Alkalinity

Alkalinity or A_T is a measure of the acid neutralizing capacity (ANC) of a solution.

This neutralizing capacity is equal to the stoichiometric sum of the bases in solution. In the natural environment carbonate alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of carbonate rocks and presence of carbon dioxide in the atmosphere. Other common natural components that make up alkalinity include borate, hydroxide, phosphate, silicate, nitrate, and sulphide. Solutions produced in a laboratory may contain a virtually limitless number of bases that contribute to alkalinity. Alkalinity is usually given in the unit mEq/l.

Incorrectly, alkalinity is sometimes used interchangably with basicity. For example, the pH of a solution can be lowered by the addition of CO₂, this will reduce the basicity; however, but the alkalinity will remain unchanged (see example below).

Theoretical treatment of alkalinity

In typical seawater:

A_T = + 2 4⁻" target="_blank" >^*_T" target="_blank" >+ 4⁻³" target="_blank" >^*_T" target="_blank" >+ 3⁻" target="_blank" >^*_T" target="_blank" >− ^*

(subscript T indicates the total concentration of the species in the solution as measured. This is opposed to the free concentration, which takes into account the significant amount of ion pair interactions that occur in seawater)

Alkalinity can be measured by titrating a sample with a strong acid until all the buffering capacity of the aforementioned ions is consumed. This point is functionally set to pH 4.5. At this point, all the bases of interest have been protonated to the zero level species, hence they no longer cause alkalinity. For example, the following reactions take place during the addition of acid to a typical seawater solution:

HCO₃⁻ + H⁺ → CO₂ + H₂O

CO₃⁻² + 2H⁺ → CO₂ + H₂O

B(OH)₄⁻ + H⁺ → B(OH)₃ + H₂O

OH⁻ + H⁺ → H₂O

PO₄⁻³ + 2H⁺ → H₂PO₄⁻

HPO₄⁻² + H⁺ → H₂PO₄⁻

+ H⁺ → [Si(OH)₄⁰

It can be seen from the above protonation reactions that most bases consume one proton (H⁺) to become a zero level species, thus increasing alkalinity by one per mole of base. CO₃⁻² however, will consume two protons before becoming a zero level species (CO₂), thus it increases alkalinity by two per mole of CO₃⁻². and ^*_T.

Example problems

Sum of contributing species

The following equations demonstrates the relative contributions of each component to the alkalinity of a typical seawater sample. Contributions are in μmol/kg-H₂O and are obtained from A Handbook of Methods for the analysis of carbon dioxide parameters in seawater "^*,"(Salinity = 35, pH = 8.1, Temp = 25°C).

A_T = + 2 4⁻" target="_blank" >^*_T" target="_blank" >+ 4⁻³" target="_blank" >^*_T" target="_blank" >+ 3⁻" target="_blank" >^*_T" target="_blank" >− 4⁻" target="_blank" >^*" target="_blank" >− [HF

Phosphates and Silicate, being nutrients are typically negligible. At pH = 8.1 and [HF are also negligible. So,

A_T = + 2 4⁻" target="_blank" >^*_T" target="_blank" >+ ^*

A_T = 1830 + 2*270 + 100 + 10 − 0.01

A_T = 2480 μmol/kg−H₂O

Addition of CO₂

The addition (or removal) of CO₂ does not contribute to alkalinity. This is unexpected as it is likely (depending on pH) dissociation into bicarbonate or carbonate - both contributing species to alkalinty. However the net reaction produces the same number of positively contributing species as negative contributing species. ie

At neutral pH's:

CO₂ + H₂O → HCO₃⁻ + H⁺

At high pH's:

CO₂ + H₂O → CO₃⁻² + 2H⁺

Dissolution of carbonate rock

The dissolution (or precipitation) of carbonate rock has a strong influnce on the alkalinity. This is because carbonate rock is composed of CaCO₃ and its dissociation will add Ca⁺² and CO₃⁻² into solution. Ca⁺² will not influence alkalinty, but CO₃⁻² will increase alkalinity by 2 units.

CaCO_3(s) → Ca⁺² + CO₃⁻²

External links

Holmes-Farley, Randy. "Chemistry and the Aquarium," Advanced Aquarist's Online Magazine. Alkalinity as it pertains to salt-water aquariums.
DOE (1994) "^*,"Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. Version 2, A. G. Dickson & C. Goyet, eds. ORNL/CDIAC-74

Chemical oceanography | Acid-base chemistry

Alcalinidade