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Definitely one of the most important and one of the most frequently studied physical-chemical parameters of seawater is the salinity. As originally conceived, salinity was to provide a measure of the mass of salt per unit mass of seawater. The saltiness of ocean waters has been recognized throughout recorded history. Many of the early investigations on salt in the ocean focused on the measurement of those salts.
 Robert Boyle (1627-1691)
The paper was the result of a general interest in natural waters rather than advancement of seawater chemistry (Wallace, 1980). His measurements of the salt in seawater were done both by evaporating a pound of seawater and by precipitating the salt. He favored the latter process and recommended the use of silver nitrate to determine the sweetness of all waters (Boyle, 1693). For the next century, no systematic studies of sea water salts were done using a common analytic scheme.
 Antoine Lavoisier (1743-1794)  Joseph-Louis Gay-Lussac (1778-1850) Evaporation-solvent-extraction continued to be the primary method of determining saltiness until Murray (1818) introduced the indirect method involving the precipitation of specific "acids and bases," then inferring the constituents of sea salt. Gay-Lussac agreed with Murray that the total salt content of seawater could be determined by an analysis such as Murray’s simply by the addition of the weights of the individually determined components; but he felt that for the determination of the absolute salt content only a simple evaporation would work: "…but it is simpler and more exact to determine it by evaporation until a deep red. This procedure is done very conveniently in a matrass whose neck is tilted at an angle of about 45° and which is stirred continually while it is over the flame, as soon as the salts begin to recipitate, in order to avoid bumping. The boiling cannot throw anything outside,and the residue yielded weighs exactly the weight of the saline substances".  Alexander Marcet (1770-1822)  Georg Forchhammer (1794-1865) Between 1819-1822 Alexander Marcet performed some of the first measurements of the concentrations of the major salts in seawater, and he also invented a sampling bottle capable of retrieving samples directly from the ocean depths.He discovered that the highly precise and accurate measurement of the chemical composition of seawater could be accomplished by gravimetric analysis. Marcet also proposed that seawater contained small quantities of all soluble substances and that the relative abundances of some were constant (a theory later to be known as Marcet's Principle).
The concept of salinity was introduced by the Danish chemist Johann Georg Forchhammer in 1865. Georg Forchhammer found that the ratio of major salts in samples of seawater from various locations was constant. This constant ratio is known as Forchhammer's Principle, or the Principle of Constant Proportions. One of the most interesting and most recent his scientific work is "On the Constitution of Sea Water at Different Depths and in Different Latitudes" (1863)
 John Buchanan (1844-1925)  HMS Challenger
Studies of ocean circulation, which were carried out later in 19th century, involved investigation the distribution of salinity. Attempts were made to measure salt content by heating to remove the water from the sample by evaporation. Simple drying was accompanied by losses of violatile compounds and the hydroscopic nature of the thick residue made the measurement of its weight very difficult. A dry residue method was offered as a solution; the seawater sample was evaporated and dried to a stable weight at 480° C after processing with hydrochloric acid. On this basis salinity was defined as "the total amount of solid material in grams contained in one kilogram of seawater when all the carbonate has been converted to oxide, all the bromine and iodine replaced by chlorine, and all the organic material oxidized".
 William Dittmar (1833-1892) In 1889 the International Council for the Exploration of the Sea named Martin Knudsen as chairman of a commission to study the problem of determination the salinity of seawater. Based on the premise of constancy of ionic ratios in seawater, the commission defined a "chlorinity" that could be determined by a simple volumetric titration using silver nitrate, to be used as a measure of salinity. Knudsen and his colleagues made measurement on samples of seawater from the different regions of the World Ocean and on the basis of comparation of nine determinations of salinity and chlorinity, proposed the formula:
S = 0.03 + 1.805Cl (1)
which served oceanographers for the next 65 years.
In his proposal Knudsen stressed the importance of the measurement of salinity of seawater in physical, climatological and biological investigations and mainteined that the measurement could be carried out by titration with an accuracy of 0.04 ppm but that was not generally achieved by the method them in use. Usually a few titrations were carried out by weighing and all volumetric titration were then referred to these. Knudsen pointed out that titration by weighing was at that time a fairly difficult operation and the errors in salinity determination were usually as high as 0.1-0.2 ppm. Obviously, better accuracy could be obtained in salinity determination if all water samples were examined in one laboratory but Knudsen realized that this would not be very practical. Instead he proposed that all interested nations should contribute to the establishment of an institution for procuring standard water. This institution would prepare (and standartize in terms of its chlorinity) the standard water and distribute samples to interested laboratories, together with a statement of the physical and chemical qualities (properties) of standard.
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