Alkalinity

The alkalinity defined the acid-binding capacity of soils, rocks and natural waters. The degree of alkalinity is dependent on the amount of basic action ions contained mainly the content of carbonates from. Therefore, one also distinguishes between total alkalinity and carbonate alkalinity. From the alkalinity results in the ability of a mixture or a solution to bind oxonium ions or hydrogen ions. The alkalinity of a solution can be quantitatively determined in the presence of an indicator, by titration with an acid. One calls this size therefore acid-binding capacity.

Buffer capacity of water

Carbonate alkalinity

The buffer capacity of natural water is mainly determined by the level of dissolved carbon dioxide, bicarbonates and carbonates.

The reference system is the alkalinity of the CO2 system. It forms in the water, the components CO32 - ( carbonate ), HCO3- ( bicarbonate or bicarbonate ) and H2CO3 ( carbonic acid, negligible). Thus the whole of the CT CO2 dissolved in water can be represented as follows:

[CO2 *] is the dissolved CO2.

This yields the following definition of the carbonate alkalinity AC:

The factor 2 in [ CO32 - ] explained by the fact that per mole of CO2 is the acid-binding capacity twice as large. In the neutral pH range, the carbonate alkalinity by the concentration of HCO3- and CO32- is determined as [ OH - ] and [ H3O ] are small and have opposite signs.

The analytical determination is usually carried out by titration with hydrochloric acid to pH 4.3 or until the color of the indicator methyl orange. If the initial pH of a sample is above 8.3, it must additionally as intermediate consumption of acid to pH 8.3 ( phenolphthalein as indicator ) will be listed and placed twice in approach. The consumption of acid to pH 8.3, is then available for the conversion of carbonate to bicarbonate. The ( more ) acid consumption to pH 4.3 represents the conversion of any bicarbonate in free carbon dioxide. This size is in the German language as " acid - binding capacity " ( " SBV " ) and are usually given in meq / l.

As an alternative to acid titration can also Gesamtcarbonatsystem be determined by ion chromatography or capillary electrophoresis and computationally resolved against the pH value in the components of the carbonate system.

Total alkalinity

In the sea water are more ions for the acid-binding capacity of importance. Therefore, it is expected the total alkalinity. Which ions are taken into account is different depending on the definition. Dickson defined, for example, bases of weak acids (pK > 4.5 at 25 ° C) as part of the total alkalinity AT:

Since the concentrations can be neglected by many ions, suggest Zeebe and Wolf Gladrow to consider a simplification only Tetrahydroxyborat addition to the carbonate alkalinity:

However, this depends on the otherwise known composition of the sample material. Thus, e.g. the water in fish - circulation systems or wastewater in sewage treatment plants have high levels of ammonia and phosphates, so this would be negligible in this case, by no means.