Chemistry of Absorbents
Three formulations of carbon dioxide absorbents are commonly available:
soda lime, Baralyme, and calcium hydroxide lime (Amsorb). Of these agents, the
most commonly used is soda lime. All serve to eliminate carbon dioxide from the
breathing circuit with various degrees of efficiency.
By weight, the approximate composition of "high-moisture" soda
lime is 80% calcium hydroxide, 15% water, 4% sodium hydroxide, and 1% potassium hydroxide
(an activator). Small amounts of silica are added to produce calcium and sodium
silicate. This addition produces a hard compound and reduces dust formation. The
efficiency of the soda lime absorption varies inversely with the hardness; therefore,
little silicate is used in contemporary soda lime. Sodium hydroxide is the catalyst
for the carbon dioxide absorptive properties of soda lime.[114]
[115]
Baralyme is a mixture of approximately 20%
barium hydroxide and 80% calcium hydroxide. It may also contain some potassium hydroxide.
Calcium hydroxide lime is one of the newest commercially available carbon dioxide
absorbents. It consists primarily of calcium hydroxide and calcium chloride and
contains two setting agents: calcium sulfate and polyvinylpyrrolidine. The latter
two agents enhance the hardness and porosity of the agent.[116]
The most significant advantage of calcium hydroxide lime over other agents is its
lack of sodium and potassium hydroxides. The absence of these chemicals eliminates
the undesirable production of carbon monoxide and the nephrotoxic substance known
as compound A.[117]
The size of the absorptive granules has been determined by trial
and error, which represents a compromise between resistance to airflow and absorptive
efficiency.[118]
The smaller the granules, the
more surface area is available for absorption. However, air flow resistance increases.
The granular size of soda lime and Baralyme in anesthesia practice is between 4
and 8 mesh, a size at which resistance to airflow is negligible. Mesh refers to
the number of openings per linear inch in a sieve through which the granular particles
can pass. A 4-mesh screen means that there are four 0.25-inch openings per linear
inch. An 8-mesh screen has eight 0.125-inch openings per linear inch.[114]
The absorption of carbon dioxide by soda lime is a chemical process,
not a physical process. Carbon dioxide combines with water to form carbonic acid.
Carbonic acid reacts with the hydroxides to form sodium (or potassium) carbonate
and water. Calcium hydroxide accepts the carbonate to form calcium carbonate and
sodium (or potassium) hydroxide. The equations for the reaction steps are as follows
- CO2
+ H2
O ⇔ H2
CO3
- H2
CO3
+ 2NaOH (KOH) ⇔ Na2
CO3
(K2
CO3
) + 2H2
O + Heat
- Na2
CO3
(K2
CO3
) + Ca(OH)2
⇔ CaCO3
+ 2NaOH (KOH)
Some carbon dioxide may react directly with Ca(OH)2
,
but this reaction is much slower.
The reaction with Baralyme differs from that of soda lime because
more water is liberated by a direct reaction of barium hydroxide and carbon dioxide.
- Ba(OH)2
+ 8H2
O + CO2
⇔ BaCO3
+ 9H2
O + Heat
- 9H2
O + 9CO2
⇔ 9H2
CO3
Then by direct reactions and by KOH and NaOH,
- 9H2
CO3
+ 9Ca(OH)2
⇔ CaCO3
+ 18H2
O + Heat
