Waterguide - Alkalinity

What is alkalinity?

Alkalinity refers to the ability of an aqueous solution to maintain its pH value when acid is added. It is also called acid binding capacity or buffer capacity. The SI unit is mmol/l, but the (outdated) designation in degrees of German hardness °dH is also still in widespread use.

What determines the alkalinity?

The alkalinity value is mainly determined by the ions that have an alkaline effect in the water, i.e. by acid-neutralising substances (alkalis).

  • Hydrogen carbonate (HCO3-)
  • Carbonate (CO32-)
  • Hydroxide (OH-)

Alkalinity falls into two categories: total alkalinity and carbonate alkalinity.

Carbonate alkalinity

As carbon dioxide dissociates into several products (including hydrogen carbonate and carbonate) when it comes into contact with water, the carbonate alkalinity is indirectly linked to the amount of dissolved carbon dioxide.

The quantity of carbonate ions and bicarbonate ions in a solution is measured. If the pH value is > 8.3, the carbonate ion is the buffer. If the value is below 8.3, the bicarbonate ion is the buffer.

Total alkalinity

Total alkalinity refers to the total quantity of alkali ions in the water. The carbonate alkalinity is part of the total alkalinity.

How is alkalinity measured?

Carbonate alkalinity: The carbonate alkalinity is measured through titration with an acid up to a pH value of 8.3.

Total alkalinity: The total alkalinity is measured through titration with an acid of pH 8.3 to pH 4.5.

The quantity of titrations can be converted.

Alkalinity in natural waters and specific applications

The alkalinity of natural waters varies depending on the geological formation of the area from which the water originates. Waters with a limestone formation typically have a fairly high alkalinity, while waters with a granite formation have a low alkalinity.

Alkalinity is an important measured value in fishkeeping. If the alkalinity is too low, the addition of acid can cause the pH level to fluctuate, which can damage the organisms in the aquarium.

It is similar with swimming pools. The alkalinity has to be set to a certain value in order to avoid pH fluctuations and create the optimum reaction conditions for disinfection agents.

In agriculture on the other hand, if the alkalinity is too high, it can cause certain nutrients to become chemically bound, meaning they are no longer available to the plants. That can lead to deficiency symptoms despite there being a seemingly sufficient amount of nutrients.

Setting the right alkalinity is also essential for various industrial applications (e.g. surface treatment and electroplating) to create the right process conditions and, in particular, to avoid workpieces or equipment from corroding.

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