| Identification
of metallic and non-metallic ions
Francine
Taylor-Campbell, Contributor
 |
| Students
watch and cheer on the artistes who were a part of the Star's 'No Violence in
Schools' tour at the Spanish Town High School on Thursday, December 7, 2006. -
Ricardo Makyn/Staff Photographer | Metallic
ions or cations are identified based on the colour and solubility of the hydroxides
formed from their reaction with aqueous ammonia and sodium hydroxide. Non-metallic
ions or anions are generally identified based on the evolution and identification
of gases produced when the compounds are heated or reacted with acids. The colour
and solubility of precipitates formed on reaction with barium ions and ammonia
(for the halide ions) are also used. Summary
of the reactions of the metallic ions Let's
summarise the reactions of the metallic ions: Pb2+,
Al3+, Ca2+,
Zn2+, Fe3+,
Fe2+, NH4+
and Cu2+ with
aqueous ammonia and sodium hydroxide. Fe2+
produces a dirty green precipitate, which is insoluble in excess aq NH3
and NaOH. Fe3+
produces a rusty brown precipitate, which is insoluble in excess aq NH3
and NaOH. Cu2+
forms a blue precipitate, which is insoluble in excess aq NaOH, but soluble
in excess aq NH3.
These
cations can clearly be identified based on their colours alone. Al3+
and Pb2+ form
white precipitates when reacted with aq NaOH and NH3
. In aq NaOH, the precipitates are soluble in excess, while in aq NH3,
they are insoluble in excess. As can be seen, Al3+
and Pb2+ give
the same results, so to differentiate between them, other reactions must be taken
into consideration. Pb2+
will form precipitates with I-, Cl- and SO42-
ions (PbI2 (yellow), PbCl2 (white),
and PbSO4 (white)) while Al3+
will not. Two
other cations form white precipitates. Zn2+
forms a white precipitate which is soluble both in excess aq NaOH and NH3.
Ca2+ forms
a white precipitate which is insoluble in excess NaOH, but does not form a precipitate
when treated with ammonia . The
NH4+ ion can
be identified based on the evolution of ammonia (NH3) gas
when warmed with aq. NaOH. The
identification of the anions is a lot more complicated. The ones we are concerned
with are CO32-,
SO42-,
SO32-,
NO3-
, Br-, Cl- and I-. One
way to do this is to heat the compound and identify the gas produced. CO32-
====== (heat) produces the gas CO2 NO3-
===== (heat) gives off O2 and/or NO2
SO32-
===== (heat) gives off SO2 These
gases can be identified by their characteristic tests: CO2
is weakly acidic and turns lime water (Ca(OH)2) milky. O2
is neutral and relights a glowing splint. NO2
is a red-brown/yellow-brown acidic gas. SO2
is an acidic gas which turns acidified potassium manganate (VII) solution from
purple to colourless and changes acidified potassium dichromate (VI) from orange
to green. NH
3 is an alkaline gas which forms dense, white fumes with hydrogen chloride gas.
Some
anions react with diluted and concentrated acids (if they are added to the solid).
CO32-
reacts with diluted and concentrated acid to release CO 2. SO
3 2- forms SO 2 gas on reaction with dil and concentrated acid. If
concentrated H2 SO4 acid is added
to a solid containing the halide ions (I- , Cl- , and Br- ), the following gases
can be formed: HI(g), which decomposes to I2;
HCl(g); and HBr, which decomposes to Br2.
The
halide ions, in solution, can be identified differently based on their reaction
with silver nitrate solution (AgNO3) and the solubility
of the precipitate formed in ammonia. Cl-
forms a white precipitate with AgNO3 solution, which
is soluble in aq NH3 . Br-
forms a cream precipitate with AgNO3, which is partially
soluble in aq NH3 . I-
forms a light-yellow precipitate which is insoluble in aq NH3
. Finally,
some anions form precipitates with a solution of Ba2+
ions. CO32-
forms a white precipitate, which dissolves on addition of acid to form
CO2. SO32-
forms a white precipitate, which dissolves on addition of acid to give
SO2. SO42-
forms a white precipitate, which is insoluble in diluted acid. Francine
Taylor-Campbell is an independent contributor. | 
