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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of COPPER Cations by Precipitation Reactions
LAST UPDATE: 31-Jul-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 Copper ions in solution by simple precipitation reactions. Copper is quite easy to distinguish since some of its unique (mainly bluish) precipitates. With the following set of tests it will be easy to confirm Copper without requiring complex procedures or sophisticated equipment.


As mentioned, the tests are simple precipitation reactions. A solution of a Copper salt (Copper Sulphate) 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 Copper insoluble compound or formation of a complex since Copper is a transitional metal.

Cu++ X (aq) + 2 Na+ Y- (aq) ===> Cu++ 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 Copper, since other metal salts can give the same results. The verification of 4 or 5 such tests will be enough to confirm Copper in an unknown sample.


In 10ml testtubes, 4mls of Copper 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 Copper salt of which 24 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 Y
12 Potassium Iodate
13 Ammonium Phosphate Y
14 Sodium TetraBorate (Borate) Y
15 Sodium Salicylate
16 Sodium Benzoate Y
17 Tannic Acid Y
18 Sodium Malate sol.
19 Sodium Methanoate sol. Y
20 Sodium Ethanoate Y
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
28 Potassium Dichromate
29 Sodium Tungstate Y
30 Ammonium Molybdate
31 Sodium BiSelenite Y
32 Potassium Thiocyanate Y


01: Sodium Hydroxide

a) A PURE BLUE ppt was formed, insoluble in xs.

b) On heating the ppt turned to a black colour

The insoluble blue hydroxide was precipitated. This was oxidized by heat to to black Copper Oxide.

02: Ammonium Hydroxide

a) A SKY BLUE ppt was formed on adding very little Ammonia.

b) On adding some ammonia in xs, the ppt dissolves into a characteristic DEEP BLUE solution. c) Heating this solution formed some BLACK ppt. Interestingly, heating the initial blue ppt didn't turned to black as with the hydroxide. Is a Hydroxide formed here?

Since heating the sky blue ppt, didn't turned to black, it is very unprobable that the ppt is a common hydroxide, but some sort of hydrated hydroxide. On adding xs, the deep blue TetrAmmineCopper(II) complex ion was formed. [ Cu(NH3)4 ] 2+

03: Sodium Carbonate

a) A LIGHT BLUE (CELESTE) ppt was formed, insol in xs.

b) Heating turned the ppt black. Precipitation of Copper Carbonate, oxidized by heat to black Copper Oxide.

04: Sodium Thiosulphate

a) On adding the Thiosulphate, the blue sol turns GREEN (due to a mixture of Yellow (by the rection) + Blue (copper sulphate not reacted).

b) On heating, the green solution turns to a LIGHT BROWN ppt, then BROWN, DARK BROWN and finally BLACK ppt. The Copper Thiosulphate could have been ppt or is it black copper oxide?!

05: Sodium Sulphite (or Sodium Metabisulphite)

a) A GREEN BROWN ppt is formed. The more sulphite added, the more YELLOW BROWN is the ppt. However on xs sulphite, the ppt dissolves to a PALE PINK/CLEAR solution.

b) On heating, the solution turns completely COLOURLESS. The sulphite was inititialy precipitated which dissolved in xs due a complex transitional reaction.

06: Sodium Sulphide

a) A DARK BROWN ppt, insoluble in xs was formed immediately

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

07: Potassium Iodide

a) Initially a MUSTARD BROWN ppt (mixture) was formed.

b) On heating, PURPLE vapour of Iodine was seen to be liberated, and the mixture colour turned to DARK BROWN. Filtering resulted in a DARK BROWN solution (of Iodine) and a CREAM/MAGNOLIA PPT was left on the filter paper. Iodine was liberated and White/Magnolia Copper(I)Iodide was precipitated

08: Ammonium Phosphate

a) A GELATINOUS SKY BLUE ppt was formed.

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

09: Sodium Tetraborate

a) A SKY BLUE PPT was formed.

b) On heating the ppt intensified slight to a more blue colour. Copper Borate was precipitated, increased in rate by heating.

10: Sodium Salicylate

a) the solution turned to a green/olive green colour

b) Colour slightly darkened, but no precipitation or great change. A complex reaction here, perhaps.

11: Sodium Benzoate

a) A LIGHT BLUE (CELESTE) ppt was formed.

b) More ppt was formed on heating

Insoluble Copper Benzoate was precipitated.

12: Tannic Acid

a) A DIRTY GREY-BROWN ppt was formed.

b) On heating the ppt turned more to a BROWN/YELLOW ppt. Insoluble Copper Tannate was precipitated.

13: Sodium Malate

a) The clear blu solution darkened slightly to a bluish solution, but no ppt or particular colour change.

b) No further reaction on heating or standing out. Considered as a negative reult since no ppt was formed.

14: Sodium Methanoate

a) No reaction initially

b) Only on heating for few minutes, some LIGHT BLUE ppt was formed

Slow precipitation of Copper Methanoate.

15: Sodium Ethanoate

a) No reaction initially

b) Only on heating for few minutes, some LIGHT BLUE/CYAN ppt was formed

Slow precipitation of Copper Ethanoate. (Similar reaction to the methanoate)

16: Sodium Citrate

a) The clear blue solution darkene to BLUE solution but no ppt was formed even after heating strongly

Same reaction as with the malate. This is not an indicative test, and regarded as negative.

17: Sodium Silicate

a) A BLUE ppt was formed at once

b) No further reaction on heating or standing

Copper Silicate was precipitated as a blue solid.

18: Potassium Ferro(II)Cyanide


b) On heating no reaction took place. Another straight forward precipitation

19: Potassium Ferri(III)Cyanide

a) This resulted in DIRTY BROWN-GREEN ppt

b) On heating + standing no further reaction took place. Another Precipitation reaction

20: Sodium Vanadate

a) A GREEN YELLOW ppt was formed.

b) On heating the ppt turned more YELLOW, hence a YELLOW, slightly GREEN mixture. On filtering this it was found that this mixture consisted of a CLEAR YELLOW solution + a YELLOW-GREEN ppt. Complex reaction, perhaps the Vanadium salt (yellow solution) and the Copper Vanadate precipitation were formed.

21: Sodium Tungstate

a) A CYAN ppt was formed

b) No further changes on heating. Formtion of the solid BLUE GREEN Copper Tungstate

22: Ammonium Molybdate

a) No colour change initially.

b) On heating the blue colour turned slightly cloudy

Not quite an indicative test!? Regarded as a negative result.

23: Sodium Biselenite

a) LIGHT GREEN-CYAN ppt was deposited

b) No further reactions on heating

Copper selenite ppt was formed

24: Potassium ThioCyanate

a) Initially a GREEN solution was formed. In the period of 1 minute, the solution turned to a GREEN ppt, then GREEN GREY, GREY and BAIGE ppt.

b) On heating, the baige ppt turned into a BAIGE-GREY/LILAC ppt On filtering it was found that the mixture consist of a CLEAR YELLOW solution and a unique WHITE-LILAC ppt

An excellent test for Copper, since the Thiocyanate is quite unreactive with other metals, and this reaction is particular to copper.


Copper is a transitional metal, and forms many coloured precipitates and a few distinct complexes.

It forms those particular BLUE or SKY BLUE ppts with Ammonium Hydroxide, Sodium Hydroxide and Sodium Carbonate, in which the former results in a deep blue solution on xs Ammonia, and the latter two, turns Black (copper Oxide) on heating.

These tests are already unique to Copper, but further confirmatory test include primarily the white lilac ppt (+yellow sol) with Thiocyanate, and then the Reddish Brown ppt with ferro(II)Cyanide, the green ppt with the Sulphite, soluble in xs to a pink/colourless solution, and also the Brown Black ppt with the Sulphide.

Finally, there are many other addition tests which forms mostly a Blue or cyan ppt, unlike other metals under test.

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