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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of MAGNESIUM Cations by Precipitation Reactions
LAST UPDATE: 02-May-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 magnesium ions in solution by simple precipitation reactions. Magnesium 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 Magnesium without requiring complex procedures or sophisticated equipment.


As mentioned, the tests are simple precipitation reactions. A solution of a Magnesium salt (Magnesium 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 Magnesium insoluble compound.

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

One type of reaction is not enough, to confirm the presence of Magnesium, since other metal salts may 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 Magnesium cations. However the verification of 4 or 5 such test will be enough to confirm Magnesium in an unknown sample.


In 10ml testtubes, 4mls of Magnesium 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 Magnesium salt of which only 7 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
07 Sodium Sulphide Y
08 Sodium Fluoride
09 Sodium Chloride
10 Potassium Bromide
11 Ammomium Iodide
12 Potassium Iodate Y
13 Ammonium Phosphate
14 Sodium TetraBorate (Borate)
15 Sodium Salicylate
16 Sodium Benzoate
17 Tannic Acid
18 Sodium Malate sol.
19 Sodium Methanoate sol.
20 Sodium Ethanoate
21 Sodium Citrate
22 Sodium Tartarate Y
23 Sodium Silicate Y
24 Potassium Ferro(II)Cyanide
25 Potassium Ferri(III)Cyanide
26 Sodium Vanadate
27 Potassium Permanganate
28 Potassium Dichromate
29 Sodium Tungstate
30 Ammonium Molybdate
31 Sodium BiSelenite
32 Potassium Thiocyanate


01: Sodium Hydroxide

a) This gave a GELETINOUS WHITE PPT. (Z125, 013)insoluble in exess Hydroxide

b) No reaction on heating

Insoluble white Magnesium Hydroxide was formed

02: Ammonium Hydroxide

a) This gave a GELETINOUS WHITE PPT. insoluble in exess Ammonia

b) No reaction on heating. Insoluble white Magnesium Hydroxide was formed

03: Sodium Carbonate

a) A WHITE PPT was immediately formed

b) No reaction on heating or exess

Insoluble white Magnesium Carbonate was formed.

04: Sodium Sulphide

a) A FAINT WHITE / TRANSPERENT ppt was formed slowly, and after left standing for some minutes.

b) No visible change on heating or adding xs Sulphide. Magnesium Sulphide was precipitated as a white solid. The sulphide confirms itself as being an anion which precipitates most metals from their solution, after the carbonate and hydroxide.

05: Sodium Fluoride

a) A faint white ppt was formed, which did not intensified on standing.

b) No further reaction on heating. White Magnesium Fluoride was precipitated

06: Ammonium Phosphate

a) A CLOUDY WHITE PPT was immediately formed

b) No reaction on heating or exess

Insoluble white Magnesium Phosphate was formed. This had two curious properties. 1: The insoluble ppt. sank to the bottom layer of the test tube very fast; in about 5 seconds. 2: It had some explosive effect. When heated, it broke (exploded!) 2 test-tubes, and when using a harder glass tube, violent bumps and splashes resulted. However, on heating the solid alone, no explosive effect was detected

07: Sodium Silicate

a) A WHITE ppt was formed immediately.

b) No further reaction on heating or standing

White insoluble Magnesium Silicate was precipitated


The main confirmative tests for magnesium is the formation of a gelatinous white precipitate formed with Sodium Hydroxide and Ammonium Hydroxide, remaining undissolved on exess. The phosphate is of no help since there are many white insoluble phosphates such as Calcium, Zinc, and Aluminium.

0Thus Magnesium can be identified more from the lack of precipitates, rather from those few it forms.

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