Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO2 retention and carbon dioxide poisoning.
The main components of soda lime are
- Calcium hydroxide, Ca(OH)2 (about 75%)
- Water, H2O (about 20%)
- Sodium hydroxide, NaOH (about 3%)
- Potassium hydroxide, KOH (about 1%).
During the administration of general anaesthesia, the gases expired by a patient, which contain carbon dioxide, are passed through an anaesthetic machine breathing circuit filled with soda lime granules. Medical-grade soda lime includes an indicating dye that changes color when the soda lime reaches its carbon dioxide absorbing capacity.
To ensure that a soda lime canister (CO2 absorber) is functioning properly, it should not be used if the indicating dye is activated. Standard anaesthesia machines typically contain up to 2 kg of soda lime granules.
Lithium hydroxide (LiOH) is the alkali hydroxide with the lowest molecular weight (24 g/mol; Li: 7 g/mol) and is therefore used as CO2 absorbent in space flights since the Apollo programme to spare weight at launch. During Apollo 13 flight, the crew sheltered in the lunar module started suffering from high CO2 levels and had to adapt spare absorbent cartridges from the Apollo capsule to the LEM system.
Recent generation of CO2 absorbents have been developed to reduce the risk of formation of toxic by-products as a result of the interaction between the absorbent and inhaled anesthetics. Some absorbents made from lithium hydroxide (LiOH) are also available for this purpose.
Exhaled gas must be passed through a carbon dioxide scrubber where the carbon dioxide is absorbed before the gas is made available to be breathed again. In rebreathers the scrubber is a part of the breathing loop. Color indicating dye was removed from US Navy fleet use in 1996 when it was suspected of releasing chemicals into the circuit. In larger environments, such as recompression chambers or submarines, a fan is used to maintain the flow of gas through the scrubbing canister.
The overall reaction is:
- CO2 + Ca(OH)2 → CaCO3 + H2O + heat (in the presence of water)
The reaction can be considered as a strong-base-catalysed, water-facilitated reaction.
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 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2) – hence a catalyst)
This sequence of reactions explains the catalytic role played by sodium hydroxide in the system and why soda lime is faster in chemical reactivity than calcium hydroxide alone. It reacts much more quickly and so contributes to a faster elimination of the CO2 from the rebreathing circuit. The formation of water and the moisture from the respiration also act as a solvent for the reaction. Reactions in aqueous phase are generally faster than between a dry gas and a dry solid.
Analogy with the alkali-silica reaction
A quite didactic parallelism can be drawn between the catalytic role of sodium hydroxide in the process of carbonation of soda lime and the role of sodium hydroxide played in the alkali-silica reaction, a slow degration process causing the swelling and the cracking of concrete containing aggregates rich in reactive amorphous silica. In a very similar way, NaOH greatly facilitates the dissolution of the amorphous silica. The produced sodium silicate then reacts with the calcium hydroxide (portlandite) present in the hardened cement paste to form calcium silicate hydrate (abbreviated as C-S-H in the cement chemist notation). This silicification reaction of Ca(OH)2 on its turn continuously releases again sodium hydroxide in solution, maintaining a high pH, and the cycle continues up to the total disappearance of portlandite or reactive silica in the exposed concrete. Without the catalysis of this reaction by sodium or potassium soluble hydroxides the alkali-silica reaction would not proceed or would be limited to a very slow pozzolanic reaction. The alkali silica reaction can be written like the soda lime reaction, by simply substituting CO2 by SiO2 in the reactions mentioned here above as follows:
reaction 1: SiO2 + NaOH → NaHSiO3 (silica dissolution by NaOH: high pH) reaction 2: NaHSiO3 + Ca(OH)2 → CaSiO3 + H2O + NaOH (C-S-H precipitation and regeneration of NaOH) sum (1+2): SiO2 + Ca(OH)2 → CaSiO3 + H2O (global reaction = Pozzolanic reaction catalysed by NaOH)
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