Chapter 19 Laboratory: Qualitative Analysis 341
solution cloudy, establishing that carbonate ions are
present in the sample.
- Warm the solution gently to drive off any remaining sulfur
dioxide gas and carbon dioxide gas. (Do not boil the
solution or heat it strongly; excessive heat can oxidize the
iodide ion to elemental iodine.)
- Add a few drops of 0.1 M silver nitrate solution to the
solution in the test tube. A precipitate confirms the
presence of one or more of the halide anions.
- Continue adding 0.1 M silver nitrate until no further
precipitation occurs. The precipitate contains all of the
halide anions present in the sample. Allow the precipitate
to settle completely.
- Carefully decant the supernatant liquid or use a pipette
to draw it off. Get as much of the liquid as possible, while
leaving all of the precipitate in the test tube. Transfer
the supernatant liquid to another test tube. That liquid
contains any sulfate and phosphate ions present in the
sample.
Wash the precipitate from step 8 with a few mL of distilled
water. Allow the precipitate to settle completely. Draw
off or decant the wash water, and discard it, keeping the
precipitate.
Add 5 mL of 6 M aqueous ammonia to the tube containing
the precipitate. Stir or swirl the tube. Any silver chloride
present in the precipitate dissolves in the 6 M aqueous
ammonia. Silver bromide and silver iodide are insoluble
in 6 M aqueous ammonia, and so remain as a solid
precipitate.
Draw off or decant the supernatant fluid and transfer it to
another test tube. Add about 5 mL of 6 M nitric acid to the
new test tube to neutralize the 6 M aqueous ammonia.
Silver chloride forms a white precipitate. Discard the
contents of this test tube.
Add 4 mL of 15 M (concentrated) aqueous ammonia
to the precipitate remaining from step 11. Stir or swirl
the tube. Any silver bromide present in the precipitate
dissolves in the 15 M ammonia. Silver iodide is insoluble
in 15 M ammonia and so remains a solid light yellow
precipitate, confirming the presence of the iodide anion.
Draw off or decant the supernatant fluid and transfer it to
another test tube. Add about 10 mL of 6 M nitric acid to
that test tube to neutralize the 15 M aqueous ammonia.
Silver bromide forms a cream-colored precipitate. Discard
the contents of this test tube.
Draw off or decant about 4 mL of the supernatant liquid
and transfer it to another test tube. (You may discard
the remaining supernatant fluid and the precipitate from
step 15.) This liquid must contain only phosphate ions,
because all of the other anions have been removed by
earlier tests.
Add about 2 mL of 6 M nitric acid to the supernatant liquid;
stir to mix the solution. Add about 2 mL of 0.5 M ammonium
molybdate solution to the test tube. An insoluble precipitate
will form.
FIGURE 19-2: A white precipitate upon addition of barium nitrate
confirms the presence of sulfate ions
dISoALp S : dispose of all waste materials by
neutralizing the acids with sodium bicarbonate and then
flushing the neutralized solutions down the drain with
plenty of water.
The test tube containing the supernatant fluid from step 9
includes the sulfate and phosphate ions present in the
sample. Add a few drops of 0.1 M barium nitrate solution
to the supernatant fluid. A white precipitate confirms the
presence of sulfate ions. Continue adding barium nitrate
until no further precipitation occurs.
FIGURE 19-3: A yellowish precipitate upon addition of ammonium
molybdate confirms the presence of phosphate ions
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