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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of CERIUM Cations by Precipitation Reactions
LAST UPDATE: 18-Jul-98
VERSION HISTORY: 1.0, 1.1 ( Context updates)
V2.0 ( Text and formatting update - Sep-2009)


This is an account on how to detect Cerium ions in solution by simple precipitation reactions. Cerium is not so easy to distinguish but thanks to some unique precipitates, it becomes not so hard to confirm it presence of it's ions in solution. Unfortunately, cerium forms a pale yellow ppt on heating or standing, and hence the following tests are prone to several false positive results, since of this ppt. With the following set of tests it will be easy to confirm Cerium without requiring complex procedures or sophisticated equipment.


As mentioned, the tests are simple precipitation reactions. A solution of a Cerium salt (Ammonium Cerium (IV) 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 Cerium insoluble compound or formation of a complex since Cerium 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 Cerium, since other metal salts can give the same results. The verification of 4 or 5 such tests will be enough to confirm Cerium in an unknown sample.


In 10ml testtubes, 4mls of Cerium 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 Cerium salt of which 22 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)
15 Sodium Salicylate Y
16 Sodium Benzoate
17 Tannic Acid Y
18 Sodium Malate sol. Y
19 Sodium Methanoate sol.
20 Sodium Ethanoate
21 Sodium Citrate Y
22 Sodium Tartarate Y
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 Y
29 Sodium Tungstate Y
30 Ammonium Molybdate
31 Sodium BiSelenite Y
32 Potassium Thiocyanate

Reactions marked with an (*) are propably false precipitate formers due the formation of a Pale Yellow ppt formed on heating the solution of Ammonium cerium sulphate, or leaving it to stand, without the addition of any reagent! It is for example very surprizing that a ppt was resulted with the sulphate anion, since the soluble cerium salt used in this identification test is in fact Cerium sulphate (which is soluble.) Hence these reactions are not reported in the results


01: Sodium Hydroxide

a) A GELATINOUS WHITE/CREAM ppt insoluble in xs was formed

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

02: Ammonium Hydroxide

a) A GELATINOUS WHITE/CREAM ppt insoluble in xs was formed

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

03: Sodium Carbonate

a) A PALE YELLOW/GELATINOUS ppt was formed.

b) No change on heating. Precipitation of Cerium Carbonate. The colour of the carbonate and hydroxide is identical and this leads in the suspect that these are in fact not actuall ppt but a false positive result! On the other hand, all three ragents (sodium hydroxide, ammonium hydroxide, and sodium carbonate) could have led to the formation of the pale yellow Cerium Oxide.

04: Sodium Thiosulphate

a) A YELLOW/CEAM ppt was formed which did dissolved to a colourless solution in xs.

b) On heating (or standing) a FAINT WHITE ppt was formed. Complex reactions are involved here.

05: Sodium Sulphite (or Sodium Metabisulphite)

a) A YELLOW/LIGHT ORANGE ppt was formed, which dissolved on shaking or staning. On adding fyrther sulphite solution, a yellow ppt was resulted which did not dissolve

b) No important changes on heating. Cerium Sulphite is perhaps the final yellow insoluble solid, prior to some soluble complex formation.

06: Sodium Sulphide

a) A BAIGE (TEA+MILK) PPT was immediately formed.

b) No further reactions on heating, or standing out. Cerium Sulphide was ppted at once. The absence of that pale yellow colour and the straight forward precipitation makes this a good test for detecting cerium

07: Sodium Fluoride

a) A WHITE ppt was formed on adding sufficient fluoride

b) Was not heated due the formation of that pale yellow ppt

Propably the fluoride was precipitated?!

08: Potassium Iodide

a) An ORANGE/YELLOW ppt was initially formed. This is noticed to consists of a YELLOW/ORANGE solution and a yellow ppt. On standing further or heating the solution turns to Iodine brown and most of the ppt disappears. Complex reaction(s) with the liberation of Iodine.

09: Ammonium Phosphate

a) A GELATINOUS WHITE ppt was formed.

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

10: Sodium Salicylate

a) A DIRTY GREEN/BROWN ppt was formed

b) On heating, the precipitate turned more GREENISH, but not a very different colour change was involved. A good straight forward precipitation, whith an unique result for Cerium. The Cerium Salicylate has been precipitated

11: Tannic Acid

a) A BROWN/ORANGE ppt was formed immediately, which dissolves in xs forming a dark brown solution.

b) On heating the brown solution is formed rapidly

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

12: Sodium Malate/Malic Acid

a) The pale clear yellow solution of Ammonium Cerium(IV) Sulphate darkens slightly to a yellow solution on adding the malate ions.

b) As the solution is slightly heated, a MILKY WHITE/CREAM ppt was imemdiately formed. This is considered as a positive result, and actually the precipitation of the Cerium Malate(?) is taught to have occurred. This is because this ppt is different from the false pale yellow ppt obtained on just heating the cerium salt. It was different particularly in it's milky colour, and also the clear solution turned immedaitely to a milky colour after only 2-3 seconds the tube was placed in the water bath.

13: Sodium Citrate

a) This gave slightly different results on repeating, depending the concentration of the cerium salt. Generally, a yellow/orange solution is formed. If not in xs citrate, a ppt was even formed (but this ppt is not always given).

b) However, in each case, on heating (and in xs citrate) an ORANGE solution is always resulted. This makes a very good test to identify cerium since an orange solution was never obtained by the citrate anion with other metals.

14: Sodium Tartarate

a) The colour intensified to a yellow/orange solution, but no ppt.

b) On heating a pale yellow ppt was formed. Not a very indicative result but since it is one of the few reactions brought forward by the tartarate, the slight change was written.

15: Sodium Silicate


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

16: Potassium Ferro(II)Cyanide

a) A BRIGHT YELLOW ppt was observed to form initially. On xs, this was partially dissolved to a YELLOW/LIME GREEN solution. On standing, the few ppt that was present, fell to the bottom, and it was observed that a WHITE ppt in a LIME GREEN/YELLOW solution was resulted.

b) On heating strongly for some minutes, white ppt turned to a PALE BLUE (CELESTE) colour, on further standing (2 hours) the solution was completely LIME GREEN in both the heated and unheated tubes. Complex reactions, with several intermediates here.

17: Potassium Ferri(III)Cyanide

a) A YELLOW/LIGHT BROWN ppt was initially obtained.

b) On heating a light GREY/GREEN ppt + a DARK BROWN SOLUTION was formed. This is another great test to detect cerium ions. Again complex and more than one reactions are possible to take place here.

18: Sodium Vanadate

a) A yellow solution without any precipitation was formed.

b) On heating no reaction took place. Very difficult to say, if a reaction had actually took place.

19: Potassium Dichromate

a) A YOLK YELLOW ppt and an ORANGE SOLUTION (perhaps xs dichromate) was formed.

b) No particular change on heating apart from increasing the rate of precipitation. Yellow Cerium Dichromate was precipitated

20: Sodium Tungstate

a) A MILKY LEMON YELLOW ppt was formed. (On filtering the solution was clear yellow not colourless (perhaps some unreacted cerium was left)

b) No further changes on heating. Since being milky and intense in colour this reaction is not considerd as that pale yellow ppt regarded as a false positive. Hence it is propable that the Cerium Tungstate was precipitated.

21: Sodium BiSelenite

a) A YOLK YELLOW ppt was immediately formed.

b) No reaction on heating. Cerium Biselenite precipitation.

22: Potassium ThioCyanate

a) A MILKY WHITE / BAIGE ppt was formed

b) On heating many of the ppt seemed to have dissolved, but some white ppt was still left, even in xs rhiocyanate. Solution was colourless (slight pink)

The precipitation of Cerium Thiocyanate. (*) The following gave that pale yellow ppt which is considered as a false ppt due to, hydrolysis, oxidation, or any complex reaction which is taking place. SULPHATE - CHLORIDE - BROMIDE - IODATE - BORATE - PERMANGANATE(?) BENZOATE - METHANOATE - (carbonate/hydroxude??!)


Cerium first metal of the Lactanide series, and takes part in quite complex reactions, and some colured ppts. In general white or pale yellow ppt are involved

For instance, it forms a PALE YELLOW/WHITE ppt with the Sodium Hydroxide, Ammonia solution and Sodium Carbonate, all insoluble in xs. More tests to confirm Cerium at this stage are needed.

The most unique tests are the ORANGE solution with xs (+Heat) with Sodium Citrate, GREEN BROWN ppt wth Sodium Salicylate, the BROWN ppt sol in xs with Tannic Acid, and the coloured ppts with Ferro(II)Cyanide (LIME GREEN/YELLOW SOL + WHITE/BLUE ppt on heating) and with the Ferro(III)Cyanide (DARK BROWN SOLUTION + GREY GREEN PPT on heating).

One can also mention the BAIGE ppt obtained with Sodium Sulphide and a WHITE/BAIGE ppt with the Thiocyanate, the YELLOW Selenite and Dichromate (the latter also with lead though), the LEMON YELLOW ppt with the Tungstate and finally the Iodine sol obtained with Potassium Iodide.

As already enphazied one must be careful not to misinterpret the pale yellow ppt which is obtained on heating or leave to stand a solution of cerium salt on its own (without adding a reagent).

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