of water.

While in the high temperature condition, the number of dissolved gas will be decrease. In this condition, gas will improve its ability to move and escape from the water. So, dissolved gas in seawater will increase in the low salinity and low temperature condition. The non conservative gas examples are oxygen and carbon dioxide. Their level will be affected by the organism in the sea itself. Oxygen comes to sea water form the atmosphere. Previously, photosynthesis process in plants will produce the oxygen to sea surface water. Oxygen is used for the animal’s respiration.

On the other hand, carbon dioxide is the material used for photosynthesis. Animals will release carbon dioxide in their respiration system to atmosphere. This carbon dioxide can reat with the water to create bicarbonate and ions of carbonate. The chemical reaction is as follows:

CO2 + H2O –> HCO3(-) + H(+) + CO3(2-) + 2H(+)

The reaction above will determine the level of pH in the seawater. This is another important part in the chemical analysis of seawater.

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Nutrients Uses in Seawater

Besides the dissolved water, there’re also substances which are non conservative in the seawater. They are the nutrients. Nutrients have the vital role as the fertilizers for growing some plants in sea like algae. Some popular types of nutrients in seawater are nitrate, silicate and phosphate. The nutrients which have positive impact toward the plant are nitrogen (both in NO3, NO2 or NH4), phosphate (P in PO4), potassium (K), Sulfur (S), magnesium (Mg) and Calcium (Ca). Another substance which is great is iron (Fe).

Iron is helpful to grow plant as it’s the essential enzymes component. Unfortunately, iron is greatly available in soil but rarely found in the sea water. The common level of Fe in seawater is only 0.0034 ppm. Meanwhile, iron is important for the sea plant like plankton. Iron can be the great nutrient for growing plankton.

In short, seawater has some basic chemical properties based on its containment. Seawater can have different amount of concentration based on its geographical location, depth, temperate and so on. However, most of ocean has the salinity level of 35%.

Seawater has the ions containment which consist of some major ion substances as mentioned above. The ratio of salt ion in the seawater will remain the same in the ocean, as has been proved by experiment for more than 1.5 billion years. Besides the salt ions, seawater also contain dissolve gases and nutrient which can impact the organism in the ocean. From the article, we can see that the chemical analysis of seawater is important to improve our understanding about sea, its chemical containment and its water properties.

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