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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of MERCURY Cations by Precipitation Reactions
LAST UPDATE: 13-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 Mercury ions in solution by simple precipitation reactions. Mercury is very easy to distinguish since it has many of its compounds in the solid form, hence precipitate from solution, which they dissolve in xs to a colourless solution. With the following set of tests it will be easy to confirm Mercury without requiring complex procedures or sophisticated equipment. It should ne noted that Mercury and it's compounds are very poisoinous substances and great care should be taken in performing such tests.


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

Hg2+ X2- (aq) + 2 Na+ Y- (aq) ===> Hg2+ Y2- (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 Mercury, since other metal salts can give the same results. The verification of 4 or 5 such tests will be enough to confirm Mercury in an unknown sample.


In 10ml testtubes, 4mls of Mercury solution was placed. Note that when the white sulphate is added water, the crystals turns yellow. Some of this remains undissolved even after heating, and on standing, the solution starts tp precipitates. To this clear sol. of mercury sulphate, about 2mls 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 Mercury salt of which 18 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 Y
13 Ammonium Phosphate
14 Sodium TetraBorate (Borate)
15 Sodium Salicylate Y
16 Sodium Benzoate
17 Tannic Acid Y
18 Sodium Malate sol.
19 Sodium Methanoate sol. Y
20 Sodium Ethanoate
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
30 Ammonium Molybdate Y
31 Sodium BiSelenite Y
32 Potassium Thiocyanate


01: Sodium Hydroxide

a) An ORANGE/YELLOW PPT was formed, insoluble in xs.

b) On heating colour change slightly to more YELLOWISH color. The insoluble Mercury Oxide was precipitated.

02: Ammonium Hydroxide

a) Unlike the sodium hydroxide, this gave a plain WHITE ppt. insoluble in xs

b) No further reaction on heating or standing

The hydroxide or a complex solid was preipitated by Ammonia

03: Sodium Carbonate

a) An ORANGE/LIGHT BROWN ppt was formed which turned YELLOW on adding more carbonate

b) Heating the dense white ppt gave no particular change, else becoming more yellowish in color

Precipitation of Mercury Carbonate, which decomposed to the yellow Oxide on heating.

04: Sodium ThioSulphate

a) An ORANGE/BROWN ppt was given, which darkened to BROWN ppt on xs

b) On heating, a GREY, then BLACK ppt resulted, with the evolving of sulphur smell. The Mercury ThioSulphate could have been ppt here, but seems to be a complex reaction involved here. Sulphur seems to be liberated.

05: Sodium Sulphite (or Sodium Metabisulphite)

a) No particular reaction initially (on xs solution turns turbid)

b) When heated, a MID-GREY ppt was formed. Precipitation of the insoluble Mercury Sulphite could have took place.

06: Sodium Sulphide

a) A BROWN, DARK BROWN, and finally BLACK BROWN ppt was formed on adding the Sulphide. On adding in xs, the black ppt dissolved and a clear green solution was left (contamination?!)

b) No further reactions on heating, or standing out. Mercury Sulphide was ppted at once, which dissolved in xs. In many books Mercury Sulphide is described as a bright red solid ?!.

07: Sodium Fluoride

a) A BRIGHT YELLOW ppt was formed which dissolved to a colourless solution on xs fluoride.

b) No reaction on heating. One of the rare reactions with a fluoride - a Fluoride ppt which dissolves in xs.

08: Potassium Iodide

a) A WONDERFUL PINK/PEACH-RED (SCARLET) PPT was immediately formed. On xs this dissolved to a colourless solution

b) No further reaction on heating. Scarlet coloured Mercury Iodide was precipitated out, which dissolved in xs forming the HgI4- colourless ions. An excellnt test for Mercury 2+ ions

09: Potassium Iodate

a) A MILKY WHITE ppt soluble in xs was resulted

b) No reaction on heating or standing. Precipitation of white Mercury Iodate.

10: Sodium Salicylate

a) No reaction initially

b) THICK MILKY white ppt only on heating. Mercury Salicylate was precipitated out on heating.

11: Tannic Acid

a) No reaction initially.

b) A BAIGE/LIGHT BROWN ppt was formed on heating

The Mercury Tannate was precipitated out only on heating.

12: Sodium Methanoate

a) No reaction initially

b) On heating, a WHITE ppt was formed, which turned to light grey on further heating

Propably precipitation of Mercury Methanoate.

13: Sodium Silicate

a) A BRIGHT YELLOW ppt was formed, which then dissolved on xs on adding xs silicate.

b) No further reaction on heating or standing

Mercury Silicate was precipitated, which formed a soluble com-lex ion on xs silicate. A rare occassion in which a solid Silicate dissolves in xs.

14: Potassium Ferro(II)Cyanide

a) A WHITE ppt was formed.

b) On heating, a BLUE/CYAN ppt/sol was obtained. It is not clear if solid or dissolved substance. Against light it is opaque, but when filtered no residue was left. Suspension of very fine particles perhaps?

Very complex reaction, and similar to Zirconium.

15: Potassium Ferri(III)Cyanide

a) A DIRTY YELLOW ppt was formed, which on xs did dissolve to a CLEAR YELLOW solution (or dissolved to a colourless solution with the xs Ferro(III)cyanide forming the yellow solution)

b) On heating the yellow ppt, a DARK GREEN (as grass) solution was formed. Heating the clear yellow solution resulted in a FAIRLY DARK BROWN solution. Another precipitation reaction which dissolves in xs. This time quite different from Zirconium.

16: Sodium Vanadate

a) A YELLOW solution was immediately formed. No ppt on xs

b) No particular change on heating apart from some orange deposits

Complex reaction, perhaps the Vanadium salt (yellow solution) was formed. Quite common that the vanadate forms a yellow solution.

17: Ammonium Molybdate

a) A CREAM/MILKY WHITE ppt resulted on heating

Formation of the solid Mercury Molybdate

18: Sodium Biselenite

a) A DENSE MILKY WHITE ppt was formed. On xs this ppt dissolved to a colourless solution.

b) No further reactions on heating

Another reaction in which the ppt formed dissolved in xs. This time white Mercury Selenite ppt dissolved to a colourless solution.


Mercury is a transitional metal, and forms many precipitates of which, compared with other metals, do dissolve in xs to form colourless solutions. However, strange enough, the ppts formed with Sodium Hydroxide and Ammonia solution do not dissolve in xs.

It forms a light yellow ppt with the Sodium Hydroxide, and a white ppt with Ammonium hydroxide, (both insoluble in xs) The Carbonate is Orange - Mustard in colour.

The most unique test for mercury (2+) ions is the scarlet ppt with the Iodide, soluble in xs. One can also join the bright yellow ppts formed with the Fluoride and the Silicate which both dissolves in xs to a colourless solution

Other precipitates that dissolve to a colourless solution in xs reagent are obtained with the Selenite (milky white -> colourless), Sulphide (Black/Brown -> clear green), Iodate (milky white -> colourless) and finally but not least the unique dirty yellow ppt obtained with Ferro(III)Cyanide, which dissolves to a colourless or clear yellow solution.

The Ferro(II)Cyanide, reaction is very similar to zircorium, hence the formation of a cream/pale yellow ppt which turns to cyan on heating. As can be seen, Mercury forms many distinct precipitates to be easily reckognized from.

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