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C-10
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
2 mL has been added. Stir this solution and record your observations. Compare your observations
with the possibilities given in Table 2.
NOTE:
The NaOH test for the cation must be done on all samples; however, once a positive test is
obtained to identify the cation, do not proceed with any further analysis, as further testing
will give confusing results.
3.
To distinguish barium from magnesium, add a few drops of 1.0 M H2SO
4
to a fresh 1 mL sample of
your unknown hydrate solution. (WARNING: H2SO
4
is mildly corrosive.) Formation of a white
precipitate confirms the presence of barium.
NOTE: MgSO
4
is soluble in water, whereas BaSO
4
is not.
4.
If no precipitate appears to have formed after adding NaOH, the ion is likely sodium (Na
+
) or
potassium (K
+
).
Note however, there may be a very slight amount of the Ba(OH)2 precipitate that is NOT
distinctly visible; therefore, do step 3 above before proceeding to the flame test.
To distinguish between Na
+
and K
+
ions, a flame test can be carried out. A nichrome wire, obtained
from your laboratory instructor, is used for this test.
First, clean the nichrome wire thoroughly by washing it with water and then, alternately, dipping the
wire into concentrated HCl (in a test tube) and heating the wire in the hottest part of the Bunsen
burner flame until the wire gives little or no color to the flame. Repeat this several times until the wire
is clean. Place a solid crystal of your unknown hydrate on the loop of the clean wire. Heat the
crystal in the flame and observe any change in color of the flame. Record your observations. When
you have completed this test, immediately clean the nichrome wire and return it to your
laboratory instructor.
NOTE: As soon as you have identified both the ions present in your unknown, report them to your
laboratory instructor. You will be given one mark for each ion correctly identified. If you have
incorrectly identified an ion, you may repeat some of the procedures. If you correctly identify the ion
at this point, you will be given partial marks.
Before leaving the laboratory, check to make sure that all of your equipment is clean, and back in your drawer.
All communal equipment must be cleaned and returned to the designated location in the laboratory. Remember
to turn in the original copy of the Observations Sheet to your laboratory instructor before leaving.
