Soda lime is amixture ofsodium hydroxide (NaOH) andcalcium oxide (CaO). It is used in granular form within recirculating breathing environments likegeneral anesthesia and itsbreathing circuit,submarines,rebreathers, andhyperbaric chambers andunderwater habitats. Its purpose is to eliminatecarbon dioxide (CO
2) frombreathing gases, preventingcarbon dioxide retention and, eventually,carbon dioxide poisoning.[1][2] The creation of soda lime involves treatingslaked lime with a concentratedsodium hydroxide solution.
The primary components of soda lime include:calcium oxide (CaO) constituting approximately 75%,water (H
2O) accounting for around 20%, sodium hydroxide (NaOH) making up about 3%, andpotassium hydroxide (KOH) present at approximately 0.1%.
Duringgeneral anaesthesia, a patient'sexhaled gases, containing carbon dioxide, pass through ananaesthesia machine'sbreathing circuit, containing asoda lime canister filled with soda lime granules.[1] Medical-grade soda lime includes an indicating dye that changes color when it reaches its carbon dioxide absorption capacity. To ensure proper functioning, acarbon dioxide scrubber (or soda lime canister) should not be used if the indicating dye is activated. Standard anesthesia machines typically contain up to 2 kilograms (4.4 lb) of soda lime granules.[citation needed]
Recent carbon dioxide absorbents have been developed to minimize the risk of toxic by-product formation resulting from the interaction between the absorbent andinhaled anesthetics, likehalothane. Some absorbents, including those made from lithium hydroxide, are available for this purpose.[citation needed]
In space flights,lithium hydroxide (LiOH) is used as a carbon dioxide absorbent due to its lowmolecular weight (Na: 23 g/mol; Li: 7 g/mol), saving weight during launch. During theApollo 13 flight, high carbon dioxide levels in theLunar Module led the crew to adapt spare absorbent cartridges from the Apollo capsule to the Lunar Excursion Module (LEM) system.
Exhaled gas in a breathing circuit must pass through a carbon dioxide scrubber where carbon dioxide is absorbed before the gas is circulated for breathing again. Inrebreathers, this scrubber is integrated into the breathing loop.[2][3] However, in larger settings likerecompression chambers or submarines, a fan is employed to ensure a continuous flow of gas through the scrubbing canister. The use of indicating dye inUnited States Navy fleet applications ceased in 1996 due to concerns about potential chemical releases into the circuit.[4]
The overallchemical reaction is:
Eachmole of CO2 (44 g) reacts with one mole ofcalcium hydroxide (74 g) and produces one mole of water (18 g).
The reaction can be considered as a strong-base-catalysed, water-facilitated reaction.[5]
The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:
This sequence of reactions explains thecatalytic role played bysodium hydroxide in the system and why soda lime is faster in chemical reactivity than calcium hydroxide alone.[6] The moist sodium hydroxide impregnates the surface and theporosity of calcium hydroxide grains with a highspecific surface area.[7] It reacts much more quickly and so contributes to a faster elimination of the carbon dioxide from the rebreathing circuit. The formation of water by the reaction 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. Soda lime is commonly used in closed-circuitdiving rebreathers and in theanesthesia breathing circuit in anesthesia systems.[8][9]
The same catalytic effect by thealkali hydroxides (function of the Na2Oeq content ofcement) also contributes to the carbonation ofportlandite by atmospheric CO2 inconcrete although the rate of propagation of thereaction front is there essentially limited by the carbon dioxidediffusion within the concrete matrix lessporous.[10]
A similar reaction to above, also catalysed by sodium hydroxide, is the alkali–silica reaction, a slow degradation process causing the swelling and the cracking ofconcrete containingaggregates rich in reactiveamorphous silica. In a very similar way, sodium hydroxide greatly facilitates the dissolution of the amorphous silica. The producedsodium silicate then reacts with the calcium hydroxide (portlandite) present in the hardenedcement paste to formcalcium silicate hydrate (abbreviated as C-S-H in thecement chemist notation). Thissilicification reaction of calcium hydroxide 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 thecatalysis of this reaction by sodium- or potassium-soluble hydroxides, the alkali–silica reaction would not proceed or would be limited to a very slowpozzolanic reaction. The alkali–silica reaction can be written like the soda lime reaction, by simply substituting carbon dioxide bysilica dioxide 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 |
{{cite journal}}: CS1 maint: multiple names: authors list (link){{cite book}}:ISBN / Date incompatibility (help)