Many (too many) decades ago in physical chemistry lessons, I seem to recall that given the enthalpy changes of reactions, and maybe phase diagrams, one could work out what equilibrium a reaction would reach.

I'm contemplating doing an experiment in much reduced pressure, by no means a hard vacuum, around 1 kPa, that might be achievable by flushing the volume with CO2, and then connecting a container of NaOH as a sorption pump, (both of which I have) as an alternative to buying a refrigerant servicing pump. What's the lowest system pressure that's thermodynamically possible this way, at room temperature?

  • $\begingroup$ It is perhaps not as trivial. NaOH will almost instantaneously absorb all the carbon dioxide in the beginning, then you will have sodium carbonate. Sodium carbonate can further absorb carbonate dioxide to form sodium hydrogen carbonate. So you have multiple reactions. $\endgroup$
    – AChem
    Jul 21, 2021 at 14:28

1 Answer 1


Two carbon dioxide scrubbers come to mind: lithium hydroxide and soda-lime.

(https://en.wikipedia.org/wiki/Lithium_hydroxide)"Lithium hydroxide is used in breathing gas purification systems for spacecraft, submarines, and rebreathers to remove carbon dioxide from exhaled gas by producing lithium carbonate and water:

2 LiOH•H2O + CO2 → Li2CO3 + 3 H2O or

2 LiOH + CO2 → Li2CO3 + H2O

The latter, anhydrous hydroxide, is preferred for its lower mass and lesser water production for respirator systems in spacecraft. One gram of anhydrous lithium hydroxide can remove 450 cm3 of carbon dioxide gas. The monohydrate loses its water at 100–110 °C."

The anhydrous form is preferred because of lower molecular weight, but I doubt that it is entirely anhydrous - not after the first CO2 reacts. I wonder if you would get all the CO2 out, but replace it with water - which could have a reduced vapor pressure because it is being absorbed by the lithium hydroxide.

(https://en.wikipedia.org/wiki/Soda_lime) "The main components of soda lime are

Calcium oxide, CaO (about 75%)

Water, H2O (about 20%)

Sodium hydroxide, NaOH (about 3%)

Potassium hydroxide, KOH (about 1%).

The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:

  1. CO2(g) → CO2(aq) (CO2 dissolves in water – slow and rate-determining)
  2. CO2(aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
  3. NaHCO3 + CaO→ CaCO3 + NaOH (NaOH recycled to step 2 – hence a catalyst)"

The phrase the bothers me is that "CO2 dissolves in water - slow and rate-determining." I wonder if the vacuum would pull of the water before the CO2 had a chance to react with the NaOH. I think the reason soda-lime is used rather than solid NaOH is that the NaOH has very little surface area for interaction with the CO2; soda-lime is more powdery.

As M. Farooq points out, the calculations are not trivial. I don't want to sound negative; give it a go. My point was that NaOH is not generally used to absorb CO2, but rather, soda-lime. But it has that water content that might interfere with your vacuum. Lithium hydroxide - at least from the equations - might devolve into LiOH.H2O plus Li2CO3. There may be other options.


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