Soda lime

Regarding the soda lime in a circle system, the following statements are true EXCEPT

which one?

a) The granule diameter is 3 to 4mm

b) If it contains Titan Yellow, the colour change will be from deep pink when fresh to

off-white when exhausted

c) It will contain water even in an unopened packet prior to use

d) It will always contain calcium hydroxide

e) As it is used there will be a steady decline in the amount of sodium hydroxide

present

Answer: e

Explanation

Soda lime is made up of calcium hydroxide (approximately 95% and always present),

sodium hydroxide and sometimes some potassium hydroxide (e.g. in Baralyme). In

addition there will be some silicates for binding, water (even prior to first use) and a

colour indicator. Titan Yellow causes the soda lime to turn from deep pink to off-white

(fresh to exhausted), and Ethyl Violet causes a change from white to purple (fresh to

exhausted). Soda lime is capable of absorbing approximately 25 L of CO2 per 100 g.

Carbon dioxide reacts with sodium (and potassium) hydroxide to form sodium carbonate.

This then reacts with calcium hydroxide to reform the sodium hydroxide and

produce calcium carbonate. Thus it is only at the very end of soda lime’s active life that

the sodium hydroxide concentration falls. This reaction is exothermic and produces

water, both of which may be beneficial especially at low flows as it will humidify and

warm the inspired gases.