It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods.
The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes.
The 1st analytical group of cations consists of ions which form insoluble chlorides.
While AgCl dissolves in ammonia (due to the formation of the complex ion [Ag(NH3)2]+), Hg2Cl2 gives a black precipitate consisting of a mixture of chloro-mercuric amide and elemental mercury.
Furthermore, AgCl is reduced to silver under light, which gives samples a violet colour.
The silver ammonia complex can react with bismuth ions and iodide to generate orange or brown Ag2BiI5 precipitate.
Its purpose is to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone.
If dilute acid is not used, the early precipitation of 4th group cations (if present in solution) may occur, thus leading to misleading results.
Usually it is done by passing hydrogen sulfide over the test tube for detection of 1st group cations.
Ammonium chloride is added to ensure low concentration of hydroxide ions.
The 4th analytical group of cations includes ions that precipitate as sulfides at pH 9.
The reagent used is ammonium sulfide or Na2S 0.1 M added to the ammonia/ammonium chloride solution used to detect group 3 cations.
Magnesium can also be distinguished from other cations in this group by adding sodium hydroxide to drive the pH to 11 or higher, which selectively precipitates Mg(OH)2.
After addition of the acid, chlorides, bromides and iodides will form precipitates with silver nitrate.
When the salt is heated with K2Cr2O7 and concentrated H2SO4, red vapours of chromyl chloride (CrO2Cl2) are produced.
Nitrate ion is confirmed by adding an aqueous solution of the salt to FeSO4 and pouring concentrated H2SO4 slowly along the sides of the test tube, which produces a brown ring around the walls of the tube, at the junction of the two liquids caused by the formation of Fe(NO)2+.
[4] Upon treatment with concentrated sulfuric acid, oxalates yield colourless CO2 and CO gases.
These gases burn with a bluish flame and turn lime water milky.
Modern techniques such as atomic absorption spectroscopy and ICP-MS are able to quickly detect the presence and concentrations of elements using a very small amount of sample.
The test can distinguish between copper (Cu), iron (Fe), and calcium (Ca), zinc (Zn) or lead (Pb).
While this test is useful for telling these cations apart, it fails if other ions are present, because most metal carbonates are insoluble and will precipitate.
In addition, calcium, zinc, and lead ions all produce white precipitates with carbonate, making it difficult to distinguish between them.