Marcet’s Principle

The concept of salinity was introduced by Georg Forchhammer in 1865. From extensive analyses of seawater samples, he was able to demonstrate the validity of Marcet s principle for the most abundant of the salt ions chloride, sodium, calcium, potassium, magnesium, and sulfete. Thus, he recognized that the salinity of seawater could be inferred from the easily measurable chloride concentration or chlorinity. The details of this relationship were worked out by Martin Knudsen, Carl Forch, and S. E L. Sorenson between 1899 and 1902. With the international acceptance of their equation relating salinity to chlorinity (S%o = 1.805 Cl%o + 0.030), the standardization necessary for hydro-graphic research was provided. A slight revision in this equation (S%o = 1.80655 Cl%o) was made in 1962 by international agreement. 

This constancy in relative ion concentration was first postulated by Alexander Marcet in 1819 and, hence, is known as Marcet s Principle or the Rule of Constant Proportions. Formally stated, it says that regardless of how the salinity may vary from place to place, the ratios between the amounts of the major ions in the waters of the open ocean are nearly constant. ... 

The relative constancy of major (and many minor) elements in seawater is referred to as the rule of constant proportions or Marcet s principle. These elements are considered to be conservative elements, whereby changes in their concentrations reflect the addition or loss of water through physical processes. The remaining elements in seawater are termed nonconservative because they remain in constant proportion due to biological or chemical processes. 

Marcet s principle constituents of seawater are found in relatively constant proportions in the oceans, indicating that the residence times of these elements are long (thousand to millions of years)—highly indicative of nonreactive behavior. 

The chemistry of the carbonic acid system in seawater has been one of the more intensely studied areas of carbonate geochemistry. This is because a very precise and detailed knowledge of this system is necessary to understand carbon dioxide cycling and the deposition of carbonate sediments in the marine environment. A major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a constant ionic medium. This concept is based on the observation that the salt in seawater has almost constant composition, i.e., the ratios of the major ions are the same from place to place in the ocean (Marcet s principle). Possible exceptions can include seawater in evaporative lagoons, pores of marine sediments, and near river mouths. Consequently, the major ion composition of seawater can generally be determined from its salinity. It has been possible, therefore, to develop equations in which the influence of seawater composition on carbonate equilibria is described simply in terms of salinity. 

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