To understand the impact of pH on water, it is important to understand what the pH is. The acronym pH is a unit of measurement that defines the amount of free hydrogen and hydroxyl ions in the solution. When water is highly concentrated in hydrogen ions, it is acidic [0 - 7]. On the contrary, when it is highly concentrated in hydroxyl ions, it is basic [7 - 14].

The importance of considering the pH when looking at the chemical evolution of water is that the pH can be affected by the chemicals present in or added to the water. In addition, the pH of the water will have an impact on the solubility and biological availability of the chemical constituents in the water.

In other words, the presence of hydrogen ions (acidic) or hydroxyl ions (basic) will affect the solubility of materials in the water. This can lead to significant differences in the composition of a sample. For example, the presence of heavy metals in an acidic solution presents a greater risk than in a basic solution since their solubility increases proportionally to the acidity of the solution. It is much the same with chemical elements such as phosphorus, which will change form depending on the pH of the water.

The buffer capacity of water

Buffer capacity is the ability of water to resist changes in pH imposed by the addition of a base or an acid. In its purified state, water has no buffering capacity, so its pH is strongly affected by the addition of an acid or base solution.

The lack of buffering capacity of purified water is due to the fact that the buffering capacity of a water sample is caused by reactions between the "buffers" and the added acid or base. In other words, the "buffering" agent reacts with the added product to neutralize the pH change.

Examples of buffering agents include:

- Citric acid with sodium citrate;

- Ammonium hydroxide with ammonium chloride;

- Acetic acid with sodium acetate.

Carbonic gases, water and pH

The significant increase in carbon dioxide (CO2) emissions over the last few decades has had a much greater impact on water than we think. When CO2 comes into contact with a water source and dissolves in it, a decrease in pH is noticed.

This decrease in pH is due to the fact that some of the dissolved CO2 reacts with H2O to form carbonic acid (H2CO3). Moreover, this acid can also dissociate to form hydrogen ions (H+) and carbonate ions (CO3-2).

The impact of CO2 dissolving in water affects water all over the world since traces of CO2 are found everywhere. It is mainly due to this phenomenon that the pH of oceans, lakes and rivers has been decreasing for the last few decades. Moreover, the phenomenon of acid rain can be explained by these reactions since the CO2 in the atmosphere dissolves in the water droplets during their fall.

Finally, to come back to the buffering capacity of water, the impact of CO2 dissolution varies according to the contaminants in the water. In other words, water with a buffering agent will have a lower pH than purified water.