Bismuth halogen complexes
Bismuth in its +3 oxidation state forms complexes with the halogen ions in their -1 oxidation state. With chloride and bromide, colorless complexes are formed. When iodide is added, then colorful complexes are formed. Iodide also forms a black precipitate with bismuth. Bromide, with some iodide mixed in, forms a light brown precipitate with bismuth.
An interesting thing, shown by this set of experiments, is that bismuth also forms mixed-halogen coordination complexes, e.g. chloro-iodo-bismuth complexes.
Required
chemicals:
-
bismuth nitrate or basic bismuth nitrate
-
dilute nitric acid, containing 5 to 10% of HNO3 by weight
-
dilute hydrochloric acid, containing 5 to 10% of HCl by weight
-
sodium chloride
-
potassium bromide
-
potassium iodide
Required
equipment:
- test tubes
Safety:
- Dilute nitric acid and dilute hydrochloric acid are corrosive.
Disposal:
- Although bismuth is not particularly toxic, it is considered a heavy metal. Just to be on the safe side, bring waste to a waste processing facility.
Different experiments with bismuth complex formation
This experiment actually is a set of experiments.
In the first experiment it is shown
that bismuth forms a precipitate with iodide, which on addition of more iodide
redissolves with the formation of a brightly colored iodo-complex of bismuth.
This behavior resembles the behavior of mercury, which also forms a soluble iodo-complex
with excess iodide.
In the second experiment, it is
shown that bismuth iodide also dissolves in solutions with excess chloride. It
is also shown that when iodide is added to a solution of bismuth in excess
chloride-solution, no precipitate is formed at all, but the iodide replaces
chloride ligands (at least one, but most likely more than one, when sufficient
iodide is present).
In the third experiment, it is
shown that bismuth also forms mixed bromo-iodo complexes, which resemble the
chloro-iodo complex very much.
In a final experiment, it is shown
that with small amounts of bromide and iodide a mixed halogen precipitate is
formed, which has properties close to the properties of bismuth iodide, but
which has a different color.
Discussion of results
With a small amount of iodide, bismuth (III) forms a black precipitate of bismuth (III) iodide:
Bi3+(aq) + 3I–(aq) → BiI3(s)
When more iodide is present, then the precipitate dissolves again and a deep orange/red complex is formed:
BiI3(s) + I–(aq) → BiI4–(aq)
The reactions, as shown above, only occur in fairly strong acidic media. Even at moderate acidity, the bismuth ion already hydrolyses and a white precipitate of a hydrous basic bismuth salt is precipitated.
When bismuth nitrate is dissolved in dilute nitric acid, then plain Bi3+(aq) ions are formed, but in dilute hydrochloric acid BiCl4–(aq) is formed. The latter complex is colorless. On addition of iodide, however, this complex is changed to a mixed halogen complex BiClnI4-n–(aq) and no precipitate of BiI3 is formed. The ion BiClnI4-n–(aq) is yellow. In one of the experiments, the color of this ion can nicely be compared with the orange color of the BiI4–(aq) ion. The exact value of n is not known (at least not by the author).
With bromide, a similar reaction occurs and ions of the form BiBrnI4-n–(aq) are formed. Again, the precise value of n is not known (by the author).
When only a small amount of a mix of bromide and iodide is present, relative to the amount of bismuth (III) ions, then a light precipitate of bismuth (III) iodide/bromide is formed, which dissolves on addition of more iodide or bromide.