CHEMISTRY EXPERIMENT 163
SPOKANE FALLS COMMUNITY COLLEGE
QUALITATIVE ANALYSIS OF AN UNKNOWN CATION SOLUTION
OBJECTIVES
1. To observe chemical reactions of selected cations, barium, calcium, magnesium.
2. To analyze an unknown solution for the presence of one or more of the ions observed.
3. To develop a data table, flow chart or analysis scheme.
4. To develop laboratory skills, including the use of a centrifuge, litmus paper, process of decanting
and flame testing.
DISCUSSION
Qualitative analysis is the branch of chemistry that is concerned with identifying particular ions present
in a substance. Selecting a reagent that will react differently with the ions in solution separates the ions
and identification is then made. For example, barium ions can be precipitated (form a solid) with
ammonium sulfate while calcium and magnesium do not react. A flow-chart, data table, or analysis
scheme is used to order the sequences used in carrying out the analysis.
In this experiment you will separate and identify Ba+2, Ca+2, and Mg+2. First, a known solution of all
three anions will be analyzed to develop techniques, observe reactions, and develop an analysis
sequence. Second, an unknown solution will be analyzed to determine the cation(s) present.
TERMINOLOGY AND TECHNIQUES
Centrifugation-Separation of solids from liquids by subjecting them to fast rotation.
Centrifuge-A machine for centrifugation consisting of a rapidly rotating container.
Centrifuging Techniques-Always operate the centrifuge with an even number of test centrifuge tubes
containing equal volumes of liquid placed opposite each other. If only one tube needs to be centrifuged,
balance the centrifuge with another tube filled to same volume with solvent.
Precipitate. The deposit of an insoluble salt in a solution as a result of a chemical reaction.
Decant. Pouring off the top liquid layer.
Supernate, supernatant. The top liquid layer, the liquid above a precipitate.
Litmus paper. Litmus paper is used to determine whether a solution is basic or acidic. Acidic solutions
turn blue litmus paper red, basic solutions turn red litmus paper blue. A stirring rod is placed in the
solution and then touched to the litmus paper.
Flame test. Flame testing used to confirm the presence of an ion. A wire is dipped into a solution, held
under a hot flame and the resulting color is observed. Many elements produce colored flames, sodium is
yellow, potassium is violet, and copper is green.
Semi micro. Semi micro method uses small test tubes and dropper amount reagents.
Do the analysis of the known solution first, and then obtain your unknown. Please note the unknown
solutions have been individually assigned.
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Flow chart for the Separation and Analysis of Barium, Calcium and Magnesium
STEP 1
(NH4)2SO4
1
Ba+2, Ca+2, Mg+2
centrifuge
STEP 2
(NH4)2C2O4
1
2
+2
+2
Ca , Mg
Decant
_______ ppt
HCl
centrifuge
STEP 3
Na2HPO4 + NaOH
1
2
Mg+2
3
______ ppt
______Flame Test
(CONFIRMS Ba+2)
HCl
centrifuge
2
3
Discard
______ Flame Test
(CONFIRMS Ca+2)
HCl & Magnesium
Indicator
3
NaOH
Red Litmus
3
______
(CONFIRMS Mg+2)
3
centrifuge
centrifuge
_____spot
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PROCEDURE: Read and fill in the blanks before obtaining necessary equipment.
Obtain three small test tubes and a flame test wire from the stock room.
A.
Analysis of a Known Solution.
1. Separation and Identification of cation #1 in a known solution of Ba+2, Ca+2, Mg+2.
a) Place 10 drops of the solution labeled “Known Cation Solution” into test tube #1. Add
ten drops of ammonium sulfate (NH4)2SO4, and mix by gently shaking or with a stir rod.
Write a net ionic equation for each possible precipitation reaction that can occur, look up
Ksp for each.
Reaction 1__________________________ Ksp _________________
Reaction 2 _________________________ Ksp__________________
Reaction 3 _________________________ Ksp __________________
The white precipitate that will form in the “known cation solution” is
___________________ (cation #1)
b) Centrifuge the precipitate, remembering to balance the test tube with a blank, for 30-60
seconds. Remove the tube and test for completeness of precipitation by adding one more
drop of ammonium sulfate. If no new precipitate forms, the test is complete. Pour off the
supernate into test tube #2 and save it for Step 2.
What cations are left in the supernatant solution? ______________________
Add five drops of 6M hydrochloric acid, HCl, into test tube one and stir until most of the
precipitate redissolves. Clean the flame test wire with the HCl, and dip it into the
solution. Place the wire over the hot part of the flame from a Bunsen burner and record
the color.
What color flame do you expect to see? ________________________
Barium burns with a green flame, Calcium with a brick red flame. Magnesium does not
burn with a distinctive color flame.
2. Separation and identification of cation #2 and cation #3.
a) Add 10 drops of ammonium oxalate, (NH4)2C2O4, to the solution in test tube#2.
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Write equations for each possible precipitation reaction that can occur. Look up Ksp’s for
each.
Reaction #1 ____________________________
Ksp __________________
Reaction #2 ____________________________
Ksp___________________
The white precipitate that will form in the “Known cation solution” is ________________
(cation #2).
b) Centrifuge and test for completeness of precipitation by adding one more drop of
ammonium oxalate. Pour off the supernate into test tube #3.
What cation is left in the solution? __________________________
c) Add five drops 6M hydrochloric acid to test tube #2 and stir or shake to dissolve. Clean
the flame test wire with HCl and dip it into the solution. Place the loop over the hot part of
the flame of your Bunsen burner and record the color of the flame.
What color flame do you expect to see?____________________________
3. Identification of cation #3 in a known solution.
a) Add 10 drops sodium monohydrogen phosphate Na2HPO4, to the solution in test tube #3.
Add sodium hydroxide NaOH until the solution is distinctly basic.
What is/are the possible precipitate(s) present? _______________________________
b) Centrifuge and discard the supernate.
c) Dissolve the precipitate in test tube #3 with 6M HCl. Add a few drops of “Blue Lake”
indicator, then add enough NaOH to make the solution distinctly basic.Centrifuge.
A blue “lake” precipitate confirms the presence of cation #3. What is the cation?
____________
Complete the Flow Chart on page 3 for an overview of the analysis of your known compound. Make a
similar chart or table for your unknown analysis, have the Unknown Analysis chart checked before you
get your unknown. Be careful and descriptive with your observations. The chart will be handed in
with the lab and must indicate procedures used to identify you unknown.
Perform the procedure used for the “Known Cation Solution” on your unknown solution.
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Name_________________________________
Unknown #________
Data Sheet
Qualitative Analysis of an Unknown Cation
Results of addition of (NH4)SO4.
Known
Unknown
If flame test was performed after this step, describe the results.
Known
Unknown
Results after addition of (NH4)2C2O4.
Known
Unknown
If flame test was performed after this step, describe the results.
Known
Unknown
Results after addition of Na2HPO4 + NaOH.
Known
Unknown
Results after addition of HCl and indicator followed by NaOH.
Known
Unknown
Cation(s) determined in unknown sample: ______________________________________
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Flow Chart Unknown Cation Solution
STEP 1
(NH4)2SO4
1
Unknown Solution
centrifuge
STEP 2
(NH4)2C2O4
1
2
+2
+2
Ca , Mg
Decant
_______ ppt
HCl
centrifuge
STEP 3
Na2HPO4 + NaOH
1
2
3
______ ppt
______Flame Test
HCl
centrifuge
2
3
Discard
______
Flame Test
HCl & Magnesium
Indicator
3
NaOH
Red Litmus
3
______
(CONFIRMS Mg+2)
3
centrifuge
centrifuge
_____ spot
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Questions
1. Design a separation scheme for a solution containing Ag+, K+, and Al+3. Draw a flow chart showing
your proposed analysis scheme below.
2. An unknown solution of barium, calcium, and magnesium group did not give a precipitate with
(NH4)2SO4 solution. Upon addition of (NH4)2C2O4 solution to the unknown a white precipitate
formed. To the supernatant liquid was added a solution of Na2HPO4 and NaOH, but no precipitate
resulted. State the ion(s) present in the unknown.
__________________________________
3. An unknown solution of barium, calcium, and magnesium group gave a precipitate with (NH4)2SO4
solution. Upon addition of (NH4)2C2O4 solution to the supernatant liquid, no precipitate formed, but
upon addition of Na2HPO4 and NaOH. a white precipitate formed. This precipitate was dissolved in
HCl solution; then indicator reagent with NaOH solution was added to give a blue “lake”. State the
ion(s) present in the unknown solution.
___________________________________
Proposed Analysis Scheme
Unknown Letters & Spokane River water analysis, if time permits:
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