alkalinity

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The alkalinity defines the acid binding capacity of soils, rocks and natural water. The degree of alkalinity depends on the amount of basic ions contained , mainly the carbonate content . A distinction is therefore made between total alkalinity and carbonate alkalinity . From the alkalinity is the ability of a substance mixture or a resulting solution , oxonium or - hydrogen ions bind. The alkalinity of a solution can be determined quantitatively by titration with an acid in the presence of an indicator . This is why this quantity is also called acid-binding capacity .

Buffer capacity of water

Carbonate alkalinity

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

The reference system for alkalinity is the CO 2 system. In the water it forms the components CO 3 2− ( carbonate ), HCO 3 - ( hydrogen carbonate or bicarbonate ) and H 2 CO 3 ( carbonic acid , negligible). The total amount of CO 2 C T dissolved in water can thus be represented as follows:

C T = [CO 2 * ] + [HCO 3 - ] + [CO 3 2− ]

[CO 2 * ] is the dissolved CO 2 .

This results in the following definition of the carbonate alkalinity A C :

A C = [HCO 3 - ] + 2 · [CO 3 2− ] + [OH - ] - [H 3 O + ]

The factor 2 for [CO 3 2− ] is explained by the fact that the acid binding capacity is twice as great for each mole of CO 2 . In the neutral pH range , the carbonate alkalinity is determined by the concentration of HCO 3 - and CO 3 2− , since [OH - ] and [H 3 O + ] 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 methyl orange indicator changes . If the initial pH value of a sample is above 8.3, the acid consumption up to pH 8.3 ( phenolphthalein as an indicator) must also be noted as an intermediate stage and added twice. The acid consumption up to pH 8.3 then stands for the conversion of carbonate into hydrogen carbonate. The (further) acid consumption up to pH 4.3 stands for the conversion of any hydrogen carbonate into free carbon dioxide. This quantity is in the German language as " S äure- B indungs- V referred Property Assets" ( "SBV") and usually in mEq / l specified.

As an alternative to acid titration, the total carbonate system can also be determined by ion chromatography or by capillary electrophoresis and resolved into the components of the carbonic acid system by calculation using the pH value.

Total alkalinity

In sea water, other ions are important for the acid binding capacity. Therefore the total alkalinity is used for the calculation. Which ions are taken into account varies depending on the definition. For example, Dickson defined bases of weak acids (pK> 4.5 at 25 ° C) as part of the total alkalinity A T :

A T = [HCO 3 - ] + 2 * [CO 3 2− ] + [OH - ] - [H 3 O + ] + [B (OH) 4 - ] + 2 * [PO 4 3− ] + [HPO 4 2− ] - [H 3 PO 4 ] + [SiO (OH) 3 - ] + [NH 3 ] -…

Since the concentrations of many ions can be neglected, Zeebe and Wolf-Gladrow propose to consider only tetrahydroxyborate in addition to the carbonate alkalinity as a simplification :

A T ≈ [HCO 3 - ] + 2 · [CO 3 2− ] + [OH - ] - [H 3 O + ] + [B (OH) 4 - ]

This depends on the otherwise known nature of the material to be measured. So z. B. the water in fish farming systems or wastewater in sewage treatment plants have high levels of ammonia and phosphates, so that these would by no means be neglected in this case.

literature

  • Richard E. Zeebe, Dieter A. Wolf-Gladrow: CO 2 in Seawater: Equilibrium, Kinetics, Isotopes . Elsevier, 2001, ISBN 978-0-444-50579-8 .
  • K. Grasshoff, Manfred Ehrhardt, Klaus Kremling: Methods of Seawater Analysis . 3. Edition. Wiley-VCH, 2002, ISBN 978-3-527-29589-0 .

Individual evidence

  1. ^ Wissenschaft-Online-Lexika: Entry on alkalinity in the lexicon of geosciences .
  2. ^ AG Dickson, C. Goyet: Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water ( Memento of March 3, 2012 in the Internet Archive ), Chapter 2, p. 3.
  3. ↑ " I have found at least twenty different definitions of alkalinity ", quotation from a carbon cycle modeler in: Richard E. Zeebe, Dieter A. Wolf-Gladrow: CO 2 in Seawater: Equilibrium, Kinetics, Isotopes . Elsevier, 2001, ISBN 978-0-444-50579-8 .
  4. ↑ AG Dickson: An exact definition of total alkalinity and a procedure for the estimation of alkalinity and total inorganic carbon from titration data . Deep-Sea Research 28A, 609-623, doi: 10.1016 / 0198-0149 (81) 90121-7 .
  5. ^ Richard E. Zeebe, Dieter A. Wolf-Gladrow: CO 2 in Seawater: Equilibrium, Kinetics, Isotopes . Elsevier, 2001, ISBN 978-0-444-50579-8 .