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SODA LIME CANISTER.
This is used in a special circuit which allows the expired gas from the patient to be put back through the system via this Soda Lime canister, the soda-lime removes the carbon dioxide, which is a waste gas.
The remaining gas goes back to the patient, so any anaesthetic agent or oxygen that the patient breathes out can be used again, this means that the anaesthetist can use less anaesthetic agent, nitrous oxide or oxygen during the operation thus making it cheaper.
The soda-lime starts off as one colour and changes to another colour as it gets saturated with carbon dioxide.
The colour change will vary from one make to another, this is the time to throw it away and fill up with new.
Soda lime is a mixture of 90 % calcium hydroxide with 5 % sodium hydroxide and 1 % Potassium Hydroxide, with silicates to prevent powdering.
It is essential for effective absorption that moisture (14 -15 %) be incorporated within the granule.
The Hydroxides combine with the carbon dioxide in the presence of water to form carbonates.
The chemical change involved in absorption results in heat production.
The temperature may reach 60 centigrade in the part of the canister in which active absorption is taking place.
The end products of the reaction are water and the carbonate of the respective metals.
It can absorb about 20 % of its own weight of Carbon dioxide.
Storing soda-lime in its container does not interfere with its efficiency.
Durasorb is an improved soda-lime with a prolonged effective life which does not over heat. Its pink colour turns to white as it is used up.
Baralyme (barium hydroxide lime, USP) is 80 % calcium hydroxide with 20 % barium octohydrate. It is said to be less caustic, and to produce less heat than soda-lime.
Pink granules change to purple when exhausted.
The highest permissible concentration of Carbon dioxide in the anaesthetic system is 0.2 %, so it is important that the soda-lime is kept fresh.
A 1 lb. canister will last about 6 hours of intermittent use, or 2 hours continuous use.

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