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Qualitative Analytical Chemistry

Qualitative Analytical ChemistryDr Mark SelbyE-Block E413D (GP)m.selby@qut.edu.au

1Baia Mare Tailings Dam Disaster

More than 1,400 tons of fish have died as a result.2In 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?Wont the compounds or ions interfere with each other? Making identification difficult or impossible?Qualitative Analysis3To 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 dont interfere with each other.Separation Schemes4Qualitative Analysis: used to separate and identify cations and/or anions in an unknown substance or solutionthe semimicro level used of qualitative analysis employs methods used to detect 1-2 mg of an ion in 1 - 5 mL of solutionmacro 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 Schemes5The 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/hydroxides6Example 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.7Step 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 ProblemIron (III) hydroxide (red-brown ppt) is basic and doesnt dissolve in excess NaOH. This allows the Fe(OH)3 and Al(OH)3 to be separated.

8Step 5: to the test tube containing Al(OH)- (aq) add acid until the solution is around pH 9. Test forAl3+ using aluminon reagent.

Example ProblemThe 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.

9Tutorial 2Practical ManualSeparation Scheme10Problem: 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 ions11Hypothetical 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.12Hypothetical 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.13The 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)14Pb2+ is the only ion to form a ppt with HClPb2+ 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 NaOHNi2+ is the only ion to dissolve in excess NH3Insoluble hydroxides (ppts) of all 4 ions (Pb2+, Al3+, Ni2+, and Fe3+ ) dissolve in excess HCl

Deductions(from the preceding tables of results)15Use 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 ionsThere are at least 3 possible satisfactory solutions to this problem (but only 1 of these solutions is optimal)

Separation Flow Chart16Group I: Ag+, Hg22+, Pb2+Precipitated in 1 M HClGroup II: Bi3+, Cd2+, Cu2+, Hg2+, (Pb2+), Sb3+ and Sb5+, Sn2+ and Sn4+Precipitated in 0.1 M H2S solution at pH 0.5Group III: Al3+, (Cd2+), Co2+, Cr3+, Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+Precipitated in 0.1 M H2S solution at pH 9Group IV: Ba2+, Ca2+, Mg2+Ba2+, Ca2+, and Mg2+ are precipitated in 0.2 M (NH4)2CO3 solution at pH 10; the other ions are solubleGroup V: K+, Na+, NH4+; the remaining soluble ions

Group Separations for Cations(a generalized separation scheme)17Once 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 cations18Test 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)19Spot PlatesPure 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.

20Hach Portable Water Monitoring Kit

21Scheme of classification The methods available for detection of anions are not as systematic as described above for cationsNo 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 cationsEssentially, 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 anions22Class 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 anions23Class 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 anions24Discuss 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 duri


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