You can deoxygenate water using four main approaches: bubbling an inert gas through it (sparging), pulling oxygen out with a vacuum, adding a chemical that reacts with dissolved oxygen, or using specialized membranes. The right method depends on your volume of water, how low you need the oxygen level to go, and what equipment you have available. Fresh water at room temperature (20°C) holds about 6.4 milliliters of dissolved oxygen per liter, and at 0°C that rises to about 10.3 ml/L. Your goal with any deoxygenation method is to drive that number as close to zero as practical.
Why Oxygen Leaves Water
Dissolved oxygen behaves according to Henry’s Law: the amount of gas dissolved in water is proportional to the pressure of that gas above the water’s surface. Normal air is about 21% oxygen, so water exposed to air will absorb oxygen until it reaches equilibrium. To pull oxygen out, you either reduce the oxygen pressure above the water (by replacing air with another gas or pulling a vacuum), chemically consume the oxygen, or pass water through a membrane that selectively lets gas escape. Temperature also matters. Warmer water holds less dissolved oxygen, so heating your water before or during treatment helps any method work faster.
Sparging With Nitrogen or Argon
Sparging is the most common lab and industrial method. You bubble an inert gas, typically nitrogen or argon, through the water. As the bubbles rise, dissolved oxygen diffuses from the water into the bubble and gets carried out. The rate of oxygen removal depends on bubble size (smaller is better), gas flow rate, water depth, and tank volume.
For a simple setup, you run gas through a diffuser stone or frit submerged at the bottom of your container. A sintered glass frit produces fine bubbles that maximize the contact area between gas and water, speeding up oxygen transfer. In a typical lab beaker, sparging with nitrogen for 15 to 30 minutes brings dissolved oxygen down to low parts-per-million levels. For larger tanks, engineers use a design equation that relates oxygen concentration to tank height, water volume, nitrogen flow rate, and time. The key insight: taller water columns give bubbles more contact time, so deep tanks deoxygenate more efficiently than shallow ones at the same flow rate.
Argon works identically to nitrogen in principle but costs more. The practical advantage of argon is that it’s denser than air, so it forms a protective blanket on the water surface after sparging, slowing re-oxygenation. For most purposes, nitrogen is the cost-effective choice.
Vacuum Degassing
Pulling a vacuum over water lowers the total gas pressure, which shifts the equilibrium and causes dissolved oxygen (along with other gases) to come out of solution. This method removes all dissolved gases, not just oxygen, which can be an advantage or disadvantage depending on your application.
Lab-scale systems using membrane vacuum pumps can reach pressures of 4 to 60 millibar inside a degassing chamber. At 4 mbar, researchers have achieved up to 99.9% oxygen removal. At moderate vacuum (around 60 mbar), you can bring oxygen down to 10 to 20 parts per million in under a minute for small volumes. Getting below that requires both stronger vacuum and longer residence time. Below about 4 mbar, oxygen reabsorption through seals and fittings becomes significant, so there’s a practical floor to how low you can go without extremely tight equipment.
For home or small-shop use, a hand vacuum pump with a sealed flask can noticeably reduce dissolved oxygen, though you won’t reach the sub-ppm levels that lab equipment achieves. Shaking or stirring the water while under vacuum helps gas escape faster.
Chemical Deoxygenation
If you need the oxygen gone but don’t have gas cylinders or vacuum equipment, chemical scavengers are an option. The two most accessible are sodium sulfite and ascorbic acid (vitamin C).
Sodium Sulfite
Sodium sulfite reacts with dissolved oxygen to form sodium sulfate, which is harmless in most applications. The stoichiometric ratio requires about 7.9 mg of sodium sulfite per 1 mg of dissolved oxygen, but in practice you need an excess to drive the reaction to completion. At room temperature (around 100°F/38°C) with only the exact stoichiometric amount, oxygen removal is slow and incomplete. Using a tenfold molar excess of sulfite removes 95% of dissolved oxygen within 10 seconds at room temperature. At higher temperatures, the reaction speeds up dramatically: at 200°F, 95% removal happens in 10 seconds even at stoichiometric ratios.
A small amount of cobalt chloride (used as a catalyst) accelerates the reaction significantly at lower temperatures, though this introduces metal ions into your water. For applications where trace metals are acceptable, such as boiler feedwater treatment, catalyzed sodium sulfite is the industry standard.
Ascorbic Acid
Ascorbic acid reacts with dissolved oxygen in a reaction that’s pH-dependent. In a solution buffered to around pH 4.7 with a concentration of 0.2 molar ascorbic acid, oxygen is removed in less than 5 minutes. The reaction follows first-order kinetics with respect to both oxygen and ascorbic acid, meaning higher concentrations of ascorbic acid speed things up proportionally. Below pH 4, the reaction slows because hydrogen ions interfere. This method is popular in electrochemistry labs where sulfite ions would interfere with measurements, and it’s food-safe, which matters for brewing and beverage applications.
Membrane Contactors
Hollow fiber membrane contactors pass water along one side of a gas-permeable membrane while vacuum or a sweep gas runs along the other side. Dissolved oxygen migrates through the membrane and gets removed. Pilot-scale systems using hydrophobic hollow fiber membranes have reduced dissolved oxygen below 10 micrograms per liter (essentially near-zero for most purposes). These systems scale well: a membrane area of 40 square meters can handle about 1 cubic meter of water per day.
Warmer feedwater improves performance substantially. In one study, increasing water temperature from 10°C to 25°C cut the dissolved oxygen in the output from 1,200 to 400 micrograms per liter using the same membrane area. Membrane systems are mainly used in industrial settings like district heating, semiconductor manufacturing, and pharmaceutical production where continuous, low-maintenance deoxygenation is needed.
Boiling
The simplest method is also the least precise. Boiling water drives out dissolved gases, including oxygen. A rolling boil for 10 to 15 minutes removes most dissolved oxygen. The limitation is that as soon as the water cools and contacts air, oxygen starts dissolving back in. If you boil water for deoxygenation, transfer it to a sealed container while still hot, leaving minimal headspace. This approach works for home brewing, aquarium experiments, or any situation where you need reasonably low oxygen without specialized equipment.
Keeping Oxygen Out After Removal
Deoxygenated water re-absorbs oxygen rapidly when exposed to air, especially if disturbed. Turbulence, splashing, and even pouring water between containers can reintroduce significant oxygen in seconds. Still water re-oxygenates more slowly but will eventually return to equilibrium with the atmosphere.
To maintain low oxygen levels, store deoxygenated water in sealed containers with minimal headspace. If you must work with it in open air, blanket the surface with nitrogen or argon gas. Use tubing and transfer lines rather than pouring. For critical applications, keep a slow nitrogen purge flowing over the water surface during use. Every connection point, valve, and seal is a potential entry point for oxygen, so the tighter your system, the longer your water stays deoxygenated.
Choosing the Right Method
- Small lab volumes (under 1 liter): Nitrogen sparging through a glass frit for 20 to 30 minutes, or vacuum degassing at low mbar pressures. Both reach sub-ppm oxygen levels reliably.
- Home brewing or food applications: Boiling with immediate sealed transfer, or ascorbic acid addition if trace acidity is acceptable.
- Industrial boiler water: Sodium sulfite with a cobalt catalyst, dosed continuously into the feed line.
- Continuous flow systems: Membrane contactors or inline vacuum degassers, which operate without consumable chemicals.
- Quick and approximate: Boiling for 10 to 15 minutes removes the bulk of dissolved oxygen without any special supplies.