The document discusses qualitative analytical chemistry techniques for separating and identifying cations and anions in aqueous solutions. It provides examples of separation schemes based on differences in solubility rules of metal hydroxides and oxides. Confirmatory tests are described to verify the presence of specific cations like Fe3+, Al3+, Pb2+, and Ni2+ after separation. The document also discusses approaches for distinguishing between common anions using chemical reactions.

1. Qualitative Analytical Chemistry
Dr Mark Selby
E-Block E413D (GP)
m.selby@qut.edu.au

2. Baia Mare Tailings Dam Disaster
More than 1,400
tons of fish have
died as a result.

3.  In the first week we considered the problem of
qualitatively identifying a single compound/ion on the
basis of its chemical properties by their characteristic
chemical reactions.
 What if the compound/ion we seek to identify is
mixed with other compounds or ions?
 Won’t the compounds or ions interfere with each
other? Making identification difficult or impossible?
Qualitative Analysis

4.  To answer this problem: separation schemes have
been employed to separate cations from one another
(into groups).
 Then to apply confirmation tests to verify the
identity of the individual cations that have been
group separated.
 After group separation, the separated cations
generally don’t interfere with each other.
Separation Schemes

5.  Qualitative Analysis:
 used to separate and identify cations and/or anions in an
unknown substance or solution
 the “semimicro” level used of qualitative analysis employs
methods used to detect 1-2 mg of an ion in 1 - 5 mL of solution
 “macro” level is approx. 0.1 M solutions and 1 – 10 mL volume
(Practical 212.1)
 The classical method (Vogel) involves separation of ions into 5
groups using (in part) the differing solubilities of sulfides under
acid or alkaline conditions.
 H2S is used to generate the insoluble sulfides. H2S is extremely
toxic and unpleasant (smells like rotten eggs).
Separation Schemes

6.  The separation scheme that we will develop here is
based upon the differences between amphoteric and
basic oxides/hydroxides.
 Recall that amphoteric oxides dissolve in both acid
and alkaline solutions.
 Basic oxides/hydroxides only dissolve in acid
solutions.
Amphoteric Oxides/hydroxides

7.  Example problem: we have a solution containing both
Fe(III) and Al(III) cations. Design a separation scheme to
separate and identify both the Fe(III) and Al(III) cations.
 Step 1: Add a few drops of NaOH solution to precipitate
both the Fe(III) and Al(III) cations as their hydroxides.
 Step 2: Since Al(IIl) is amphoteric continue to add NaOH
until it all dissolves.
Example Problem
Aluminium hydroxide (white ppt)
is amphoteric and dissolves to
yield a clear solutions when either
excess acid or alkaline solution is
added.

8.  Step 3: we now have Fe(OH)3 in
the precipitate and Al(OH)- in the
supernatant solution. Use a
centrifuge to separate all the
Fe(OH)3(s) to the bottom and then
pipette all of the Al(OH)-(aq) into a separate test tube. Rinse and
repeat.
 Step 4: To the test tube containing the Fe(OH)3(s), add strong acid
to dissolve the precipitate an then perform a confirmatory test for
Fe(III) cations (potassium ferrocyanide  Prussian blue).
Example Problem
Iron (III) hydroxide (red-brown ppt) is
basic and doesn’t dissolve in excess
NaOH. This allows the Fe(OH)3 and
Al(OH)3 to be separated.

9.  Step 5: to the test tube containing
Al(OH)- (aq) add acid until the
solution is around pH 9. Test for
Al3+ using aluminon reagent.
Example Problem
The confirmatory test for Al(III) is the
formation of a gelatinous “red lake”
precipitate when aluminon reagent is
added and the solution is made slightly
alkaline.

10. Tutorial 2
Practical Manual
Separation Scheme

11. Problem: Design a separation scheme to separate the following
4 cations from an aqueous mixture:
Al3+ Fe3+ Ni2+ Pb2+
Using only the following reagents:
6M HCl 6M NH3 6M NaOH
Once the ions have been separated, to identify each of the ions
using confirmatory tests.
Problem: Separation of ions

12. Hypothetical separation scheme for the ions above: Suppose you have a
reagent X that you know will precipitate Al3+ and Fe3+ as insoluble salts but not
Ni2+ or Pb2+. You can show the results on a flow chart such as the one below:
Hypothetical Separation Scheme
In the laboratory you can then centrifuge the solid precipitate (containing
AlX and FeX), and the supernatant liquid (now containing only the ions Ni2+ and
Pb2+) can be decanted into another test tube.

13. Hypothetical Separation Scheme
(cont)
You have now reached the point where at
least Fe3+ and Al3+ are in separate test
tubes. The next step would be to test for
the presence of Al3+ in the solution and to
redissolve FeX so that you can test for Fe3+.
Suppose you now find another
reagent Y that will dissolve AlX but
not FeX. This will enable you to
separate Al3+ from Fe3+, and the
next part of the flow chart would
now look like the diagram shown.

14.  The hypothetical scheme above has allowed us to separate
Fe3+ and Al3+ from the other two ions and to find a way to
test for the presence of the iron(III) and aluminium ions in
your unknown.
 The next step would be to work out a method of
separating Ni2+ and Pb2+ from one another and testing for
their presence.
 This hypothetical scheme is for illustrative purposes only.
The task now is to find a scheme that will actually solve
this problem based upon the information given in the
previous lecture(s).
Hypothetical Separation Scheme
(cont)

15.  Pb2+ is the only ion to form a ppt with … HCl
 Pb2+ and Al3+ both dissolve in excess … NaOH but Ni2+
and Fe3+ do not (they remain behind as ppts)
 All 4 ions (Pb2+, Al3+, Ni2+, and Fe3+ ) form insoluble
hydroxides (ppts) with …. NH3 and with … NaOH
 Ni2+ is the only ion to dissolve in excess … NH3
 Insoluble hydroxides (ppts) of all 4 ions (Pb2+, Al3+,
Ni2+, and Fe3+ ) dissolve in excess … HCl
Deductions
(from the preceding tables of results)

16.  Use the information from the preceding deductions to
construct a flow chart for the separation of the 4 ions
(Pb2+, Al3+, Ni2+, and Fe3+ ) using the 3 reagents (NaOH, NH3,
HCl)
 The intended outcome of the flow chart is to separate
each cation from the original mixture into 4 separate test
tubes as aqueous ions
 There are at least 3 possible satisfactory solutions to this
problem (but only 1 of these solutions is “optimal”)
Separation Flow Chart

17.  Group I: Ag+, Hg2
2+, Pb2+
Precipitated in 1 M HCl
 Group II: Bi3+, Cd2+, Cu2+, Hg2+, (Pb2+), Sb3+ and Sb5+, Sn2+
and Sn4+
Precipitated in 0.1 M H2S solution at pH 0.5
 Group III: Al3+, (Cd2+), Co2+, Cr3+, Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+
Precipitated in 0.1 M H2S solution at pH 9
 Group IV: Ba2+, Ca2+, Mg2+
Ba2+, Ca2+, and Mg2+ are precipitated in 0.2 M (NH4)2CO3 solution
at pH 10; the other ions are soluble
 Group V: K+, Na+, NH4
+; the remaining soluble ions
Group Separations for Cations
(a generalized separation scheme)

18.  Once the cations have been separated into groups, chemical
tests can be carried out in order to confirm the existence of each
cation.
 Test for aluminium, Al3+: Using a few drops of the solution
containing Al3+, make it strongly acidic with 6 M HCl. Add 1 drop
of aluminon dye, and then add 6 M NH3 dropwise until the
solution is basic to litmus paper. If present, Al3+ will form a
gelatinous precipitate of Al(OH)3 that absorbs the red dye to
give what is commonly called an "aluminium lake.“
 Test for iron(III), Fe3+: Make the solution acidic with 6 M HCl.
Add 1 drop of K4[FeII(CN)6] solution (potassium ferrocyanide). A
blue precipitate of KFeIII[FeII(CN)6] (usually called "Prussian
blue") confirms the presence of iron(III).
Confirmatory tests for cations

19.  Test for lead, Pb2+: Make the solution to be tested neutral.
Then add 2 drops of 0.2 M K2CrO4. Mix and centrifuge. A
yellow precipitate of PbCrO4 indicates the presence of lead
ion.
 Test for nickel, Ni2+: Make the solution basic with 6 M NH3.
Add several drops of a solution of the ligand
dimethylglyoxime and mix well. If Ni2+ is present, a
strawberry-red precipitate will form.
 Additional, test for copper, Cu2+: Add 6 M NH3, a pale blue
precipitate of copper(II) hydroxide that forms initially is
soluble in excess NH3 to give a deep royal-blue solution of
the ammine complex [Cu(NH3)4]SO4.
Confirmatory tests for cations (cont)

20. Spot Plates
Pure white porcelain plate
for ease of observing
reactions with color
changes
• Identify ions based
upon their
characteristic chemical
reactions
• Use specific reagents
which produce a
characteristic reaction
e.g., using a spot plate.

21. Hach® Portable Water Monitoring Kit

22.  Scheme of classification – The methods available for
detection of anions are not as systematic as described
above for cations
 No entirely satisfactory scheme has yet been proposed
which separates the common anions into major groups,
with the unequivocal separation of each group into its
independent constituents - as is the case with cations
 Essentially, the process adopted is to divide anions into
Class A – those that involve identification by volatile
products obtained upon treatment with acids - and Class B
- those dependant on reactions in solution.
Tests for anions

23. Class A
those anions reacting with dilute hydrochloric or dilute
sulfuric acid:
 carbonate, hydrogen carbonate, sulfite, thiosulfate, sulfide,
nitrite, hypochlorite, cyanide and cyanate.
those anions reacting with concentrated sulfuric acid:
 fluoride, chloride, bromine, iodide, nitrate, chlorate (danger!),
perchlorate, permanganate (danger!), bromate, borate,
ferrocyanide, ferricyanide, thiocyanate, formate, acetate,
oxalate, tartrate, and citrate.
Tests for anions

24. Class B
those anions undergoing precipitation reactions:
 Sulfate, phosphate, phosphite, hypophosphite,
arsenate, chromate, dichromate, silicate, salicylate,
benzoate and succinate.
those anions undergoing oxidation-reduction reactions:
 manganate, permanganate, chromate, and dichromate.
Tests for anions

25.  Discuss how you would distinguish between sulfate and sulfite anions in
an aqueous solution. Use relevant chemical equations to illustrate your
reasoning.
 You are given an unlabelled solution which may contain either nitrate or
nitrite anions. How would you determine whether the anion is NO2
- or
NO3
- ? Use relevant chemical equations in your explanation.
 Sodium bromide was rumoured to have been used during WWII as an
agent for temporary sterilisation of servicemen. How could you
determine if your table salt (sodium chloride) had been replaced by
sodium bromide? Use chemical equations in your answer.
 How would you distinguish between sodium carbonate and sodium
hydrogen carbonate using a simple chemical test? Use chemical
equations in your answer.
Example exam questions

26. Brown ring test:
 Iron(II) sulfate solution is added to a
nitrate solution in a test tube. Then
2-3 mL of concentrated H2SO4 is
poured down the side of the test tube
so that the acid forms a layer below the nitrate mixture.
 A brown ring will form where the liquids meet.
 The brown coloration is due to the transient formation of
[Fe(NO)]SO4.
 Upon shaking and warming nitric oxide is released and
light-green iron(II) sulfate remains in the test tube.
Confirmatory test for nitrate

27. Interferences:
 Bromides and iodides interfere with the brown ring test. The test
is also unreliable in the presence of chromates, sulfites,
thiosulfates, iodates, cyanides, thiocyanates, ferro- and ferri-
cyanides. All of these interferences can be removed with
addition of nitrate-free Ag2SO4 and filtering the insoluble silver
precipitate that forms.
 Nitrites react similarly to nitrate. Nitrites can be removed by
adding sulfamic acid (HO.SO2.NH2) to decompose it:
HO.SO2.NH2 + NaNO2 ↓ N2 + NaHSO4 + H2O
Confirmatory test for nitrate