SEATTLE CENTRAL COMMUNITY COLLEGE
DIVISION OF SCIENCE AND MATHEMATICS
QUALITATIVE ANALYSIS OF CATIONS
INTRODUCTION
The goal of inorganic qualitative analysis is identification of the cations and anions present in
material of unknown composition. Qualitative analysis is based largely on the characteristic
reactions of ions in solution.
The solution equilibrium properties an ion - strength as Brønsted-Lowry acid/base, redox behavior,
salt solubility and tendency to form complex ions - provide an alternative to the periodic table in
categorizing the properties of the aqueous ions. This organization is the basis for inorganic
qualitative analysis. The reactions of inorganic qualitative analysis provide excellent examples of
the usefulness of equilibrium principles to obtain desired results from chemical reactions.
The associated laboratory work also provides practice in observing chemical reactions and
interpreting observations based on theory. You will exercise judgment and decide a course of action
for both confirmatory and contradictory results.
Qualitative analysis can be carried out cook book style, merely by carefully following the
directions. Although that approach can often lead to the correct answer, it is dull and time
consuming. However, if you take advantage of every clue, use your knowledge to draw conclusions,
and view qualitative analysis as a challenge, you will save much time and have more fun than
someone who blindly follows directions. You will also learn more.
DEFINED TERMS
KNOWN
A solution containing ions that is used as a standard to compare reactions
when working with the unknown. Known reagents are colored coded.
UNKNOWN
The material to be analyzed. There will be one unknown solution for each
group. The unknowns are available from the stockroom. It is very important
to record your unknown number and affix the unknown sticker in your lab
notebook.
GROUP REAGENT In cation analysis, the group reagent reacts with all the cations of that group.
Group reagents are color coded.
PRECIPITATE
A solid formed by a chemical reaction when two solutions are mixed.
SUPERNATANT
A solution which lies above the precipitate after centrifugation.
RESIDUE
What remains after part of a solid has dissolved or after a solution has been
completely evaporated (dry).
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BALANCED
When using the centrifuge, two test tubes of the same size containing the
same volumes must always be placed across each other.
BEAKER BATH
A test tube rack placed in a beaker used for a water bath.
GETTING STARTED
Laboratory Techniques
The secret to a successful analysis is organization. Keep your lab notebook neat and legible.
Record pertinent information in your notebook, not on scrap pieces of paper towel. Make sure
glassware is clean and clearly labeled. Keep your lab desk clean and orderly. Conveniently arrange
the most frequently used items, such as clean test tubes, your wash bottle, stir sticks, Pasteur pipets
and test papers. Keep dirty test tubes and other articles in one place. Clean and rinse these at the
first opportunity so that a stock of clean equipment is always ready. To rush through experimental
work with the idea of understanding it later will NOT save you time!
Volume Measurements
Use only the small volumes specified. Use the qual tubes as your reaction vessels. The 24-well plate
works well as a test tube rack for the qual tubes. You can estimate most volumes by counting drops
(25-30 drops per mL for the Pasteur pipets). Use the 10 mL graduated cylinder for the more
accurate volumes required.
Solution Handling
Do not use Pasteur pipets to dispense reagents.
The consequence of contaminating the reagents cannot be emphasized enough!
Keep concentrated HCl (12 M) and concentrated NH3 (15 M) apart. The gas from each will react to
give a solid salt that could interfere with your tests. Keep both reagents stoppered when not in use.
All reactants must be mixed. Use a disposable wood stirring stick to mix. Never use your thumb as
a stopper. Never vigorously shake test tubes that are stoppered.
Unless instructed otherwise, the method of heating solutions is by water bath heated on a hotplate.
Use of Test Papers
Moisten the wood stir stick with the solution to be tested. Touch the test paper with the moistened
stick. Using this method you can run several tests using one strip of pH paper. The test paper must
be read while wet.
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Separation of a Precipitate
The centrifuge is used for the separation of a solid from solution. The centrifuge must be balanced
otherwise breakage and possible loss of sample can occur. Always have a test tube opposite yours
with the same volume of liquid. Separate the supernatant from the precipitate by decanting or
removing the supernatant with a Pasteur pipet. To wash a centrifuged precipitate, pour off the
supernatant. Mix the precipitate with 1 to 2 mL of deionized water depending on the amount of
precipitate, centrifuge again, and pour off the washing.
Waste
All wastes go in designated, labeled waste jars.
Flow Charts
Processing the known for each group gives valuable information as to where on a flow chart each of
the ions separate. Once completed, a flow chart is an invaluable tool in an unknown analysis.
Work Station
All qual analysis will be performed in the hood which will be assigned. Two students will share a
hood with the exception of the two smaller hoods.
Reagents and Qual kits
Qual kits will be checked out for the quarter. Reagents will be placed in the hoods and should be
returned to its original state of organization. Each bundle is color coded and rubber banded – red
for acids, green for bases and group reagents. You will not be given the next batch of reagents if
the hood is left unclean and reagents disorganized.
The Analysis
The cation analysis is divided into four groups. You will test one unknown for each of the three
cation groups. Report which cations are present and absent.
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THE SOLUBLE AND CHLORIDE INSOLUBLE CATIONS
Group I NH4+, Na+, K+, Ag+, Bi+3 and Cu+2
Silver Ion
The silver ion is precipitated as silver chloride, AgCl, with HCl. To prevent the formation
of the soluble silver chloride complex, [AgCl2]-,the addition of excess HCl must be
avoided.
AgCl(S) + Cl-(aq)  [AgCl2]Confirmation that the precipitate formed is AgCl is done by dissolving the precipitate in
ammonia solution forming [Ag(NH3)2]+ complex.
AgCl(s) + 2 NH3(aq)  [[Ag(NH3)2]+(aq) + Cl-(aq)
Because the above reaction is in equilibrium, addition of acid will react with the NH3,
shifting the equilibrium to the left and precipitating back to AgCl.
Ammonium Ion
The test for the ammonium ion takes advantage of the equilibrium below.
NH3 + H2O  NH4+(aq) + OH-(aq)
The addition of hydroxide shifts the equilibrium to the left side of the equation. The
reaction is heated to drive the ammonium gas driven from the system. If a moist red
litmus is placed on top of the reaction vessel, the ammonia gas turns the red litmus paper
blue.
Flame Test for sodium and potassium ions
Flame tests (bulky yellow for Na+ and fleeting violet for K+) are sensitive, but
contamination of sodium presents problem. It is important to realize that sodium is present
in practically everything. The glassware used to contain the solutions is made of sodium
silicate. Flame tests of solution will inevitably show the presence of sodium ion. It is critical
when performing a flame test for sodium ion that a known solution first be tested and to
note the intensity of the flame. An unknown solution that contains sodium will have the
same intensity as the known solution. The strong, persistent yellow flame covers up the
color the potassium ion.
To detect the potassium ion in the presence of sodium ion the flame will have to be viewed
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through a cobalt glass. The cobalt blue glass filters out the yellow light wavelengths of the
sodium flame but not that of other colored wavelengths.
Flow diagram for Qual 1 cations
Na+, K+, NH4+, Ag+,Cu+
CaO, (NH4)2CO3
∆,H2O
NaOH
NH3
HCl
(gas)
AgCl
Na+
K+
NH3
[ Ag(NH3)2]
HNO3
AgCl(s)
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Cu+
[Cu(NH3)4]+2(aq)
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PROCEDURE
1. Dispense 20 drops (1 mL ) of the Group I known solutions into a test tube and set
aside. The known solution contains the entire Group I cations.
Flame tests for sodium and potassium ions
2. Clean a flame wire loop by heating the loop on the hottest part of the flame. The
length of time to clean the wire loop is dependent on what solution was tested last. If
the wire loop burns yellow orange, dip the wire loop in 6 M HCl and continue to burn
the loop. Prop the wire loop on a ring and proceed to the next step while the
contaminant burns off.
3. Prior to performing a flame test remove interfering ions by transferring the 2 mL of the
Group 1 known to an evaporating dish. Add solid CaO while stirring until basic to
litmus. Add a pinch of (NH4)2CO3. Heat to moist residue on a hotplate. Do not allow to
go dry. Add 1 mL of deionized water and transfer to a small beaker.
4. Conduct flame tests. Observe flame with and without cobalt glass.
Confirmation of the silver ion.
5. Add 6M HCl drop wise to one tube set aside from step1until precipitation is complete.
Record your observation.
6. To check for completeness of precipitation, centrifuge and add a drop 6M HCl.
Precipitation is complete when no re-precipitation is observed. Centrifuge again if
more precipitation occurred. Separate the supernatant and save for step 11.The
supernatant could contain Bi+3 and Cu+2.
7. Wash the precipitate with DI water, centrifuge and discard the wash. Dissolve some of
the AgCl by adding 1mL of 15M NH3. Centrifuge and decant the supernatant which
contains [[Ag(NH3)2]+(aq). To the silver solution add 6M HNO3 drop wise until the
solution is acidic. Note any re-precipitation of the AgCl. Record observations. Discard
the excess AgCl.
Confirmation of the ammonium ion
8. Dispense 5 drops of Group 1 solution to a qual tube and add 20 drops of 6M NaOH.
9. Place a moistened red litmus paper on the mouth of the test tube.
10. Place the test tube in a warm water bath. A change in the litmus from red to blue
confirms the presence of ammonia.
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The addition of aqueous NH3 to the supernatant containing the copper ions precipitates forms a
deep cobalt blue complex, [Cu(NH3)4]2+ .
Cu2+ + 4 NH3(aq) →
[Cu(NH3)4]2+(aq)
Unknown Determination of Group 1 Cations
Repeat the experiment substituting your unknown for the Group 1 known.
The unknown can also be determined by running both the Group 1 known and unknown side
by side.
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THE INSOLUBLE HYDROXIDES
Group II - Al+3, Mn+2,,Zn+2,Ni+2,Fe+3
Zn and aluminum ions are separated as aqueous [Al(OH)4-] and [Zn(OH)4 -2] from the cation
precipitates of Mn+2, Ni+2, Fe+3. A strong base added to a solution containing the five
cations form gelatinous hydroxides of Mn+2, Ni+2, Fe+3 but Zn+2 and Al+3 hydroxides are
amphoteric and re-dissolve in excess base. The solutions, [Al(OH)4-] and [Zn(OH)4 -2], are
separated from the precipitates of Mn+2, Ni+2, Fe+3 and analyzed.
With the exception of aluminum and zinc, the elements in this group are transition metals
with incompletely filled d subshell. This leads to a variety of oxidation states. Most of these
elements have a great tendency to form colored complex ions. Careful observation of the
color of the unknown solutions and of the colors produced along the way can provide useful
clues in the identification of this group. But remember, colors can be misleading.
Group II Flow Chart
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PROCEDURE
Separation of zinc and aluminum from hydroxides precipitate
1. Add 1mL of known solution II into a qual tube and add 15 drops of 6 M NaOH.
Centrifuge and test for completeness of precipitation. Transfer the supernatant to another
test tube with a Pasteur pipet. Save the supernatant for step 9.
2. Wash the precipitate and discard the washing. Dissolve the precipitate with a minimum
amount of concentrated HNO3.
Test for Mn+2
Manganese ion is treated with sodium bismuthate, a strong oxidizer. The bismuthate ion,
BiO3-, oxidizes the Mn+2 to the soluble purple permanganate, MnO-4. The color change may
be fleeting but nevertheless confirms the presence of Mn+2.
2 Mn+2 + 5 BiO3- + 14 H+ → 2 MnO4- + 5 Bi+3 + 7 H2O
3. Transfer 5 drops of the solution from #2 into another qual tube save the remaining
solution for step 5.
4. To the 5 drops from step 3, add a scoopula tip full of NaBiO3. Note the appearance of a
purple color which sometimes can be fleeting. This confirms the presence of Mn+2.
Separation and confirmation of Fe+3
Iron cation is separated from the nickel cation with the addition of solutions of NH4Cl and
NH3. Fe+3 forms a brown precipitate, Fe(OH)3 and a soluble blue hexammine complex
[Ni(NH3)6]+2 forms in solution.
Fe+3 + 2 NH3(aq) + 3 H2O → Fe(OH)3(s) + 3 NH4+
Ni+2 + 6 NH3(aq) → [Ni(NH3)6]+2
The Fe(OH)3 precipitate is dissolved in acid and confirmed by the addition of ammonium
thiocyanate.
Fe+3 + SCN- → [FeSCN4]5. Transfer 12 drops from the solution saved from step 3.
6. Add 5 drops of 4 M NH4Cl and then drops of concentrated NH3 until the solution has a
pH of 10. Centrifuge and separate the supernatant from the precipitate. Save the
supernatant for step.8.
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7. Dissolve the precipitate with 6 M HCl and add the 6 drops of 0.1 M NH4SCN. A blood
red solution due to the thiocyanatoiron (III ) complex confirms the presence of Fe+3.
Confirmation of Ni+2
The confirmation of the nickel ion is the precipitation of the hexaamminenickel(II)
complex,[Ni(NH3)6]+2 with dimethylglyoxime (DMG).
Ni+2 + 6NH3  [Ni(NH3)6]+2 (aq)
[Ni(NH3)6]2+ + 2 H2DMG  Ni(HDMG)2(S) + 2NH4+ + 4NH3
strawberry red
8. Check to make sure that the pH is 8, if too basic add 6M HNO3 to the supernatant from
step 6. Make sure that the pH is 8 and then add 5 drops of dimethylglyoxime. The
appearance of a pink precipitate confirms the presence of Ni+2.
Separation of Al+3 from Zn+2
The supernatant containing [Al(OH)4-] and [Zn(OH)4 ]-2 when acidified reforms aqueous
Al+3 and Zn+2. Ammonia is added to reprecipitate Al+3 to Al(OH)3 (s) while Zn+2 forms the
soluble tetraammine complex, [Zn(NH3)4]+2.
[Al(OH)4]-(aq) + 4H+  Al+3 + 4 H2O
↓ 3 NH3
Al(OH)3(s) + 3 NH4+ + H2O
[Zn(OH)4 -2](aq) + 4 H+  Zn+2 + 4 H2O
↓ 4 NH3
[Zn(NH3)4]+2 (aq)
Separation and Confirmation of Al+3
The adjustment of the pH of [Al(OH)4]-(aq) to mild alkaline produces precipitate, Al(OH)3, an
opaque, gelatinous precipitate. The addition of the aluminon (aluminum reagent) dyes the
precipitate pink to red.
Al+3 + 3 NH3(aq) + 3 H2O + aluminon  Al(OH)3∙aluminon(s) + NH4-
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9. Acidify the supernatant from step 1 with 6 M HCl using a litmus paper to ensure acidity.
Add drops of 6 M NH3 until pH 8-9 is reached.
10. Heat in a boiling bath to digest the gelatinous precipitate. Centrifuge and separate the
supernatant and save for step 13.
11. Wash the precipitate twice with hot deionized water and discard the washings.
12. Add 6 M HNO3 until the precipitate just dissolves. Add 5 drops of aluminon and 3 M
NH3 until the solution is pH 9. The presence of a strawberry red precipitate indicates the
presence of Al3
Confirmation of Zn+2
When potassium hexacyanoferrate (II), K2[Fe(CN)6] is added to an acidified solution of
[Zn(NH3)4]+2 , a light green precipitate of K2Zn3[Fe(CN)6]2 (s) forms, confirming the presence of
Zn+2.
3 [Zn(NH3)4]+2(aq) + 4 H+  3 Zn+2 + 4 NH3(aq)
↓ 2 K2[Fe(CN)6]
K2Zn3[Fe(CN)6]2 (s)
13. To the supernatant from step 10, add 1 M HCl until pH 4 is reached. Add 3 drops of 0.2 M
K2[Fe(CN)6] and stir. A very light green precipitate that is slow to form confirms the
presence of Zn+2. Centrifuging might be necessary to visualize the precipitate.
Unknown Determination of Group II Cations
Repeat the experiment substituting your unknown for the Group II known.
The unknown can also be determined by running both the Group 1I known and unknown side
by side.
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.
THE CARBONATE INSOLUBLE CATIONS
Group III - Ca+2, Sr+2, Ba+2
The elements comprising the carbonate group are quite similar and have many properties in
common. Their components exhibit only one stable oxidation state; they do not form amphoteric
hydroxides, being distinctly basic; and they do not readily form complex ions. The separation of the
alkaline earth elements depends almost entirely on differences in the solubilities of their salts,
which show a regular gradation through the periodic table.
The group reagent used to precipitate the alkaline earth ions is ammonium carbonate, (NH4)2CO3.
Since this is a salt of weak base and of a weak acid, it will hydrolyze in solution to a marked extent,
forming the hydrogen carbonate ion and ammonia.
NH4+ + CO32-  HCO3- + NH3
This results in a lower concentration of the carbonate ion, on which the precipitation of insoluble
carbonates depends. To prevent this, and thereby increase the carbonate ion concentration, the
precipitation is carried out in a strong aqueous ammonia solution.
The confirmation tests for each of the three Group III cations are the precipitates that form with the
test reagents.
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Group III Cations
Ba+2, Sr+2, Ca+2
HNO3
∆
HCl
NH3, NH4Cl
(NH4)CO3
BaCO3
SrCO3
CaCO3
CH3COOH
NH3
NH4CH3COO
Ethanol
K2CrO4
BaCrO4
SrCrO4(aq)
CaCrO4(aq)
CaCrO4(aq)
SrCrO4(s)
Na2C2O4
CaC2O4
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.
PROCEDURE
1. Place 1/2mL of the known III solution in an evaporating dish or crucibles. Add 5 drops of
15M HNO3 and heat until dense white vapors of ammonium salts are no longer evolved.
The evaporating dish must not be baked red hot.
2. Cool and add 10 drops of 6M HCl and 10 drops of water. Stir until the residue is dissolved
and decant to a test tube. Rinse the evaporating dish with 5 to 10 drops of water and add the
rinse to the test tube.
3. If the solution is not clear, centrifuge and discard the residue.
4. Add 1 drop of 6M NH4Cl. Adjust the pH to 9 with 15M NH3. Add 5 drops of (NH4)2CO3
until precipitation is complete. Warm in a water bath to coagulate the precipitate. Centrifuge
and separate and discard the supernatant.
5. Wash the precipitate, centrifuge and discard the washings.
Separation of barium from calcium and strontium
6. Dissolve the precipitate by adding a minimum of 3M acetic acid drop wise while stirring..
Add half as many more drops of 3M ammonium acetate. Warm the solution in a water
bath. Add 1M K2CrO4 drop wise to the solution until precipitation is complete. Centrifuge
and separate the supernatant. Save the supernatant
Confirmation of Barium
7. The formation of the precipitate, BaCrO4, confirms the presence of Ba+2.
Separation and Confirmation of Calcium from Strontium
Separation of aqueous strontium chromate, SrCrO4, is performed by the addition of ethanol.
The decrease of SrCrO4 solubility in ethanol allows the compound to come out of solution. The
solid SrCrO4 confirms the presence of Sr+2.
8. Neutralize the solution of calcium and strontium with 3M NH3 until basic. Add a volume of
ethanol equal to the volume of the original solution. Centrifuge and separate the supernatant.
Save the supernatant for step 9
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Confirmation of Calcium
Ca2+ reacts with oxalate ion, C2O4-2, to form a white precipitate, CaC2O4. A positive reaction
is a generous amount of precipitate. Minimal precipitation should be considered negative.
9. Make the supernatant basic to pH 9 using 3M NH3. Add 5 drops of 0.25M (NH4)2C2O4. Heat
the solution in a boiling water bath. Allow a few minutes for complete precipitation.
Centrifuge and discard the supernatant. Wash the precipitate with 10 drops of water and a
drop of (NH4)2C2O4. Centrifuge and discard washings. The precipitate should be white.
Unknown Determination of Group 1II Cations
Repeat the experiment substituting your unknown for the Group 1 known.
The unknown can also be determined by running both the Group 1 known and unknown side
by side.
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