Soda Lime Flashcards
What is soda lime?
Soda lime is a granulated hydrated lime, used for the absorption of carbon dioxide in anaesthetic breathing systems.
What is the composition of soda lime?
Soda lime is made up of 80% calcium hydroxide, less than 4% sodium hydroxide, 16% water, and a fractional concentration of an indicator dye. There are also silicates in trace amounts that harden the granules.
It used to contain potassium hydroxide, but this was removed in 2000.
What are the dyes used in soda limes, and what are the colour changes they undergo?
The indicator dyes used in soda lime are either phenolphthalein, which has a colour change from red or pink to white; or ethyl violet, which has a colour change from white to purple.
The dyes are pH indicators, so colour change occurs when the soda lime becomes partially exhausted, CO2 dissolves and carbonic acid levels increase, causing the pH to drop.
As the colours change in opposite directions, it is important to know which dye is being used.
What are the series of reactions that occur in soda lime, and can you draw them?
CO2 + H2O > H2CO3
H2CO3 + 2 NaOH > Na2CO3 (soluble) + 2 H2O (+heat)
Na2CO3 + Ca(OH)2 > CaCO3 (insoluble) + 2 NaOH (+heat)
H2CO3 + Ca(OH)2 > CaCO3 + 2 H2O [very slow rate]
As heat is produced in these reactions, some Ca(OH)2 breaks down to produce CaO (calcium oxide) + H2O.
Can you summarise the overall chemical reaction that occurs in soda lime?
The overall reaction that occurs between calcium hydroxide and carbon dioxide, in the presence of water, and with sodium hydroxide as an activator, is:
CO2 + Ca(OH)2 > CaCO3 + H2O
What are the benefits to using soda lime in a breathing system?
Soda lime is primarily used to absorb carbon dioxide, thereby allowing the use of low flows and rebreathing of gases without carbon dioxide accumulation.
This technique allows for fresh gas flows equivalent to only the basal oxygen requirements, resulting in a much more efficient breathing system, with less waste, less pollution, and reduced cost.
The reactions involved in the absorption of CO2 result in production of heat and water. Therefore gases passing through are both warmed and humidified.
What are the drawbacks to soda lime?
Soda lime decomposes some inhalational anaesthetic agents.
Sevoflurane, and to a lesser extent Halothane, are slightly decomposed by soda lime.
The reaction between sevoflurane and soda lime produces a number of compounds: A, B, C, D, E, G [NOT F]
The compound of concern is compound A. It has been shown to be toxic in rats, causing renal, hepatic and cerebral damage. There is no evidence to suggest it has these effects in humans. Even at very low flows, the maximal concentrations are about 30ppm. Potential toxic levels are estimated to be 150-200ppm.
If soda lime becomes dried out under warm conditions, and volatile agents containing the CHF2 moiety (which includes enflurane, isoflurane, and desflurane) are used with it, then carbon monoxide may be produced. This can happen as most anaesthetic machines continue to deliver 200ml/min FGF, even when the gases are turned off. This dries out the soda lime.
Trichloroethylene is an old inhalation agent, which when decomposed by soda lime produced a potent neurotoxin called dichloroethylene resulting in neurological damage particularly affecting the facial and trigeminal nerves.
The colour changes which occur in the indicator dyes can be misleading, resulting in use of exhausted soda lime.
