WATER ALKALINITY: WHAT IT IS, DETERMINATION AND IMPORTANCE - CHEMISTRY - 2025

The alkalinity of water is its resistance to a change in pH due to the addition of acidic substances or liquids. This characteristic is often confused with basicity. The addition of CO ₂, for example, can cause a decrease in pH (basicity) without changing the alkalinity.

In fresh water, alkalinity is mainly due to the contribution of compounds such as carbonate (CO ₃ ^2-), bicarbonate (HCO ₃ ^-) and hydroxyl (OH ^-). In seawater, the contribution of boron hydroxide (BOH ^4-), silicates (SiO ₄ ^2-) and phosphates (PO ₄ ^3- and HPO ₄ ^2-) must be added.

Groundwater, an example of highly alkaline water. Source: Max Pixel.

The alkalinity of water is usually expressed in mEq / L, corresponding to the amount of acid used in its titration: hydrochloric or sulfuric. It is also usually expressed as mg CaCO ₃ / L, or part per million (ppm), even if other salts are present.

This characteristic of water is usually associated with its hardness, since calcium and magnesium carbonates contribute to alkalinity. While calcium and magnesium, that is, their metal cations Ca ²⁺ and Mg ²⁺ respectively, are the elements responsible for the hardness of water.

What is the alkalinity of water?

It is the ability of water to neutralize the acidic substances that may be incorporated into it, thus avoiding a decrease in its pH. This buffering action is due to the presence of weak acids and their conjugate bases.

Bases can react with acids to become electrically neutral, that is, uncharged species.

HCO ₃ ^- + H ⁺ <=> CO ₂ + H ₂ O

The bicarbonate (chemical equation above) reacts with the hydrogen ion to become carbon dioxide, an uncharged compound. One mole of HCO ₃ ^- represents a molar equivalent. Meanwhile, carbonate (CO ₃ ^2-) represents two molar equivalents.

Groundwater

Groundwater carries compounds from acid rains, including sulfuric acid. The presence of carbon dioxide from the atmosphere that dissolves in water can also form carbonic acid.

Acids act on limestone rocks, rich in calcium and magnesium carbonates, causing their dissolution. This causes the accumulation of carbonate and bicarbonate in the water, mainly responsible for its alkalinity.

2 CaCO ₃ + H ₂ SO ₄ → 2 Ca ²⁺ + 2HCO ₃ ^- + SO ₄ ^2-

Adding an acid (above) causes an increase in alkalinity as long as more bicarbonate is produced than the hydrogen left over from the previous reaction.

When alkaline groundwater comes into contact with the atmosphere, it loses carbon dioxide and carbonate precipitates, which lowers alkalinity. A dynamic equilibrium is then established between the atmosphere, the water and the carbonaceous minerals.

Under the conditions that exist in surface waters, the contribution of carbonate to alkalinity decreases, and bicarbonate becomes the maximum contributor to it.

Sea water

In addition to carbonate, bicarbonate, and hydroxyl and hydrogen ions, other compounds contribute to the alkalinity of the water. These include borates, phosphates, silicates, conjugated bases of organic acids and sulfates.

Anaerobic processes such as dinitrification and sulfate reduction occur in the ocean and sea, which have a contribution of 60% of the alkalinity of the water. These processes consume hydrogen, thus causing an increase in pH, in addition to originating N ₂ and H ₂ S.

In general, anaerobic processes cause an increase in alkalinity. On the contrary, aerobic processes produce a decrease in it. In surface waters, in the presence of oxygen, there is a process of degradation of organic matter carried by the water.

As it degrades, H ⁺ is produced, which is carried into the water, producing a decrease in alkalinity.

Environmental pollution causes, among other consequences, the melting of the polar cap, which results in an increase in the volume of seawater. This causes a dilution of the compounds responsible for the alkalinity of seawater, and therefore its decrease.

Units

Water alkalinity is usually reported as mg CaCO ₃ / L, although calcium carbonate is not the only compound present, nor is it the only contributor to water alkalinity. The mg / L of carbonate can be converted to mEq / L by dividing by 50 (approximate equivalent weight of CaCO ₃).

Determination

It is determined by titrating the bases present in water with a strong acid. The most used acids are 0.1 N hydrochloric and 0.02 N sulfuric.

50 mL of the water to be titrated is measured in a volumetric flask, placing that volume of water in a 250 mL Erlenmeyer flask. A mixture of indicators is often used, commonly phenolphthalein and methyl orange. The acid is placed in a burette and is poured drop by drop into the water that is being titrated.

If the alkalinity of the water is greater than 9.6 at the beginning of the titration with the acid, a variation in coloration attributable to phenolphthalein will not be observed. Then, when the pH decreases between 9.6 and 8.0, the appearance of a currant color will be observed, which disappears when the pH falls from 8.0 during the titration.

Stages of the degree

During the first stage, the carbonate is titrated, a reaction outlined in the following equation:

CO ₃ ^2- + H ₃ O ⁺ <=> HCO ₃ ^- + H ₂ O

As the acid continues to be added during the titration, the color of the titrated solution turns orange due to the change that the methyl orange undergoes, indicating that the carbonate forms and the other bases have been totally consumed.

In the final stage, only carbonic acid remains:

HCO ₃ ^- + H ₃ O ⁺ <=> H ₂ CO ₃ + H ₂ O

This occurs at pH 4.3 - 4.5, called the CO ₂ equivalence point. This is the existing compound and the alkalinity of the water becomes "zero". If the water is heated, there will be bubbling of CO ₂ from the decomposition of H ₂ CO ₃.

The volume of acid required to reach the equivalence point for CO ₂ is a measure of the total alkalinity of the water.

Importance

The existence of the alkalinity of the water is a mechanism of protection of the environment to limit the damage that can be caused to the aquatic flora and fauna, by the influx of wastewater or acid rains capable of modifying the pH where they live.

Coral reefs are seriously damaged by an increase in the acidity of seawater. The alkalinity of the water limits the extent of this damaging action, neutralizing excess acidity and allowing the maintenance of a pH compatible with life.

It has been estimated that the alkalinity of the water must have a minimum value of 20 mg as CaCO ₃ / L, a limit to guarantee the maintenance of aquatic life.

The knowledge of the value of the alkalinity of the water can guide about the amount of sodium or potassium carbonate and lime necessary for the precipitation of calcium as carbonate when the hardness of the water is reduced.

References

1. Day, RA and Underwood, AL (1989). Quantitative Analytical Chemistry. 5 ^ta edition. Editorial Prentice-Hall Hispanoamericana, SA
2. Wikipedia. (2019). Alkalinity of water. Recovered from: es.wikipedia.org
3. Mr. Brian Oram. (2014). The Role of Alkalinity Citizen Monitoring. Recovered from: water-research.net
4. National Superintendency of Sanitation Services. (sf). water analysis: alkalinity.. Recovered from: bvsper.paho.org
5. Bonilla Alvaro. (2017). The alkalinity of water and its effect on substrates. Recovered from: intagri.com
6. Goyenola Guillermo. (2007). Determination of total alkalinity.. Recovered from: imasd.fcien.edu.uy