Experiment 3 - Systematic Qualitative Analysis of Cations

Group I

Ag+, Pb2+

Group II

Cu2+, Bi3+, Sn2+

Group III

Zn2+, Mn2+, Fe3+, Co2+, Ni2+

Group IV

Ba2+, Sr2+, Ca2+, Mg2+

Group V

Na+, K+

Lead ion (in the form of plumbous ion) and silver ion

are the only Group I cations selected for this procedure

Lead ion (in the form of plumbous ion) and silver ion

both give white precipitate with chloride ions

3M HCl

used as the chloride source

Separation of PbCl2 from AgCl

effected with hot water

PbCl2

becomes more soluble at elevated temperature. This converts back lead to ionic form

Chromate ions (CrO4²- )

produces a yellow precipitate

Addition of 3M NH3 to a precipitate of AgCl

the precipitate dissolves forming a complex of silver and ammonia

Addition of 3M HNO3

acidifies this solution and destroys the complex (silver & ammonia)

Cations of Group II

are precipitated as sulfides with hydrogen sulfide in an acidic condition

Hydrogen sulfide

from the hydrolysis of thioacetamide in aqueous solution when heated

Cupric ions

precipitate as black (CuS)

Bismuth ions

precipitate as dark brown (Bi2S3)

Stannous (tin) ions

precipitate as brown (SnS)

Dilute NaOH solution

dissolves SnS (the only Group IIB in the selected cations) because of its acidic nature

CuS and Bi2S3

both having basic characters (Group IIA), are not affected by the addition of NaOH, and therefore, remain in the precipitate

Sulfides of Copper and Bismuth

are oxidized by dilute nitric acid

Sulfur

the sulfides are oxidized to _______, and the cations are brought into solution

Excess ammonia

the bismuth ion is precipitated as Bi(OH)3 by _________ in the presence of cupric ion

Tetraamine complex

the cupric ions remain in the solution as they are converted to a _________ with a deep blue color

Addition of HCl

resulting Bi(OH)3 precipitate is dissolved by the ___________, resulting into a mixture containing Bi³+ and Cl-

White BiOCl

Placing this mixture in water produces ___________ which is seen as white cloudiness

Deep Blue

color produced by the complex ion, Cu(NH3)4²+

Addition of NH3

the deep blue color produced by the complex ion, Cu(NH3)4²+, formed upon the ____________ to a solution of copper ions

Alkaline Supernatant Liquid

contains tin in the form of a soluble complex anions, Sn(OH)6²- and SnS3²-

When this solution is acidified,

the complex ions are destroyed and the tin reprecipitates back as a sulfide

Addition of excess HCl

dissolves SnS into a chloride complex

Addition of magnesium metal

converts the Sn4+ back to Sn2+

Addition of mercuric chloride

produces a white or gray precipitate of mercurous chloride (Hg2Cl2) or metallic mercury (Hg)

Adding ammonia buffered by NH4Cl

The requirement of the precipitation of Group III cations is that it must be carried out in a basic condition brought about by ___________

Ferric ion (Fe3+)

This precipitates ____________ that may be present to its hydroxide

Soluble ammonia complexes

This converts nickelous, cobaltous, and zinc ions into

Manganous ions

Have no apparent change with the addition of ammonia

Treated with H2S

the mixture of these ammonia complexes, Fe (OH)3, and manganese ions are converted into sulfides when

Sulfides of iron, manganese, and zinc

dissolves readily in HCl bring back these ions to solution

Nickel and cobalt sulfides

dissolve slowly in HCl, but readily in HNO3

Water-soluble anion

Treatment of the solution that contains these ions with H2O2 and excess NaOH converts Zn2+ (Group IIIA cation) into

Fe3+, Co2+ and Ni2+

are precipitated as hydroxides

Manganese ions

are oxidized from +2-oxidation state to +4-oxidation state, and is precipitated as a hydrated oxide

Ammonia complex Zn(NH3)4²+

Treatment of the solution containing soluble zinc complex with ammonia leaves the zinc ion in the form of an

White precipitate of ZnS

If this solution is acidified with acetic acid (HAc) and treated with H2S, a ____________ is produced

Concentrated HCl

Readily dissolves hydroxides of iron, cobalt, and nickel, and hydrated manganese dioxide

Dilute HCl

Sodium bismuthate, when added to a cold solution of manganous ions containing

Excess thiocyanate ions (SCN-)

Ferric ions reacts with

Acetic acid and KNO2

Cobaltous ions react with

Dimethylglyoxime (C4H8O2N2)

Nickelous ion reacts with

Purple-colored permanganate ion (MnO4-)

Sodium bismuthate, when added to a cold solution of manganous ions containing dilute HCl, oxidizes Mn2+ to the __________

Blood red complex

Ferric ions reacts with excess thiocyanate ions (SCN-) to produce

Olive-yellow colored

Cobaltous ions react with acetic acid and KNO2 in a multi-step reaction to form an ___________ potassium cobaltinitrite

Voluminous red precipitate

Nickelous ion reacts with dimethylglyoxime producing a _____________ of nickel dimethylglyoxime

Carbonates

the cations of Group IV, except magnesium ion, are precipitated as __________ using (NH4)2CO3 in basic condition (NH3-NH4Cl buffer)

Decomposition of (NH4)2CO3

this reaction requires heating, but it needs to be ensured that the reaction should not be boiled because boiling causes __________ to NH3 and CO2

Incomplete precipitation

Decomposition of (NH4)2CO3 to NH3 and CO2 results to an

Acetic acid (HAc)

the resulting carbonates can dissolve readily in

Potassium chromate (KCrO4)

precipitates yellow barium chromate from an acetic acid solution of barium ions

BaCrO4

not sufficiently volatile to give a satisfactory flame test for the barium ion

Chromic ion

If BaCrO4 is evaporated with concentrated HCl, the chromate ion is reduced to

Yellow green color

When heated in the Bunsen flame, barium salts, especially the chloride, impart a ____________ color to the flame

Ammonium sulfate (NH4)2SO4

precipitates white SrSO4 from an acetic acid solution of strontium

Triethanolamine

first added to ensure that the calcium ion is not precipitated together with strontium ion

SrSO4

sufficiently soluble in concentrated HCl to give a flame test for the strontium ion

Crimson Red Color

When heated in a Bunsen flame, strontium salts, especially the chloride, impart a _________ color to the flame

Ammonium sulfate (NH4)2C2O4

Precipitates white CaC2O4 from an acetic acid solution of calcium ion

CaC2O4

Is sufficiently soluble in concentrated HCl to give a flame test for the calcium ion

Brick Red color

When heated in bunsen flame, calcium salts, especially the chloride, impart a __________ color to the flame

Dibasic ammonium [(NH3)2HPO4)]

Precipitated magnesium ions from an ammoniacal solution as white, crystalline magnesium ammonium phosphate

White, gelatinous magnesium hydroxide

NaOH precipitates __________ from a solution of magnesium ions

NaOH precipitate

Has the property of adsorbing certain organic dyes to form colored lakes

Blue lake

One such dye is p-nitrobenzeneazoresorcinol (S and O reagent), which forms a _________ with Mg(OH)2

White, gelatinous Mg(OH)2

In the absence of this dye, the formation of the __________ is already a sufficient confirmatory test already

No chemical tests

The procedure above offers __________ for either the sodium or the potassium ions

This ____________ is because most salts of sodium and potassium are too soluble to be adequate for precipitation tests

More satisfactory

The flame tests are __________ than any precipitation tests for sodium and potassium ions