pH of Seawater — Acidic or Alkaline?

Water is one of the essential resources available on the planet. About 71% of the earth’s surface is covered with water. It is present in oceans, lakes, seas, rivers, air, ice caps, glaciers, and even human bodies.

Approximately 97 % of the water on earth is saline water, and only 3% is fresh water. Seawater contains 96.5% water, 2.5% salts, and 1% of other dissolved substances. Large quantities of magnesium, bromine, sodium chloride are obtained from seawater.

In this article, we will study whether seawater is acidic or alkaline in nature.

So, is seawater acidic or alkaline? Seawater is alkaline in nature. The pH of seawater is >7 and is slightly alkaline due to dissolved basic minerals and the natural buffering from carbonates and bicarbonates in the water.

The salt content in seawater is indicated by salinity. Salinity is the amount of salt dissolved in 1 Kg of seawater and is expressed in parts per thousand.

The most common ions in seawater are chloride ions, magnesium ions, sulfate ions, sodium ions, potassium ions, and calcium ions.


Why is Seawater Alkaline?

A sample is alkaline when pH>7, acidic when pH<7, and neutral at pH of 7. pH of a sample changes with temperature.

For seawater, pH~8.1 at the surface. The pH of water first decreases and then increases with depth. The pH is least in the area where the concentration of carbon dioxide is maximum.

The marine ecosystem is very dynamic, and we cannot pen down all the sources responsible for alkalinity. We only focus on the major contributors.

There are many rocks and minerals which are basic. The water can easily erode their surfaces, leading to the dissolving of minerals.

• The relative amount of carbonate and bicarbonate ions also affects the pH. The buffering mechanism has been discussed in subsequent sections.


Why is Seawater more Alkaline than Freshwater?

Freshwater has relatively less concentration of basic salts and minerals. The source of water in freshwater bodies is relatively acidic.

Seawater is denser because of the presence of a large number of salts. The freezing point of seawater is less than freshwater because of the large amount of non-volatile impurities.

The pH of Freshwater Vs. Seawater

The pH of freshwater lies in the range of 6.5 to 8. The exact value depends on the surroundings like soil, rocks, etc.

The pH of seawater is ~8.1.


Why is the water salty in the ocean but not in rivers?

The rivers and lakes are not salty, but oceans are. This is because water escapes from the oceans only by evaporation, leaving salts and minerals behind.

There are many more minerals from the rocks on the ocean bed. The water becomes more concentrated with these salts and hence tastes salty.

While the water in freshwater bodies is mainly due to rains, the water from rain is unsalted. Rivers are constantly moving, and they collect salts and minerals from rocks on the way. This water goes into oceans while the water in rivers replenishes from rain.

Only those lakes are salty, which do not combine with oceans at any point and are not running.

Must Read: Is Salt Water a Compound


Ocean Buffer System

Buffer solutions are commonly used in the laboratory to maintain a constant pH. Buffer is something that resists a change in pH.

The pH of seawater has been almost the same for many years due to the carbonate buffer system in surface water.

Ocean water has many gases dissolved in it. The major ones are N2, O2, and CO2.

The ocean can exchange these gases with the atmosphere, which means that these gases can go back into the atmosphere, and the ocean can absorb atmospheric gases.

This rate of absorbing and escaping gases is almost the same such that the ocean system is in equilibrium.


How does the carbonate buffer system work?

Of all the dissolved gases, our focus is now on carbon dioxide.

When carbon dioxide enters the ocean, it reacts with water to form carbonic acid, further dissociating to release H+ ions and bicarbonate ions.

These bicarbonates can be converted to carbonates by marine organisms for making their shells.

Ocean Chemistry &amp; Acidification – Time Scavengers

Further dissociation of bicarbonate ions leads to the formation of carbonate ions and H+ ions.

Marine organisms use these carbonate ions for making their shells. Ideally, the pH of the water should decrease due to free hydrogen ions, but this is not the case.

Bicarbonate to carbonate ion

Carbonate ions also react with water to form hydroxide ions and bicarbonate ions.

This leads to an increase in the concentration of bicarbonate ions, and hence the reaction goes in a backward direction due to Le Chatelier’s principle.

According to Le Chatelier’s principle, the reaction proceeds in the forward direction if the amount of reactant is more or the amount of product is less.

The reaction proceeds in a backward direction if the amount of product is more or the amount of reactant is less.

As depicted in the diagram, weak acid (carbonic acid) and its conjugate base (bicarbonate ions) are present.

The concentration of conjugate bases increases due to another reaction of carbonate with water. This increased concentration of bicarbonate ions pushes the dissociation of carbonic acid in the backward direction.

Carbonic acid is unstable, and it further dissociates to water and carbon dioxide.

The ocean can absorb a large amount of carbon dioxide from the atmosphere without causing a change in acidity.


What Causes Ocean Acidification?

ocean acidification | Definition, Causes, Effects, Chemistry, &amp; Facts | Britannica

Ocean acidification is an effect of climate change resulting from excess carbon dioxide in the atmosphere. It is one of the significant environmental problems and needs urgent attention.

The water in oceans is slightly basic. It has been observed that the pH is decreasing to a neutral value due to the uptake of carbon dioxide by the ocean water.

When the uptake of carbon dioxide occurs in a limited amount such that it is buffered, the pH does not decrease.

The pH decreases when the uptake of carbon dioxide is in excess amount. This decrease in pH of ocean water is referred to as ocean acidification.

The buffering takes around 1000 to 100000 years. The rate of uptake of carbon dioxide is much greater than the rate of buffering.

In the last 200 years, ocean water has become 30% more acidic.

The pH has decreased from 8.2 to 8.1. the difference in pH is only 0.1 unit, but the concentration of H+ increases logarithmically.

All the activities that increase carbon dioxide content in the atmosphere result in ocean acidification. Some of these activities are burning fossil fuels and cutting forests.

Ocean acidification has a negative impact on the marine ecosystem. Marine organisms need a specific pH for their survival. The shells are becoming weaker due to the low pH.

Due to the change in pH, some organisms are unaffected, while growth has increased in some and decreased in others.

There is a threat to food security for humans as many commercially important marine species will be in danger due to low pH. Also, the capacity of oceans to absorb carbon dioxide decreases.

To have a better understanding of the concept of ocean acidification, one can have a look at this illustrative video 


Factors affecting pH of the water

pH tells us the concentration of H+ present in a given solution.

pH= – log [H+]

Acidic substances have [H+] > [OH-], and pH lies in the range of 0 to 7.

Basic substances have [H+] < [OH-], and pH lies in the range of 7 to 14.

Neutral substances have [H+] = [OH-] and pH=7

pH scale

The significant factors affecting the relative concentration of H+ and OH- are-

The concentration of carbon dioxide- There is an increase in acidic character when the carbon dioxide is dissolved in excess amount.

Acid Rains- Rainwater becomes acidic due to pollutants in the environment like sulfur dioxide and nitrogen oxide.

These pollutants can be because of burning fossil fuels or from the chimneys of industries. This rainwater makes the water body acidic.

You must read out article on the pH of acid rain.

Dissolved Minerals- There are many types of rocks and minerals underwater. The composition of bedrock or type of soil in the vicinity of the waterbody significantly impacts the pH.

For instance, limestone increases the alkalinity because of its basic character. Granite does not affect pH.

Temperature– As the temperature of water increases, pH decreases. At 25°C, neutral water has a pH of 7 and at 100°C neutral water has a pH of 6.14.

Wastewater- Many industries dispose of their waste by throwing it in oceans. There are different types of industries, and each industry has a different type of waste.

So, wastewater can either decrease or increase the pH.



Seawater contains a large amount of salt.

It is alkaline due to dissolved basic minerals and buffering mechanism of carbonate and bicarbonate ions.

The pH of freshwater is less than seawater.

Ocean water is salty, but water in rivers and lakes is not.

The buffering of carbon dioxide takes around 1000 to 100000 years.

The rate of addition of carbon dioxide is much faster than the rate of buffering, due to which the pH of ocean water is decreasing.

The pH of a waterbody depends on various factors.

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