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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of ALUMINIUM Cations by Precipitation Reactions
LAST UPDATE: 31-Jul-98, 30-May-99
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 aluminum ions in solution by simple precipitation reactions. Aluminium is not very easy to distinguish since most of its compounds are soluble, and those which are not are white. With the following set of tests it will be easy to confirm Aluminium without requiring complex procedures or sophisticated equipment.


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

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


In 10ml testtubes, 4mls of Aluminium 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
13 Ammonium Phosphate Y
14 Sodium TetraBorate (Borate) Y
15 Sodium Salicylate
16 Sodium Benzoate Y
17 Tannic Acid
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
26 Sodium Vanadate Y
27 Potassium Permanganate
28 Potassium Dichromate
29 Sodium Tungstate Y
30 Ammonium Molybdate
31 Sodium BiSelenite Y
32 Potassium Thiocyanate


01: Sodium Hydroxide

a) A MILKY WHITE PPT was formed

b) On xs the white ppt DISSOLVED forming again a clear solution. c) No reaction on heating

White insoluble Al(OH)3 was formed. On xs Hydroxide, the soluble Sodium Aluminate complx was formed, Na [Al(OH)4] - (hence the clear solution.)

02: Ammonium Hydroxide

a) A MILKY WHITE PPT was formed

b) The white precipitate DID NOT DISSOLVED on adding exess ammonia solution. c) No further reaction on heating

Formation of the insol White Aluminium Hydroxide

03: Sodium Carbonate

a) A WHITE PPT was immediately formed (n.b. not intense white)

b) No reaction on heating or exess

Insoluble white Aluminium Carbonate was formed.

04: Sodium Thiosulphate

a) This gave a little ammount of WHITE PPT, which intensified on standing after 5 minutes.

b) No reaction on exess c) When heated, a thicker white ppt was formed with the release of gas having a pungent smell of bad-eggs

On heating the reaction was accelerated giving a thicker white ppt, propably of Aluminium Thiosulphate. The reaction also produced H2S, detected from its charecteristic smell.

05: Sodium Sulphite (or Sodium Metabisulphite)

a) A FAINT WHITE ppt was formed.

b) No further reaction on heating or standing. Solid Aluminium Sulphite was formed. No ppt resulted with the Metabisulphite

06: Sodium Sulphide

a) A MILKY WHITE precipitate was immediately formed. On adding many xs sulphide, the ppt did dissolve (leaving minute grey solid traces)

b) No further reaction on heating. Precipitation of the White Aluminium Sulphide, which dissolves in very xs sulphide to a soluble complex ion.

07: Ammonium Phosphate

a) A CLOUDY WHITE PPT was immediately formed

b) No reaction on heating or exess

Insoluble white Aluminium Phosphate was formed.

08: Sodium TetraBorate

a) A GELATINOUS NEARLY TRANSPERENT ppt is formed. Initially noted by the stationary bubbles in solution. On xs, the ppt comes slightly whiter

b) No action on heating apart perhaps getting slightly WHITER in color. Aluminium TetraBorate is precipitated.

09: Sodium Benzoate

a) An INTENSE MILKY WHITE ppt was formed.

b) No reaction on heating or exess

A thick white ppt of the extremely insoluble Aluminium Benzoate.

10: Sodium Methanoate (Formate)

a) No immediate visible reaction took place

b) On heating a faint white ppt was formed. Insoluble Aluminium Formate was formed

11: Sodium Ethanoate (Acetate)

a) A FAINT GELATINOUS WHITE PPT was formed after a few minutes (about 5 mins.)

b) No reaction on exess or heating. Insoluble Aluminium Acetate was formed

12: Sodium Silicate

a) A FAINT WHITE ppt was formed

b) A WHITE PPT was intensified on heating

White, insoluble Aluminium Silicate was formed.

13: Potassium Ferro(II)Cyanide

a) No immediate colour change when cold, but a faint blue sol may develop...

b) On heating >AND ADDING 1 DROP of SULPHURIC ACID< a deep blue solution is obtained which precipitates on standing for a long time. If some NaOH is added the the blue solution becomes colourless, and on adding again some acid, the blue colour reforms. Complex reactions taking place here. This reaction takes place only in acidic conditions, otherwise the change is usually not detected.

14: Sodium Vanadate


b) On heating the reaction was accelerated and a YELLOW/ORANGE resulted. On filtering this actually showed to consist of a YELLOW ppt in an ORANGE solution. c) No reaction on XS

A complex reaction, perhaps forming a vanadium salt (orange) and Vanadium Aluminate the yellow ppt.

15: Sodium Tungstate

a) A MILKY WHITE precipitate was immediately formed.

b) No further reaction on heating, or adding xs Tungstate. Aluminium Tungstate was formed which is a white insoluble solid.

16: Sodium Biselenite

a) A WHITE PPT was immediately formed

b) No reaction on heating or exess

Insoluble white Aluminium Selenite was formed.


First, a solution of aluminate from the aluminium ions was prepared as follows. A stong solution of an Aluminium salt was prepared in a 50ml beaker. Slowly, (preferably dropwise) conc. Sodium Hydroxide was added until a thick white ppt was formed. At this stage Aluminium Hydroxide was produced. More NaOH was added dropwise until the solution is just gelatinous/faint white ppt. At this stage no more NaOH was added, and the solution was filtered off from the Aluminium Hydroxide left. The filtrate was the aluminate solution (with some unreacted Aluminium Sulphate, which will not interfer with the following tests.
Test for aluminate ions:
About 4mls of the following solutions was prepared and added 2mls - 4mls Sodium Aluminate solution to each.
No. . REAGENT NAME Precipitate Colour
01 Iron (III) Sulphate: Brown ppt.
02 Zinc Chloride: Thick White ppt.
03 Copper Sulphate: Blue sky ppt.
04 Manganous Sulphate: White/cream ppt.
05 Cobalt Chloride: Wine/lilac ppt.
06 Potassium Chromic Sulphate: Blue ppt.
07 Ammonium Nickel Sulphate: Lime green ppt.
08 Tin (II) Chloride: White ppt.
N.B. no physical change on heating or adding exess aluminate solution These colurs have same characteristics as precipitation with Hydroxide solution, but few of the above can clearly show that the colour change was responsible due to the reaction with the Aluminate ions not the Hydroxide, (or just he hydroxide). The precipitation of the Aluminate and hydroxide of the corresponding cations is compared below.


Heating the blue ppt formed by Aluminate or Hydroxide < Aluminate: No colour change results. Hydroxide: The blue ppt turns blackHydroxide oxidised to black Copper Oxide


Heating the lilac ppt formed by Aluminate or Hydroxide Aluminate: No colour change results. Hydroxide: The lilac ppt turns blue - Hydroxide oxidised to blue cobalt Oxide


Add NaOH or Aluminate in exess Aluminate: Blue ppt remains on exess; no reaction takes place Hydroxide: Blue ppt turns to a dark green solution due to a complex chrome ion.


Leave to stand for few minutes Aluminate: No visible change in colour Hydroxide: On standing the white hydroxide darkens to a cream/brown colour in area which are in contact with air, thus at the surface and top part of the tube.


This is a very delicate and sensitive test, which is of great use to confirm the Aluminium ions in solution, but must be used carefully as one can interpret false positive results.

When an aluminium solution is mixed with Potassium Ferro(II) Cyanide (PF2C) in an acidified solution, an unique, blue solution or ppt is formed. Confirmatory and sensitivty tests were performed as follows.

Confirmatory tests

This is a simple comparitive and quantitative test to demostrate that the blue colour formed is due the Aluminium ions.
  1. 30mls solution of PF2C is prepared in a 50ml beaker.
  2. 3mls of this solution is pipetted in 5 clean, dry test tubes.
  3. The following regents + analytes are added to the test tubes as follows:
    1. TestTube 1: PF2C only (Negative control)
    2. TestTube 2: PF2C + 1 drop of conc Sulphuric Acid
    3. TestTube 3: PF2C + 1 drop of conc Sulphuric Acid + Sodium Sulphate
    4. TestTube 4: PF2C + + (no acid) + + Aluminium Sulphate
    5. TestTube 5: PF2C + 1 drop of conc Sulphuric Acid + Aluminium Sulphate
  4. Any colour change within 5 mins is noted.
  5. The tubes are heated together in a boiling water bath for 3mins.
The following results were obtained in the 5 tubes after heating:
  1. TestTube 1: Colourless pale yellow solution (no reaction)
  2. TestTube 2: Lime Green solution + green ppt (after cooling)
  3. TestTube 3: Lime Green solution + green ppt (after cooling)
  4. TestTube 4: Intense blue solution, with no or little ppt
  5. TestTube 5: Blue solution, which on standing it seetles as a light blue ppt.

A faint blue solution was being formed before heating in tube 5.

From the results it seems that in the presence of Al3+, a blue solution is formed, which in acid seems to be accelerated, or even takes a different complex reaction. The deep blue sol in 4 is quite different from the blue ppt in 5.

Interestingly, adding NaOH to the blue sol, makes it turn colourless!!!
Aluminium sulphate solution reacts with Potassium Ferro(II)Cyanide in an acidified solution forming a blue complex solution or blue ppt on heating.

Sensitivity tests

The test is performed with gradualy decreasing concentration of Aluminium solution to determine wether a visible reaction takes place in very small concentrations
  1. In one testtube 4mls of aluminium solution (about 0.5g with 4mls) was prepared. In another 4 test tubes, 2ml of water were pipetted.
  2. 2mls of the aluminium solutiom were transferred into one of the testtubes with water, and this is repeated for the rest 3 test tubes, hence performing a binary serial dilution. From the last test tube, 2mls of solution were discarded.
  3. Therefore, the dilutions formed were:
    1. X (neat)
    2. X/2
    3. X/4
    4. X/8
    5. X/16 (X = initial concentration)
  4. To each test tube, 3ml of Potassium Ferro(II)Cyanide solution and 1 drop of conc H2S04 were transferred.
  5. The tubes are heated together in a boiling water bath for 3mins, and the colour changed was noted after cooling down (for ppt to settle)
The following results were obtained:
  1. X : Medium blue ppt in clear sol
  2. X/2 : Same colour as above but less ppt
  3. X/4 : Slightly darker ppt and faint blue solution
  4. X/8 : Little dark blue ppt and blue solution
  5. X/16: as above but solution is deeper blue.
This shows that a visible reaction takes place also in small Al3+ concentrations, hence the test is quite well sensitive.

It is also obvious that with a higher concentrations of Al3+, the reaction is that of forming a sea blue ppt, while little conc, a deep blue solution, instead of ppt is formed. Concentrations in betwwen results in both reactions.

Looking it another way round, one can say that on excess PF2C, a deep blue complex solution is formed. However on adding xs PF2C on the sea blue ppt, this did not dissolved into a deep blue sol.


The first indicative test for Al3+ is the formation of a white ppt with NaOH and Ammonia, with the dissolving on exess NaOH, but not with exess ammonia. Then, the blue ppt or deep blue solution (depending conc of Al3+) with acidified Potassium Ferro(II)Cyanide, or the Yellow-Orange precipitate obtained from the Vanadate ion are valuable unique tests to confirm Al, since Tin, Zinc, Lead and Bismuth have different or no reactions with these two. For instance LEAD and ZINC forms a white ppt with the ferroCyanide. Additionaly, the intense white ppt with Sodium Benzoate, and a the white ppt with the ThioSulphate are enough to further confirm the Aluminium cation.

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