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Saturation Point of Hydrogen in Water

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General information

The saturation point of Hydrogen (H2) gas in water at room temperature with no external force (pressure) is just under 1.6 mg/L or 1.6 parts per million (ppm). Typically most ionizers machines operating under no pressure will only get a fraction of this, with well-maintained ionizers often retaining levels as low as 0.1 mg/L (0.1 ppm) and the best models struggling to achieve 1 mg/L (1 ppm). Often these machines produce adequate amounts of gas, however, they produce it in a manner that encourages the gas to form large bubbles and immediately dissipate into the air.

 

Saturation

Saturation of a gas in water is defined as when the pressure of the gas above the solution is equal to (That is, at equilibrium) with the pressure of the gas in the solution. The saturation depends on the partial pressure of the gas.

For example, if you place a glass of pure water with absolutely no gases dissolved in it on a benchtop and let it sit for a while, then the gas in the air around the glass (e.g. oxygen, nitrogen, carbon dioxide, etc.) will begin to dissolve into the water until the amount of gas going into the water is equal to the amount of gas going out of the water.

Saturation is generally talked about in terms of either the concentration of a gas obtained at its normal atmospheric partial pressure (Say H2) or at the concentration obtained if the gas above the solution is only the pure gas of interest at a pressure equal to one atmosphere (atm). A pressure of 1 atm is used because that is the normal atmospheric pressure at sea level.

The table below shows the concentration of the dissolved gases at saturation if their atmospheric pressure was one atm (at SATP). The equilibrium concentration (saturation) of some common atmospheric gases in water at a partial pressure of one atmosphere (atm). The values were calculated using Henry’s law.

GasHenry’s Constant (KH) at 25 °C. (Latm/mol)Concentration in Water *
mmol/Lmg/L
Nitrogen (N2)1639.340.6117.10
Oxygen (O2)769.231.3041.60
Carbon dioxide (CO2) **29.4134.001496.43
Neon (Ne)2222.220.459.10
Helium (He)2702.700.371.50
Hydrogen (H2)1282.050.781.57

* All calculations are done at 1 atm of the pure gas.

** This species participates in acid-base reactions when dissolved in water (That is, CO2 +H2O =>H2CO3), and as such it is not an ideal gas and deviates from Henry’s law.

 

The half-life of H2 in Water

Just like opening a can of fizzy drink (soda), as soon as the H2 water is exposed to normal atmospheric gases and pressure, the concentration of H2 decreases until it is at equilibrium with the partial pressure of H2 in the atmosphere, which would be a concentration of 8.67 x 10-7 mg/L. Because hydrogen gas is the smallest molecule in the universe, it will also be able to diffuse through all plastic and many other containers. Hydrogen, therefore, has the highest effusion rate of all gases.

The rate of H2 exsolution and dissipation from the water is directly affected primarily by temperature, agitation, and surface area. A 500 mL open container of dissolved Hydrogen water has a half-life of about two hours. Therefore, if left out in the open with no turbulence at room temperature with an initial H2 concentration of 1.6 mg/L, the concentration would likely be around 0.8 mg/L after two hours. However, the dissipation rate is not exactly linear.

 

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