CHEMISTRY :
Metal Cation Identification
Z I N C
| PROJECT CODE: |
02.08 |
| SECTION: |
METAL CATION IDENTIFICATION |
| PROJECT TITLE: |
ID of ZINC Cations by Precipitation Reactions |
| RELEASE DATE: |
31 JULY 1997 |
| LAST UPDATE: |
31-Jul-98 |
| VERSION HISTORY: |
1.0, 1.1, 1.2 ( Context updates)
V2.0 ( Text and formatting update - Sep-2009) |
INTRODUCTION:
This is an account on how to detect zinc ions in solution by simple precipitation reactions. Zinc is quite easy to distinguish since many of its compounds are insoluble, however many are white in color, end so it is not so easy to distinguish between other amphoteric metal cations such as Barium, Aluminium, Lead, etc. With The following set of tests it will be easy to confirm zinc without requiring complex procedures or sophisticated equipment.
PRINCIPLE
As mentioned, the tests are simple precipitation reactions. A solution of a zinc salt (Zinc Chloride sol.) was mixed with an equal ammount of another solution, which give a physical change, usually a colour change due to a precipitation of the zinc insoluble compound.
Zn X- (aq) + Na+ Y- (aq) ===> Zn+ Y- (s) + Na+ X- (aq)
(s) Solid precipitate forming a colour change in soultion
One type of reaction is not enough, to confirm the presence of zinc, since other metal salts 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 zinc cations. However the verification of 4 or 5 such test will be enough to confirm zinc in an unknown sample.
PROCEDURE
In 10ml testtubes, 4mls of zinc solution was placed. To this, about 2mls of solution of the following compounds all having different anions (-ve) was added. If desired, the mixture was heated gently to increase rate of reaction or added in exess to detect further complex reactions, usually the dissolving of the ppt just formed.
The following compounds was mixed with the zinc salt of which 16 produced a valuable result. These are marked with an Y in the React Column .
RESULTS
a) A GELATINOUS WHITE ppt was formed, which dissolved on adding xs NaOH. Plenty of NaOH was required to dissolve the ppted Hydroxide
b) Explosive on heating direct on the flame.
Zinc Hydroxide was precipitated, which formed a soluble complex on adding xs NaOH - [ Zn(OH)4 ] 2-
a) A MILKY WHITE ppt was formed which was soluble in xs Ammonia. A little ammount of xs Ammonia were enough to dissolve the dense ppted Hydroxide.
b) No reaction on heating.
Zinc Hydroxide was formed, which reacted easily with further Ammonia forming a complex anion, TetraAmmineZinc ion. [ Zn(NH3)4 ] 2+
a) A GELATINOUS WHITE ppt was immediately formed. (Insoluble in xs)
b) Heat had no effect on the ppt.
Zinc Carbonate was ppted.
a) No visible reaction was detected as the sulphite sol was dropped in the Zinc solution
b) On heating a FAINT WHITE ppt was formed and intensified on standing.
Propably the precipitation of Zinc Suphite is slow and heating increased the rate of reaction.
N.B. The Metibisulphite produced no reaction with Zinc, neither on heating.
a) A DENSE WHITE ppt was formed.
b) No further reactions on heating, or standing out.
White Zinc Sulphide was ppted at once.
a) A VERY FAINT WHITE ppt was formed.
b) On heating, more ppt was formed forming a WHITE color
As many other metal fluorides, Zinc Fluoride is an insoluble white solid.
a) No PPT was initially formed, but on xs Iodate, a FAINT WHITE PPT began to form.
b) Heating didn't increased the rate of reaction to a large extent.
Could be the formation of a solid Zinc transition complex, or that of Zinc Iodate which is sparingly soluble in water. Could be a false +ve result too.
a) A WHITE PPT was formed.
b) Explosive on heating and no reaction in xs Phosphate.
White Zinc Phospahet was ppted.
a) A THICK TRANSPERENT GELATINOUS ppt was formed at once.
b) On heating, the ppt became more white.
Insoluble Zinc Borate was ppted.
a) A CREAM WHITE ppt was immediately formed. On adding XS Tannic Acid, the ppt dissolved forming a clear solution.
b) Heat had no effect
The precipitated Zinc Tannate formed a transitional complex with xs Tannate. This is a good test for the Zinc cation, since if not Zinc only, very, very few metals produces a soluble complex with the Tannate anion. Excellent as an identification test for Zinc cations.
a) A GELATINOUS WHITE ppt was formed.
b) No further reaction on heating or standing.
Insoluble Zinc Silicate was precipitated.
a) A CREAM/MAGNOLIA ppt was formed.
b) On standing, the CREAM/MAGNOLIA ppt turned to a WHITE ppt
Insoluble Zinc FerroCyanide was formed. Some sort of oxidation could have taken place on standing, leading to the colour change from cream to white.
a) A YELLOW ORANGE precipitate was formed, insoluble in any xs Cyanide
b) On heating + standing for a long time, an orange ppt sank to the bottom of the tube, leaving a faint yellow solution.
Formation of solid Zinc Ferri(III)Cyanide or complex
a) This formed a BRIGHT ORANGE ppt. By filtering, it was found out the mixture consisted of a yellow ppt and an orange solution. Hence actually the ppt was yellow not orange.
b) No further reaction on heating, standing or adding xs.
This is the only coloured ppt that Zinc formed. Here 2 reactions might have taken place, which are the precipitation of yellow Zinc Vanadate, and the formation of an orange soluble Vanadium salt.
a) A MILKY WHITE ppt was formed immediately, which was insol in xs.
b) No reaction on heating or standing.
White Zinc Tungstate was precipitated. As with the Vanadate, it is possible that a Tungsten salt was formed, but since being colourless, it is not detected by a colour change.
a) A gelatinous white ppt was immediately formed.
b) No reaction on heating or xs Biselenite apart from settling to the bottom, and became more whiter in colur.
Insoluble Zinc Selenite was formed
CONCLUSION
Zinc is characterized by the dissolving of the white ppt in xs NaOH AND in xs Ammonium Hydroxide solution. Although the white hydroxide ppt of Tin, Lead, and Aluminium are soluble in xs NaOH, they are not soluble in xs Ammonia solution as that of Zinc is. Hence this is the first indication of the presence of Zinc cation.
Thus is obviously not enough, hence further confirmatory tests are required. The ones which stand out are that with the Vanadate producing an orange sol + yellow precipitate solid and with the Ferro(II)Cyanide and Ferri(III)Cyanide producing a cream white ppt and a brown ppt respectively. Also the cream white ppt with Tannic Acid, soluble in xs, is the final confirmatory test for Zinc.
Finally the lack of reaction of Zinc, with some reagents is unique, as other metal cation react, with the exception of Magnesium.
Metal Cation ID Section Links
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