Transcript

Qualitative Analytical ChemistryDr Mark Selby

E-Block E413D (GP)

[email protected]

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

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

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 cationsgenerally don’t interfere with each other.

Separation Schemes

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

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

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 NaOHuntil it all dissolves.

Example Problem

Aluminium hydroxide (white ppt) is amphoteric and dissolves to yield a clear solutions when eitherexcess acid or alkaline solution is added.

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.

Step 5: to the test tube containing Al(OH)- (aq) add acid until the solution is around pH 9. Test forAl3+ 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.

Tutorial 2Practical Manual

Separation Scheme

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

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 (containingAlX and FeX), and the supernatant liquid (now containing only the ions Ni2+ and Pb2+) can be decanted into another test tube.

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.

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)

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)

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

Group I: Ag+, Hg22+, 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)

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

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)

Spot Plates

Pure white porcelain plate for ease of observing reactions with colorchanges

• Identify ions based upon their characteristic chemical reactions

• Use specific reagents which produce a characteristic reaction e.g., using a spot plate.

Hach® Portable Water Monitoring Kit

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

Class Athose 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

Class Bthose 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

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

Brown ring test: Iron(II) sulfate solution is added to a

nitrate solution in a test tube. Then2-3 mL of concentrated H2SO4 is

poured down the side of the test tubeso 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

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


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