Alkalinity testing of water: P and M measurements

The type of alkalinity found in water depends on the pH, such as P-alkalinity (phenolphthalein) and M-alkalinity (methyl orange), which can be determined using a Metrohm 794 Titrino

Water that contains high levels of alkalinity has a bitter taste.

De-alkalisation removes alkaline salts from water supplies to reduce scale formation, reduce the chance of corrosion and remove the bitter taste.

Most natural waters contain alkalines that result from the attack by carbon dioxide on various alkaline minerals it contacts.

Limestone, in particular, is dissolved by carbonic acid (carbon dioxide dissolved in water) to yield calcium and magnesium bicarbonate.

These salts are referred to as temporary hardness due to their ability to decompose and form scale upon heating.

Carbon dioxide, which is corrosive to metal surfaces, is also formed.

Usually, several ions in water supplies contribute to the alkalinity.

Keeping it simple, alkalinity is due to the presence of bicarbonate, carbonate and hydrate (hydroxide) ions.

However, ions such as phosphate and silicate potentially may contribute to alkalinity.

The type of alkalinity found in water depends on the pH, such as P-alkalinity (phenolphthalein) and M-alkalinity (methyl orange).

The P and M alkalinity values are determined by measuring the acid neutralising ability of a water supply to a set pH value.

P-alkalinity is measured down to a pH of 8.3.

The M-alkalinity is measured down to a pH of 4.3.

The M-alkalinity is also called the total alkalinity.

Alkalinity is normally measured in parts per million (ppm) as calcium carbonate.

The different types of alkalinity present can be calculated from the P and M values reported.

Analysis can be performed manually by using the indicators above.

However, this is subjective as individual people will "see" the endpoint differently.

A much better way to analyse for P and M alkalinity is by an autotitrator, for example a Metrohm 794 Titrino, which can titrate to the correct pH, notably 8.3 and 4.3.

The titrant used is normally 0.1 Mol/L HCl, and a combined pH electrode is used as the indicator electrode to follow the path of the reaction and to ascertain the endpoints.

The Aquatrode Plus electrode has been specially developed for the titrimetric determination of the alkalinity (P and M values) in weakly buffered solutions such as drinking water.

Its pH glass membrane is made of a new type of special glass that is characterised by very short response times.

The shape of the electrode fixed ground sleeve diaphragm has also been optimised.

As a result it has been possible to increase the reproducibility of the electrolyte outflow while at the same time minimising the dependence on the stirring speed.

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