Early Determination of Salinity: from Ancient Concepts to Challenger Results

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Empedocles

The saltiness of seawater has been recognized throughout recorded history.  Theories about origin of the seawater and nature of saltness of the sea have first been formulated by ancient philosophers. In the 6th century BC Greek scientists abandoned mythological interpretations of the universe in favour of explanations relying on natural causes.  Famous pre-Socratic philosopher and poet Empedocles (490-430BC) is known as a founder of the cosmogenic theory of the four classical elements. But he was also the first philosopher, who left such poetical definition of the seawater origin: ”The Sea is the Sweat of the Earth ” (The Fragments, Book1, p.179) Another philosopher said that all the earth was at first surrounded with moisture, some of which later formed the sea in a process of drying which would ultimately end in the loss of the sea (attributed to Anaximander and Diogenes of Appolonia). One more philosopher simply attributed the saltness to the earth that the water picked up as it came in contact with the earth as it ran over it, just as water strained through ashes is known to be salty. The sea itself was explained by the accumulation of the run-off (attributed to Metrodorus of Chios and Anaxagoras).

 

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Aristotle (384-322BC) summarised Greek philosophers views by saying: “At first the Earth was surrounded by moisture. Then the sun began to dry it up, part of it evaporated, and is the cause of winds while the remainder formed the seas. So the seas are being dried up. Others say that the sea is a kind of sweat exuded by the earth when the sun heats it, and that this explains its saltness, for all sweat is salt. Others say that the saltness is due to the earth. Just as water strained through ashes becomes salt, so the sea owes its saltness to the mixture of earth with similar properties.” Aristotle mentioned that salt water was heavier and more dense than fresh water, and salt water would seek a lower level. Aristotle was aware that the sea contained other than just salt and commented on both its salt and bitter taste. Aristotle apparently was the first person to have noticed and attempted to explain the bitter quality of sea water – a point which did not occur in the literature for at least two thousand years later. More specifically he was the first to mention something other then salt and water in seawater. The salt from the sea (prepared by evaporation) was weaker in saltiness and was generally not as white and less lumpy than “normal” salt.  He added: “Why is the sea salty and bitter? Is it because the juices in the sea are numerous? For saltness and bitterness appear at the same time.” One of Aristotle’s experimental proofs that saltness is due to the admixture of some substance was with the use of a completely closed wax vessel. This container was lowered into the sea and: “then the water that percolates through the wax sides of the vessel is sweet, the earthy stuff, the admixture of which makes the water salt, being separated of as it were by a filter.” Aristotle’s attempts to explain the saltness of the sea were hardly altogether clear and the wax container experiment is just one simple example. Later confusion and misconceptions did arise concerning this subject area. From a compositional standpoint, Aristotle tried to answer the questions: why the sea water was salty, why water which is naturally fresh became salt, and what was the nature of the material that caused the bitter taste present in sea water.

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Roman natural philosopher and naval commander Pliny the Elder (25-76AC) in his fundamental work “Naturalis Historia” in Chapter 104 “Why the sea is salt”, presents Aristotele’s discoveries and gives qualitative description of salinity distribution with depth: “Hence it is that the widely-diffused sea is impregnated with the flavour of salt, in consequence of what is sweet and mild being evaporated from it, which the force of fire easily accomplishes; while all the more acrid and thick matter is left behind; on which account the water of the sea is less salt at some depth than at the surface.” In his explanation Pliny followed Aristotle, and helped to open up magnificent arena for the scholastics of the Middle Ages to dispute in. Pliny appears to have been the first person to give an early quantitative estimate for the amount of salt in sea water by which one could make sea water: “If more than a sextarius of salt is dropped into four sextari of water, the water is overpowered, and the salt does not dissolve. However, a sextarius of salt and four sextari of water give the strength and properties of the saltest sea. But it is thought that the most reasonable proportion is to compound the measure of water given above with eight cyathi of salt. This mixture warms the sinews without chafing the skin.” As in a case of Aristotle, Pliny thought the salt content should be greater at the surface due to the loss of water here. Yet along with Aristotle Pliny knew that salt water was more dence than fresh water, and he indicates that patches of fresh water can be found floating on the surface of the sea.

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Another Roman and contemporary of Pliny was the philosopher  Lucius Seneca (3BC-65AD). Seneca’s view as to the nature of the world appeared in his Quaestiones Naturales. Seneca was a keen observer, and much of the Quaestiones Naturales present his own observations with more originality than, for example, Pliny. Seneca had noticed that the water level and the salinity of the sea remained constant even though water was constantly being added by rivers and rain. The constancy was, he believed, due to the evaporation of the sea’s waters. He believed that saline waters could be filtered by earth and attributed the formation of calcereous tuffs to this action. Seneca thought that the world in the beginning was characterized by a primordial ocean, and the substances dissolved therein separated out over some space of time.  Although he know that solubility of a substance was in some way related to the water’s temperature and that the temperature of the sea varied, he seems to have believed that the ocean’s saltness was a constant.

Bishop Watson says: “There are few questions respecting the natural history of the globe which have been discussed with more attention, or decided with less satisfaction, than that concerning the primary cause of the saltness of the sea. The solution of it had perplexed the philosophers before time of Aristotle; it surpassed his own great genius, and those of his followers who have attempted to support his arguments have been betrayed into very ill grounded conclusions concerning it.  Father Kircher, after having consulted three and thirty authors upon the subject, could not help remarking, that the fluctuations of the ocean itself were scareely more various then opinions of  men concerning the origin of its saline impregnation”.

 

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In the Renaissance period Leonardo da Vinci (1452-1519) in his famous  The Notebooks in Note 946 “Refutation of the Pliny’s theory of the saltness of the sea” polemicize  with Pliny: “Pliny says, that the water of the sea is salt because the heat of the sun dries up the moisture and drinks it up; and this gives to the wide stretching sea the savour of salt. But this cannot be admitted, because if the saltness of the sea were caused by the heat of the sun, there can be no doubt that lakes, pools and marshes would be so much the more salt, as their waters have less motion and are of less depth; but experience shows us, on the contrary, that these lakes have their waters quite free from salt. Again it is stated by Pliny that this saltness might originate, because all the sweet and subtle portions which the heat attracts easily being taken away, the more bitter and coarser part will remain, and thus the water on the surface is fresher than at the bottom; but this is contradicted by the same reason given above.  Again, it has been said that the saltness of the sea is the sweat of the earth; to this it may be answered that all the springs of water which penetrate through the earth, would then be salt. But the conclusion is, that the saltness of the sea must proceed from the many springs of water which, as they penetrate into the earth, find mines of salt and these they dissolve in part, and carry with them to the ocean and the other seas, whence the clouds, the begetters of rivers, never carry it up. And the sea would be salter in our times than ever it was at any time; and if the adversary were to say that in infinite time the sea would dry up or congeal into salt, to this I answer that this salt is restored to the earth by the setting free of that part of the earth which rises out of the sea with the salt it has acquired, and the rivers return it to the earth under the sea.”

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Scientific work on ocean salts was first done by the celebrated British natural philosopher Robert Boyle in 1674 with his publication of ”Observations and Experiments in the Saltness of the Sea“ :
The Cause of the Saltness of the Sea appears by Aristotle’s Writings to have busied the Curiosity of Naturalists before his time; since which, his Authority, perhaps much more than his Reasons, did for divers Ages make the Schools and the generality of Naturalists of his Opinion, till towards the end of the last Century, and the beginning of ours, some Learned Men took the boldness to question the common Opinion; since when the Controversie has been kept on foot, and, for ought I know, will be so, as long as `tis argued on both sides but by Dialectical Arguments, which may be probable on both sides, but are not convincing on either. Wherefore I shall here briefly deliver some particulars about the Saltness of the Sea, obtained by my own trials, where I was able; and where I was not, by the best Relations I could procure, especially from Navigators.”

After all, (says he,) it may be observed, that we are inquiring into the cause of a phenomenon, which it may be said had no secondary cause at all. It is taken for granted, in this disquisition, that the water which covered the globe in its chaotic state, was not impregnated with salt as at present, but quite fresh: now this is an opinion concerning a matter of fact, which can never be proved either way; and surely we extend our speculations very far, when we attempt to explain a phenomenon, primeval to, or coeval with, the formation of the earth.”

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This sensible writer then states the different experiments which have been made to discover the saltness of the sea, round the shores of Britain; and proposes the following simple method of ascertaining it with tolerable certainty:”As it is not every person who can make himself expert in the use of common means of estimating the quantity of salt has g contained in sea water, I will mention a method of doing it, which is so easy and simple, that every common sailor may understand and practise it; and which, at the same time, from the trials I have made of it, seems to be as exact a method as any that has yet been thought of. – Take a clean towel, or any other piece of cloth; dry it well before the sun or before the fire, then weigh it accurately, and note down its weight; dip it in the sea water, and, when taken out, wring it a little till it will not drip when hung up to dry; weigh it in this wet state, then dry it in the sun or at the fire, and when it is perfectly dry, weigh it again: the excess of the weight of the wetted cloth above its original weight, is the weight of the sea water imbibed by the cloth; and the excess of the weight of the cloth after being dried, above its original weight, is the specific gravity of the salt retained by the cloth; and by comparing this weight with the weight of the sea water imbibed by the cloth, we obtain the proportion of salt contained in that species of sea water.” Whoever undertakes to ascertain the quantity of salt contained in sea water, either by this or any other method, would do well to observe the state of the weather preceding the time when the sea water is taken out of the sea; for the quantity of salt contained in the water near the surface, may be influenced, both by the antecedent moisture, and the antecedent heat of the atmosphere. And this leads to the consideration of a question proposed by Aristotle, – Why are the upper parts of the sea Salter and warmer than the lower? Some philosophers, admitting the fact, have followed him in attempting to explain it; whilst others have thought themselves authorized by experiment to deny the truth of the position; and those, perhaps, will argue with the greatest justness, who shall affirm that it is neither generally to be admitted, nor generally to be rejected, but that the sea in some places, and under certain circumstances, is salter and warmer at the surface, than at any considerable depth beneath it, while in many others the reverse is true. The question consists of two parts, betwixt which, though there probably is a connection, yet it is not so necessary a one as to hinder us from considering each part by itself.With regard to the use of this salt property of sea water, it is observed, that the saltness of the sea preserves its waters pure and sweet, which otherwise would corrupt, and emit a stench like a filthy lake, and consequently that none of the myriads of creatures which now live therein could exist. From thence also the sea water becomes much heavier, and therefore ships of greater size and burden are safely borne thereon. Salt water also does not freeze so soon as fresh water, hence the seas are more free for navigation.

Boyle measured and compiled a considerable set of data for variations in the saltness of surface seawater. He personally made a series of observations on the water of the English Channel, collecting it from various depth, and observing its specific gravity. He also designed an improved piece of equipment for sampling seawater at depth, but the depths at which it was used were modest: 30 m with his own instrument, 80 m with another, similar sampler.  Boyle investigated the saltness of the water by a number of processes: he tried the estimation of total solids by direct evaporation and ignition, but not being satisfied with result, he ultimately took the density as an index of the saltness, and determined this either by means of a glass hydrometer, by weighting in a phial which was afterward weighted when full of distilled water, or by weighting a piece os sulphyr in distilled water and sea water consecutively.His measurements of the salt in seawater were done also by precipitating the salt. He 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.

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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.

The Histoire Physique de la Mer was written by Marsilli and published in Amsterdam in 1725. ImagePrior 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.

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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

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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)

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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).

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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.

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The most comprehensive early study of the composition of seawater was that made by W.Dittmar (1884) on 77 samples collected by chemist J.Y.Buchanan during the Challenger Expedition(1872-1876).The physical and chemical investigations conducted by Mr.J.Y.Buchanan, during the three and half years’ cruise of H.M.S. Challenger, are among th most important and valuable of the Expedition.Mr.Buchanan collected daily, with much care, samples of the surface water, and determined the specific gravity. At all Stations, a slip water bottle was attached to the sounding line, and the specific gravity of the specimen of bottom water thus collected was also ascertained. At every Station, where practicable, waters were collected from intermediate depths at 25, 50, 100, 200, 300, 400, and 800 fathoms from the surface, with a stop-cock water bottle attached to a separate sounding line, under Mr. Buchanan’s personal supervision. The specific gravity of these waters was also determined.The routine chemical work of the Laboratory consisted in boiling out the gases from, and in determining the carbonic acid in, as many samples as possible. A very large number of samples of sea-water were collected from the surface, bottom, and intermediate depths, and preserved in glass stoppered bottles.

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These were either sent to home along with other collections from various ports touched at during the Expedition, or brought home by the ship.It is difficult to any one, except those who actually witnessed the daily work at sea, to form an abequate idea of the labor, skill, and continious effort required to carry these observations in all sorts of weather, and to form, and bring home successfully, collections and observations like those which have resulted from Mr. Buchanan’s exertions.Shortly after return to England, Mr. Buchanan analysed a number of the samples of gas which had been boiled out from the waters on board ship. As Mr.Buchanan was subsequently unable to proceed with the chemical work connected with the Expedition, the reminder of the gas samples (along with the results of those analysed), the water samples, and Mr.Buchanan’s official journals, were entrusted by late sir C.Wyville Thomson to Professor W.Dittmar, F.R.S., with a request that he would undertake certain analyses of the gas samples and the waters. Professor Dittmar forwarded reports on his analyses at various times to the late Editor during the years 1878-81.

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In the year 1882, Professor Dittmar undertook, at my request, to complete the gas and water analyses, and to prepare a Report on the whole of this investigations into the Composition of Ocean-Water, embracing the work done on board ship by Mr.Buchanan. “ John Murray, Editorial Notes to Report of the Scientific Results of the Voyage of HMS Challenger during the years 1873-76, Physics and Chemistry,V1.

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With the exeption of the early analysis as to the purity of a certain well-known soap, no results of an analytical laboratory have recieved more publicity and attention than the classical results obtained at the University of Edinburgh by Dittmar on the waters collected by J.Y.Buchanan. Since then many investigators have become interested in the chemistry of the sea.

“The importance of this result cannot be over emphasized, as upon it depends the validity of chlorinity-salinity-density relationship and, hence, the accuracy of all conclusions based on distribution of density where the latter is determined by chemical or indirect physical methods, such as electrical conductivity…” Sverdrup, Johnson, Fleming