QUALITATIVE ANALYSIS FOR CAPE CHEMISTRY8

QUALITATIVE ANALYSISThe primary objective of qualitative analysis is the identification of the constituents of a substance, be it a single solid or a mixture of solid substances, a solution, a gas or a mixture of gases. This identification usually involves the conversion of the unknown into some other compound which can be identified by its known characteristic properties. For example an unknown compound can identified as being a sulphate if it produces, on reaction with a mixture of barium chloride solution and hydrochloric acid, a white precipitate. The reactions carried out are called tests.Tests may be divided into preliminary and confirmatory. Preliminary gives an indication of what may be present where as confirmatory as the name suggests, confirms what was indicated to be present.Preliminary TestsSome preliminary tests are listed below:NB. Some preliminary tests may serve as confirmatory tests.(1) AppearanceNote the colourAppearanceDeductions

White solid or colourless solutionProbable absence of transition elements

Coloured solid or coloured solutionProbable presence of transition elements

Black solidProbable presence of a sulphide or oxide

Some characteristic colours of aqueous solutions of ions containing transition elements:IonColour

Cr3+(aq)Green, blue green or pale violet

CrO42-(aq)yellow

Cr2O72-(aq)orange

Cu2+(aq)Blue, blue green or green

Fe2+(aq)Pale green, pale blue green

Fe3+(aq)Yellow brown, brown

Mn2+(aq)Very pale pink

Ni2+(aq)green

The underlined are the more common colours.(2) Flame TestsFlame tests are carried out using nichrome wire ( cleaned with conc. HCl ). It identifies the presence of cations.The cleaned wire is dipped into the solid sample, then placed in the flame. A characteristic colour of the flame is observed. Some are listed in the table below:

CationColour of Flame

Cu2+Bright green

Na+yellow

Ca2+Brick red

Ba2+Yellow green

K+ lilac

(3) Action of Heat ( on solids )Observation(s)Inference(s)

Liquid (water) condenses at top of tube which either turnsa) blue CoCl2 paper pinkb) white anhydrous CuSO4 bluewhen GENTLY heated.Solid is a hydrate or contains water of crystallization

When STRONGLY heated:Colourless, odourless gas (CO2) is evolved which turns lime water milky.Colouless, pungent gas (NH3) which turns moist red litmus blue or form dense white fumes with HCl(g)Colourless, choking odour gas (SO2 ) which turns orange H+/K2Cr2O7(aq) green or purple H+/KMnO4(aq) colourless

Reddish brown, pungent odour gas (NO2) which turns moist blue litmus redPurple vapour

No reaction ( recorded as no visible change)

Purple black solid formed (I2) on the cooler parts of the tubeSolid is a carbonate or a bicarbonate

Solid contains NH4+ ionSolid contains either S2-, SO32-, or SO42- ion

Solid contains NO3- ion

Iodine I2.

Solid is an ionic compound with high lattice energy

Solid contains iodide (I-) ion

(4) Action of dilute HClObservation(s)Inference(s)

Effervescence, gas (CO2) is evolved which turns lime water milky.Substance contains CO32- .

Colourless, choking odour gas (SO2 ) which turns orange H+/K2Cr2O7(aq) green or purple H+/KMnO4(aq) colourlessSubstance contains SO32-.

Reddish brown, pungent odour gas (NO2) which turns moist blue litmus red.Substance contains NO2-.

(5) Action of conc. H2SO4 ( on solids)Observation(s)Inference(s)

Colourless, lachrymatory gas (HCl) which eithera) fumes in moist airb) turns moist blue litmus red orc) forms dense white fumes with ammonia gasRed brown gas (Br2) which turns moist blue litmus red then bleaches it.Purple vapour (I2(g)) which bleaches litmus and which turns starch solution dark blue.Solid contains chloride (Cl-) ion

.Solid contains bromide (Br-) ion

Solid contains iodide (I-) ion

(6) Action of conc. HCl ( on solids)Observation(s)Inference

Greenish yellow gas (Cl2) which bleaches litmusSolid contains PbO2.

Confirmatory TestsFor some common anionsAnionTestObservation

SO32- Add acid followed by KMnO4Add Iodine solutionColour change from purple to colourlessColour change from brown to colourless

NO3-Add conc. H2SO4 then warm.Reddish brown gas (NO2)

PO43- Add HNO3 followed by ammonium molybdate solution.Yellow precipitate

For some cationsCationTestObservation

Pb2+ Add dilute HCl(aq)Add K2CrO4(aq)White ppt., soluble on heating.Yellow ppt., soluble in dilute HNO3(aq)

Fe2+ Add potassium hexacyanoferrate(II) soln.Add potassium hexacyanoferrate(III) solnPale blue ppt.Dark blue ppt.

Fe2+ Add potassium hexacyanoferrate(II) soln.Add Ammonium thiocyanate solutionDark blue ppt.Light red solution

Reactions of cations with NaOH(aq), NH3(aq) and Na2CO3(aq)The cations of concern: Cr3+, Al3+,Pb2+,Zn2+ , Ba2+,Ca2+,Mg2+ ,Mn2+, Fe2+, Fe3+, Ni2+,NH4+,Cu2+.Introduction:The reactions of the cations with these reagents will forma)hydroxides in the case of NaOH(aq) and NH3(aq) &b)carbonates in the case of Na2CO3(aq)A precipitate will form if the compounds formed are insoluble in water. It is therefore important to understand and memorize the solubility of hydroxides and carbonates.Only group I hydroxides ( these include for sodium and potassium) and hydroxides of barium and strontium of group II are soluble.Only the carbonates of group I ( these include for sodium and potassium) and ammonium carbonate are soluble.Some insoluble hydroxides are amphoteric (i.e. they can react with both acids and bases). These will react with excess strong bases to form soluble salts i.e. they will dissolve in the excess alkali to form solutions. Examples of ions that will form amphoteric hydroxides are Cr3+, Al3+,Pb2+,Zn2+.Some insoluble hydroxides will form more stable soluble complexes with excess ammonia molecules in solution. Examples of ions whose hydroxides will form stable soluble complexes with excess aqueous ammonia are Cu2+, Zn2+ and Ni2+.Precipitates and solutions which are coloured usually contain a transition element, so the ions that may be inferred are Cr3+, Mn2+, Fe2+, Fe3+, Ni2+ and Cu2+.Precipitates which are white and solutions which are colourless usually contain no transition elements, so one can infer the absence of Cr3+, Mn2+, Fe2+, Fe3+, Ni2+ and Cu2+. Carbonate precipitates are usually formed with either of the following:a) No further change. Examples include those formed from Pb2+, Zn2+, Ba2+,Ca2+, and Mg2+.b)Effervescence as CO2 is evolved. Usually the reactions of small cations with large charges will cause this. The solutions of these ions are acidic. Examples include Fe3+ and Al3+.c)The carbonates formed turned black on heating. Recall that some carbonates decompose on heating to form the oxide and carbon dioxide. It is the oxide formed which is black. Examples include those formed from Cu2+ and Ni2+.

CationEffect of NaOH(aq)Effect of NH3(aq)

Pb2+

White ppt.; soluble in excess to form colourless solution.Pb(OH)2(s) + 2OH-(aq)Pb(OH)42-(aq) white Plumbate ion (or leadate ion)White ppt.; insoluble in excessPb2+(aq) + 2OH-(aq). Pb(OH)2(s) white

Al3+

White ppt.; soluble in excess to form colourless solution.

whiteAl(OH)3(s) + OH-(aq)Al(OH)4-(aq) Aluminate ionWhite ppt.; insoluble in excessAl3+(aq) + 3OH-(aq). Al(OH)3(s) white

Zn2+

White ppt.; soluble in excess to form colourless solution.Zn(OH)2(s) + 2OH-(aq)Zn(OH)42-(aq)white zincate ionWhite ppt.; soluble in excess

Tetrammine zinc (II) ion( Stable complex)Zn(OH)2(s) + 4NH3(aq). Zn(NH3)42+(aq) + 2OH-(aq)

Cr3+Grey green ppt.; soluble in excess to form green solution.Cr(OH)3(s) + OH-(aq)Cr(OH)4-(aq) grey green green

Grey green ppt.; insoluble in excessCr3+(aq) + 3OH-(aq). Cr(OH)3(s) Grey green

Mg2+White ppt.; insoluble in excess.Mg2+(aq) + 2OH-(aq). Mg(OH)2(s)White ppt.; insoluble in excess.Mg2+(aq) + 2OH-(aq). Mg(OH)2(s)

Cation

Effect of NaOH(aq)

Effect of NH3(aq)

Ca2+

White ppt.; insoluble in excess.Ca2+(aq) + 2OH-(aq). Ca(OH)2(s) white

No ppt. [OH-] furnished by NH3(aq) is insufficient to cause the product of the ion concentrations,[Ca2+][OH-]2, to exceed Ksp(Ca(OH)2), so no precipitation.

Ba2+White ppt.; insoluble in excess ONLY with concentrated NaOH(aq).Ba2+(aq) + 2OH-(aq). Ba(OH)2(s) WhiteReason: Ba(OH)2 is quite soluble in water. However in concentrated NaOH(aq), the product of the ion concentration,[Ba2+][OH-]2, is EXCEEDS the Ksp(Ba(OH)2), so precipitation occursNo ppt..

Cu2+Light blue ppt.; insoluble in excess.Cu2+(aq) + 2OH-(aq). Cu(OH)2(s)blue light blue( Turns black on standing the suspension)Cu(OH)2(s) CuO(s) + H2O(l)Light blue ppt.; soluble in excess forming a dark blue solution.Cu(OH)2(s) + 4NH3(aq) Cu(NH3)42+(aq) + 2OH-(aq)light blue dark blue Tetraammine copper (II) ion ( stable complex )

CationEffect of NaOH(aq)Effect of NH3(aq)

Ni2+

Pale green ppt.; insoluble in excess.Ni2+(aq) + 2OH-(aq). Ni(OH)2(s)green pale greenPale green ppt.; soluble in excess to form a green solution.Ni(OH)2(s) + 6NH3(aq) Ni(NH3)62+(aq) + 2OH-(aq)pale green green hexaammine nickel(II) ion (stable complex)

NH4+

No ppt. Ammonia is produced on heating.Not applicable

Fe2+Dirty green ppt.; insoluble in excessFe2+(aq) + 2OH-(aq) Fe(OH)2(s)green dirty green( Turns red-brown on exposure to air; oxidized to Iron (III))Dirty green ppt.; insoluble in excessFe2+(aq) + 2OH-(aq) Fe(OH)2(s)green dirty green( Turns red-brown on exposure to air; oxidized to Iron (III))

Fe3+Red-brown ppt.; insoluble in excessFe2+(aq) + 2OH-(aq) Fe(OH)2(s)pale green dirty greenRed-brown ppt.; insoluble in excessFe2+(aq) + 2OH-(aq) Fe(OH)2(s)pale green dirty green

Mn2+Off white ppt. ;insoluble in excess. Precipitate turns light brown on exposure to air ; it is oxidized to manganese (III).Mn2+(aq) + 2OH-(aq) Mn(OH)2(s)pink off whiteOff white ppt. ;insoluble in excess. Precipitate turns light brown on exposure to air ; it is oxidized to manganese (III).Mn2+(aq) + 2OH-(aq) Mn(OH)2(s)pink off white

CationEffect of Na2CO3(aq)

Pb2+Zn2+Ba2+Ca2+Mg2+ All forms a white ppt. with no other change.General equation: M2+(aq) + CO32-(aq) MCO3(s) white

Al3+

White ppt., effervescence, evolution of CO2 Al3+(aq) + CO32-(aq) Al2CO3(s) white CO32-(aq) + H+(aq) H2O(l) + CO2(g) ( from Al3+(aq) )

Fe3+Red-brown ppt., effervescence, evolution of CO2. Fe3+(aq) + CO32-(aq) Fe2CO3(s) Yellow-brown Red-brown CO32-(aq) + H+(aq) H2O(l) + CO2(g) ( from Fe3+(aq) )

Fe2+

Cr3+Dirty-green ppt. with no other change. Fe2+(aq) + CO32-(aq) FeCO3(s) green dirty-green

Cr3+(aq) + CO32-(aq) Cr2CO3(s) green dirty-green

Cu2+

Ni2+Light-blue ppt.; turns black on heating. Cu2+(aq) + CO32-(aq) CuCO3(s) blue light-blue CuCO3(s) CuO(s) + CO2(g) black

Light-green ppt.; turns black on heating. Ni2+(aq) + CO32-(aq) NiCO3(s) green light-green NiCO3(s) NiO(s) + CO2(g) black

Text Frank Weise, 2009. All rights reserved.