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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of COBALT Cations by Precipitation Reactions
LAST UPDATE: 01-Aug-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 Cobalt ions in solution by simple precipitation reactions. Cobalt is fairly easy to distinguish since many of its different colour compounds. With the following set of tests it will be easy to confirm Cobalt without requiring complex procedures or sophisticated equipment.


As mentioned, the tests are simple precipitation reactions. A solution of a Cobalt salt (Cobalt Chloride) was mixed with an equal ammount of another solution, in which a physical change is noted, usually a colour change due to a precipitation of the Cobalt insoluble compound or formation of a complex since Cobalt is a transitional metal.

Co++ X (aq) + 2 Na+ Y- (aq) ===> Co++ Y (s) + 2 Na+ X- (aq)
(s) Solid precipitate forming a colour change in soultion

One type of reaction is not enough, to confirm the presence of Cobalt, since other metal salts can give the same results. The verification of 4 or 5 such tests will be enough to confirm Cobalt in an unknown sample.


In 10ml testtubes, 4mls of Cobalt 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 were mixed with the Cobalt salt of which 15 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
06 Sodium (Metabi)Sulphite Y
07 Sodium Sulphide Y
08 Sodium Fluoride
09 Sodium Chloride
10 Potassium Bromide
11 Ammomium Iodide
12 Potassium Iodate
13 Ammonium Phosphate Y
14 Sodium TetraBorate (Borate) Y
15 Sodium Salicylate
16 Sodium Benzoate
17 Tannic Acid
18 Sodium Malate sol.
19 Sodium Methanoate sol. Y
20 Sodium Ethanoate
21 Sodium Citrate
22 Sodium Tartarate
23 Sodium Silicate Y
24 Potassium Ferro(II)Cyanide Y
25 Potassium Ferri(III)Cyanide Y
26 Sodium Vanadate Y
27 Potassium Permanganate Y
28 Potassium Dichromate
29 Sodium Tungstate
30 Ammonium Molybdate
31 Sodium BiSelenite Y
32 Potassium Thiocyanate


01: Sodium Hydroxide

a) BLUE ppt was initially formed with changes into VIOLET-BLUE, GREY BLU, GREY, GREY PINK, LIGHT BROWN/PINK within a minute of standing

b) Heating results in the above change to occur rapidly, and hence the final LIGHT BROWN-PINK c) PPT was insoluble in xs. The insoluble Hydroxide was precipitated. This seems to undergoe various Co oxidizing states (characteristic of transition metals) while standing.

02: Ammonium Hydroxide

a) BLUE/CYAN PRECIPITATE was initially formed within the few drops of Ammonia. On adding more drops, the colour changed to CYAN, and finally to GREEN ppt. On filtering, a GREEN ppt was left, and a CLEAR YELLOW BROWN solution was given off.

b) No reaction on heating. A diffeent reaction from the Sodium hydroxide here. Very difficult to say what reactions are going on. The coloured solution is propably a Cobalt complex solution, [ Co(NH3)6 ] 2+ whereas the ppt is a hydrate form of hydroxide with a different oxidation state given with NaOH.

03: Sodium Carbonate

a) LILAC/PURPLE ppt was formed

b) On heating the ppt turns more to a MAGENTA colour. Precipitation of Cobalt Carbonate.

04: Sodium Sulphite (or Sodium Metabisulphite)

a) A LIGHT PURPLE ppt was formed. On filtering it was found out that a clear ORANGE/PINK (Peach col.) Solution was given, and PURPLE ppt was left on the filter paper, which after some minutes was oxidized to a Brown colour.

b) No important changes on heating. Cobalt Sulphite may have been precipitated together with the possible formation of a complex solution.

05: Sodium Sulphide

a) A CHARCOAL BLACK ppt was formed

b) No further reactions on heating, or standing out. Cobalt Sulphide was ppted at once.

06: Ammonium Phosphate

a) A GELATINOUS VIOLET ppt was formed.

b) On heating, no colour changes seem to happen. The insoluble Cobalt phosphate was precipitated out.

07: Sodium Tetraborate

a) Initially a LILAC PPT was formed

b) On heating the ppt turned to a BLUE/Violet colour. Cobalt Borate was precipitated. Perhaps oxidation has taken place on heating, to bring about the colour change. This is a good test for the Cobalt

08: Sodium Methanoate

a) Few magenta/purple ppt on xs methanoate in a pink solution of unreacted Cobalt chloride.

b) On heating strongly, no particular change took place

This is considered as a negative result since few pt was formed, even on xs.

09: Sodium Silicate

a) A GELATINOUS MAGENTA ppt was formed.

b) On heating, the colour turned into a BLUE/Violet ppt. Insoluble Cobalt Silicate was precipitated.

10: Potassium Ferro(II)Cyanide

a) A GRASS GREEN ppt was formed on adding few drops of Ferro(II)Cyanide. On adding xs, the green colour transformed into an OLIVE/GREEN-YELLOW ppt. Filtering resulted in no coloured solution.

b) On heating the OLIVE GREEN ppt remained. It is propably a simple precipitation of the green Cobalt Ferro(II)Cyanide.

11: Potassium Ferri(III)Cyanide

a) A DARK RED-BROWN ppt was formed. On filtering this upon the initial drops of Ferri(III)cyanide, it was evident the the brown colour was due a ppt. On adding xs Cyanide salt, the brown colour remained but when filtered, the solution obtained was the same brown colour. Only few ppt was left over. So one can say that in xs, the ppt dissolved. Strangely, the mixture didn't seem to be a solution when placed against light.

b) On heating + standing no further reaction took place. Fairly complex reaction here, which is regarded as a precipitation soluble in xs Ferri(III)Cyanide.

12: Sodium Vanadate

a) A LILAC/PALE VIOLET ppt was formed.

b) No further changes on heating

Precipitation of Cobalt Vanadate.

13: Potassium Permanganate

a) The colour of the solution turned slight to red, but no ppt formed.

b) On heating, Some black ppt was formed, but not all the permanganate was used, even if cobalt was in xs. Hence the violet solution of the permanganate was still there. The interior sides of the tube got brown and only on filtering the Black ppt can be noted easily. Propably a slow precipitation reaction of the Black solid Cobalt permanganate.

14: Sodium Biselenite

a) No reaction initially, even upon adding xs BiSelenite

b) On heating strongly a PURPLE/LILAC ppt was formed. Cobalt selenite ppt was formed only on heating

15: Potassium ThioCyanate


a) No ppt, but the solution colour turned from pink to purple (darker pink?)

b) No particular effect on heating

Not a quite a good qualitative test. In Inorganic Qualitative Analysis, Richard hahn states that this should give a dark blu complex of [ Co(SCN)4 ] 2-


Cobalt is a transitional metal, which is quite easy to detect from its colourful (purple/violet mostly) precipitate copmounds it forms.

It forms a BLUE/VIOLET ppt which then turns to PINK on standing with Sodium Hydroxide, and a Blue/GREEN ppt with Ammonium Hydroxide, both insoluble in xs.

Further confirmatory tests are:
The RED/BROWN ppt/sol formed with Potassium Ferri(III)Cyanide solution, the green ppt formed with Ferri(II)Cyanide, the black Sulphide, the Purple ppt which turns blue on heating with the Borate, and a range of PURPLE/LILAC ppt obtained by various reagents such as Carbonate, Silicate, Sulphite, and Vanadate are unique to Cobalt. The Sulphite, additionally produced a PURPLE ppt which turned brown on standing on a filter paper, and also a CLEAR PEACH/PINK solution, after filtering the ppt off.

These are enough for detecting the presence of Cobalt ions in solution.

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