light from above, due to the same mechanisms that color the sky at midday and dusk.

Thiosulfate ions react with iodine to give tetrathionate ions:

:2 S₂O₃^2- + I₂ -> S₄O₆^2- + 2 I^-

This reaction is key for iodometry.With bromine (X = Br) and chlorine (X = Cl), thiosulfate