Chromium is searched on the third analytic group as Cr +3.
The chromium hydroxide is precipitated together with the hydroxides of iron and aluminum for alkalization of the solution with an ammonium buffer / ammoniacal nitrogen (NH 4 Cl / NH 4 OH).
Subsequently, chromium and aluminum are separated from the iron on the basis of their amphoteric behavior.
Furthermore, it also takes the opportunity to oxidize Cr (III) to Cr (VI) much easier to analyze.
- Treatment with soda and water ossigentata
Both Cr +3 that CrO 4 2- are able to color the solution. Already when we report in solution the hydroxide with the soda, we should be able to distinguish a more or less pale greenish coloration imparted by Cr +3. Similarly, after the latter oxidized to chromate ion with the hydrogen peroxide we should distinguish the typical yellow color of the chrome ion. This clue is already perceived as very indicative (NaOH and H 2 O 2 are added colorless solutions).
Chromium recognition
Already the treatment with soda and hydrogen peroxide we made to separate aluminum and chromium in iron (which remains as precipitated as hydroxide) should have given us an indication of the presence of chromium. If the solution has become yellow, this test is already in itself a positive test. There are other species that color in the yellow solution. For further confirmation, however, we can do other wise. The one we already know.
1) treatment with lead acetate (CH 3 COO) 2 Pb
This essay is very easy, because it exactly reproduces one of identification papers that we have seen the first analytic group to search for the lead. In that case after making the reaction environment suitable we added a salt containing the anion chromate. Now we seek the chrome we do exactly the opposite, that we add a soluble lead salt.
(See research of lead first analytical group )
Also in this case we need to create a slightly acidic pH, so as not to precipitate the lead hydroxide and not to transform all the chrome dichromate. Using acetic acid to a pH in the neighborhood of 5 (litmus test).
Also in this case we can confirm the nature of the precipitate verifying the solubility in strong bases (the precipitate dissolves because the lead, amphoteric, is risolubilizza as piombito).
2) Assay Baresville
It 'a quite particular article, which is at the base of the not entirely known reactions but which in fact is very specific to the chrome ion. It is thought that the formation of a compound wherein the chromium has higher state of oxidation to 6, perhaps a chromium peroxide.
From a practical point of view is so proceeds. The solution, for alkaline soda, is brought to pH 3 by addition of nitric or sulfuric acid (non-randomly). Then added a few ml of diethyl ether or isoamyl alcohol (just enough to form a two-phase system) and add a few drops of hydrogen peroxide. Plugging the tube with the finger is stirred rapidly to extract the new compound in the organic phase. The assay is sensitive but must be performed correctly in all its parts. Requires a determined pH, it must be done cold and very quickly, to avoid that the new compound may be decomposing.
Warning: the English translation of this article must be completed.
