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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of CADMIUM 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 Cadmium ions in solution by simple precipitation reactions. Cadmium is fairly easy to distinguish since many of its precipitate compounds it forms, however most of them are white. With the following set of tests it will be easy to confirm Cadmium without requiring complex procedures or sophisticated equipment.


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

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


In 10ml testtubes, 4mls of Cadmium 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 Cadmium salt of which 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
06 Sodium (Metabi)Sulphite Y
07 Sodium Sulphide</TD> 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
18 Sodium Malate sol.
19 Sodium Methanoate sol.
20 Sodium Ethanoate
21 Sodium Citrate Y
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 Y
31 Sodium BiSelenite Y
32 Potassium Thiocyanate


01: Sodium Hydroxide

a) A WHITE ppt insoluble in xs was formed

b) Heating results in no colour change. The insoluble Hydroxide was precipitated.

02: Ammonium Hydroxide

a) A WHITE ppt was formed which did dissolve immediately on adding xs ammonia

b) No reaction on heating, apart from the formation of a few white ppt. Again Cadmium Hydroxide was precipitated which formed a complex soluble ion on adding xs ammonia. [ Cd(NH3)4 ] 2+

03: Sodium Carbonate

a) A MILKY WHITE ppt was formed.

b) No change on heating. Precipitation of Cadmium Carbonate.

04: Sodium Sulphite (or Sodium Metabisulphite)

a) A MILKY WHITE ppt was formed

b) No important changes on heating. Cadmium Sulphite is also a White insoluble solid.

05: Sodium Sulphide

a) A BRIGHT YELLOW PPT was immediately formed. On xs the colour intensifies to YOLK YELLOW precipitate

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

06: Potassium Iodate

a) A MILKY WHITE PPT was formed immediately

b) No further reaction on heating. White Cadmium Borate was precipitated.

07: Ammonium Phosphate

a) A GELATINOUS WHITE ppt was formed.

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

08: Sodium TetraBorate:

a) A WHITE ppt was formed

b) No further reaction on heating

The Borate ppt was given

09: Sodium Methanoate

a) Faint white ppt was formed

b) On heating white ppt intensified slightly

This test is considered as a negative result since liitel ppt was given!?

10: Sodium Citrate

a) A WHITE ppt was formed

b) No further reaction on heating, but gave the impression that it was going to dissolve?!

Possibly the Cadmium citrate is an insoluble white solid

11: Sodium Silicate


b) No further reaction on heating or standing. Insoluble Cadmium Silicate was precipitated.

12: Potassium Ferro(II)Cyanide

a) A MILKY WHITE ppt was formed

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

13: Potassium Ferri(III)Cyanide

a) This resulted in YELLOW CREAM ppt

b) On heating + standing no further reaction took place. Precipitation of the Cadmium Ferri(III)Cyanide

14: Sodium Vanadate

a) A MILKY WHITE ppt was formed

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

15: Sodium Tungstate

a) A THICK WHITE ppt was formed

b) No further changes on heating. Cadmium Tngstate was precipitated

16: Ammonium Molybdate

a) No colour change initially

b) On heating a WHITE ppt was formed. Cadmium Molybdate precipitation.

17: Sodium Biselenite

a) A MILKY WHITE ppt was formed

b) On heating no reaction took place. The white Cadmium selenite ppt was formed on heating


Cadmium is a transitional metal, but which unlike most metals of this family, it does not form many coloured ppt or complex solutions

It forms a white ppt with the Sodium Hydroxide (insoluble in xs) and a white ppt, with Ammonia, whih is soluble in xs. (Same as zinc). More tests are required to confirm cadmium at this stage.

The most unique tests are the yellow precipitate with the SULPHIDE, and the yellow ppt with the FERRO(III)CYANIDE. All other ppt are white in colour, of which the most uncommon is that with the citrate. Further white ppt are obtained with the VANADATE, MOLYBDATE, TUNGSTATE, and SELENITE.

Unfortunately there are no other unique tests here.

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