BT_SQ 1.15 Outline how CO2 is absorbed in a circle system, and the hazards associated with CO2 absorption.
The other day I saw a picture of some jacaranda trees on a Facebook post, and (tragically) it reminded me of the colour of the ethyl violet pH indicator in our CO2 absorbers.
I also recently did a tutorial on equipment for the registrars at my hospital, who were all panicking about the chemistry of soda lime.
For both these reasons, I though I’d do a post on soda lime / CO2 absorption.
No doubt you’ve realised that the need for chemical CO2 absorption is unique to anaesthesia (i.e. it’s not needed/used in ICU). This is because we use circle breathing systems which permit rebreathing. And the reason for this is to conserve N2O and volatile agents, by allowing them to be rebreathed. Without a rebreathing circuit, the cost of N2O and volatiles would be astronomical. To permit safe rebreathing, the patient’s exhaled CO2 has to be eliminated.
When you first look at the soda lime equations, you can get a bit daunted. Your goal should be to look beyond the equations, so that you understand what is happening. Simply writing out equations in an exam, with no explanation, will score hardly any marks. You could pass a question on this topic without using the actual equations, but by providing an explanation in words of what is happening. (Ideally, you should be able to do both).
NB…. the above equations are not balanced. I’ve done this deliberately to make them look simpler. If you want to balance them, you will need to add some numerical coefficients (large numbers to the left of some of the chemical formulae).
T / F fresh soda lime is about 15% water – without water, the reactions can’t start
T / F A strong base (sodium hydroxide) is needed to speed things up. Reaction #1 is very slow. But #1 will go faster if its product (carbonic acid) becomes one of the reactants in a very fast reaction (reaction #2).
T / F the product of reaction #2 is sodium bicarbonate, which reacts with calcium hydroxide (another strong base) in reaction #3
T / F the NaOH produced in reaction #3 is recycled through reaction #2 to keep things going
T / F the CO2 absorber becomes “exhausted” when all the calcium hydroxide is used up
T / F the basic aim of the CO2 absorber is to turn CO2 into chalk (calcium carbonate)
Is NaOH really necessary? If you feel inspired by this post, you might like to read about “Amsorb”. What advantages does it have over soda lime? Is its CO2 absorption capacity the same?
Does soda lime really “absorb” CO2? Can you think of a better word?