Artificial seawater
Artificial seawater is a mixture of dissolved mineral salts that simulates seawater. Artificial seawater is primarily used in marine biology and in marine and reef aquaria, and allows the easy preparation of media appropriate for marine organisms. From a scientific perspective, artificial seawater has the advantage of reproducibility over natural seawater since it is a standardized formula. Artificial seawater is also known as synthetic seawater and substitute ocean water.
Industrial and laboratory formulations
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.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 as ASTM G31, ASTM G44, ASTM G78, and the international standard ISO 11130 for corrosion of metals and alloys under controlled conditions.
By contrast, aquarium and aquaculture formulations of artificial seawater—sold under trade names such as Instant Ocean and Reef Crystals—are designed to sustain marine organisms rather than to model corrosion processes. 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 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 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 | Ionic reproducibility, conductivity, chloride/sulfate ratio |
| Longevity in use | Weeks to months | Hours to days |
Example
The tables below present an example of an artificial seawater preparation devised by Kester, Duedall, Connors and Pytkowicz. The recipe consists of two lists of mineral salts, the first of anhydrous salts that can be weighed out, the second of hydrated salts that should be added to the artificial seawater as a solution.| Salt | Molecular weight | g kg−1 solution |
| Sodium chloride | 58.44 | 23.926 |
| Sodium sulfate | 142.04 | 4.008 |
| Potassium chloride | 74.56 | 0.677 |
| Sodium bicarbonate | 84.00 | 0.196 |
| Potassium bromide | 119.01 | 0.098 |
| Boric acid | 61.83 | 0.026 |
| Sodium fluoride | 41.99 | 0.003 |
| Salt | Molecular weight | mol kg−1 solution |
| Magnesium chloride | 203.33 | 0.05327 |
| Calcium chloride | 147.03 | 0.01033 |
| Strontium chloride | 266.64 | 0.00009 |
While all the compounds listed in the recipe above are inorganic, mineral salts, some artificial seawater recipes, such as that of Goldman and McCarthy, also add trace solutions of vitamins and organic compounds needed by marine organisms.