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Feature Article

Developments in the Standardization of Salinity
A Historical Review of 150 Years of Salinity Research and The Future Applications of Current Standardization Methods

By Paul Ridout
Ocean Scientific International Ltd.
Havant, England

Salinity is one of the most measured parameters in oceanography. Its importance has long been recognized in studies of water mass movements in the open ocean. Improving knowledge of sea-surface salinity is leading to a better estimation of the global hydrological cycle, information that ultimately will contribute to a better understanding of climate change.

As a result of global warming, increased amounts of freshwater are being evaporated from the ocean surface and transported to higher latitudes, giving rise to increased surface salinities in some areas of the oceans. The Ocean Observations Panel for Climate and its predecessors examined the usefulness of sea-surface salinity data in the context of climate change detection. They state that 'at high latitude, sea-surface salinity is known to be critical for decadal and longer time scale variations associated with deep-ocean overturning and the hydrological cycle.'

Salinity is important in other aspects of marine science. Sonar is influenced by water density and hence seawater salinity needs to be known for applications such as seabed mapping, submarine detection and bathymetry. Salinity also has a considerable influence as an ecological factor on marine organisms and can affect algal blooms, movement of fish stocks, shellfish productivity and aquaculture.

This article provides a historical review on the development and evolution of salinity standardization, the current work in producing standard seawater at Ocean Scientific International Ltd. (OSIL) and a discussion of ongoing work and developments in the field.

Early History of Salinity Research
Early work in measuring salinity involved techniques that utilized weighing after evaporation, solvent extraction and precipitation. In 1865, scientist Johan Georg Forchhammer introduced the term salinity and the concept of measuring one parameter, chloride (in reality, total halide), from which the salinity could be calculated. His work was supported further by William Dittmar, who analyzed more than 75 samples from the Challenger expedition (1872 to 1876) to establish the theory of 'constant composition of seawater.' Further work by Danish physicist and hydrographer Martin Knudsen resulted in a new definition, stating that salinity was, 'the total amount of solid material in grams contained in one kilogram of seawater when all of the carbonate has been converted to oxide, all the bromine and iodine replaced by chlorine and all the organic material oxidized.'

Toward the end of the 19th century, Knudsen prepared sealed glass tubes of seawater to standardize the silver nitrate solutions used in Danish hydrographic work. In 1899, when a preparatory conference took place in Stockholm to establish the International Council for the Exploration of the Seas (ICES), Knudsen submitted a proposal for an international institution for the procurement of standard water. Key features of his proposal were the collection of open Atlantic seawater, detailed determination of the total salinity and testing of sealed glass tubes for maintaining the integrity of the standard. The Norwegian explorer Fridtjof Nansen proposed to the conference that a central laboratory be set up in connection with ICES for the supply of standard seawater.

Salinity Research in the Early 20th Century
In 1900 about 80 tubes of 'Standard Water No. VI' and random samples were investigated for chlorinity and specific gravity by the Danish chemist Søren Peder Lauritz Sørensen. When Nansen relinquished this responsibility in 1908, Knudsen agreed to direct the Standard Seawater Service on behalf of the ICES. In 1914, he assumed personal responsibility for its operation.

In 1947, at the age of 76, Knudsen suggested that the Association Internationale d'Oceanographie Physique (AIOP) take over responsibility for the Standard Seawater Service in order to secure its future. Soon after, Knudsen died, and at the request of AIOP, Helge Thomsen took over administrative responsibility with Knudsen's assistant Frede Hermann preparing and calibrating the standards.

Hermann eventually took over full responsibility and the requirement for standards increased dramatically as oceanographic science expanded around the world. Scaling up of the seawater collection, processing and calibration was necessary to meet the demand, which peaked at around 30,000 ampoules of standard per annum.

Responsibility for the service passed in 1974 to Fred Culkin, a U.K. marine chemist who had collaborated with Hermann on chlorinity titrations for several years. With support from the International Association for the Physical Sciences of the Ocean (IAPSO) all the equipment, which included a 5,000-liter tank, circulation pumps, filling manifold and stocks of ampoules were transferred to the Institute of Oceanographic Sciences (IOS) in Wormley, England.

Much of the knowledge and analytical techniques involved in the production of this important standard had been transferred via word of mouth and handwritten notes, so when Hermann died a year or so later, Fred Culkin essentially was the only person who had the knowledge to produce standard seawater.

By way of mitigation and to ease the workload, IOS employed the author of this article as an understudy. At that time, standard seawater was still calibrated using a high-precision potentiometric titration of the seawater total halide with silver nitrate solution to give chlorinity. To continue this article please click here.

Paul Ridout, a marine chemist, worked for the Institute of Oceanographic Sciences at Wormley, England, before launching his company, Ocean Scientific International Ltd., in 1989. He is the director of the IAPSO Standard Seawater Service and is actively involved in the measurement of salinity, both at sea and in the laboratory.

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