Standard soda lime used in anaesthesia circle absorbers contains about 80 percent calcium hydroxide along with sodium and potassium hydroxide activators. What is the purpose of adding silica to the mixture?

• A It catalyses the reaction between carbon dioxide and calcium hydroxide
• B It hardens the granules and reduces dust formation ✓
• C It acts as a colour indicator for absorbent exhaustion
• D It absorbs water produced during the neutralisation reaction

Explanation

Silica (kieselguhr) is added to soda lime as a binding agent that hardens the granules and minimises alkaline dust, which would otherwise be carried into the circuit and irritate the airway. The carbon dioxide absorption chemistry depends on calcium hydroxide with sodium and potassium hydroxide accelerating the initial reaction. Colour change is produced by dyes such as ethyl violet, not silica, and water is deliberately included in the granules because dry absorbent degrades volatile agents.

Reference: Miller's Anesthesia, 9th ed.

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