It is vital to be able to reproduce measurements of the physical and chemical properties of seawater, and many years ago this was done using ‘Copenhagen standard seawater’. This was a sample of seawater carefully adjusted to match the standard parameters for the major dissolved ions. Back in those days the primary means of comparing seawater from one area to another was a standard titration of the chloride content, and each laboratory would carefully calibrate their titration against the Copenhagen standard to ensure reproducibility (in practice, usually scientists would perpare a large number of secondary standards from the Copenhagen standard for this purpose, to reduce cost).

This titration measured chlorinity, which was empirically related to the salinity of the seawater, but by only measuring one ion, the relationship was not very accurate or dependable under all circumstances. In the 1960-70’s titrimetry was gradually replaced by conductivity measurements, this was a measurement that was influenced by all of the ions present in the seawater sample, and as importantly, was a lot easier and quicker to perform. The Copenhagen standard seawater continued, however, to be used for standardising the conductivity measurement, though this was strictly designed for the earlier chlorinity procedure. Today we are placing every more reliance on the accuracy of our measurements of the thermodynamic properties of seawater. We now need very much greater precision and accuracy to follow tiny changes in surface seawater compositions, which reflect how the water responds to changes in temperature.

As a consequence a new standard has just been introduced. This uses the concept of ‘Absolute Salinity’, and should help researchers correct for experimental artefacts estimated to contribute up to 1°C differences in current sea-surface models.

Ref: CSIRO Australia (2009, July 20). Science Adopts A New Definition Of Seawater. ScienceDaily.
Retrieved July 20, 2009, from www.sciencedaily.com