Identification of Anions and Cations in Solution
Purpose:
To identify the ions in an unknown solution through the application of chemical tests.
Background:
Detectives in mystery novels often rush evidence from the crime scene to the lab for analysis. In
this experiment, you will become a chemical detective. You will conduct laboratory analysis to
determine the composition of an unknown solution. The process of determining the composition
of a sample of matter by conducting chemical tests is called qualitative analysis. Solutions of
unknown ions can be compared to the results given by known ions, in the same tests. By
conducting the appropriate tests and applying logic, the identities of the ions present in an
unknown solution can be determined.
The analyses you perform are based upon the idea that no two ions produce the same set of
chemical reactions. Each ion reacts in its own characteristic way. In this experiment, you will
observe several types of chemical reactions commonly used as tests in qualitative analysis.
These reactions will produce visible evidence that a reaction is occurring, including a color
change, the evolution of a gas, and the formation of a precipitate-a solid product. As you do this
experiment, remember that careful observation and logical reasoning are the keys to being a good
detective. Who knows what ions lurk in your unknown solutions?
Equipment:
Safety goggles
2 Medium test tubes
Test tube rack
Bunsen burner
Red litmus paper
Test tube holder
Hot plate
250-mL Beaker
Wash bottle with distilled water
Chemicals:
Part A. Anions
0.1 M Silver nitrate, AgNO3
0.1 M Sodium sulfate, Na2SO4
0.1 M Barium chloride, BaCl2
0.1 M Sodium bicarbonate, NaHCO3
0.5 M Ammonium molybdate, (NH4)2MoO4
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0.1 M Sodium chloride, NaCl
1.0 M Sodium phosphate, Na3PO4
6 M Nitric acid, HNO3
6 M Hydrochloric acid, HCl
Unknown anion solution
Part B. Cations
0.1 M Iron(III) sulfate, Fe2(SO4)3
0.1 M Sodium chloride, NaCl
0.1 M Potassium thiocyanate, KSCN
0.1 M Calcium nitrate, Ca(NO3)2
3M sulfuric acid, H2SO4
0.1 M Ammonium nitrate, NH4NO3
0.1 M Potassium chloride, KCl
6 M Sodium hydroxide, NaOH
0.1 M Sodium oxalate, Na2C2O4
Unknown cation solution
Safety concerns:
1. Nitric acid, hydrochloric acid, and sodium hydroxide are corrosive and can cause severe
injury. Never cover the opening of a test tube with your finger when mixing chemicals in
the tube.
2. Silver, barium, and oxalate compounds are poisonous. Avoid contact with these chemicals.
3. Silver nitrate will stain skin and clothing
4. Exercise care when working with a hot water bath.
Pre-lab questions: answer these in your lab notebook
1. What is qualitative analysis?
2. In this experiment, you will be given an unknown solution containing ions, and your job will
be to figure out what ions are in your unknown. What will you be comparing your unknown
to?
3. What might you see as evidence that a chemical reaction is taking place in your experiment?
4. You will be using 2 test tubes for each step of the procedure. The first thing you will do is
place your unknown in one test tube. What will you place in the other?
5. List the 4 known solutions for the Anion tests.
6. List the 3 known solutions for the Cation tests.
Procedure:
As you perform the experiment, record your observation in the data table. In testing for different
ions in steps 3-13 you will always begin with two test tubes. For each step you must add 1 mL
(~20 drops) of the known solution to one test tube, and 1 mL (~20 drops) of your unknown
solution to a second test tube. The name of the known solution is given in each step. Always
rinse the test tubes several times with water after each step is performed. Some lab groups will
be doing part A on day one, and some will be doing part B on day one. You will switch for day
two.
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Part A. Testing for Anions
1. Set up a hot water bath for use in step 6. Half-fill a 250ml beaker with water, and place it on
a hot plate.
2. Thoroughly clean all the test tubes used in this experiment, rinsing them well with distilled
water. Record the number of the anion unknown solution you will be testing.
3. Test for chloride ion, Cl . Known solution is sodium chloride CAUTION: Nitric
acid is corrosive. Do not put your finger over the top of the test tube to cover it. Add 20 drops
of 6 M nitric acid to each tube containing the solutions to be tested and flick the tubes to mix.
Add 10 drops of silver nitrate to each tube and flick to mix. Record observations. Rinse your
test tubes thoroughly, first with tap water, then with distilled water.
4. Test for sulfate ion, SO42 . Known solution is sodium sulfate. CAUTION: Hydrochloric
acid is corrosive. Add 20 drops of 6 M hydrochloric acid to each tube and mix. Add 10 drops of
barium chloride to each tube and mix. Record observations. Rinse your
test tubes thoroughly, first with tap water, then with distilled water.
5. Test for bicarbonate ion, HCO3 . Known solution is sodium bicarbonate. Carefully observe
the test tubes as you add 20 drops of 6 M hydrochloric acid to each tube. Record observations.
Rinse your test tubes thoroughly, first with tap water, then with distilled water
6. Test for phosphate ion, PO43 . Known solution is sodium phosphate. Add 20 drops of 6 M
nitric acid and 10 drops of ammonium molybdate solution to each tube and mix. Place the tubes
in a hot water bath and heat for five minutes. Record observations.
7. Thoroughly rinse all test tubes and place on the wooden pegs at the big sink to dry. Put all the
chemicals back in the white bin.
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Part B. Testing for Cations
8. Review the general directions at the beginning of the Procedure section. Set up a hot water
bath for use in step 10. Half-fill a 250ml beaker with water, and place it on a hot plate.
Thoroughly clean all the test tubes used in this experiment, rinsing them well with distilled
water. Record the number of the cation unknown solution you will be testing.
9. Test for iron(III) ion, Fe3+. Known solution is iron(III) sulfate. Add 5 drops of sulfuric acid
and 5 drops of potassium thiocyanate solution to each tube. Flick gently to mix. Record
observations. Rinse your test tubes thoroughly, first with tap water, then with distilled water
10. Test for calcium ion, Ca2+. Known solution is calcium nitrate. To each tube, add 10 drops
of sodium oxalate. Warm the tubes in a boiling water bath for a few minutes. Record your
observations. Rinse your test tubes thoroughly, first with tap water, then with distilled water
13. Test for ammonium ion, NH4+. Known solution is ammonium nitrate. Caution: Sodium
hydroxide can cause burns. To each test tube add 3 drops of 6 M sodium hydroxide. Hold the
tube containing the ammonium nitrate solution with a test tube holder. Gently warm the tube
along its sides using a back-and-forth motion through a “cool” burner flame. Do not allow the
solution to boil. Caution: At all times make sure that the opening of the tube is pointed away
from other people. Hold a moistened piece of red litmus paper near the mouth of the test tube
(be careful not to touch the sides of the test tube with the litmus paper. The test will be spoiled if
the solution contacts the litmus paper.) Record the changes you observe. Carefully fan the
vapors coming out of the tube towards your nose with your hand. Cautiously sniff the vapors.
Record your observations. Repeat the procedure for your unknown solution.
14. Thoroughly rinse all test tubes and place on the wooden pegs at the big sink to dry. Put all
the chemicals back in the white bin.
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Data:
Anion unknown number: ______
Cation unknown number: _______
Data Table:
Ion tested
Test result for the known
solution
Test result for the
unknown solution
Is the ion
present in the
unknown?
Chloride
Sulfate
Hydrogen
carbonate
Phosphate
Iron (III)
Calcium
Ammonium
Conclusion questions:
1. List the anions present in your unknown
2. List the cations present in your unknown.
3. Sometimes a false negative might occur with a qualitative test. That means that the ion
was supposed to be in the sample, but the test did not produce a positive result. What do
you think might cause that result?
4. List some sources of error that might have occurred with your lab.
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