seafriends.org.nz-the chemical composition of seawater

8/10/2019 Seafriends.org.Nz-The Chemical Composition of Seawater

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seafriends.org.nz http://www.seafriends.org.nz/oceano/seawater.ht

The chemical composition of seawater

. By Dr J Floor Anthoni (2000, 2006)www.seafriends.org.nz/oceano/seawater.htm(best viewed in a window as wide as a page. Open links in a new tab.)

In order to understand the sea, some of its chemical properties are important. This page details thechemical composition of sea water, salinity, density, its dissolved gases, carbon dioxide and pH aslimiting factor. Chemical elements in sea water do not exist on their own but are attracted topreferential ions of opposite charge: sulphur will occur mainly as sulphate, sodium as sodiumchloride, and so on.

Detailed composition: abundance of the elements in seawater

Salinity: the main salt ions making the sea saltyDensity: the density of sea water depends on temperature and salinity

Dissolved gases: the two important gases to life, oxygen and carbondioxide. Limiting hydrogen ions

and ocean pH.

Bicarbonate: the life of dissolved carbon dioxide in the sea.

Related chapters:

global climate: learn about global climate step by step, from a very wide perspective.

Is global warming real or fraudulent? (140p) Must-read!

acid oceans: are oceans becoming more acidic? How does it work? Threat or fraud?

(60p) Must-read!

abundance of the elements of lifein the universe, earth, sea and organisms.

table of units & measures: units, measures, conversion constants, world dimensions,

and much more.

periodic table: the periodic table of elements, complete with elementary chemistry

and interesting facts.

soil/ecology: the main biomes of the land and their carbon sinks. How does soil

work? Sustainability? What to do against erosion? (large)

the Dark Decay Assay: new discoveries of the plankton ecosystem. pH as most

important limiting factor.

.

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Detailed composition of seawaterat 3.5% salinity

ElementHydrogen H2OOxygen H2OSodium NaClChlorine NaClMagnesium Mg

Sulfur SPotassium KCalcium CaBromine Br

At.weight1.00797

15.999422.989835.45324.312

32.06439.10240.0879.909

ppm110,000883,00010,80019,4001,290

90439241167.3

ElementMolybdenum MoRuthenium RuRhodium RhPalladium PdArgentum (silver) Ag

Cadmium CdIndium InStannum (tin) SnAntimony Sb

At.weight0.09594101.07102.905106.4107.870

112.4114.82118.69121.75

ppm0.010.0000007..0.00028

0.00011.0.000810.00033

Helium HeLithium LiBeryllium BeBoron BCarbon CNitrogen ionFluorine F

Neon NeAluminium AlSilicon SiPhosphorus PArgon ArScandium ScTitanium TiVanadium VChromium CrManganese MnFerrum (Iron) FeCobalt CoNickel Ni

4.00266.9399.013310.81112.01114.00718.998

20.18326.98228.08630.97439.94844.95647.9050.94251.99654.93855.84758.93358.71

0.00000720.1700.00000064.45028.015.513

0.000120.0012.90.0880.450


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Salinity and the main salt ions The salinity of sea water (usually 3.5%) is made up by all the dissolved salts shown in the above table.Interestingly, their proportions are always the same, which can be understood if salinity differences are caused byeither evaporating fresh water or adding fresh water from rivers. Freezing and thawing also matter.

Salinity affects marine organisms because the process of osmosis transports water towards a higher concentration

through cell walls. A fish with a cellular salinity of 1.8% will swell in fresh water and dehydrate in salt water. So,

saltwater fish drink water copiously while excreting excess salts through their gills. Freshwater fish do the opposite b

not drinking but excreting copious amounts of urine while losing little of their body salts.

Marine plants (seaweeds) and many lower organisms have no mechanism to control osmosis, which makes them

very sensitive to the salinity of the water in which they live.

The main nutrients for plant growth are nitrogen (N as in nitrate NO3 -, nitrite NO2-, ammonia NH4+), phosporus (P a

phosphate PO43-) and potassium (K) followed by Sulfur (S), Magnesium (Mg) and Calcium (Ca). Iron (Fe) is an

essential component of enzymes and is copiously available in soil, but not in sea water (0.0034ppm). This makes iro

an essential nutrient for plankton growth. Plankton organisms (like diatoms) that make shells of silicon compounds

furthermore need dissolved silicon salts (SiO2) which at 3ppm can be rather limiting.

The main salt ions that make up 99.9% are the following:

chemical ion valence concentrationppm, mg/kg

part ofsalinity %

molecularweight

mmol/kg

Chloride Cl -1 19345 55.03 35.453 546

Sodium Na +1 10752 30.59 22.990 468

Sulfate SO4 -2 2701 7.68 96.062 28.1

Magnesium Mg +2 1295 3.68 24.305 53.3

Calcium Ca +2 416 1.18 40.078 10.4

Potassium K +1 390 1.11 39.098 9.97

Bicarbonate HCO3 -1 145 0.41 61.016 2.34

Bromide Br -1 66 0.19 79.904 0.83

Borate BO3 -3 27 0.08 58.808 0.46

Strontium Sr +2 13 0.04 87.620 0.091

Fluoride F -1 1 0.003 18.998 0.068

By adding the mol in last column up, multiplied by respective valences, like: -546 +468 -56.2 +106.6 + .... one ends

up with almost 0, suggesting that the above values are about right. During the Challenger Expedition of the 1870s, it

was discovered that the ratios between elements is nearly constant although salinity (the amount of H2O) may vary.

Note that the figures above differ slightly in differing publications. Also landlocked seas like the Black Sea and the

Baltic Sea, have differing concentrations.


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This world map shows how the salinity of theoceans changes slightly from around 32ppt(3.2%) to 40ppt (4.0%). Low salinity is found incold seas, particularly during the summerseason when ice melts. High salinity is foundin the ocean 'deserts' in a band coinciding withthe continental deserts. Due to cool dry airdescending and warming up, these desert

zones have very little rainfall, and highevaporation. The Red Sea located in thedesert region but almost completely closed,shows the highest salinity of all (40ppt) but theMediterranean Sea follows as a close second(38ppt). Lowest salinity is found in the upperreaches of the Baltic Sea (0.5%). The DeadSea is 24% saline, containing mainlymagnesium chloride MgCl2. Shallow coastalareas are 2.6-3.0% saline and estuaries 0-3%.

Making sea saltSea salt is made by evaporating sea water, but this is not straight-forward. Between 100% and 50% first thecalcium carbonate (CaCO3= limestone) precipitates out, which is chalk and not desirable. Between 50% and 20%,gypsum precipitates out (CaSO4.2H2O), which also tastes like chalk. Between 20% and 1% sea salt precipitates(NaCl) but going further, the bitter potassium and magnesium chlorides and sulfates precipitate, which is to beavoided, unless for health reasons. In commercial salt production, the water is led through various evaporationponds, to achieve the desired result.Note that this process has also happened where large lakes dried out, laying down the above salts in the abovesequence. Note that normal sea water is undersaturated with respect to all its salts, except for calcium carbonate

which may occur in saturated or near-saturated state in surface waters.An artificial salt solution of 3.5% (35ppt) is made by weighing 35g of salt in a beaker and topping it up with freshwater to 1000g.

DensityThe density of fresh water is 1.00 (gram/ml or kg/litre) but added salts can increase this. The saltier the water, thehigher its density. When water warms, it expands and becomes less dense. The colder the water, the denser itbecomes. So it is possible that warm salty water remains on top of cold, less salty water. The density of 35ppt

saline seawater at 15C is about 1.0255, or s (sigma)= 25.5. Another word for densityis specific gravity.


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The relationship between temperature, salinityand density is shown by the blue isopycnal(of

same density) curves in this diagram. In red,green and blue the waters of the major oceansof the planet is shown for depths below -200metre. The Pacific has most of the lightestwater with densities below 26.0, whereas theAtlantic has most of the densest waterbetween 27.5 and 28.0. Antarctic bottom wateris indeed densest for Pacific and Indianoceans but not for the Atlantic which has a lotof similarly dense water.

Dissolved gases in seawaterThe gases dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrationsdepend on each gas' solubility, which depends also on salinity and temperature. As salinity increases, the amountof gas dissolved decreases because more water molecules are immobilised by the salt ion. As water temperatureincreases, the increased mobility of gas molecules makes them escape from the water, thereby reducing theamount of gas dissolved.

Inert gases like nitrogen and argon do not take part in the processes of life and are thus not affected by plant and

animal life. But non-conservative gases like oxygen and carbondioxide are influenced by sea life. Plants reduce the

concentration of carbondioxide in the presence of sunlight, whereas animals do the opposite in either light or

darkness.

gasmolecule

% inatmosphere

% in surfaceseawater

ml/litresea water

mg/kg (ppm)in sea water

molecularweight

mmol/kg

Nitrogen N2 78% 47.5% 10 12.5 28.014 0.446

Oxygen O2 21% 36.0% 5 7 31.998 0.219

Carbondioxide CO2 0.03% 15.1% 40 90 * 42.009 2.142


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Argon 1% 1.4% . 0.4 39.948 0.01

One kg of fresh water contains 55.6 mol H2O* also reported as 80 mg/kg

Please note that these figures may be incorrect as too many different values have been published

In the above table, the conservative gases nitrogen and argon do not contribute to life processes, even though

nitrogen gas can be converted by some bacteria into fertilising nitrogen compounds (NO3, NH4). Surprisingly the

world under water is very much different from that above in the availability of the most important gases for life: oxyge

and carbondioxide. Whereas in air about one in five molecules is oxygen, in sea water this is only about 4 in everythousand million water molecules. Whereas air contains about one carbondioxide molecule in 3000 air molecules, in

sea water this ratio becomes 4 in every 100 million water molecules, which makes carbondioxide much more

common (available) in sea water than oxygen. Note that even though their concentrations in solution differ due to

differences in solubility (ability to dissolve), their partial pressures remain as in air, according to Henry's law, except

where life changes this. Plants increase oxygen content while decreasing carbondioxide and animals do the reverse

Bacteria are even capable of using up all oxygen.

All gases are less soluble as temperature increases, particularly nitrogen, oxygen and carbondioxide which become

about 40-50% less soluble with an increase of 25C. When water is warmed, it becomes moresaturated, eventually

resulting in bubbles leaving the liquid. Fish like sunbathing or resting near the warm surface or in warm water outfall

because oxygen levels there are higher. The elevated temperature also enhances their metabolism, resulting in fastgrowth, and perhaps a sense of wellbeing.

Likewise if the whole ocean were to warm up, the equilibrium with the atmosphere would change towards more

carbondioxide (and oxygen) being released to the atmosphere, thereby exacerbating global warming.

Since the volume of all oceans is 1.35E21 kg (see table of units & measures) and CO2 concentration is 9E-5 kg/kg

(90ppm), it follows that the total amount of CO2 in all oceans is 12.2E16 kg = 121,000 Pg (Mt) and the partial carbon

amount (12/42) = 34,700 Pg (600Pg in surface waters + 7000Pg in mid waters + 30,000Pg in deep ocean =

37,600Pg [1]). Compare this with the amount of carbon in soil and vegetation (1301 + 664 = 1965 Pg, see

soil/ecology) and the carbon in the atmosphere, about 1 kg per square metre over 510E6 km2 = 510E12 kg = 510

Pg (700Pg [1]). It follows that the ocean is a very large reservoir of carbondioxide, also called Dissolved Inorganic

Carbon (DIC). In addition to this, it contains Dissolved Organic Carbon (DOC) of unknown quantity. The difference

between DIC and DOC is an arbitrary particle size of 0.45m which passes DIC through filtration paper. This

definition does not distinguish our newly discovered slush(incompletely decomposed biomolecules) as DOC. See

our DDA section.
http://www.seafriends.org.nz/dda/index.htmhttp://www.seafriends.org.nz/enviro/soil/ecology.htmhttp://www.seafriends.org.nz/books/units.htm


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What is dissolved, particulate, inorganic and organic carbon?Carbon is a miraculous element located in the middle of the Periodic Table, next to nitrogen, which is also asurprising element. Elements to the left are basic with positive valence (attracting free electrons) and those to theright are acidic with negative valence (owning loose electrons). Carbon with a valence of 4 can bind with both sidesof the table and with itself. When combined with hydrogen, it forms long chains of organic molecules likeCH3.CH2.CH2......X where the end group X gives it the character of an alkane (CH3), alcohol (OH), acid (COOH),aldehyde (COH), amino (NH2), and so on. The organic carbon chains can form loops and bonds with otherelements, all being organic compounds. Only few inorganic carbon compounds are known, of which carbondioxide(CO2) is by far the most common. Natural gas or methane (CH4) is either the last inorganic molecule or the first

organic molecule. So it is safe to say that dissolved inorganic carbon is CO2, particularly since it dissolves soreadily in water.

All biomolecules that make up the structure of an organism are organic (except for salts in body liquids), and whenthese are decomposed, the leftover molecules are also organic, except for inorganic nutrients and CO2, for thewhole purpose of decomposition is to turn organic molecules into inorganic nutrients and CO2 for plants. Allbiomolecules can be transported by being dissolved in water. When an organism dies and decomposes, most of itsorganic molecules end up in solution as dissolved organic carbon (DOC), molecules that are very much smallerthan the smallest of organisms (viruses).

Plankton organisms are classified by size from femtoplankton (smaller than 0.2m), picoplankton (0.2-2m) tomegaplankton (0.2-2m). Note that the wavelength of visible light is 0.4-0.7m, which means that organismssmaller than 1m are not visible under a light microscope (all viruses and most bacteria). What all this means is

that measuring the biomass of plankton is almost impossible. For practical reasons, scientists decided thatanything passing through fine filtration paper (0.45m) is dissolvedand all that is retained is particulate.Unfortunately this marks a substantial amount of particulate biomass as dissolved.

Phytoplankton consists of organisms from bacteria to diatoms and large dinoflagellates (like sea spark, Noctilucascintillans). Their biomass can be estimated by measuring their chlorophyl (green pigment) from lightmeasurements. However, other pigments (brown, red) are also common and the amount of chlorophyl is only asmall part of biomass. So, even quantifying the amount of phytoplankton is almost impossible.

The bottom line is that the boundaries between dissolved, particulate, inorganic and organic are rather vague. Alsothe functional difference between producers (phytoplankton) and decomposers (most bacteria) is seldomacknowledged.
http://www.seafriends.org.nz/books/periodi.htm


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Deep sea temperature, oxygen & nutrientsIn general the ratios between the variouselements in seawater is constant, exceptwhere modified by life. In this diagram one cansee how light penetrates no deeper than 150mfor photosynthesis. Indeed at 800m, the oceanis pitch dark. In the surface mixed layer abovethe thermocline, water mixes sufficiently tosustain life. Gas exchange with the

atmosphere is near-perfect such that theoxygen concentration in the water is inequilibrium with the atmosphere. But it rapidlydecreases below 50-75m as photosynthesisdeclines while animals use up most oxygen. Ataround 800m oxygen levels reach a minimum(as also carbondioxide levels reach amaximum, not shown). Towards the deep andbottom water, oxygen levels increase slightlydue to an influx of cold bottom water from thepoles. Due to lack of oxygen, deep sea fishcannot be very active.

The coloured curves for phosphate and nitrate show how these nutrients are almost completely used near the surfa

and how they gradually become available in the thermocline layer. Note how the Atlantic Ocean ends up with less

nutrients than the Pacific and Indian oceans.

The temperature curve shows the general idea of staying relatively high and constant in the mixed layer, then

declining rapidly in the thermocline layer until reaching a near constant temperature of +3C in deep and bottom

water. The maximum surface temperature of course depends on many factors, like latitude and season.

Note that the concentration of CO2 in the atmosphere has increased from 280 ppm in 1850 to 360 ppm in 1998, and

is still rising. It is estimated that about 50% of anthropogenic CO2 has been absorbed by the oceans. Because the

upper atmosphere is bombarded by cosmic rays, some of the nitrogen atoms become radioactive isotopes C-14 wit

a half life of 5730 years. Once incorporated into organisms, its radioactivity decays slowly, allowing scientists tocalculate the age of organic substances. Fossil fuels which have been underground for over 60 million years, have

lost nearly all their radioactive carbon isotopes, and in this manner CO2 from burning fossil fuels can be distinguishe

from normal CO2 circulation. The diagrams below shows how fossil carbondioxide is absorbed by the oceans.


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Radioactive Carbon-14 As cosmic rays bombard the outer atmosphere, they are slowed down by the thin gases there. With their energy ofbillions of electron-Volt (eV) they produce fast neutrons that gradually slow down to that of thermalneutrons. At aheight of about 9-15km, these neutrons collide with nitrogen-14 (normal nitrogen), producing radioactive carbon-14(carbon with one extra neutron). The total amount of C-14 produced each year is about 9.8kg for the whole Earth,or about 1 atom C-14 for 1 trillion (1E-12) normal C-12 atoms. Nuclear tests have almost doubled the quantity inthe atmosphere in a peak (year 1964) that is gradually becoming normal again as the peak is absorbed byorganisms and the ocean. Radioactive carbon decays back to nitrogen by emitting an electron (beta radiation) atthe initial rate of 14 disintegrations per minute per gram carbon. The C-13 carbon isotope which is not radioactive,

occurs for about one in every 100 atoms C. The age of organic remains can thus be measured by counting betaradiation from disintegrating atoms, but a much more sensitive method is by counting all C14 atoms by massspectrometry.Because of its slow decay rate of 50% in 5700 years, the total amount of C-14 in the atmosphere, biosphere andoceans is much higher than 10kg. According to Libby (1955) who invented carbon dating, the distribution of carbonand carbon-14 is as follows:

carbon reservoir percentage

CO2 dissolved in oceans 87.5

Dissolved Organic Carbon (DOC) in oceans 7.1

Biosphere, all living organisms 4.0

Atmospheric CO2 1.4


10/12

Note that at a pH of 7.0 (neutral water) only 0.1 mol/kg (10-7) of water is dissociated into positive hydrogen ions H

and negative hydroxyl ions OH

-

. In the ocean where a pH of around 8 is found, this becomes even less at 0.01mol/kg, which makes hydrogen ions twenty times scarcer than oxygen and 200 times scarcer than

carbondioxide. It explains how important the pH is to the productivity of aquatic ecosystems. Visit our latest plankto

discoveries in the Dark Decay Assay section where this limiting factor was quantified in freshwater lakes.
http://www.seafriends.org.nz/dda/index.htm


11/12

This world map of ocean acidity shows that ocean pH varies from about 7.90 to 8.20 but along the coast one mayfind much larger variations from 7.3 inside deep estuaries to 8.6 in productive coastal plankton blooms and 9.5 intide pools. The map shows that pH is lowest in the most productive regions where upwellings occur. It is thoughtthat the average acidity of the oceans decreased from 8.25 to 8.14 since the advent of fossil fuel (Jacobson M Z,2005).

Carbondioxide as bicarbonateCarbondioxide binds loosely with water to form bicarbonate:

CO2 + H2O H2CO3 H+ + HCO3- H++ H++ CO32-

in the ratios CO2 & carbonic acid H2CO3 = 1%, bicarbonate HCO3-= 93%, carbonate CO32-=6%. These variantof CO2 (species) add up to the total amount of Dissolved Inorganic Carbon (DIC), which also includes a smaller

amount of Dissolved Organic Carbon (DOC) that passes filtration techniques.The symbol means 'in equilibrium with'.

These forms of carbon are always in close equilibrium with the atmosphere and with one another. When one talks

about dissolved carbondioxide, it is the slightly acidic bicarbonate. When the concentration of CO2 in the atmospher

increases, presumably also the concentration in the ocean's surface increases, and this works itself through to the

right in above equation.

Photosynthesis of organic matter is often simplified as: CO2 + H2O + sunlight => CH2O +O2, which happens only i

the sunlit depths to 150m and down to where the sea mixes.


12/12

The average composition of marine plants is: H:O:C:N:P:S = 212:106:106:16:2:1 which comes close to CH2O.

Respiration is often simplified as : CH2O => CO2 + H2O + energy, which can happen at all depths, depending on th

amount of food sinking down from above.

Therefore the concentrations of oxygen and carbondioxide vary with depth. The surface layers are rich in oxygen

which reduces quickly with depth, to reach a minimum between 200-800m depth. The deep ocean is richer in oxyge

because of cool and dense surface water descending from the poles into the deep ocean.

It is thought that the carbondioxide in the sea exists in equilibrium with that of exposed rock containing limestoneCaCO3. In other words, that the element calcium exists in equilibrium with CO3. But the concentration of Ca

(411ppm) is 10.4 mmol/l and that of all CO2 species (90ppm) 2.05 mmol/l, of which CO3 is about 6%, thus 0.12

mmol/l. Thus the sea has a vast oversupply of calcium.

[1] Report of the Royal Society (June 2005): Ocean acidification due to increasing atmospheric carbon dioxide

http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13539 (1MB)

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