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{{Short description|Chemical mixture for absorbing carbon dioxide}}
[[File:Drägersorb Soda Lime.jpg|thumb|Soda lime canister used in anaesthetic machines to act as a carbon dioxide scrubber. ]]
'''Soda lime''', a [[Mixture (chemistry)|mixture]] of [[sodium hydroxide]] (NaOH) and [[calcium oxide]] (CaO), is used in granular form within recirculating breathing environments like [[general anesthesia]] and its [[breathing circuit]], [[submarines]], [[rebreathers]], and [[hyperbaric chamber]]s and [[underwater habitat]]s. Its purpose is to eliminate [[carbon dioxide]] ({{Chem|C|O|2}}) from [[breathing gas]]es, preventing [[carbon dioxide retention]] and, eventually, [[carbon dioxide poisoning]].<ref name=Barash>{{cite book
 |author     = Andrews, J. Jeff
 |title      = Clinical Anesthesia
 |editor1    = Paul G. Barash
 |editor2    = Bruce F. Cullen
 |editor3    = Robert K. Stoelting
 |chapter    = Anesthesia Systems
 |publisher  = Lippincott Williams & Wilkins
 |location   = United States
 |edition    = 5th
 |date       = 1 September 2005
 |pages      = 1584
 |isbn       = 978-0-7817-5745-4
 |chapter-url        = http://www.isbnlib.com/isbn/0781757452/Clinical-Anesthesia-Clinical-Anesthesia--Barash
 |archive-url = https://web.archive.org/web/20110713061326/http://www.isbnlib.com/isbn/0781757452/Clinical-Anesthesia-Clinical-Anesthesia--Barash
 |url-status   = dead
 |archive-date = 13 July 2011
 |access-date = 1 July 2010
}}</ref><ref name=Brubakk>{{cite book |title=Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. |last=Brubakk |first=Alf O. |author2=Tom S. Neuman  |year=2003 |publisher=Saunders Ltd. |location=United States |isbn=978-0-7020-2571-6 |pages=800 }}</ref> The creation of soda lime involves treating [[slaked lime]] with a concentrated [[sodium hydroxide]] solution.

== Chemical components ==
The primary components of soda lime include: [[calcium oxide]] (CaO) constituting approximately 75%, [[water]] ({{Chem|H|2|O}}) accounting for around 20%, sodium hydroxide (NaOH) making up about 3%, and [[potassium hydroxide]] (KOH) present at approximately 0.1%.

==Anaesthesia==
{{Further information|Carbon dioxide scrubber}}
During [[general anaesthesia]], a patient's [[Exhaled gas|exhaled gases]], containing carbon dioxide, pass through an [[anaesthesia machine]]'s [[breathing circuit]], containing a [[soda lime canister]] filled with soda lime granules.<ref name="Barash" /> Medical-grade soda lime includes an indicating dye that changes color when it reaches its carbon dioxide absorption capacity. To ensure proper functioning, a [[carbon dioxide scrubber]] (or soda lime canister) should not be used if the indicating dye is activated. Standard anesthesia machines typically contain up to {{Convert|2|kg}} of soda lime granules.{{citation needed|date=February 2022}}

Recent carbon dioxide absorbents have been developed to minimize the risk of toxic by-product formation resulting from the interaction between the absorbent and [[inhaled anesthetics]], like [[halothane]]. Some absorbents, including those made from lithium hydroxide, are available for this purpose.{{citation needed|date=February 2022}}

==Space flight==
In space flights, [[lithium hydroxide]] (LiOH) is used as a carbon dioxide absorbent due to its low [[molecular weight]] (Na: 23 g/mol; Li: 7 g/mol), saving weight during launch. During the [[Apollo 13]] flight, high carbon dioxide levels in the [[Lunar Module]] led the crew to adapt spare absorbent cartridges from the Apollo capsule to the Lunar Excursion Module (LEM) system.

==Rebreather use==
Exhaled gas undergoes a crucial process: it must pass through a carbon dioxide scrubber where carbon dioxide is absorbed before the gas is circulated for breathing again. In [[rebreathers]], this scrubber is integrated into the breathing loop.<ref name=Brubakk/><ref name=rebreather2.0>{{cite journal |author=Richardson, Drew; Menduno, Michael; Shreeves, Karl (eds). |title=Proceedings of Rebreather Forum 2.0. |journal=Diving Science and Technology Workshop. |publisher=[[Diving Science and Technology]] |year=1996 |pages=286 |url=http://archive.rubicon-foundation.org/7555 |archive-url=https://web.archive.org/web/20080915044304/http://archive.rubicon-foundation.org/7555 |url-status=usurped |archive-date=September 15, 2008 |access-date=2009-03-18 }}</ref> However, in larger settings like [[Recompression chamber|recompression chambers]] or submarines, a fan is employed to ensure a continuous flow of gas through the scrubbing canister. Notably, the use of color indicating dye in [[United States Navy]] fleet applications ceased in 1996 due to concerns about potential chemical releases into the circuit.<ref>{{cite journal |vauthors=Lillo RS, Ruby A, Gummin DD, Porter WR, Caldwell JM |title=Chemical safety of U.S. Navy Fleet soda lime |journal=Undersea Hyperb Med |volume=23 |issue=1 |pages=43–53 |date=March 1996 |pmid=8653065 |url=http://archive.rubicon-foundation.org/2238 |archive-url=https://web.archive.org/web/20071116131405/http://archive.rubicon-foundation.org/2238 |url-status=usurped |archive-date=November 16, 2007 |access-date=2009-03-18}}</ref>

==Chemical reaction==

The overall [[chemical reaction]] is:

:CO<sub>2</sub> + Ca(OH)<sub>2</sub> → CaCO<sub>3</sub> + H<sub>2</sub>O + heat (in the presence of water)

Each [[Mole (unit)|mole]] of CO<sub>2</sub> (44 g) reacts with one mole of [[calcium hydroxide]] (74 g) and produces one mole of water (18 g).

The reaction can be considered as a strong-base-catalysed, water-facilitated reaction.<ref name="Pelc_1921">[https://patentimages.storage.googleapis.com/84/2d/e0/19cc8f7e593d70/US1447568.pdf 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.]</ref>

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

:1) <chem> CO2(g) <=>> CO2(aq)</chem>            (CO<sub>2</sub> dissolves in water – slow and [[Rate-determining step|rate-determining]]),

:2) <chem>CO2(aq) + NaOH -> NaHCO3</chem>      (bicarbonate formation at high pH),

:3) <chem>NaHCO3 + Ca(OH)2 -> CaCO3 + NaOH + H2O </chem>  (NaOH recycled to step 2 – hence a [[catalyst]]).

This sequence of reactions explains the [[Catalysis|catalytic]] role played by [[sodium hydroxide]] in the system and why soda lime is faster in chemical reactivity than calcium hydroxide alone.<ref name="Samari_2019">{{cite journal|last1=Samari|first1=Mohammad|last2=Ridha|first2=Firas|last3=Manovic|first3=Vasilije|last4=Macchi|first4=Arturo|last5=Anthony|first5=E. J.|title=Direct capture of carbon dioxide from air via lime-based sorbents|journal=Mitigation and Adaptation Strategies for Global Change|year=2019|volume=25|pages=25–41|issn=1381-2386|doi=10.1007/s11027-019-9845-0|doi-access=free}}</ref> The moist sodium hydroxide impregnates the surface and the [[porosity]] of calcium hydroxide grains with a high [[specific surface area]].<ref name="Sevcik_2016">{{cite journal|last1=Ševčík|first1=Radek|last2=Mácová|first2=Petra|last3=Sotiriadis|first3=Konstantinos|last4=Pérez-Estébanez|first4=Marta|last5=Viani|first5=Alberto|last6=Šašek|first6=Petr|title=Micro-Raman spectroscopy investigation of the carbonation reaction in a lime paste produced with a traditional technology|journal=Journal of Raman Spectroscopy|volume=47|issue=12|year=2016|pages=1452–1457|issn=0377-0486|doi=10.1002/jrs.4929|bibcode=2016JRSp...47.1452S}}</ref> It reacts much more quickly and so contributes to a faster elimination of the carbon monoxide 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 [[rebreather|diving rebreathers]] and in the [[anesthesia breathing circuit]] in anesthesia systems.<ref name="Adriani_1941">{{cite journal|last1=Adriani|first1=J.|last2=Byrd|first2=M. L.|year=1941|title=A study of carbon dioxide absorption appliances for anesthesia: The canister|journal=Anesthesiology |volume=2|issue=4|pages= 450–455|doi=10.1097/00000542-194107000-00009 |doi-access=free}}</ref><ref name="Freeman_2014">{{cite book|first1=Brian S.|last1=Freeman|first2=Jeffrey S.|last2=Berger|year=2014|url=http://accessanesthesiology.mhmedical.com/content.aspx?aid=1102566097|title=Anesthesiology Core Review: Part One Basic Exam. Chapter 17: Absorption of Carbon Dioxide|publisher=McGraw-Hill Education|access-date=22 April 2020|via=Access Medicine}}</ref>

The same catalytic effect by the [[alkali hydroxide]]s (function of the Na<sub>2</sub>O<sub>eq</sub> content of [[cement]]) also contributes to the carbonation of [[portlandite]] by atmospheric CO<sub>2</sub> in [[concrete]] although the rate of propagation of the [[Reaction–diffusion system|reaction front]] is there essentially limited by the carbon dioxide [[diffusion]] within the concrete matrix less [[porosity|porous]].<ref name="Verbeck_1958">{{cite journal|last1=Verbeck|first1=G.|title=Carbonation of hydrated Portland cement|year=1958|journal=STP205-EB Cement and Concrete (West Conshohocken, PA: ASTM International|pages=17–36|doi=10.1520/STP39460S|isbn=978-0-8031-5667-8}}</ref>

==Analogy with the alkali–silica reaction==
{{See also|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 [[Construction aggregate|aggregate]]s rich in reactive [[amorphous silica]]. In a very similar way, sodium hydroxide 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 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 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 carbon dioxide by [[silica dioxide]] in the reactions mentioned here above as follows:

:{| style="white-space:nowrap;"
|-
! style="text-align:left;" | reaction 1:
| &nbsp; || align = "right" | SiO<sub>2</sub> + NaOH || &nbsp; || → || &nbsp; || NaHSiO<sub>3</sub>|| &nbsp; ||silica dissolution by NaOH:<br />high pH
|-
! style="text-align:left;" | reaction 2:
| &nbsp; || align = "right" | NaHSiO<sub>3</sub> + Ca(OH)<sub>2</sub> || &nbsp; || → || &nbsp; || CaSiO<sub>3</sub> + H<sub>2</sub>O + NaOH || &nbsp; &nbsp; ||C-S-H precipitation<br />and regeneration of NaOH
|-
! style="text-align:left;" | sum (1+2):
| &nbsp; || align = "right" | SiO<sub>2</sub> + Ca(OH)<sub>2</sub> || &nbsp; || → || &nbsp; || CaSiO<sub>3</sub> + H<sub>2</sub>O || &nbsp; &nbsp; ||global reaction:<br />[[Pozzolanic reaction]] catalysed by NaOH
|}

==See also==
* [[Carbon dioxide scrubber]]
* [[Alkali-silica reaction|Alkali–silica reaction]]
* [[Soda–lime glass]], a type of glass made from silica, soda, lime and aluminum oxide

==References==
{{Reflist}}

==External links==
* {{usurped|1=[https://web.archive.org/web/20120908112233/http://archive.rubicon-foundation.org/xmlui/search?query=Sodalime Publications on sodalime in diving operations]}}
* [https://web.archive.org/web/20100928053857/http://molecularproducts.com/pdf/Sofnolime%20Issue%2012.pdf Sofnolime MSDS] Example of a commercial soda lime product that is used in diving and medicine
* [https://web.archive.org/web/20100922200304/http://molecularproducts.com/pdf/technical-library/An%20Introduction%20to%20Sofnolime%20Technical%20Article.pdf An Introduction to Sofnolime (Technical Article)]

[[Category:Hydroxides]]
[[Category:Calcium compounds]]
[[Category:Sodium compounds]]
[[Category:Anesthetic equipment]]
[[Category:Rebreather components|*]]
[[Category:Catalysis]]