The determination of seawater alkalinity

Corals and other organisms deposit calcium carbonate in their skeletons and other body parts. In order to do this they must generate calcium and carbonate at the surface of the growing calcium carbonate crystal. This will have an affect on seawaters alkalinity.

Alkalinity is defined in different ways for different applications. In the chemistry of natural waters, there are several types of alkalinity that are encountered. Each of these is a measure of how much acid (H⁺) is required to lower the pH to a specific level. This is known as "total alkalinity" (TA). TA is defined as the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO₃^-] and carbonate [CO₃^-] could be converted to carbonic acid [H₂CO₃]. This is called the carbonic acid equivalence point or the carbonic acid endpoint.

The alkalinity of sea water (defined as the number of mg.-equivalents of hydrogen ion neutralised by one litre of sea water at 20°C.) is primarily due to bicarbonates, with lesser amounts of borates, phosphates, etc., forming a buffer system of pH about 8. Sea water is about as alkaline as 0.002 N sodium bicarbonate solution determined as follows.100-ml. samples of sea water of various ionic strengths is titrated with 0.01 M hydrochloric acid, using a combined pH electrode with in built temperature measurement. The equivalence point (about pH 4.5) is taken, and the number of milliliters of titrant at this endpoint is used in the alkalinity calculation.

This alkalinity determination can be carried out easily and very accurately with the Metrohm 809 Titrando, in conjunction with an 801 magnetic stirrer, and a 10 ml dosing unit. The Metrohm Aquatrode plus (6.0257.000) is the electrode of choice for such a difficult matrix.

As expected, temperature plays an important part in such titrations so the analysis is carried out in a jacketed titration vessel, so the temperature can remain constant.

The instrument is controlled with Metrohm’s very own tiamo titration software which in turn archives all the relevant data from these titrations.