Early Determination of Salinity: from Ancient Concepts to Challenger Results

Louis Ferdinand Marsilli (1658-1730)
Louis Ferdinand Marsilli (1658-1730)

Count Louis Ferdinand Marsilli (1658-1730) was the first actual marine scientist. For some years he served as a military officer aboard ship in the Mediterranean. Throughout most of this time he spent his leisure in the study of the sea. The methods Marsilli chose to analyze sea water were essentially those of Boyle’s. The chemical substances that Marsilli used were spirit of salt ammoniac and oil of tartar. Rather than using process of simple evaporation he, too, used distillation of known weight of sea water to produce the dry salt of the sea water. This was not uncommon by this time. These residues he weighted. Over period of time, in spite of all the care he took with the balance, he became convinced that the hydrometer was preferable in such measurements. Aside from inconsistencies in residue weights Marsilli had a consistent lightness in weight determined by balance. The weight loss Marsilli belived to be due to a loss of salt during the distillation caused by action of fire. The fire he believed actually consumed some salt. He found the same inconsistencies and lightness in using the balance to check saltness of artificially prepared sea water.

Histoire Physique de la Mer
Histoire Physique de la Mer

The Histoire Physique de la Mer was written by Marsilli and published in Amsterdam in 1725. Prior to the Histoire Physique de la Mer , no book existed that dealt solely with the sea from the scientific standpoint. This book of 173 pages with many accompanying plates covered all of the aspects of the sea. The first part dealt with the sea’s basin, the second with the water itself, the third the movements of the water, the fourth and the fifth marine plants and animals. Topics such as the nature of the bottom, saltness, temperature, density, currents and color were treated. In the study of these he commonly made use of the hydrometer, microscope and balance. For density determinations at sea he used the hydrometer, as the balance was not reliable on board ship. The Histoire Physique de la Mer is truly remarkable book. As with most creative thinkers, Marsilli’s work is a complex mixture of the old and the new. While it was ahead of its time from the standpoint of ocean study its chemistry was characteristic of the period in which it was written. The use of data tables was not new in water analyses although Marsilli first used them in reporting of the large number of sea water analyses along with the location from which each sample was taken, and accompanying tide, current, temperature and time data, much like modern station data. He felt that many of the ocean parameters influenced other and took great care in his measurement. Prior to Marsilli virtually all sea water samples were performed by people other than the sampler and brought usually some distance to the analyst.

Antoine Lavoisier (1743-1794)
Antoine Lavoisier (1743-1794)

Late in the 18th century, Antoine Lavoisier (1743-1794) used evaporation with a solvent extraction to obtain data for his analysis of sea water. In 1772 he wrote a paper on the use of alcohol in mineral water analyses. In it he chose to include the first analysis of sea water ever published. Sea water, according to Lavoisier, was a mineral water, but the most complicated one that he had examined.  The analysis of sea water was essentially this. Lavoisier evaporated the total volume of water slowly to driness by means of a “feu de lampe” in a “capsule de verre”. In the drying process “selenite” and “sel gypseux” were precipitatednaturally as the water became more concentrated. These salts were removed, dried and weighted. Alcohol was then added to the final dried saline mass and the “sel marin a base de sel d’Epsom” dissolved in it. The existing residue was then heated with a two-to-one mixture (by volume) of alcohol and water until completely dissolved. “Sel de Glauber” and “sel d’Epsom” crystallized from the cooled solution ane were dried and weighed. The remaining alcohol-water solution contained some “sel marin” and “sel marin a base de sel d’Epsom” which was again slowly evaporated, dried and weighed.  Six years after the paper on the use of alcohol in water analyses Lavoisier wrote a short paper on the analysis of water from the Dead Sea. The procedure for analysis of the Dead Sea water was essentially the same as that used in the sea water analysis previously. In the course of this analysis Lavoisier used eight different mixtures of alcohol and water. Lavoisier is recognized as a major contributor to the chemistry of sea water, altough he wrote only one article solely on this subject and included only one other analysis of sea water in his writings.  Lavoisier was familiar with the precipitate formed in salt solution by the addition of “dissolution d’argent” (silver nitrate) and he, of course, knew that precipitate was “lune cornee” (luna cornea:silver chloride).  He used this test as a rough indication of the saltness of the liquids, but did not regard this test as useful. Torbern Bergman (1774) used evaporation and precipitation to carry out a detailed examination of all natural waters and developed a list of the substances that he had identified in sea water. He introduced the technique of weighing the precipitated salts to determine their concentrations (Wallace, 1980

Joseph Louis Gay-Lussac  (1778–1850)
Joseph Louis Gay-Lussac (1778–1850)

Joseph Louis Gay-Lussac  (1778–1850) used titrimetry to develop simple and accurate methods to determine the salts and concluded that the salt concentrations of open sea water were constant everywhere. Most of Gay-Lussac’s comments on the sea are contained in the article “Note sur la Salure de l’Ocean Atlantique” (Note on the Saltness of the Atlantic Ocean). A number of seawater samples were gathered from the middle of the English Channel by Gay-Lussac himself. It not only shows his willingness to go to sea but his understanding that the chemist should take his own samples whenever possible. 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 Murrays 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“.On the basis of his own values and those of others, he concluded that everything indicated that seawater contained at least “trois centimes et demi (three and a half percent) of salt matter.. ” After much consideration, Gay-Lussac decided that the “salureâ” of the great ocean has very small variations, if it is not the same everywhere (1817).This is extremely important. This is the first precise pronouncement that the salinity of the open ocean (specifically the Atlantic) is nearly the same” (Wallace,2004)

Alexander Marcet (1770-1822)
Alexander Marcet (1770-1822)

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. As Marcet summarized his method of analysis, the procedure was:

1.To ascertainthe quantity of saline matter contained in  a known weight of the water under examination, desiccated in a uniform and well defined mode; and to compare it with the specific gravity of the water.

2.To precipitate the muriatic acid from a known weight of the water, by nitrate of silver.
3. To precipitate thesulfiric acid by nitrate of barytes, from another similar portion of water.
4. To precipitate the lime from the water.
5. To precipitate the magnesia from the clear liquor remaining after the separation of the lime, which is best effect by phosphate of ammonia, or of soda, with the addition of carbonate of ammonia.

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).

Johann Georg Forchhammer (1794-1865)
Johann Georg Forchhammer (1794-1865)

The concept of salinity was introduced by the Danish chemist Johann Georg Forchhammer (1794-1865) in 1865. Forchhammer worked under great disadvantages: his samples of water were brought home by seafaring men from different parts of the world in corked bottles, and they were necessarily all taken from the surface or immediately beneath it. Forchhammer did not attempt to determine quantitatively all the elements that occur in sea water, but confined himself to the very accurate estimation of the principal salt components, such as  chlorine, sulphuric acid, magnesia, lime, potash and soda. 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 his scientific work “On the Constitution of Sea Water at Different Depths and in Different Latitudes” (1863) made an era in the history of ocean chemistry.

Matthew Maury (1806-1873) is perhaps the first person to have oceanography as a full-time occupation. Between 1842 and 1855, this American naval officer assembled ships logs and charts into one large atlas of oceanic winds and currents. He also produced maps for efficient sailing, known as “Sailing Directions”(1854). In early 1855 the first edition of Maury’s book, “The Physical Geography of the Seas”, was published:  “The salts of  the sea, as its solid ingredients may be called, can neither be precipitated on the bottom, nor taken up by the vapors, not returned again by the rains to the land; and but for the presence in the sea of certain agents to which has been assigned the task of collecting these ingradients again, in the sea they would have to remain.”

Matthew F. Maury (1806-1873)
Matthew F. Maury (1806-1873)
The salt content for 1000 gramm of seawater given by Maury:
Chloride of sodium -27.1 g
Chloride of magnesium 5.4 g
Sulphate of magnesia – 1.2 g
Sulphate of lime – 0.8 g
Chloride of potassium – 0.4 g
Carbonate of lime – 0.2 g
Bromide of magnesia – 0.1 g
Leaving residium of – 2.9 g
Water – 962 g

Maury was no chemist and he didn’t analyse sea water himself. Presumably, data come from authors such as Von Bibra.

The following quotes from Maury are significant: “As a general rule, the sea is nearly of a uniform degree of saltness, and the constituents of sea water are as constant in their proportions as are the components of the athmosphere”. The constancy of the ocean’s saltness and especially of that of the specific salts was strongly asserted.  Largely due to the strenght of his reputation, Maury, although not a chemist, was more responsible for the belief in the constancy of salt content and proportion of constituents of sea water than anyone else before him.