Soda lime (8006-28-8)

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%).

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. Color indicating dye was removed from US Navy fleet use in 1996 when it was suspected of releasing chemicals into the circuit. In larger environments, such as recompression chambers or submarines, a fan is used to maintain the flow of gas through the scrubbing canister.

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).