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C-9
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 small beaker.
Record the color on your Observations Sheet. Carry out and record your observations for 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
-
). (WARNING: 6.0 M HCl is corrosive.)
2.
To a 1 mL sample of the hydrate solution, add 1 mL of 6.0 M HCl and 1 mL of
1.0 M BaCl2. A white precipitate (BaSO
4
) confirms the presence of the sulfate anion (SO
4
2-
).
3.
To another 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. Then wash the precipitate with water and discard the wash liquid in the metal
ion waste container.
B.
Cation Analysis.
1.
Be sure to record 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.
