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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Water Analysis  /  Total Alkalinity

Total Alkalinity

Total alkalinity of a water body refers to its ability to neutralise a strong acid, ie. its buffering capacity. Although the alkalinity may in theory be caused by any weak acid anion it is usually only carbonate, or more strictly bicarbonate, alkalinity that is important in freshwaters (Wetzel and Likens 1991).

The measurement of total alkalinity is achieved by titrating a known volume of a strong acid (eg. 1.6N H2SO4 against the water sample until all the carbonate has been used. This equivalence end point can be identified using a Bromocresol-green Methyl-red indicator. The end point of pH 4.5 is recommended for waters containing elevated levels of phosphate.

In the field a 100ml sample of water is collected in a well rinsed measuring cylinder and transferred to a 250 ml conical flask. To this 6-8 drops of Bromocresol-green Methyl-red indicator are added turning the sample blue-green. The titration can be performed using a Hach digital titrator (model 16900-01) with 1.6N H2SO4 as the titrant until the sample turns to light violet grey (pH 4.5). The total alkalinity is then read directly from the digital display and expressed as mg/L CaCO3. This should be repeated three times to give a mean value.

The titration can also be carried out in the laboratory using a standard burette and a standardised acid solution.

The end point of the titration can also be determined potentiometrically - this method is recommended for people with impaired colour vision.


American Public Health Association (1989) Standard Methods for the Examination of Water and Wastewater (17th ed.). American Public Health Association, Washington DC. 1550pp.

Wetzel, R. G. & Likens, G. E. (1991) Limnological Analysis. (2nd ed.) Springer-Verlag, Berlin. 391pp.