Determination of the alkalinity of seawater

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 20C) is primarily due to bicarbonates, with lesser amounts of borates, phosphates, etc, forming a buffer system of about pH8.

Sea water is about as alkaline as 0.002N sodium bicarbonate solution determined as follows.100ml samples of sea water of various ionic strengths is titrated with 0.01M hydrochloric acid, using a combined pH electrode with in built temperature measurement.

The equivalence point (about pH4.5) is taken, and the number of milliliters of titrant at this endpoint is used in the alkalinity calculation.

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 Tiamo titration software which in turn archives all the relevant data from these titrations.