CHM 101/102
Spot Tests:
Another Qualitative Analysis
General Chemistry 101/102
Laboratory Manual
University of North Carolina Wilmington
Laboratory Manual
CHM 101/102
Spot Tests: Another Qualitative Analysis
•
Purpose
 To identify several ions present in an unknown mixture:
Cl- (chloride), SCN- (thiocyanate), C2H3O2- (acetate),
PO43- (phosphate), CO32- (carbonate), CrO42- (chromate),
SO42- (sulfate), and NH4+ (ammonium).
•
 This analysis differs from other Qualitative Analysis
experiments because we are not separating the ions from
the mixture, therefore interferences between ions need to be
considered.
Safety Considerations
 Do not allow acids or bases to come in contact with your
skin. Avoid skin contact with silver nitrate as it will leave a
dark stain for several days.
 Place all waste solutions in the appropriately labeled
containers in the hood.
Laboratory Manual
CHM 101/102
Spot Tests
•
General Procedures
 Use clean glassware and deionized water to avoid
contaminating your sample.
 Perform tests on “known” solutions and your unknown
solution simultaneously. Keep in mind that your
unknown is diluted compared and may be colored by
the presence of several ions. Therefore, positive results
might have slightly different appearances between the
known and your unknown.
 Some spot test reactions occur slowly. If no reaction is
apparent after the addition of reagent, heat the reaction
mixture in a hot water bath according to instructions
given.
 Stir well after addition of reagent using a clean stirring
rod or disposable transfer pipet.
 Perform tests in the order given as described in the
Laboratory Manual
Laboratory Manual
CHM 101/102
Spot Tests
•
Test 1: Carbonate Ion
• You will first test the “known”
solution by placing 5 drops of
the 1 M Na2CO3 solution in a
micro test tube (or spot plate)
and add 5 drops of 6 M HCl. The
formation of bubbles indicates
release of carbon dioxide gas.
• Repeat the test using 5 drops of
your unknown and adding 5
drops of 6 M HCl (do NOT add
the 1 M Na2CO3 solution as this
is the “known” solution).
Laboratory Manual
CHM 101/102
Spot Tests
•
Test 2: Thiocyanate Ion
• This basis of the thiocyanate spot
test is to identify the thiocyanate
ion, SCN- , by its reaction with
iron(III) to form a blood-red colored
complex. Add 5 drops of 6 M HCl to
5 drops of 0.5 M KSCN (the
“known”) on the spot plate. Add 1
drop of 0.1 M Fe(NO3)3 to the
solution in the spot plate. The
solution will turn red if SCN- is
present.
• Repeat the test on your unknown
solution without adding the
“known” 0.5 M KSCN solution.
Laboratory Manual
CHM 101/102
Spot Tests
•
Test 3: Sulfate Ion
• Put 5 drops of the “known”
0.5 M Na2SO4 solution in a
well of a spot plate. Add 5
drops of 6 M HCl to the plate.
Now add 1 drop of 1 M BaCl2
to the plate. A precipitate of
white BaSO4 indicates the
presence of sulfate ion.
• Repeat the test on your
unknown without using the
0.5 M Na2SO4 “known”
solution.
Laboratory Manual
CHM 101/102
Spot Tests
•
Test 4: Phosphate Ion
• Place 5 drops of the “known” 0.5 M
Na2HPO4 (sodium monohydrogen
phosphate) solution in a medium test
tube. Add 1 mL of 6 M HNO3. Now
add 1 mL of Ammonium molybdate
solution, (NH4)2MoO4 to the test tube.
The presence of phosphate ions is
confirmed by the formation of a
yellow precipitate.
• This precipitate is often slow to form
and heating in the hot water bath
may be required to initiate
precipitation.
• Repeat the test on your unknown.
Laboratory Manual
Spot Tests
•
Test 5: Chromate Ion
• Solutions containing chromate ions
are yellow when neutral or basic and
orange when acidic. The orange
color is due to the formation of the
dichromate ion (Cr2O72-).
• Add 5 drops of “known” 0.5 M K2CrO4
to a spot plate. Add 5 drops of 3%
H2O2. Now add 10 drops of 6 M HNO3
to the plate. The formation of a bluegreen color, which appears but
rapidly disappears, indicates the
presence of the chromate ion.
• Repeat this test on your unknown
without adding the 0.5 M K2CrO4
“known” solution.
Laboratory Manual
CHM 101/102
Spot Tests
•
Test 6: Chloride Ion
• Place 5 drops of “known” 0.5 M NaCl
on a spot plate. Add 5 drops of 6 M
HNO3. Now add 2 drops of 0.1 M
AgNO3 to the plate. The presence of
chloride ion is indicated by the
formation of a white precipitate of
AgCl.
• If Test 2 indicated that your unknown
contained SCN-, follow the lab
manual procedure for removing the
SCN- ion before you test your
unknown for Cl- because SCN- also
forms a white precipitate with Ag+.
• Repeat this test on your unknown
solution. Remember not to add the
“known” NaCl solution.
Laboratory Manual
CHM 101/102
CHM 101/102
Spot Tests
•
Test 7: Acetate Ion
• Place 5 drops of “known” 0.5 M
NaC2H3O2 (sodium acetate) on a spot
plate. (The acetate ion is often
abbreviated as OAc-, or as CH3COO-).
Add 5 drops of 3 M H2SO4 to the spot
plate. The presence of acetate ion is
indicated by the smell of acetic acid
(a vinegar odor) coming from the
plate. If no odor is immediately
present, repeat the test in a micro
test tube, warming it in a hot water
bath for 20 seconds and check the
odor again.
• Repeat this test on your unknown
solution. Do not add the “known”
NaC2H3O2 solution.
Laboratory Manual
•
CHM 101/102
Spot Tests
Test 8: Ammonium Ion
• This test also relies on your sense of
smell to detect the presence of a
substance. Add 5 drops of “known” 0.5
M NH4Cl to a spot plate or a micro test
tube. Add 5 drops of 6 M NaOH to the
plate and check for the presence of an
ammonia odor by gentle wafting. If no
odor is immediately apparent, warm the
test tube and check again.
• As an additional test for NH4+, place a
drop of the unknown on a spot plate
and add a drop of Nessler’s Reagent. A
reddish-brown precipitate indicates the
presence of the NH4+ ion (see lower
photo).
Laboratory Manual
• Repeat the test on your unknown.
CHM 101/102
Spot Tests
•
Interfering Ions
• If you suspect that an ion is interfering with a particular
test, make up a “known” solution that contains the ions you
think you have in our unknown and repeat the spot tests
which gave you trouble.
• Test 3 is most likely to have interference problems from the
Ba2+ ion which also forms precipitates with the chromium
and phosphate ions.
• Test 6 is most likely to have interference problems from the
Ag+ ion which forms precipitates with the phosphate,
sulfate, carbonate, chromate, and thiocyanate ions.
• Refer to the Laboratory Manual for specific instructions of
how to remove these interfering ions.
Laboratory Manual