Soda lime

Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO₂ retention and carbon dioxide poisoning.

It is made by treating slaked lime with concentrated sodium hydroxide solution.

Two types of soda lime are available in India: one is the non-standard variety usually meant for use in laboratories, and the other is Soda Lime I.P. which is fit for use on humans.

While administering general anaesthesia, the patient's expired gases which contain carbon dioxide, are passed through a circle absorber filled with soda lime granules. Medical grade soda lime has indicators which change colour when the soda lime loses its carbon dioxide absorbing capacity.

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%), and
• Potassium hydroxide, KOH (about 1%).

Scrubber in a rebreather

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. In larger environments, such as recompression chambers, a fan is used to pass gas through the 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.

steps:

1) CO₂ + H₂O → CO_{2 (aq)} (CO2 dissolves in water - slow and rate determining)

2) CO_{2 (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 (44g) reacted produces one mole of water (18g)

See also

• Lithium hydroxide

External links

• Sofnolime Example of a commercial soda lime product that is used in diving and medicine