Illustrated Guide to Home Chemistry Experiments

Chapter 19 Laboratory: Qualitative Analysis 339

LABORATORY 19 .3:

qUALITATIvE ANALySIS of INoRGANIC ANIoNS

Qualitative inorganic analysis is generally done

in two phases. In one phase, the sample is

tested to determine which anions (negatively

charged ions, usually nonmetals) are present.

In the other phase, the sample is tested to

determine which cations (positively charged

ions, usually metals) are present.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ gas burner

£ test tubes (6)

£ test tube clamp

£ ring stand

£ stirring rod (2)

£ anion sample solution (see Substitutions and

modifications)

£ sulfuric acid, 3 m (a few drops)

£ sulfuric acid, concentrated (~2 mL)

£ iron(II) sulfate (~2 g)

£ nitric acid, 3 m (~15 mL)

£ potassium permanganate, 0.02 m (one drop)

£ barium hydroxide, saturated (one drop)

£ silver nitrate, 0.1 m (a few mL)

£ aqueous ammonia, 6 m (a few mL)

£ aqueous ammonia, 15 m (a few mL)

£ barium nitrate, 0.1 m (a few mL)

£ ammonium molybdate, 0.5 m (a few mL)

Chemists have developed specific tests for all common anions.
These tests depend on chemical reactions that occur between
the anion being tested for and one or more reagents. A positive
test results in some detectable change—a precipitate, color
change, odor, evolution of gas, and so on. Some anion tests yield
positive results if any of a group of anions is present. If that test
is negative, none of the anions in that group can be present. If
that test is positive, a further test is done to determine which
anion on anions from that group is present in the sample.

In this lab, we’ll use the following tests to identify these eight
common anions:

Nitrate (No 3 – )
The nitrate anion is identified using the brown ring test.
Iron(II) (ferrous) ions are added to a portion of the sample in
aqueous solution. Concentrated sulfuric acid is then added to
the sample by carefully allowing it to run down the side of the
test tube. Because concentrated sulfuric acid is very dense,
it settles to the bottom of the test tube, under the aqueous
layer that contains the sample and ferrous ions. At the phase
boundary between the aqueous layer and the sulfuric acid layer,
a brownish or purple-brown ring forms if nitrate ion is present in
the sample. Testing for nitrate ions must be done on a separate
portion of the sample, because the nitrate test introduces
sulfate ions into the sample and the other anion tests introduce
nitrate ions in the form of nitric acid to the sample.

Sulfite (So 3 2–)
Acidifying a solution that contains sulfite ions causes
evolution of sulfur dioxide gas, which has a characteristic
sharp, choking odor familiar to anyone who has smelled the
smoke from a firecracker. The presence of sulfur dioxide can
be confirmed by its decolorizing effect on a dilute solution of
potassium permanganate.

Carbonate (Co 3 2–)
Acidifying a solution that contains carbonate ions causes
evolution of carbon dioxide gas. Although carbon dioxide

is odorless, its presence can be confirmed by testing with

a solution of barium hydroxide, which turns cloudy in the

presence of carbon dioxide.

Chloride (Cl–), Bromide (Br–), and Iodide (I–)

Adding silver nitrate solution to an acidic solution that

contains halide ions causes the corresponding silver halide

to precipitate. Silver chloride is white, silver bromide cream-

colored, and silver iodide pale yellow, but it can be difficult

to discriminate by color alone, particularly if the sample

contains a mix of halide ions. The particular silver halide or

halides present can be confirmed by testing with aqueous

ammonia. A 6 M ammonia solution dissolves silver chloride,