Colorful precipitates of metal periodates
Periodate ion is a well-known strong oxidizer, capable of oxidizing many compounds quickly. It also is used in organic chemistry for this purpose. A much less known property of periodate ion is that it forms precipitates with many transition metals, which have very unexpected colors. When one is working with the transition metals frequently, then one develops a feeling for what color of precipitates are normal for the metals (e.g. copper (II) has blue or greenish precipitates, iron (III) has brown precipitates, cobalt has blue or pink precipitates, etc.). With periodate, however, the colors are totally unexpected and the colors really are bright.
The experiments, described on this page are very simple. Just add a solution of a metal salt to a solution of sodium metaperiodate and heat a little. Below, pictures are shown of different precipitates for different metal ions.
Required
chemicals:
-
vanadyl sulfate
-
chrome alum
-
manganese sulfate
-
ferric ammonium sulfate (or ferric sulfate)
-
cobalt sulfate
-
nickel sulfate
-
copper sulfate
-
cadmium nitrate
-
mercury nitrate
-
lead nitrate
-
cerium(III) nitrate
-
cerium(IV) sulfate
Required
equipment:
- test tubes
- a means to safely heat the test tube
Safety:
- Many of the transition metal salts are toxic, especially the cadmium, mercury and lead salts.
Disposal:
- All metal waste must be brought to a proper waste processing facility. The waste should not be flushed down the drain.
Results of mixing the solutions - vanadium
With vanadyl sulfate, no precipitate is formed. The vanadyl ion quickly is oxidized by the periodate ion to the pervanadyl ion. The liquid becomes orange, due to formation of condensed vanadium (V) species. So, adding the blue vanadyl-solution to periodate-solution results in formation of an orange vanadium (V) solution.
Results of mixing the solutions - chromium
With chromium ion, no precipitate is formed when a large excess of periodate is used. In that case, the Cr3+(aq) ions quickly are oxidized to orange dichromate ion, Cr2O72-. It is remarkable how fast and complete this oxidation is. Normally, oxidizing aqueous chromium (III) to chromium (VI) is quite difficult, but with periodate as oxidizer it is easy and fast.
With a smaller quantity of solution of periodate, a nice light green precipitate is formed, with a yellow liquid above this. This yellow liquid contains chromate ion, CrO42-, but also some other chromium compound (pure chromate is lemon-yellow).
When the precipitate is poured on a white ceramic surface, then one can see that the liquid is yellow, and the precipitate is green. The green color is not like the green color of many other chromium (III) precipitates, which are more grey/blue/green.
Results of mixing the solutions - manganese
With manganese(II) sulfate, a beautiful orange/red precipitate is formed and in solution, some permanganate is formed. The colors are really nice in this case. When a large excess amount of periodate is used, then with some heating, all manganese is oxidized to permanganate. When a smaller amount of periodate is used, then only part of the manganese (II) is oxidized to permanganate and a nice and very compact precipitate is formed, which also is stable, when the liquid is heated. When the purple liquid is decanted and the precipitate is kept and swirled along the glass, then its real color can be observed very nicely.
Results of mixing the solutions - iron
With iron(III), a yellow precipitate is formed, also very different from other iron(III) precipitates. When a solution of ferric sulfate or ferric ammonium sulfate is mixed with a solution of sodium metaperiodae, then the yellow precipitate is formed quickly.
Results of mixing the solutions - cobalt
Cobalt sulfate gives a very strange olive green precipitate with sodium metaperiodate. This most likely is a cobalt(III) compound.
The picture below shows a still somewhat pink solution above the precipitate. This is an indication of that a slight excess of cobalt sulfate was used in the experiment.
Results of mixing the solutions - nickel
Nickel also gives results, which are very different from most other nickel compounds. Usually, nickel gives light green or mint green precipitates. With metaperiodate, only a small amount of precipitate is formed, and this precipitate is light dirty yellow. The left picture below shows that most nickel ions remain in solution. The right picture nicely shows the true color of the precipitate with nickel ion.
Results of mixing the solutions - copper
When a solution of copper(II) sulfate is mixed with a solution of sodium metaperiodate, then again a precipitate with remarkable color is formed. Apparently, the lemon-yellow precipitate, formed in the reaction, is quite soluble. The left picture shows the lemon-yellow precipitate, with a blue liquid above the precipitate, but there is a clearly visible green hue. Most likely this is due to mixing of the blue color of aqueous copper(II) ions with the yellow complex of copper(II) with periodate.
Results of mixing the solutions - cadmium
Cadmium is one of the metals, which gives a precipitate with metaperiodate, which is much as one would expect. Both ions are colorless, and the precipitate is white.
Results of mixing the solutions - mercury
Mercury(II) nitrate (in dilute nitric acid, otherwise the mercury(II) hydrolyses) gives a nice rust/brown precipitate with metaperiodate.
Results of mixing the solutions - lead
Lead gives a precipitate with a color, which is different than expected. The colorless lead(II) ions and the colorless metaperiodate result in formation of a yellow precipitate.
Results of mixing the solutions - cerium
Cerium exists in two oxidation states in water, being +3 and +4. It was expected that the color of the precipitate does not depend on the oxidation state, because one would expect that cerium(III) is oxidized to cerium(IV), but this seems not to be the case.
With cerium(III) nitrate, a white precipitate is formed. With cerium(IV) sulfate, a light yellow precipitate is formed, which is only slightly darker than the precipitate, formed with lead (see above).
