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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of ANTIMONY Cations by Precipitation Reactions
LAST UPDATE: 31-July-98
VERSION HISTORY: 1.0, 1.1, 1.2 ( Context updates)
V2.0 ( Text and formatting update - Sep-2009)


This is an account on how to detect Antimony ions in solution by simple precipitation reactions. Antimony is quite easy to distinguish since there are quite unique imsoluble compounds, however it has a similar amphoteric characteristics to metal cations such as Zinc, Aluminium, Lead, etc. With the following set of tests it will be easy to confirm Antimony without requiring complex procedures or sophisticated equipment.


As mentioned, the tests are simple precipitation reactions. A solution of an Antimony double salt (Potassium Antimony Tartarate) was mixed with an equal ammount of another solution, which give a physical change, usually a colour change due to a precipitation of the Antimony insoluble compound.

Sb+ X- (aq) + Na+ Y- (aq) ===> Sb+ 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 Antimony, since other metal salts can 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 Antimony cations. However the verification of 4 or 5 such test will be enough to confirm Antimony in an unknown sample.


In 10ml testtubes, 4mls of Antimony 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 Antimony salt of which only 17 produced a valuable result. These are marked with an Y in the React Column .
01 Sodium Hydroxide Y
02 Ammonium Hydroxide Y
03 Sodium Carbonate Y
04 Potassium Sulphate<
05 Sodium ThioSulphate Y
06 Sodium (Metabi)Sulphite Y
07 Sodium Sulphide Y
08 Sodium Fluoride
09 Sodium Chloride
10 Potassium Bromide
11 Ammomium Iodide
12 Potassium Iodate Y
13 Ammonium Phosphate Y
14 Sodium TetraBorate (Borate) Y
15 Sodium Salicylate
16 Sodium Benzoate
17 Tannic Acid Y
18 Sodium Malate sol.
19 Sodium Methanoate sol.
20 Sodium Ethanoate
21 Sodium Citrate
22 Sodium Tartarate
23 Sodium Silicate Y
24 Potassium Ferro(II)Cyanide Y
25 Potassium Ferri(III)Cyanide
26 Sodium Vanadate Y
27 Potassium Permanganate Y
28 Potassium Dichromate Y
29 Sodium Tungstate Y
30 Ammonium Molybdate Y
31 Sodium BiSelenite
32 Potassium Thiocyanate


01: Sodium Hydroxide

a) A MILKY WHITE PPT was formed after few seconds

b) On exess the white ppt DISSOLVED slowly forming again a clear (or partially clear) solution. c) No reaction on heating

White insoluble Antimony Hydroxide was formed slowly. On exess Hydroxide the soluble Sodium Stibnate [ Sb(OH)4 ] - was formed, and hence a the formation of clear solution.

02: Ammonium Hydroxide

a) No immediate ppt was formed but on standing A FAINT WHITE PPT start to form slowly

b) The white precipitate DID NOT DISSOLVED on adding exess ammonia solution. c) Further precipitation was formed on heating, resulting in a WHITE solution

Antimony Hydroxide was formed very slowly, where the rate of reaction was increased by heating. No soluble complex on xs Ammonia

03: Sodium Carbonate

a) No immediate ppt was formed but on standing a WHITE PPT was formed slowly.

b) No reaction on exess c) A MILKY WHITE PPT was formed on heating

Insoluble White Antimony Carbonate was formed slowly; rate increased by heating.

04: Sodium ThioSulphate

a) A FAINT RUBBER-TUBING ORANGE PPT (peach ppt) was formed slowly

b) No reaction on xs. Heat increased the precipitation. Relatively few ppt was formed. This could be either due the formation of insoluble ThioSulphate, or a further complex reaction might took place, such as the formation of the Sulphide from the Thiosulphate Anion.

05: Sodium Sulphite (or Sodium Metabisulphite)

a) No ppt was formed

b) On heating a WHITE precipitate formed. Antimony Sulphite precipitated on heating. The metibisulphite gave no ppt, neither on heating nor on standing.

06: Sodium Sulphide

a) On first sight, a rusty brown/orange ppt is formed.

b) This brown ppt dissolved in xs Sulphide forming a colourless solution. On adding the Sulphide dropwise, the following colour changes took place:

Orange ppt -> orange brown ppt -> light brown ppt -> brown ppt -> dark brown ppt -> brown ppt/solution -> Iodine brown sol/ppt -> brown sol/ppt -> orange sol -> clear yellow sol -> clear solution. Formation of the soluble complex ion [ SbS3 ] 2-

07: Potassium Iodate

a) No immediate ppt.

b) On heating (or standing for long time), a brown/orange solution was formed, with the formation of black/purple deposits at the bottom. Iodine was liberated which resulted the orange solution, and black deposits of solid Iodine.

08: Ammonium Phosphate

a) No initial ppt, but a WHITE PPT formed slowly on standing

b) A WHITE PPT was formed on heating

Insoluble white Antimony Phosphate was formed slowly, heat increasing the rate of reaction

09: Sodium Tetraborate

a) No initial ppt, but a WHITE PPT formed slowly on standing

b) A WHITE PPT was formed on heating

Insoluble white Antimony Borate was formed slowly, heat increasing the rate of reaction

10: Tannic Acid

a) A CREAM WHITE ppt was formed immediately.

b) No reaction on exess or heating. PPT floated on top

White Antimony Tannate was formed.

11: Sodium Silicate

a) A WHITE ppt fas formed. On adding xs silicate, many of the ppt dissolved leaving a very faint white ppt.

b) Heating made the ppt to dissolve rapidly. The white insoluble Antimony Silicate formed, made a soluble complex with further xs silicate.

12: Potassium Ferro(II)Cyanide

a) No ppt or visible color change was detected.

b) On heating, a yellow solution was formed, and very little white ppt was formed at the bottom of the tube. Difficult to say what reaction ,if any, might have occured.

13: Sodium Vanadate

a) A COFFEE-MILK (Cafelatte) PPT was immediatelly formed.

b) On adding xs Vanadate, the ppt darkened a little and on filtering off, the mixture appeared to consists of a LIGHT BROWN PPT, and a violet blue solution (filtrate). The addition of xs vanadate did not dissolved the COFFEEMILK precipitate. c) No further reaction on heating, perhaps incrasing up the reaction. A complex reaction resulted on the addition of xs vanadate. This is a unique test for the presence of Antimony.

14: Potassium Permanganate

a) The permanganate was decolorized by the Antimony solution. On adding more permanganet the solution turned pale yellow, oil yellow, brown and finally dark brown/black.

b) Heating speeds up the reaction, forming a DARK BROWN PPT in a colourless solution. When few Permanganate was used, the PPT was GREY BROWN in colour. c) No reaction on xs permangante, execpt from remaining unreacted forming a violet solution in the brown ppt

Propably the insoluble Permanganate of Antimony was formed.

15: Potassium Dichromate

a) On adding few drops of dichromate a series of color changes took place as follows: Colourles -> clear green/blue solution -> light green -> brilliant green -> Dark green. No ppt was formed. The dark green solution remained unreacted with NaOH, Ammonia and Carbonate, hence no free chrome ions are present, but a complex was formed.

b) No reaction on heating or standing. Complex reaction took place due the transiton chromium atom in the dichromate. This is another unique test for antimony.

16: Sodium Tungstate

a) No ppt was formed, neither on standing for few minutes.

b) However, on heating a WHITE ppt was formed. Either insol Antimony tungstate was formed, or heat decomposed one of the chemicals resulting in a precipitation with the other.

17: Ammonium Molybdate

a) A FAINT OPAQUE YELLOW ppt/suspension was slowly formed.

b) No further reaction on standing. c) On heating-boiling, a wonderful DEEP BLUE SOLUTION was slowly formed till the temperature reached the boiling point. No ppt was detected, and no colour change on cooling back. A wide range of reactions could have took place, either a complex of antimony, or molybdium ions where formed, or a complex between antmony and molybdium ?? Difficult to draw a definite conclusion.


Antimony is first characterized by the partial and slow dissolving of the white ppt in xs NaOH forming the Stibnate soluble ion. Zinc, Lead, and Aluminium, also produces a similar reaction of which Zinc can be eliminated as it forms a soluble complex with xs ammonia as well.

Further confirmatory and unusual reactions for Antimony are those with the Molybdate, Vanadate, Dichromate, and ThioSulphate. It seems that particular reactions took place with the transition anions salts (dichromate, vanadate, molybdate and permanganate). The ppt with Permanganate, and Tannate completes the confirmation of Antimony.

Note that many Antimony precipitates formed only after heating, because are slow to form. This applies surprisingly to the Carbonate, Phosphate and Ammonia (and Borate as well) which usually form a quick precipitate with most metal cations.

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