Download The Salinity of Seawater

The Salinity of Seawater
“On average,
seawater in the
world's oceans has a
salinity of about 3.5%
(35 g/L, or 0.600 M).”
- Wikipedia
Chem M3LC
R. Corn
Seawater Analysis Project
Major ionic species
in seawater
Concentrations
Cl - - 536 mM
Na+ - 457 mM
Mg2+ - 56.3 mM
Ca2+ - 10.0 mM
K+ - 9.74 mM
SO42- - 2.76 mM
Br - - 0.823 mM
We will measure six of the
top seven ions in sea water!
Salinity - A Short History
from http://salinometry.com/
Robert Boyle (1674)
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". 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).
Antoine Lavoisier (1772)
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 (1817)
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).
HMS Challenger Measurements (1884)
W.Dittmar
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.
Chlorinity Scale Standards (1889)
Martin Knudsen
Accordingly there was a need for a better method of determination total dissolved salts then the tedious and unreliable one of
evaporating a sample. In 1889 the International Council for the Exploration of the Sea named Martin Knudsen (1871-1949) 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. 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 standardize 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.
Conductivity Measurements (1893)
Hercules Tornóe
In the late 19th century a method was developed for determining the salinity of the seawater sample from measurement of its
electrical conductivity.The Norwegian chemist Hercules Tornóe (1856-1907) became a pioneer in this matter. At the meeting on 6
October 1893 of the Norwegian Academy of Science, he reported on the series of investigations which showed that salinity of the
seawater could be determined by measuring the electrical conductivity of the water.The conductivity would be determined by means
of alternating current and a telephone bridge. As conductivity is also greatly influenced by temperature, the temperature would have to
be determined with great accuracy and its influence eliminated.
New relationship between salinity, conductivity ratio and temperature were, therefore, established based on measurements carried out
on natural seawaters covering a wide salinity range.Thus the chlorinity based salinity became re-defined in terms of conductivity ratio
and at the same time the equation (1) was replaced by:
S = 1.80655 Cl
(2)
This made both salinity and chlorinity conservative properties. Equations and tables based on the new measurements were published
in 1966 by UNESCO and were widely adopted.
Chlorinity Titration
Karl-Friedrich Mohr
Chemical determination of halide content by titration was for many years the usual routine method for
determining the “sea salt” content of sea water samples.This method, which known as Mohr method (Mohr,
1856), consist of titrating a sample of seawater with silver nitrate solution of known concentration to the
point where all halides (chloride plus a small amount of bromide ) have been precipitated as silver halide, as
detected by suitable indicators or electrode systems. Potentiometric end point determination has been
utilized by several workers to enhance the precision of the silver nitrate titration. Reeburgh and Carpenter
(1964) used a differential electrochemical system for end point detection. Chlorinity was then converted to
salinity by means of equation [2], prior to the introduction of the Practical Salinity Scale 1978.
In the past fifty years, the chlorinity titration, which was time-consuming and requied a certain degree of
analytical skill, has been largely replaced by the measurement of electrical conductivity as a mean of
estimating salinity.The measured conductivity ratio is converted to the practical salinity by means of the
equation of the PSS-78.Chlorinity is now regarded as an independent chemical parameter to describe the
properties of seawater and has no defined relationship to salinity.
Development of the Practical Salinity Scale in 1978 (PSS-78)
The work aimed at the development of a uniform repeatable Practical Salinity Scale, based on electrical conductivity
measurement, was undertaken in 1975 at the request of the UNESCO-SCOR-ICES-IAPSO, Joint Panel on
Oceanographic Tables and Standards (JPOTS) in several different laboratories in four countries with radically different
measurement equipment. The PSS consolidates into a single set of equations the final experimental values as supplied
by the various laboratories.These equations define Practical Salinity in term of the conductivity ratio at 15°C and give
a mechanism for calculation of salinity from conductivity, temperature and pressure measurements. PSS-78 was
recommended for use by all oceanographers in reporting future oceanographic data.
Practical salinity, symbol S, of a sample of seawater, is defined in terms of the ratio K15 of the
electrical conductivity of the seawater sample at the temperature of 15°C and the pressure of
one standard atmosphere, to that of a potassium chloride (KCl) solution, in which the mass
fraction of KCl is 32.4356E-3, at the same temperature and pressure. The K15 value exactly equal
to 1 corresponds, by definition, to a practical salinity exactly equal to 35, The practical salinity is
defined in terms of ratio K15 by the following equation:
Aquarius: Satellite-Based Ocean Salt Measurements
The Aquarius/SAC-D (Satélite de Aplicaciones Científicas) observatory, a collaboration between
NASA and Argentina’s space agency, was designed to measure global sea surface salinity.
Aquarius was the first NASA instrument specifically designed to study surface ocean salinity
from space, and it did so at a rate of 300,000 measurements per month. It used three passive
microwave sensors, called radiometers, to record the thermal signal (emissivity) from the oceans’
top 10 millimeters (about 0.4 inches).
Aquarius: Satellite-Based Ocean Salt Measurements
The salinity sensor detects the microwave emissivity of the top 1 to 2 centimeters of ocean water – a physical
property that varies depending on temperature and saltiness.The instrument collected data in 386 kilometer-wide
(240-mile) swaths in an orbit designed to obtain a complete survey of global salinity of ice-free oceans every seven
days.The map above shows salinity near the ocean surface as measured by the Aquarius instrument on the Satélite
de Aplicaciones Científicas (SAC)-D satellite.The data depicted shows average salinity from May 27 to June 2,
2012, in a range from 30 to 40 grams per kilogram, with 35 grams being the average. Lower values are
represented in purples and blues; higher values are shown in shades of orange and red.
Aquarius: Satellite-Based Ocean Salt Measurements
Atlantic Ocean Salinity