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%).
While administering general anaesthesia, the patient's expired gases which contain carbon dioxide and are passed through an Anaesthetic machine breathing circuit filled with soda lime granules. Medical grade soda lime has indicating dye that changes color when the soda lime loses its carbon dioxide absorbing capacity.
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.
1) CO2 → 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)
Each mole of CO2 (44g) reacted produces one mole of water (18g)
- J. Jeff Andrews (1 September 2005). "Anesthesia Systems". In Paul G. Barash, Bruce F. Cullen and Robert K. Stoelting. Clinical Anesthesia (5th ed.). United States: Lippincott Williams & Wilkins. p. 1584. ISBN 0-7817-5745-2. Retrieved 1 July 2010.
- 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 0-7020-2571-2.
- 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.
- 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.