Analytical Techniques of Chemical Oceanography

In order for a chemical determination to be useful to a chemical oceanographer, it must fulfill several requirements. First, it must measure accurately the concentration of a particular substance even though it is present in minute quantities.

Secondly, the mechanics of the determination must be rugged enough to go to sea. All of the equipment must be able to function in a salt-air environment on board a ship that is rolling and pitching under severe weather conditions. Finally, the determination should be rapid. The chemist should be able to keep up with the samples as they are being taken.

Seawater is analyzed for trace-element ions with specialized microchemical techniques adapted from biochemistry and clinical chemistry. Many of these determinations cannot be adopted as standard procedure, because they cannot meet the last two requirements just listed. Automated and rapid chemical analyzers, however, have been developed for use at sea.

The concentrations of nutrient ions in seawater are determined by standard chemical techniques for use at sea. These determinations are made at the time the samples are recovered, or the water is frozen for shipment to a laboratory ashore.

The -concentrations of the major elements in seawater are usually calculated from the measured concentrations of the halide ions (chlorinity) by invoking the rule of constant proportions. This rule permits one to derive fixed ratios between each major .on and the chlorinity of seawater. The classic method of measuring the halide ion concentration is by titration with a silver salt. In practice a measured volume of seawater is titrated, and the chlorinity is calculated by referring to tables that convert the weight of silver ions to chlorinity values.

The chemical method of determining chlorinity, and hence salinity, has been largely supplanted by electrical methods. The conductometric method derives from the fact that seawater conducts electricity in proportion to the amount of ions contained (provided temperature is held constant). An instrument called a salinity bridge is used to measure the electrical conductivity of a sample of seawater under controlled temperature (see Chap. 16, page 446).

From the conductivity, the total of the major ions (salinity) is calculated. A device called a salinometer exploits the relationship between magnetic susceptibility of seawater and its salinity'. A sample of seawater is introduced into the center of a coil, and the measured change in inductance is converted to values of salinity.

The alkalinity of seawater is measured by titrating a sample of known volume with a strong acid until a neutral pH is reached. The required acid is a measure of the excess base represented by the alkalinity. Accurate and durable pH meters are used to measure the hydrogen ion concentration of seawater.