Soda lime
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.[1][2]
It is made by treating slaked lime with concentrated sodium hydroxide solution.
Contents
Chemical components
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%).
Anaesthetic use
During the administration of general anaesthesia, a patient's expired gases, 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 loses its carbon-dioxide-absorbing capacity.
To ensure that a soda lime canister (CO2 absorber) is functioning properly, it must be replaced regularly and should not be used if the indicating dye is activated.
Older anaesthesia machines came equipped will large soda lime canisters within the circle system. They could be loaded with up to 2 kg of soda lime granules.
But, with the growing use of expensive inhalation agents in anaesthesia like Desflurane & sevoflurane, and also looking to their greenhouse gas potential, low-flow general anaesthesia is gaining popularity, which necessitates a small soda lime canister usually of 0.9 kg.
Fresh soda lime and end-tidal carbon dioxide monitoring (EtCO2) are a must for low-flow general anaesthesia or else, life-threatening hypercarbia can develop in the anaesthesia circuit.
There are now newer generation CO2 absorbents which may pose less of a risk of toxic by-product as a result of interaction between the absorbent and inhaled anesthetics. Absorbents made from lithium hydroxide (LiOH) have been on the market for a decade or more.
Undersea 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)
The reaction can be considered as a strong-base-catalysed, water-facilitated reaction.
Mechanism:
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 (44 g) reacted produces one mole of water (18 g).
References
External links
- Sofnolime MSDS Example of a commercial soda lime product that is used in diving and medicine
- Publications on sodalime in diving operations
- An Introduction to Sofnolime (Technical Article)
