Artificial seawater (abbreviatedASW) is a mixture of dissolvedmineral salts (and sometimesvitamins) that simulatesseawater. Artificial seawater is primarily used inmarine biology and inmarine andreef aquaria, and allows the easy preparation ofmedia appropriate for marineorganisms (includingalgae,bacteria,plants andanimals). From ascientific perspective, artificial seawater has the advantage ofreproducibility over natural seawater since it is a standardized formula. Artificial seawater is also known as synthetic seawater and substitute ocean water.
A smaller but significant use is in laboratory or fire-fighting applications. In industrial and materials-science contexts,artificial seawater refers to a chemically defined electrolyte used to reproduce the ionic composition and conductivity of natural ocean water for corrosion testing, electrochemical research, and sensor calibration.[1]
The most widely cited formulation, ASTM D1141, specifies concentrations of sodium, chloride, sulfate, magnesium, calcium, potassium, bromide, and strontium that approximate the average composition of seawater with a salinity of about 35 g kg⁻¹. This mixture is deliberately simplified and excludes organic matter or biological nutrients so that the chemical environment is reproducible from batch to batch. It is commonly employed in laboratory methods such asASTM G31 (immersion corrosion testing),ASTM G44 (alternate immersion),ASTM G78 (crevice corrosion), and the international standardISO 11130 for corrosion of metals and alloys under controlled conditions.[2]
By contrast,aquarium andaquaculture formulations of artificial seawater—sold under trade names such asInstant Ocean andReef Crystals—are designed to sustain marine organisms rather than to model corrosion processes.[3] These products include trace nutrients such as iron, iodine, molybdenum, and zinc; carbonate–bicarbonate buffers to stabilize pH near 8.2; and sometimes vitamins or chelating agents to support biological systems. Because such ingredients can introduce organic films, complexing agents, or variable redox chemistry, aquarium formulations are unsuitable for standardized corrosion or electrochemical testing.
Artificial seawater used for corrosion studies is typically prepared with analytical-grade salts[4] and deionized or distilled water, mixed shortly before use, and replaced periodically to prevent pH drift, precipitation, or contamination. Test solutions may be aerated, deaerated, or maintained at fixed temperatures depending on the procedure. The focus is on chemical reproducibility and electrochemical representativeness, rather than biological realism.
| Aspect | Aquarium / Marine Biology Use | Corrosion and Engineering Use |
|---|---|---|
| Primary purpose | Support marine life; biological fidelity | Reproduce ionic strength and corrosivity for standardized tests |
| Standards or references | Commercial reef-salt recipes; UNESCO (1981) seawater tables | ASTM D1141; ISO 11130; ASTM G31 series |
| Typical additives | Nutrients, vitamins, trace metals, organics | None — sterile, inorganic only |
| Buffering system | Carbonate/bicarbonate to stabilize pH ≈ 8.2 | Usually unbuffered or controlled with CO₂ exclusion |
| Analytical priority | Biological health (salinity, alkalinity, Ca²⁺, Mg²⁺) | Ionic reproducibility, conductivity, chloride/sulfate ratio |
| Longevity in use | Weeks to months (with aeration) | Hours to days (freshly mixed for each exposure) |
The tables below present an example of an artificial seawater (35.00 ‰ of salinity) preparation devised by Kester, Duedall, Connors and Pytkowicz (1967).[5] The recipe consists of two lists of mineral salts, the first ofanhydrous salts that can be weighed out, the second ofhydrated salts that should be added to the artificial seawater as a solution.
| Salt | Molecular weight | g kg−1solution |
|---|---|---|
| Sodium chloride (NaCl) | 58.44 | 23.926 |
| Sodium sulfate (Na2SO4) | 142.04 | 4.008 |
| Potassium chloride (KCl) | 74.56 | 0.677 |
| Sodium bicarbonate (NaHCO3) | 84.00 | 0.196 |
| Potassium bromide (KBr) | 119.01 | 0.098 |
| Boric acid (H3BO3) | 61.83 | 0.026 |
| Sodium fluoride (NaF) | 41.99 | 0.003 |
| Salt | Molecular weight | mol kg−1 solution |
|---|---|---|
| Magnesium chloride (MgCl2.6H2O) | 203.33 | 0.05327 |
| Calcium chloride (CaCl2.2H2O) | 147.03 | 0.01033 |
| Strontium chloride (SrCl2.6H2O) | 266.64 | 0.00009 |
While all the compounds listed in the recipe above areinorganic, mineral salts, some artificial seawater recipes, such as that of Goldman and McCarthy (1978),[6] also add trace solutions ofvitamins andorganic compounds needed bymarine organisms.
The International Standard for making artificial seawater can be found atASTM International.[7] The current standard code is ASTM D1141-98[8] (the original standard was ASTM D1141-52) and describes the standard practice for the preparation of substituteocean water. The ASTM D1141-98 standard is available in a ready-made artificial seawater form or as a "Sea Salt" mix that scientists and hobbyists can prepare. Generally, the ready-made artificial seawater comes in 1 gallon and 5 gallon containers, whereas the "Sea Salt" mix comes in 20 lb pails (makes approximately 57 gallons) and 50 lb pails (makes approximately 143 gallons).
There are various applications for ASTM D1141-98 synthetic seawater includingcorrosion studies, oceaninstrument calibration and chemical processing.[9] Typically,laboratory-grade water is used for preparing synthetic seawater.[10]