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C-7
NOTE:
Before going on to Part III, remember to repeat the heating, cooling, and weighing of the
hydrate sample in Part I D until a constant mass is obtained.
Part III. Identification of Ions in Hydrate Samples
To identify the ions in the unknown hydrate (except for the CO3²
-
test), dissolve about 0.5 g (about the size of a
vitamin pill) of your unknown hydrate (from the sample bottle) in about 10 mL of water in a 50 mL beaker.
On your observation sheet, record the color of the solution and the results of the following tests.
A.
Anion analysis for CO3²
-
, SO
4
2-
, Cl
-
, Br
-
Proceed in the order outlined below. Once a positive test is obtained to identify the anion, you should not
proceed any further with the analysis, as further testing will give confusing results.
1.
To a test tube add a small amount of the solid hydrate, then add 1 mL of
6.0 M HCl. Note any gas being produced. Gas bubbles, if formed, will appear immediately, so
make your observation quickly. The evolution of a gas indicates the presence of the carbonate anion
(CO3²
-
), you may now proceed to part B. (WARNING: 6.0 M HCl is corrosive.).
2.
If the procedure in step A1 does not result in the evolution of a gas, put ~1 mL of your unkown hydrate
solution (prepared above) into a small test tube. Add one mL of 6.0 M HCl followed by the addition of 1
mL of 1.0 M BaCl2. The formation of a white precipitate (BaSO
4
) confirms the presence of the sulfate
anion (SO
4
2-
).
3.
To a fresh 1 mL sample of the unknown hydrate solution add six drops of
0.10 M AgNO3 and one drop of 6.0 M HNO3 to confirm the presence of the chloride (Cl
-
) or bromide
(Br
-
) anion. (WARNING: 6.0 M HNO3 is corrosive.) A white precipitate confirms the presence of
chloride. A cream colored precipitate confirms the presence of bromide. If you wish to check the
colors of AgCl and AgBr, run this test on very small samples of NaCl and of NaBr and compare the
results with those of your unknown. If the color of the silver precipitate is masked by the color of the
cation, decant the liquid in the metal ion waste container and add deionized water.
B.
Cation Analysis.
1.
Note the color of the solution of your unknown hydrate on your Observations Sheet. This color is due
to the metal cation. Compare this color with those listed in the metal cation table.
2.
Add 3 drops of 1.0 M NaOH to a fresh 1 mL sample of the hydrate solution. (WARNING: 1.0 M
NaOH is mildly corrosive.) Record your observations. Next, continue to add 1.0 M NaOH until about
