Soda lime

Soda lime is a mixture of NaOH & Ca(OH)2 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.[1][2]

Soda lime

It is made by treating slaked lime with concentrated sodium hydroxide solution.

Chemical components

The main components of soda lime are

Anaesthetic use

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.[1] 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 (Na: 23 g/mol; Li: 7 g/mol) and is therefore used as CO2 absorbent in space flights since the Apollo program 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.

Rebreather use

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.[2][3] Color indicating dye was removed from US Navy fleet use in 1996 when it was suspected of releasing chemicals into the circuit.[4] In larger environments, such as recompression chambers or submarines, a fan is used to maintain the flow of gas through the scrubbing canister.[2]

Chemical reaction

The overall reaction is:

CO2 + Ca(OH)2 → CaCO3 + H2O + heat (in the presence of water)

Each mole of CO2 (44 g) reacting with calcium hydroxide 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:

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.[6] The moist NaOH impregnates the surface and the porosity of calcium hydroxide grains with a high specific surface area.[7] It reacts much more quickly and so contributes to a faster elimination of the CO2 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-circuit diving rebreathers and in anesthesia systems.[8][9]

The same catalytic effect by the alkali hydroxides (function of the Na2Oeq content of cement) also contributes to the carbonation of portlandite by atmospheric CO2 in concrete although the rate of propagation of the reaction front is there essentially limited by the CO2 diffusion within the concrete matrix less porous.[10]

Analogy with the alkali-silica reaction

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 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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See also

References
  1. Andrews, J. Jeff (1 September 2005). "Anesthesia Systems". In Paul G. Barash; Bruce F. Cullen; Robert K. Stoelting (eds.). Clinical Anesthesia (5th ed.). United States: Lippincott Williams & Wilkins. p. 1584. ISBN 978-0-7817-5745-4. Archived from the original on 13 July 2011. Retrieved 1 July 2010.
  2. Brubakk, Alf O.; Tom S. Neuman (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800. ISBN 978-0-7020-2571-6.
  3. Richardson, Drew; Menduno, Michael; Shreeves, Karl (eds). (1996). "Proceedings of Rebreather Forum 2.0". Diving Science and Technology Workshop. Diving Science and Technology: 286. Retrieved 2009-03-18.CS1 maint: multiple names: authors list (link)
  4. Lillo RS, Ruby A, Gummin DD, Porter WR, Caldwell JM (March 1996). "Chemical safety of U.S. Navy Fleet soda lime". Undersea Hyperb Med. 23 (1): 43–53. PMID 8653065. Retrieved 2009-03-18.
  5. Joseph Pelc (1923). Process of treating lime-containing materials. Application filed August 30, 1921. Serial No. 496,963. Patented Mar. 6, 1923. United States, 1,447,568 Patent Office.
  6. Samari, Mohammad; Ridha, Firas; Manovic, Vasilije; Macchi, Arturo; Anthony, E. J. (2019). "Direct capture of carbon dioxide from air via lime-based sorbents". Mitigation and Adaptation Strategies for Global Change. doi:10.1007/s11027-019-9845-0. ISSN 1381-2386.
  7. Ševčík, Radek; Mácová, Petra; Sotiriadis, Konstantinos; Pérez-Estébanez, Marta; Viani, Alberto; Šašek, Petr (2016). "Micro-Raman spectroscopy investigation of the carbonation reaction in a lime paste produced with a traditional technology". Journal of Raman Spectroscopy. 47 (12): 1452–1457. doi:10.1002/jrs.4929. ISSN 0377-0486.
  8. Adriani, J.; Byrd, M. L. (1941). "A study of carbon dioxide absorption appliances for anesthesia: The canister". Anesthesiology: The Journal of the American Society of Anesthesiologists. 2 (4): 450–455.
  9. Freeman, Brian S.; Berger, Jeffrey S. (2014). Anesthesiology Core Review: Part One Basic Exam. Chapter 17: Absorption of Carbon Dioxide. McGraw-Hill Education. Retrieved 22 April 2020 – via Access Medicine.
  10. Verbeck, G. (1958). "Carbonation of hydrated Portland cement". STP205-EB Cement and Concrete (West Conshohocken, PA: ASTM International: 17–36. doi:10.1520/STP39460S.
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