Artificial seawater
Artificial seawater (abbreviated ASW) is a mixture of dissolved mineral salts (and sometimes vitamins) 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 (including algae, bacteria, plants and animals). 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
[edit]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 as ASTM G31 (immersion corrosion testing), ASTM G44 (alternate immersion), ASTM G78 (crevice corrosion), and the international standard ISO 11130 for corrosion of metals and alloys under controlled conditions.[2]
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.[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) |
Example
[edit]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 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 (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 are inorganic, mineral salts, some artificial seawater recipes, such as that of Goldman and McCarthy (1978),[6] also add trace solutions of vitamins and organic compounds needed by marine organisms.
Standard
[edit]The International Standard for making artificial seawater can be found at ASTM 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 substitute ocean 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).
Uses and applications
[edit]There are various applications for ASTM D1141-98 synthetic seawater including corrosion studies, ocean instrument calibration and chemical processing.[9] Typically, laboratory-grade water is used for preparing synthetic seawater.[10]
See also
[edit]References
[edit]- ^ ASTM D1141-98(2013), Standard Practice for Substitute Ocean Water, West Conshohocken, PA: ASTM International, 2013
- ^ "ISO 11130:2017 — Corrosion of metals and alloys — Alternate immersion test in neutral 3.5 % sodium chloride solution". International Organization for Standardization. Retrieved 15 October 2025.
- ^ "Instant Ocean Sea Salt — Product Overview". Instant Ocean Brand. Retrieved 15 October 2025.
- ^ ""ASTM D1141 Artificial Seawater" ASTM D1141-98 US Corrosion".
- ^ Kester, D. R., Duedall, I. W., Connors, D. N. and Pytkowicz, R. M. (1967). Preparation of Artificial Seawater Archived 2008-12-17 at the Wayback Machine. Limnology & Oceanography 12, 176–179.
- ^ Goldman, J. C. and McCarthy, J. J. (1978). Steady-state growth and ammonium uptake of a fast-growing marine diatom Archived 2008-12-17 at the Wayback Machine. Limnology & Oceanography 23, 695–703.
- ^ "Standard Practice for the Preparation of Substitute Ocean Water". ASTM International. Retrieved 16 June 2014.
- ^ "ASTM D1141-98 Standard Practice for Preparation of Substitute Ocean Water". ASTM International.
- ^ ""Artificial Seawater" ASTM D1141-98 Lake Products Company LLC".
- ^ "Preparation of Substitute Ocean Water". G2MT Labs. 2018-11-08. Retrieved 10 November 2018.
External links
[edit]- Artificial seawater media, Goldman & McCarthy (1978)
- Modified Artificial Seawater Media (MASM), Culture Collection of Algae and Protozoa
- Synthetic Seawaters for Aquaria and Laboratories, Calypso Publications (1979)