Reef Chemistry Question of the Day #127 Halides in metal halide lamps

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#127[/HASHTAG]

What halide is most commonly used in metal halide lights?

A. Fluoride
B. Chloride
C. Bromide
D. Iodide

Bonus: why is it there at all?

Good luck!



























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[joshbbert]

sodium iodide is the most common i believe so D

 

[beaslbob]

bonus: It there to fire the electrons?

 

[Randy Holmes-Farley]

And the answer is...D. Iodide

Sodium iodide, and potentially other metals are added to add more colors than come from mercury alone (mercury is always present). As the electrons fall back from excited states in the sodium, colored light is emitted. Most intense is the sodium "d-line" at 589 nm, which is a bright yellow.

Iodide is elected over other halides (e.g., chloride) due to the ease of the sodium iodide getting into the vapor phase and coming apart.