Qualitative Analysis of Cations

Our Objective

Our objective is to determine the cation present in a given salt.

The Theory

What is Qualitative Analysis?

Qualitative analysis is a method of Analytical chemistry that deals with the determination of elemental composition of inorganic salts. It is mainly concerned with the detection of ions in an aqueous solution of the salt.

The common procedure for testing any unknown sample is to make its solution and test this solution with various reagents for the ions present in it. Testing with various reagents gives characteristic reaction of certain ions, which may be a colour change, a solid formation or any other visible changes. There are separate procedures for detecting cations and anions, called the Cation Analysis and Anion Analysis.

Let us discuss about the Qualitative Analysis of Cations.

Qualitative Analysis of Cations

Preliminary Tests

Some preliminary tests needs to be done before doing the analysis of cations.

(a) Physical Appearance: Colour and Smell

The physical examination of the unknown salt involves the study of colour, smell and density. The test is not much reliable, but certainly helpful in identifying some coloured cations. Characteristic smell helps to identify some ions like ammonium ion.

(b) Charcoal Cavity Test

This test is based on the fact that metallic carbonates when heated in a charcoal cavity decomposes to give corresponding oxides. The oxides appear as coloured incrustation or residue in the cavity. In certain cases, the oxides formed partially undergo reduction to the metallic state producing metallic beads or scales.

Examples:

(c) Cobalt Nitrate Test (Ash test)

This test is applied to those salts that leave white residue in the charcoal cavity test. This test is based on the fact that metallic carbonates when heated in a charcoal cavity decomposes to give corresponding oxides.

The test is based on the fact that cobalt nitrate decomposes on heating to give cobalt oxide, CoO. This combines with the metallic-oxides present as white residue in the charcoal cavity forming coloured compounds.

For example, when a magnesium salt undergoes charcoal cavity test, a while residue of MgO is left behind. This on treatment with cobalt nitrate and on subsequent heating forms a double salt of the formula MgO.CoO, which is pink in colour.

Other examples are:

In addition to metallic oxides, phosphates and borates also react with cobalt oxide to form Co3(PO4)2 and Co3(BO3)2 that are blue in colour.

(d) Flame Test

Certain salts on reacting with conc. HCl forms their chloride, that are volatile in non-luminous flame. Their vapours impart characteristic colour to the flame. This colour can give reliable information of the presence of certain cations . For proceeding to this test, the paste of the mixture with conc.HCl is introduced into the flame using a platinum wire.

(e) Borax Bead Test

Borax, Na2B4O7.10H2O, on heating gets fused and loses water of crystallisation. It swells up into a fluffy white porous mass which then melts into a colourless liquid that later forms a clear transparent glassy bead consisting of boric anhydride and sodium metaborate.

Boric anhydride is non-volatile. When it is made to react with coloured metallic salt, a characteristic coloured bead of metal metaborate is formed. In those cases where different coloured beads are obtained in oxidising and reducing flames, metaborates in various oxidation states of metals are formed. For example, in oxidising flame, copper forms blue copper metaborate.

 In reducing flame, cupric metaborate is reduced to metallic copper, which is red and opaque.

Identification of Cations (Basic Radicals)

Analysis of Group Zero

This group includes ammonium ion (NH4+).

Chemical reactions involved in Group Zero analysis

(a) Sodium hydroxide test

When ammonium salt is heated with conc. NaOH, ammonia gas is evolved which gives white fumes with dil.HCl due to the formation of NH4Cl.

 (b) Nessler's reagent test

Tha ammonia gas formed by the reaction of ammonium ions with NaOH reacts with Nessler's reagent to form a brown precipitate of H2N.HgO.HgI.

Analysis of Group I

This group includes Pb2+, Ag+, Hg22+. Here we shall study only Pb2+. Group reagent for this group is dil. HCl.

Chemical reactions involved in Group I analysis

The addition of HCl to the solution will precipitate Pb2+ as lead chloride which is soluble in hot water. On cooling, the precipitate settle down as PbCl2 which is less soluble in cold water.

Confirmatory tests of Lead (II) ion (Pb2+)

(a) Potassium iodide test

Lead chloride (formed by the reaction of lead salts with dil. HCl) solution in hot water reacts with potassium iodide solution to form yellow precipitate of lead iodide.

(b) Potassium chromate test

Lead chloride (formed by the reaction of lead salts with dil. HCl) solution in hot water reacts with solution of potassium chromate to form yellow precipitate of lead chromate.

Analysis of Group II

This group includes Pb2+ and Cu2+ in IIA Group and As3+ in IIB Group.

Chemical reactions involved in Group II Analysis:

Passing of H2S gas through the acidified original solution will precipitate the radicals Pb2+,Cu2+,As3+ as their sulphides.  

Confirmatory tests for Lead (II) ion (Pb2+)

Black precipitate of PbS formed in the group analysis dissolves in 50% nitric acid due to the formation of soluble lead nitrate. On adding sulphuric acid to the soluble lead nitrate, lead sulphate precipitates. 

(a) Potassium iodide test

The soluble lead nitrate (formed by the reaction between PbS and 50% Conc. HNO3) reacts with potassium iodide to form yellow precipitate of lead iodide.

(b) Potassium chromate test

The soluble lead nitrate (formed by the reaction between PbS and 50% Conc.HNO3) reacts with potassium chromate to form yellow precipitate of lead chromate.

Confirmatory tests for Copper (II) ion (Cu2+)

Black precipitate of CuS formed in the group analysis dissolves in 50% nitric acid and a blue solution is obtained on addition excess of NH4OH. 

(a) Potassium ferrocyanide test

The above blue solution gives a chocolate brown precipitate with potassium ferrocyanide solurion.

(b) Potassium iodide test

In this test, the white precipitate is due to the formation of cuprous iodide and the brown colour of the solution is due to liberation of iodine.

Confirmatory tests of Arsenic (III) ion (As3+)

The yelllow residue of As2S3 formed in the group analysis is dissolved in conc.HNO3 forming arsenic acid.

(a) Ammonium molybdate test

Arsenic acid ( formed by dissolving As2S3 in conc.HNO3) reacts with ammonium molybdate to form yellow precipitate of ammonium arseno molybdate.

(b) Magnesia mixture test

Arsenic acid ( formed by dissolving As2S3 in conc.HNO3) reacts with magnesia mixture to form a white precipitate of Mg(NH4)2AsO4.

Analysis of Group - III

The cations present in this group are Fe2+, Fe3+, Cr3+ and Al3+. We will look at only Fe2+/ Fe3+ and Al3+.

Chemical Reactions Involved Group III Analysis

The cations in this group are precipitated as hydroxides by adding ammonium hydroxide in presence of ammonium chloride. Thus, group reagent for this group is NH4OH in the presence of NH4Cl. 

Confirmatory tests of Ferric ion (Fe3+)

The reddish brown precipitate of ferric hydroxide is dissolved in HCl due to the formation of soluble ferric chloride.

(a) Potassium ferrocyanide test

Ferric chloride (formed by dissolving ferric hydroxide in HCl) reacts with potassium ferrocyanide to form prussian blue coloured Ferric ferrocyanide.

(b) Potassium sulphocyanide test

Ferric chloride (formed by dissolving ferric hydroxide in HCl) reacts with potassium sulphocyanide to form blood red coloured Ferric sulphocyanide.

Confirmatory tests of Aluminium (III) ion ( Al3+)

(a) Lake test

Aluminium hydroxide formed in the group analysis dissolves in dil. HCl to form soluble aluminium chloride. The aluminium chloride thus formed reacts with ammonium hydroxide to reform aluminium hydroxide. Blue colour of litmus solution is adosrbed on this precipitate.

(b) Charcoal cavity/Cobalt nitrate test

In this test, aluminium oxide produced in the charcoal cavity test reacts with CoO in cobalt nitrate test to produces a blue mass due to the formation of Al2O3.CoO.

Analysis of Group - IV 

The radicals present in this group are CO2+, Ni2+,Mn2+ and Zn2+. These are precipitated as sulphide by passing H2S gas through the ammonical solution of the salt. The group reagent for this group is H2S gas in the presence of NH4Cl and NH4OH.

Chemical Reaction involved in Group IV Analysis

Passing of H2S gas through the group III solution will precipitate the radicals Co2+, Ni2+, Mn2+ and Zn2+ as their sulphides. Formation of black ppt. (CoS or NiS) indicates cobalt or nickel. Formation of buff-coloured ppt. (MnS) indicates manganese and dirty white ppt. (ZnS) indicates zinc. 

Confirmatory tests of Cobalt (II) ion (Co2+)

(a) Potassium nitrite test

Cobalt ion reacts with potassium nitrite in presence of acetic acid to form yellow precipitate of Potassium cobalti nitrite.

(b) Ammonium thiocyanate ether test

On addition of ether and a crystal of ammonium thiocyanate, a blue colour is obtained in the ethereal layer due to the formation of ammonium cobalti thiocyanate.

(c) Borax bead test

In this test Co2+ ion produces blue beads in both oxidising and reducing flames.

Confirmatory tests of Nickel (II) ion (Ni2+)

(a) Dimethyl glyoxime test

Ni2+ ions react with dimethyl glyoxime to form bright rose-red coloured Nickel-Dimethyl glyoxime complex, Ni(dmgH)2.

(b) Sodium hydroxide-bromine water test

Ni2+ ions react with excess of NaOH and bromine water to form a black precipitate of Nickelic hydroxide.

(c) Borax bead test

Ni2+ ions produce brown bead in oxidising and grey bead in reducing flames.

Confirmatory tests of Manganese (II) ion (Mn2+)

Manganese sulphide formed in the group analysis dissolves in dil.HCl forming manganese chloride, and H2S is boiled off.

(a) Sodium hydroxide - bromine water test

The manganese chloride (formed by dissolving MnS in HCl) reacts with excess of NaOH to form white precipitate of manganese hydroxide.

The white precipitate of manganese hydroxide turns brown on addition of  bromine water due to its oxidation to brown manganic hydroxide MnO(OH)2.

(b) Lead peroxide test

Manganese sulphide formed in the group analysis reacts with Conc. HNO3 and lead peroxide to form a pink coloured solution, due to the formation of HMnO4.

(c) Borax bead test

Mn2+ ions produce pinkish beads in oxidising and colourless beads in reducing flames.

Confirmatory tests of Zinc (II) ion (Zn2+)

The white precipitate of ZnS formed in the group analysis dissolves in dil. HCl forming zinc chloride, and H2S is boiled off.

(a) Sodium hydroxide test

Zinc chloride (formed by dissolving ZnS in dil. HCl) reacts with sodium hydroxide to form white precipitate of zinc hydroxide.

(b) Potassium ferrocyanide test

Zinc chloride (formed by dissolving ZnS in dil. HCl) reacts with potassium ferrocyanide to form white or bluish white precipitate of zinc ferrocyanide.

(c) Charcoal cavity/Cobalt nitrate test

In this test, zinc oxide produced in the charcoal cavity test reacts with CoO in cobalt nitrate test to produces a greenish residue due to the formation of ZnO.CoO.

Analysis of Group - V

Group V consist of three radicals: Ba2+, Sr2+ and Ca2+. These cations are precipitated as their carbonates.Group reagent for this group is (NH4)2CO3 in the presence of NH4Cl and NH4OH.

Chemical Reaction involved in Group V Analysis

When (NH4)2CO3 is added to salt solution containing NH4Cl and NH4OH, the carbonates of  Ba2+, Sr2+ and Ca2+ are precipitated.

Confirmation of Barium (II) ion (Ba2+)

The white precipitate of barium carbonate formed in the group analysis dissolves in hot dil. acetic acid due to the formation of soluble barium acetate.

(a) Potassium chromate test

Barium acetate (formed by dissolving barium carbonate in dil. acetic acid) reacts with potassium chromate to form yellow precipitate of barium chromate.

(b) Flame test

Barium imparts a grassy green colour to the flame.

Confirmation of Strontium (II) ion (Sr2+)

The white precipitate of strontium carbonate formed in the group analysis dissolves in hot dil. acetic acid due to the formation of soluble strontium acetate.

(a) Ammonium sulphate test

Strontium acetate ( formed by dissolving Strontium carbonate in dil. acetic acid ) reacts with ammonium sulphate to form a white precipitate o strontium sulphate.

(b) Flame test

Strontium imparts a crimson red colour to the flame.

Confirmation of Calcium (II) ion (Ca2+)

The white precipitate of calcium carbonate formed in the group analysis dissolves in hot dil. acetic acid due to the formation of soluble calcium acetate.

(a) Ammonium oxalate test

Calcium acetate (formed by dissolving calcium carbonate in dil. acetic acid) reacts with ammonium oxalate to form a white precipitate o calcium oxalate.

(b) Flame test

Calcium imparts brick red colour to the flame.

Analysis of Group - VI 

Chemical Reaction involved in Confirmation of Mg2+

(a) Ammonium Phosphate test

Mg2+ ions react with ammonium phosphate in presence of NH4Cl and NH4OH to form white precipitate of magnesium ammonium phosphate.

(b) Charcoal cavity/Cobalt nitrate test

In this test, magnesium oxide produced in the charcoal cavity test reacts with CoO in cobalt nitrate test to produces a pink mass due to the formation of MgO.CoO.

Learning Outcomes

  • Students understand different types of cations.
  • Students understand various tests to determine the cation present in a given salt.
  • Students understand the chemical reactions and their balanced equations that takes place during each test.
  • Students acquire the skill to perform the experiment in the real lab once they understand different steps in the procedure.

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