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Short answer
Supercritical CO2 is carbon dioxide held above its critical point, 31.03 °C (304.18 K) and 73.8 bar (7.38 MPa), per the NIST Chemistry WebBook. Above that point it is a single dense fluid, not a liquid or a gas. Its solvent strength can be tuned with pressure and temperature, which is why it is used for extraction and chromatography.
Definition and the critical point
Every pure substance has a critical temperature and critical pressure. Above both, the liquid and gas phases no longer exist as separate phases and the substance becomes a supercritical fluid. For carbon dioxide, NIST Standard Reference Database 69 lists a critical temperature of 304.18 K (±0.04 K, Suehiro 1996) and a critical pressure of 73.80 bar (±0.15 bar). The triple point is 216.58 K at 5.185 bar.
Published values differ slightly between sources. NIST itself lists other reported critical temperatures from 304.1 to 304.35 K and other critical pressures of 73.40 and 73.825 bar, and the review cited below rounds to 31.1 °C and 73.8 bar. Use the NIST value when you need a citable figure, and treat the second decimal as source-dependent.
How it works in practice
Density tuning
The solvating power of a supercritical fluid follows its density. According to a review of supercritical fluid extraction of natural bioactive compounds (PMC7504334), raising pressure at constant temperature raises density and solvating power. Raising temperature at constant pressure lowers density, but it can also raise the vapor pressure of the solute, so the net effect on extraction varies by compound.
| Change | Effect on CO2 density | Effect on extraction (as reported) |
|---|---|---|
| Pressure up, temperature constant | Increases | Solvating power increases |
| Temperature up, pressure constant | Decreases | Net effect varies: lower density, but possibly higher solute vapor pressure |
The same review notes that very high pressure can compact the plant material in the vessel and reduce yield, so more pressure is not always better.
Why co-solvents are added
Pure CO2 is nonpolar. The review reports that polar or larger molecules need a modifier such as ethanol, methanol or water, with alcohol content frequently below 10%, and that ethanol is less toxic than methanol. Highly polar substances with a molecular weight above 400 are almost insoluble in CO2 even with tuning.
Why it leaves little residue
CO2 is a gas at ambient conditions, so when pressure is released the solvent leaves the extract with minimal residue. The review also describes it as non-toxic, easy to separate and an approved food-grade solvent, and notes that its low operating temperature suits heat-sensitive compounds.
Where it is used
The same review lists decaffeination, hops and spice extracts, alkaloids, artemisinin and cosmetic ingredients as applications of supercritical CO2 extraction. In analytical labs the fluid is the mobile phase in supercritical fluid chromatography; see SFC versus HPLC for how that differs from liquid chromatography. Outside the lab, the U.S. Department of Energy has published work on supercritical CO2 as the working fluid in power-generation Brayton cycles, a separate use of the same phase.
Reported operating windows (not recommendations)
The review reports the ranges below from published studies. They show the order of magnitude of conditions used, not settings to copy.
| Parameter | Reported range |
|---|---|
| Thermolabile compounds | 35 to 60 °C at about 400 bar |
| Essential oils and terpenoids | 90 to 120 bar at 35 to 50 °C |
| Sample moisture | 4 to 14% |
| Particle size | 0.25 to 2.0 mm |
Where it matters for labs
Working above 73.8 bar means supercritical CO2 systems are pressure equipment, and CO2 itself is an exposure hazard when it is released into a room. NIOSH lists for carbon dioxide an IDLH (immediately dangerous to life or health) value of 40,000 ppm, revised down from 50,000 ppm in 1994, a recommended exposure limit of 5,000 ppm (9,000 mg/m3) as a time-weighted average, and a short-term limit of 30,000 ppm (54,000 mg/m3). Pressurized CO2 is supplied in cylinders; see the dictionary entry on compressed gas cylinder sizes, and for local exhaust and ventilation concepts see fume hood. Follow your institution’s pressure-equipment and gas-handling procedures and the instrument manufacturer’s instructions; this page does not replace them.
For downstream analysis of extracts, an evaporative detector is one option covered in evaporative light scattering detector.
Common misunderstandings
- It is not a new state of matter. It is a fluid phase of the same substance above its critical point.
- It is not a universal solvent. Pure CO2 is nonpolar and needs a modifier for polar compounds; very polar, high-molecular-weight substances stay almost insoluble.
- Hotter is not automatically stronger. At constant pressure, higher temperature lowers density, so the net effect depends on the compound.
- Residue-free does not mean hazard-free. Co-solvents add their own toxicity considerations, and CO2 has the exposure limits above.
FAQ
What is the critical point of CO2?
NIST lists 304.18 K (31.03 °C) and 73.80 bar (7.38 MPa). Other sources report values within a few hundredths of a kelvin and a few tenths of a bar.
Is supercritical CO2 a liquid or a gas?
Neither. Above the critical point there is one fluid phase with no liquid-gas boundary.
Why is supercritical CO2 used for extraction?
It is non-toxic, leaves little residue because it is a gas at ambient conditions, works at temperatures gentle on heat-sensitive compounds, and its solvating power can be adjusted through pressure and temperature.
Sources
- NIST Chemistry WebBook, SRD 69, carbon dioxide (phase change data): webbook.nist.gov, read 2026-11-08.
- NIOSH, Immediately Dangerous to Life or Health Values: carbon dioxide: cdc.gov/niosh/idlh/124389, read 2026-11-08.
- Supercritical Fluid Extraction of Natural Bioactive Compounds (review), PMC7504334: pmc.ncbi.nlm.nih.gov, read 2026-11-08.
- U.S. Department of Energy, Supercritical Carbon Dioxide Brayton Cycle: energy.gov (PDF).
