Chapter 19 Laboratory: Qualitative Analysis 343LABORATORY 19 .4:
qUALITATIvE ANALySIS of INoRGANIC CATIoNS
Qualitative analysis of inorganic cations
refers to a structured method of analyzing an
unknown solid or solution to determine whether
specific cations are present. Such an analysis
is an iterative process of separating cations
into groups based on reactions characteristic
to each group and then further analyzing each
separated group to determine which specific
cations in that group are present in the sample.
Cations are conventionally grouped as follows:
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ 2-liter soft drink bottle labeled “hazardous waste”£ test tubes (7)£ test tube rack£ eye dropper or Beral pipette (5)£ graduated cylinder, 10 mL£ stirring rod (1 or more)£ litmus paper (red and blue)£ cation sample solutions (see Substitutions and
modifications)£ primary and secondary reagents (see Substitutions
and modifications)Group I cations (Ag+, pb2+, Hg+)
Group I cations are the only cations that produce insoluble
chlorides. Group I cations can be precipitated by adding dilute
hydrochloric acid, leaving all other cations in solution.
Group II cations (Cu2+, Bi3+, Cd2+, Hg2+, As3+, Sb3+, Sn4+)
Group II cations produce extremely insoluble sulfides (Ksp <
10 –30). Group II cations can be precipitated by adding sulfide
ions at low concentration, which is most conveniently achieved
by adding thioacetamide to the acidic solution. In aqueous
solution, thioacetamide produces hydrogen sulfide gas.
Because hydrogen sulfide is a very weak acid, the equilibrium
in acidic solution stays far to the left, with most of the hydrogen
sulfide present in molecular form rather than dissociated to
form sulfide ions.
Group III cations (Al3+, Cr3+, fe3+, zn2+, Ni2+, Co2+, mn2+)
Group III cations produce slightly soluble sulfides (Ksp > 10–20).
Group III cations can be precipitated by adding sulfide ions at
high concentration, which is most conveniently achieved by
adding aqueous ammonia to the acidic thioacetamide solution
used to precipitate Group II cations until the solution is strongly
basic. The addition of ammonia drives the equilibrium to
the right, forcing more of the aqueous hydrogen sulfide
gas produced by the thioacetamide to dissociate, forming
sulfide ions.
Group Iv cations (mg2+, Ca2+, Sr2+, Ba2+)
Group IV cations produce insoluble carbonates. Group IV
cations can be precipitated by adding carbonate ions.
Group v cations (Na+, k+, NH 4 +)
Group V cations are not precipitated by chloride, sulfide, or
carbonate ions, and so are the only cations remaining in solution
after an unknown solution has been treated with those reagents.
In a standard qualitative cation analysis, the first step is to treat
the unknown with chloride ions, which causes all Group I cations
to precipitate out. The precipitate and supernatant liquid are then
separated by filtration or centrifugation. The liquid at this point
contains only Group II, III, IV, and V cations. The precipitate, which
may contain any or all of the Group I cations, is subsequently
treated with hot water. Lead chloride, which is insoluble in cold
water, is reasonably soluble in hot water, and so can be separated
from any silver chloride and/or mercury(I) chloride present in the
precipitate by removing the supernatant fluid from the precipitate
by filtration or centrifugation. The supernatant fluid is tested
with potassium chromate solution. If lead(II) ions are present,
they combine with the chromate ions to produce insoluble lead
chromate, which precipitates as a solid with a characteristic yellow
color, confirming the presence of lead(II) ions in the sample.The precipitate from the first step, which may contain silver
chloride and/or mercury(I) chloride, is then treated with aqueous
ammonia, with which silver chloride forms a soluble complex.
Any solid precipitate remaining after treatment with aqueous
ammonia must be mercury(I) chloride, and so confirms the
presence of mercury(I) ions in the sample. To confirm the
presence of silver, the aqueous ammonia is neutralized with nitric
acid. If a precipitate occurs, the presence of silver ions in the
sample is confirmed.