Soda Lime Ethyl Violet — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementSoda Lime Ethyl VioletFRCA Primary

Regarding soda lime the indicator dye changes colour when the absorbent is exhausted. Which indicator is commonly used?

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Correct answer: D — Ethyl violet

Ethyl violet changes from white to purple when soda lime is exhausted. The colour change is pH-dependent: fresh soda lime is alkaline (containing NaOH/KOH) and the dye is colourless. As CO2 reacts with the alkali the pH decreases and the dye changes colour. Some older formulations used Clayton yellow or phenolphthalein.

Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment