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CHEMISTRY : Metal Cation Identification


PROJECT TITLE: ID of BISMUTH Cations by Precipitation Reactions
LAST UPDATE: 03-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 Bismuth ions in solution by simple precipitation reactions. Bismuth is fairly easy to distinguish since it forms quite a lot insoluble and unique precipitates. With the following set of tests it will be easy to confirm Bismuth without requiring complex procedures or sophisticated equipment.


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

Bi4+ 4X- (aq) + 4Na+ 4Y- (aq) ===> Bi4+ 4Y- (s) + 4Na+ 4X- (aq)
(s) Solid precipitate forming a colour change in soultion

One type of reaction is not enough, to confirm the presence of Bismuth, 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 Bismuth cations. However the verification of 4 or 5 such test will be enough to confirm Bismuth in an unknown sample.


When making a solution of Bismuth Nitrate, a white precipitate of a Bismuth insoluble complex was formed. The Bi 4+ cations are obtained as a clear solution by dropping hydrochloric acid to a hot white solution of insoluble Bismuth complex, until a clear solution is formed. It is important not to add xs Hydrochloric acid, so that the reactions involved would be due the Bismuth ions only, and not also due the interference of the xs Hydrochloric acid.
In 10ml testtubes, 4mls of this clear Bismuth 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. If a precipitate was formed, an xs volume of the anion (eg Hydroxide) was added to determine wether the ppt is soluble in xs reagent.
The following compounds was mixed with the Bismuth salt of which 16 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 Y
09 Sodium Chloride
10 Potassium Bromide
11 Ammomium Iodide Y
12 Potassium Iodate Y
13 Ammonium Phosphate Y
14 Sodium TetraBorate (Borate)
15 Sodium Salicylate Y
16 Sodium Benzoate
17 Tannic Acid
18 Sodium Malate sol.
19 Sodium Methanoate sol.
20 Sodium Ethanoate Y
21 Sodium Citrate
22 Sodium Tartarate
23 Sodium Silicate
24 Potassium Ferro(II)Cyanide Y
25 Potassium Ferri(III)Cyanide
26 Sodium Vanadate
27 Potassium Permanganate
28 Potassium Dichromate
29 Sodium Tungstate
30 Ammonium Molybdate Y
31 Sodium BiSelenite Y
32 Potassium Thiocyanate Y


01: Sodium Hydroxide

a) A MILKY WHITE ppt was formed, which was insoluble in xs Hydroxide

b) On heating the PPT turned FAINT YELLOW (not always though!)

The insoluble white hydroxide turned to a dehydrated form of hydroxide on heating.

02: Ammonium Hydroxide

a) A MILKY WHITE ppt was formed, insoluble in xs Ammonia.

b) This hydroxide didn't turned yellow on heating !?

The Hydroxide is precipitated, but no idea why id didn't turned yellow like the hydroxide formed with NaOH.

03: Sodium Carbonate

a) A MILKY WHITE ppt was formed.

b) no further reaction on heating or standing. White Bismuth Carbonate was precipitated. However it may be that the carbonate reacted with the xs Hydrochloric Acid forming a salt, Carbon Dioxide and water, where the latter reacted and hydrolysed the Bismuth ions to the insoluble white complex, as when water is added. This could be the case because when the ppt was dried and heated over a flame, it didn't turned to the Oxide, which has a characteristic yellow/orange colour, but remained white. Bismuth is very difficult to obtain precipitation reactions.

04: Sodium ThioSulphate

a) Initially, a YELLOW precipitate/solution was formed

b) On heating an intense DARK BROWN ppt was formed which turned into BLACK GREY ppt after some more heating. Bad smell of Sulphur (or Sulphur Dioxide) was given off. The Yellow precipitate was initially caused by the xs HCl in the Bismuth solution as this was tested using HCl + Sodium Thiosulphate only as a control. However, on heating this control, it failed to form a black ppt as Bismuth solution did. This could be due the formation of Bismuth Sulphide (which is black) or Bismuth ThioSulphate.

05: Sodium (Metabi)Sulphite

a) A WHITE PPT was given after about 5-10 seconds. This took place with the Sulphite but not with the MetabiSulphite. Sulphur dioxide was evolved

b) No further rection on heating or standing, apart from a more pungent smell of Sulphor Dioxide. White and insoluble Bismuth Sulphite was precipitated. Some of the sulphur from the sulphite were oxidized to SO2 and given off.

06: Sodium Sulphide

a) A DARK BROWN/BLACK ppt ws immediately formed. On filtering, a yellow/green solution was obtained, and the black ppt remained as residue.

b) On heating strongly for a quite long time, the ppt surprising seem to disappear leaving a whitish cloudy solution. Some of the grey ppt floated on the surface, and Chlorine was given off. Bismuth Sulphide was precipitated as a black/brown solid. Chlorine from HCl was evolved, thus an oxidising agent (removal of H from HCl) is present either due to the sulphide or the Bismuth compound.

07: Sodium Fluoride

a) No ppt was formed until xs Fluoride was added where a WHITE ppt was formed. Could be a false result!

b) No further reaction. Not so clear but the Bismuth Fluoride could have been precipitated.

08: Ammonium Iodide

a) Upon adding the iodide crystals to the Bismuth (+ HCl sol.) a rapid colour change to a DARK ORANGE / RED SOLUTION took place. The color changed is confirmed to have occured due the Bismuth ions, not due the xs HCl in solution. This was done by making a control, i.e mixing HCl with Iodide sol where there was no col change until some Bismuth sol was pipetted in. Some black deposits started to form

b) On heating or standing, more solid black deposits sank to the bottom of the tube. On adding more iodide, this dissolved immediately (confirming that it is solid Iodine)

The black powder was filtered, dried and heatin on a watch glass. First, black purple fumes were given off, which is the result of Iodine sublimation. When the fumes stoped evoling, a small amount of black powder turned to yellow/red on further heating, which propably is a mixture between the red Bismuth metal itself, and its orange yellow oxide. The filtered black powder was not attacked by nitric Acid. The intense orange/yellow dye, reacted vigourously with acids and NaOH as follows:

NaOH: A white ppt started to form which dissolved on shacking back to the orange dye. On adding xs NaOH, the white ppt then remained. HCL: The dense orange dye cleared to a light orange/yellow solution

H2S04: Effervesence took place immeiately as the first drop of Acid was dropped. On few drops more of acid a purple vapour was evolved, a black ppt was formed, and a faint yellow solution remained. HNO3: Effervesence took place immediately, and black ppt in an orange brown solution resulted. The black powder deposited, is a mixture of Iodine and Bismuth, since of the red/yellow powder left on heating in a naked flame. The intense orange solution could be a complex formed between Bismuth, Iodine, xs HCl and water.

09: Potassium Iodate

a) This gave a WHITE ppt

b) On heating it turned slightly yellow

Either bismuth Iodate, or the hydrolysis of bismuth ions to subhydroxide on addition of the aques solution. Not very indicative test. This is because on heating some of the hydroxide could have been converted to the yellow bismuth oxide?!

10: Potassium Phosphate

a) A MILKY WHITE ppt was formed

b) No reaction on heating/standing

White Bismuth Phosphate precipitation.

11: Sodium Salicylate

a) This gave a very curios result. On adding the Sodium salycylate, no reaction took place apart from some white undissolved solid.

b) On heating, the solution first turned BROWN, then on further heating it turned to BLOOD RED. A characteristic organic smell was given off, and on cooling the solution would turn to BROWN/BLACK. The initial formation of white ppt is due the formation of unsoluble Salysilic Acid, displaced by the xs HCl, which is insoluble in cold water. A further reaction took place on heating resulting this red solutiton(+Smell). As a control, this was repeated with HCl only and with Nitric Acid only, and the result was different, as HCl produced a greenish solution, whereas the Nitrate produced a brown/yellow solution.

12: Sodium Ethanoate (Acetate)

a) A FAINT WHITE ppt was obtained on adding xs Ethanoate.

b) No intensification on heating or standing. This test is regarded negative, as very few ppt was obtained.

13: Sodium Silicate

a) A MILKY WHITE ppt was formed on shaking vigorously

b) No effect on heating or standing, other then speeding up the reaction. Bismuth Silicate was precipitated as a white solid.

14: Potassium Ferro(II)Cyanide

a) Initially a BRIGHT YELLOW ppt was formed which was insoluble in xs.

b) On heating (or standing for a long time), the yellow ppt turns to a GREEN/LIME ppt. On letting to settle or filter, a YELLOW/LIGHT green ppt was obtained, and the solution filtered was BLUE/GREEN. When the ppt was dried it turned to a green blue solid, and in 3 seperate testubes it was added with few drops of Conc Hydrochloric Acid, Sulphuric Acid and Nitric Acid. With Hydrochloric Acid, a dark green solution was obtained, with Sulphuric Acid a blue solution (+ some bright blue ppt) was obtained, whereas with Nitric Acid, an olive green ppt/solution was formed. Potassium Ferro(II)Cyanide with HCl alone gave a green solution with no ppt.

(15: Sodium Vanadate)

a) A YELLOW SOLUTION + RED POWDER/PPT was formed, but this was due the xs Hcl, as using HCL only produce the same colur change. This test is regarded negative, since the reaction is due only to the xs HCl in the Bismuth solution.

(16: Potassium Permanganate)

a) Some BLACK ppt was obtained.

b) Heating may have increased the rate of reatcion. This test is regarded negative, since the reaction is due only to the xs Hcl in the Bismuth solution.

(17: Sodium Tungstate)

a) A CREAM/WHITE ppt was formed. Again HCl alone also gave such a precipitate, hence this reaction is due the HCl not Bismuth ions. This is yet another false test, since the reaction took place due the xs HCl in the Bismuth solution, since HCl alone produced such a similar reaction.

18: Ammonium Molybdate

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

b) No further reactions

White Bismuth Molybdate was precipitated.

19: Sodium Biselenite

a) A THICK WHITE ppt was formed, insoluble in xs

b) No furtehr reaction on heating or standing

The insoluble Bistmuth Selenite was formed.

20: Potassium Thiocyanate

a) This provided a nice reaction, since on adding the aqueous thiocyanate, a YELLOW (sometimes in SHADES OF ORANGE too) solution was formed (no white ppt due to water)

b) On heating strongly, the solution turned to OLIVE GREEN SOLUTION c) Then on standing, BLACK-GREEN/GREY (metalic?) powder was being deposited slowly and after 1 day, the solution returned to the original yellow colour. This is an excellent bismuth test, but sometimes the white ppt is formed, in the yellow solution. Adding the thiocyante crystals to a bismuth solution is ideal to eliminate white ppt due to xs water. In my opinion the black-gray deposit is solid Bismuth metal, as it has a shiny tinge on application of light (somehow rather difficult to notice.)


The White Hydroxide of Bismuth, which is insoluble in xs NaOH, (sometimes?) bears the property of turning faint yellow on heating (in solution), and remains yellow on cooling. The white ppt with Ammonia is also insol in xs.

The Bismuth cation, is detected and confirmed primarily with the complex reaction which takes place with the Ferro(II)Cyanide and IODIDE. Another unique test is the yellow solution with THIOCYANTE, which turns green on heating strongly. The black brown Sulphide, and the black ppt with the Thiosulphate are also good confirmatory tests, although not unique.

Finally, Bismuth oxide has a characteristic yellow/orange colour, which can be obtained by burning Bismuth Hydroxide/Carbonate and watch for a such color change.

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