Once bismuth is precipitated as bismuth hydroxide and the latter is thoroughly washed, we must bring Bi3+ in solution to perform specific recognition assays.
In order to do this the hydroxide is simply treated with dilute sulfuric acid (2N). It 'a simple acid-base neutralization reaction:
Bi(OH)3 + 3H+ Bi3+ + 3H2O
Let's have a look now to some specific identification assay:
1) Reaction with potassium iodide (KI)
To the solution containing Bi3+ is added with potassium iodide. Bismuth precipitates as bismuth iodide (bismuth triiodide), black. By adding an excess of I- it will form a soluble complex (tetraiodo bismuthate) which colors the solution in a coloration range that goes from yellow-orange to violet.
2) reaction with sodium stannite
Although this essay is very simple it is also really accurate. We run the reaction directly on the precipitate of bismuth hydroxide Bi(OH)3.
The reagent is sodium stannite. It is prepared starting from SnCl2 by addition of Na(OH) dropwise. Initially tin(II) hydroxide forms, and is then added an excess of soda until the hydroxide is not solubilized again:
Sn2+ + 2OH- SnOH2
SnOH2 + 2OH- SnO22- + 2H2O
Preparing this reagent is, as you may have noticed, another good demonstration of the amphoteric nature of certains cations. Anyway, just adding one or two drops of a freshly prepared solution of stannite to our precipitate we should observe (where bismuth is present) an instant darkening due to the reduction to elementary bismuth Bi°.
2BI(OH)3 + 3SnO22- 2Bi° + 3SnO32- + 3H2O
3) Reaction with Bismuthiol
The reagent, called bismuthiol is capable of chelating Bi+3 via the electronic lone pairs of its available sulfur atoms (those sulfur atoms that are not part of rings). As a result of the formation of this complex system, it is possible to observe a red spot in solution. The reagent is extremely sensitive.