CHEMISTRY :
Metal Cation Identification
C H R O M I U M
| PROJECT CODE: |
02.20 |
| SECTION: |
METAL CATION IDENTIFICATION |
| PROJECT TITLE: |
ID of CHROMIUM Cations by Precipitation Reactions |
| RELEASE DATE: |
23 JULY 1998 |
| LAST UPDATE: |
31-Jul-98 |
| VERSION HISTORY: |
1.0, 1.1 ( Context updates)
V2.0 ( Text and formatting update - Sep-2009) |
INTRODUCTION:
This is an account on how to detect Chromium ions in solution by simple precipitation reactions. Chromium is fairly easy to distinguish since of few characteristic Blue-Green insoluble compounds. However, there is not a large variation amongst the precipitates as they are mainly all in the bluish-green-grey colour, with just few exceptions. Nevertheless, with the following set of tests it will be easy to confirm Chromium, which is one of the transitional metals, without requiring complex procedures or sophisticated equipment.
PRINCIPLE
As mentioned, the tests are simple precipitation reactions. A solution of a Chromium salt (Ammonium Chromium Sulphate) was mixed with an equal ammount of another solution, which give a physical change, usually a colour change due to a precipitation of the Chromium insoluble compound.
Cr++ 2X- (aq) + 3Na+ 2Y- (aq) ===> Cr++ 2Y- (s) + 3Na+ 2X- (aq)
(s) Solid precipitate forming a colour change in soultion
One type of reaction is not enough, to confirm the presence of Chromium, since other metal salts may give the same results. For Example with NaOH, many metals give a white ppt., and hence one can't say that the formation of a white ppt of an unknown sample with NaOH is 100% due to Chromium cations. However the verification of 4 or 5 such test will be enough to confirm Chromium in an unknown sample.
PROCEDURE
In 10ml test tubes, 4mls of Chromium solution was placed. About 2mls of reagent solution of The following compounds all having different anions (-ve) was added.
Any precipitation or colour changes are noted. If desired, the mixture was heated gently to increase rate of reaction, or added in xs perhaps the ppt formed will dissolve (especially for Ammonia and sodium hydroxide.)
The following compounds was mixed with the Chromium salt of which 14 produced a valuable result. These are marked with an Y in the React Column .
RESULTS
** N.B. ** An initial unique characteristic is that the solid salt is violet in colour and turns blue/violet in solution. This is due the Cr2+ ion, hence the +2 oxidation state. When the solution was heated, the solution turned to DEEP GREEN, due the presence of Cr3+ ions, hence +3 Oxidation State.
a) This gave an initial BLUE/LIGHT GREEN ppt, but on adding more NaOH, the ppt dissolved into a GREEN SOLUTION.
b) No reaction on heating the initial ppt, but on heating the green solution, the BLUE GREEN ppt was formed back.
Insoluble brown Chromium Hydroxide/Oxide was formed which dissolved in xs forming the green chromite complex solution - [ Cr(OH)4 ] - The latter decomposed, possibly to the oxide, on heating.
a) This gave a BlUE/GREEN ppt insoluble in xs ammonia
b) No particular reaction on heating, perhaps getting more BLUISH GREY
Insoluble Chromium Hydroxide was formed, which maybe turned into the oxide on heating.
a) A LIGHT GREYISH BLUE ppt was formed.
b) On heating, no colour change resulted.
Insoluble Chromium Carbonate was formed.
a) No reaction initially. c) Heating did formed a few light green ppt. On settling it was found that a white/yellow ppt + a green solution was obtained.
Insoluble Chromium Thiosulphate is the first prediction, but a different reaction could have took place. On heating some white/yellow sulphur was formed from the thiosulphate, and the bluish violet Cr(II) turned to the greenish Cr(III) on heating.
a) A CYAN ppt (not dark) was formed.
b) No reaction on adding xs Sulphide or boiling
This is the only Green/blue sulphide and is good test to distinguish chromium. The precipitation was not that intense as with most other sulphides.
a) A LIGHT GREEN (WHITISH) ppt was formed
b) No reaction on heating or standing.
Chromium Iodate was precipitated out.
a) A GREY/PALE GREEN PPT was formed,
b) On heating the ppt colour turned to CYAN/GREY, but no particular change at all.
Insoluble white Chromium Phosphate was produced.
a) A few Blue/green ppt suspended in unreacted blue solution of chromium was developed.
b) On heating no particular reaction or change took place.
Since lot of chromium solution was left unreacted, this test is perhaps a negative test.
a) No precipitate initially.
b) Only on heating a (WHITISH) BLUE/CYAN ppt was formed.
Insoluble Chromium Benzoate was precipitated.
a) A pale BLUE/GREEN ppt was formed, and surprisingly, it did dissolve in xs to form a GREEN solution
b) No further reaction on heating
Insoluble Chromium Silicate was formed, forming a complex ion on xs Silicate. Very rare that an insoluble silicate did dissolve in xs
a) No reaction or precipitation.
b) On heating a deep dark green solution was formed.
An insoluble complex of Chromium was propably formed on heating.
a) A FAIRLY DARK BROWN solution was formed, which on standing a brown ppt started to form.
b) On heating total precipitation occured, resulting in a BROWN ppt in a green solution. (perheps unreacted chromium salt)
Chromium Vanadate was propably formed which is brown and insoluble.
a) A THICK CYAN ppt was formed
b) No further reaction on heating.
Another straight forward precipitation, this time Chromium Tungstate.
a) A GREEN/SKY BLUE ppt was formed at once.
b) Ppt was insoluble in xs an no reaction on heating
Insoluble Chromium (bi)Selenite was formed.
CONCLUSION
The first indications of Chromium is the formation of a GREY/BLUE-GREEN ppt with dilute NaOH, soluble in few xs into a green solution. Additionally, Ammoinia, results in a rather similar coloured ppt but is insoluble in xs.
This is already a good test, and further confirmatory tests are few, since most of the formed precipitates are in the blue/green shades, and hence one cannot use a number of unique confirmatory tests.
Howver the DARK GREEN solution fromed by heated Ferro(II)Cyanide, lack of ppt with Ferro(III)Cyanide, the BLUE-GREEN Sulphide, and especially the BROWN sol (then ppt on heating) by the Vanadate are quite well unique tests for chromium, and are not exhibited by other metals.
Additionaly, one can mention the PALE BLUE/GREEN ppt sol in xs by the silicate, and the milky GREEN/CYAN Iodate, Tungstate and Selinite ppts.
Note that chromium can exist in various oxidation states, and hence the colour can range from BLUE to GREEN, including intermediate coulurs depending the conditions and phase of the reaction taking place. For example, purely Cr2+ ppt is blue, purely Cr3+ ppt is green, but half of each (hence some Cr2+ and some Cr3+) is expected to be rather blue/green (cyan) in colour.
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