Here we touch upon some more technical aspects of Seawater. While these facts are rather interesting, we cannot do justice to all there is to know and refer the scholarly-inclined reader to explore some other good books. One of these is of course René Quinton's monumental work: "L'eau de mer milieu organique" - (1912: Ed. Masson) Reprinted: Ed. ENCRE 1995. Please see our Bibliography for further reading references.
Seawater Salt Concentrations
In this field of endeavor, certain 'buzz words' are frequently thrown around, and we need to define these 'labels' in order to have a decent basis of understanding what seawater is all about.
Iso - means 'equal'. In the field of medicine, a fluid is Isotonic, when it has the same concentration of solutes as found in the blood. In case of a saline solution (seawater included), it means that an Isotonic solution is 9‰ (that's NOT 9% - that's PARTS PER THOUSAND) salt and 991‰ purified water. In a roundabout way, one might say that Isotonic seawater has only 1/3 the salt content of regular ocean water (not the Dead Sea!).
Since a red blood cell has a salt concentration of about 0.89%, putting it into hypertonic solution (>0.89%) would pull water out of the cell; putting it into a hypotonic solution (<0.89%) would cause water to flow into the cell, making it swell to bursting; putting it into an isotonic solution (=0.89%) would have no effect.
In other words, if you had a volume of 1000cc (1009g by weight) of an isotonic seawater solution, then 0.9cc would be saturated salts and 991cc would be purified water (hopefully spring water and not distilled water). So, Isotonic seawater has 9‰ (per mil) salts in solution. Because of the striking similarity between Isotonic Ocean Water and internal body fluids, notably the blood, Diluted Ocean Water has been called by various names:
- Marine Plasma
- Ocean Plasma
- Quinton Plasma
- Marine Serum
- Quinton Serum
If the salt(s) concentration is more than 9‰ then ocean water is HYPERTONIC. 21‰ seawater is sometimes called "Duplase" but it is not often used. It has roughly 2/3 the salt content of regular seawater. Ocean salinity (expressed in ‰ - per mil) is approximately equal to the weight, in grams, of salts dissolved in 1000g of seawater. This would be the salt concentration in parts-per-thousand (‰). Average ocean water has a salinity of 35g of dissolved salts. Expressed in %, it would be 96.5% pure water molecules and 3.5% of salts, dissolved gases, organic materials and undissolved particles. This means that 1000g of average seawater contains 965 grams of water and 35 grams of salts.
Hypertonic Sea Water is really an Oceanic Solution, not a true plasma or serum. This distinction should be noted when discussing these two major forms of seawater in scientific terms.
If the Ocean water solution is less that 9‰ (concentration of solutes), it is HYPOTONIC.
If ANY LARGE large amount of fluid is introduced into the human body either lower or higher than 9‰, CELL DAMAGE will be inevitable! The blood cells themselves will either: CREANATE (> 9‰ salt in solution) and crush inward on themselves or LYSE and burst (< 9‰ salt in solution). This does not apply to hypertonic seawater that is taken orally, in small doses, like mouthful at a time.
Have you ever heard of NORMAL SALINE SOLUTION? It is 9‰ salt - and that's regular table salt - NaCl. IN OTHER WORDS IT'S THAT UBIQUITOUS CLEAR FLUID THAT YOU SEE IN EVERYBODY'S ARM AT THE HOSPITAL! However, that saline solution is not equivalent to seawater any more than seawater is NaCl in purified water at 35‰! Seawater is a living substance and we explain this elsewhere in this web site.To be fair - there are other 'saline' solutions and these are discussed in detail in this document.
To convert Hypertonic seawater (35‰) to Isotonic seawater (9‰), the dilution rate is: 30% pure ocean water to 70% diluting water - by volume. Or one takes ordinary seawater and dilutes it with Spring Water by the ratio of 2:4.66 which means: 2 L of pure ocean water is mixed with 4.66 L of diluting water to produce 6.66 L of isotonic ocean water.
Chemistry of Seawater
- What is sea salt? What is salinity and how is it measured?
- Where does sea salt come from?
- Why has sea salt composition been constant for billions of years?
- What are some important nonconservative substances dissolved in seawater?
1. Salinity is related to the concentration of dissolved salts in seawater.
In the past, salinity of seawater was measured by evaporating the water and weighing the amount of salt remaining. Since that approach is difficult and inaccurate, electrical conductivity of seawater is now used to measure salinity.
- Conductivity increases as salt content of the water increases.
- Conductivity gives very accurate salinity data: 35.0000X.
- Conductivity (and temperature and depth) are measured by instruments called CTDs (Conductivity Temperature Depth). These instruments can make thousands of measurements/hour.
- Salinity, temperature, and depth (pressure) can be used to calculate density, which is important to understanding vertical circulation of the water.
- Salinity is greatest in warm, tropical surface waters, where there is more evaporation than precipitation. It is lowest where there are large inputs of freshwater from rivers.
Salinity has no units. (The PSU or "practical salinity unit" is incorrect, although frequently used.)
- Salinity is approximately equal to the weight, in grams, of salt dissolved in 1000 g of seawater. This would be the salt concentration in parts per thousand (‰).
- Average ocean water has a salinity of 35.0.
- This means that 1000 g of average seawater contains 965 g of water and 35 g of salts.
2. Salts consist of ions.
Cations have a positive electrical charge. Anions have a negative electrical charge. Salts are electrically neutral because the cation and anion charges are opposite and equal.
When salts dissolve in water, they break apart into their cations and anions. Examples are:
- Sodium chloride, NaCl, dissociates to Na+ and Cl-.
- Magnesium sulfate, MgSO4, dissociates to Mg2+ and SO42-.
3. Six major ions make up >99% of the total dissolved in seawater.
They are sodium ion (Na+), chloride (Cl-), sulfate (SO42-), magnesium ion (Mg2+), calcium ion (Ca2+), and potassium ion (K+).
4. Every naturally-occurring element has been found in seawater.
Some, however, have minuscule dissolved concentrations:
- Iron, 0.06 parts per billion (ppb)
- Lead, 0.002 ppb.
- Gold, 0.005 ppb.
- Protactinium, 0.00000005 ppb.
5. The major ions are conservative. This means that they have constant ratios, to one another and to salinity, in almost all ocean water.
Another way of saying this is that sea salts have constant composition. They almost always consist of 55% sodium ion, 31% chloride, 8% sulfate, 4% magnesium ion, 1% calcium ion, and 1% potassium ion.
The main exception is where freshwater is mixing with seawater. River water has a different composition than seawater, for example, it contains more calcium ion.
6. Sea salts mostly came from the weathering of rocks on land (the cations) and from the interior of the earth (anions).
The weathering of rock on land is a slow process of breakdown by water, with dissolved carbon dioxide, and that makes it slightly acidic.
Igneous (volcanic) rocks do not contain enough anions to be the source of mineral-laden water to the oceans. Now, sedimentary rocks are the source. In the past, volcanoes and, probably, an initial rapid release when the earth melted were the source.
- Rivers carry the dissolved ions to the ocean.
- Weathering may have been somewhat faster on the early earth, but even at the present rate it would take only about 8 to 260 million years to replace all the salts in seawater with those in the river inflow.
- The time to replace the total amount of ions in seawater with the ions in the river inflow is called the 'residence time'.
- Since this is much less than the age of the Earth and the oceans, some processes must remove the salts from seawater to keep them from building up to even higher concentration.
7. Ocean salt composition and concentration is in "steady state". This means that it does not change significantly over time.
Evidence indicates that sea salt concentration and composition has been about the same for 1.5 billion years at least. The tolerances of bacteria that probably lived 3.8 billion years before present indicate that sea salt concentration and composition were not too different, even that long ago.
The "steady state" results from the removal rate of salts from the ocean being equal to the input rate.
- This balance holds because the removal rate of salts is related to their concentration, and increases when their concentration increases.
Removal processes include:
- formation of evaporites (salt deposits left behind when seawater evaporates)
- burial of sediment porewater (the water between sediment grains)
- sediments, especially biogenic sediments, for Ca2+ (calcium ion) as calcium carbonate.
- hydrothermal vents, especially formation of the mineral chlorite within the cracks and fissures of the vents, which removes Mg2+ (magnesium ion).
8. Most of the other substances (other than sea salts) in seawater are not conservative. Their concentrations vary geographically and with depth, most often due to uptake and release by organisms.
9. Several important gases are not conservative. These include oxygen and carbon dioxide.
Oxygen dissolves in ocean surface water from the atmosphere. Photosynthesis is also a source of oxygen to ocean surface waters.
Oxygen is consumed by respiration. Rarely, animals and bacteria use all of the oxygen in sub-surface waters, which become anoxic. This can only happen if the waters are isolated from the atmosphere in some way.
Carbon dioxide is consumed during photosynthesis and released during respiration. It can also be exchanged with (dissolved from and released to) the atmosphere.
Carbon dioxide can react with water to form bicarbonate and carbonate ions.
CO2 + H2O –› HCO3- + H+ –› CO32- + 2H+
These reactions control the acidity (pH) of seawater.
Organisms use carbonate ion and calcium ion to make calcium carbonate shells, which sink after the organisms die to form calcareous sediments.
10. Another important group of nonconservative substances dissolved in seawater are the nutrients.
These are fertilizers essential for the growth of plants, including algae.
Major nutrients include nitrate, phosphate, and silicate (the latter required only by siliceous organisms).
Nutrients are depleted in surface waters, where plants grow, and are found in higher concentrations in deep waters, where the plant and animal remains that sink from surface waters decay.
Please see our Credits for the source of this information.
Detailed composition of seawater
at 3.5% salinity
This is a fairly complete analysis showing all elements as per our listed source.For an even more exhaustive elemental reference, you might want to consult this definitive listing:
Note! ppm= parts per million = mg/litre = 0.001g/kg.
source: Karl K Turekian: Oceans. 1968. Prentice-Hall
Important! You must keep in mind that the laws of osmosis, diffusion and filtration are all at work here. In all cases, the osmotic forces attract water towards areas of stronger concentration. Solutes and particles (nutrients etc.) migrate towards the intracellular areas of lesser concentration.
Therefore, a stronger solution is not necessarily a better one because it encourages a flow of water FROM the cells to the extra-cellular environment. The isotonic solution has a reputation to quickly normalize the intra-cellular environment.
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