Systematic Analysis of Cations

Analysis of metallic elements in compounds or samples is an integral part of chemical research. The purpose of this experiment is to learn the techniques to separate and identify some common cations and to understand the principles for the equilibria of precipitation and complex formation. The systematic analysis of cations is an integral part of the salt analysis (or systematic qualitative analysis).

Table of Contents

• Aim
• Theory
• Materials Required
• Procedure
• Observation and Inference
• Results
• Precautions
• Frequently Asked Questions on Systematic Analysis of Cations

Aim

To identify the cationic radicals present in an inorganic mixture of salts by performing various tests.

Theory

Qualitative analysis is the systematic approach that involves precipitation reaction to remove cations sequentially from a mixture. The behaviour of the cations toward a set of common test reagents differs from one cation to another and furnishes the basis for their separation.

The qualitative analysis of an inorganic mixture is started by first carrying out some preliminary tests. The Preliminary tests for cations are

1. Physical examination
2. Charcoal cavity test
3. Boraxbead test
4. Flame test

These tests do not give conclusive evidence yet they provide some information about the ions present in the mixture.

Qualitative analysis of cations usually consists of three stages.

1. First based on different solubility properties the cations are separated into 5 groups through the successive addition of selective precipitating reagents.
2. Second, within each group precipitated cations are separated through selective dissolution processes.
3. Third, the presence of each cation is verified through different identification tests.

The cations are classified into the following 5 groups.

1. Group I Cations (Ag⁺, Hg₂²⁺ and Pb²⁺ – insoluble chlorides):

Among the common metallic cations only three cations form insoluble chlorides with hydrochloric acid. When 6M of HCl is added to the solution, white precipitates of AgCl, Hg₂Cl₂ and PbCl₂ are formed. Other metallic cations remain in solution.

2. Group II Cations (Hg²⁺, Pb²⁺, Cu²⁺, Bi³⁺, Cd²⁺, As³⁺, Sb³⁺ and Sn⁴⁺ – insoluble sulphides in acidic medium):

After the insoluble chlorides are isolated, the pH of the solution is adjusted to 0.5 and then H₂S is added. Since the concentration of sulphide ion (s^2-) is very low at low pH, only those metallic sulphided having very low Ksp values will precipitate. Cations with larger Ksp values for their sulphides remain in solution.

3. Group III Cations (Al³⁺. Fe³⁺, Co²⁺, Ni²⁺, Cr³⁺, Zn²⁺ and Mn²⁺ – insoluble sulphides or hydroxides in alkaline medium):

After isolating the insoluble sulphides in acidic medium, the solution is made basic and the metallic sulphides having larger Ksp values such as ZnS, NiS, CoS and MnS precipitate. Moreover, since the solution is basic Al³⁺, Fe³⁺ and Cr³⁺ form insoluble hydroxides and are also separated from the solution.

4. Group IV Cations (Ca²⁺, Sr²⁺ and Ba²⁺ – carbonate precipitates):

These three metallic cations all belong to Group IIA in the periodic table of elements and therefore their chemical properties are very similar. They form soluble chlorides and sulphides and hence are separable from groups 1, 2 and 3 cations. However, their carbonates precipitate in a mixture of ammonium carbonate, ammonium chloride or ammonia solution.

5. Group V Cations (Mg²⁺, Na⁺, K⁺ and NH₄⁺):

None of the cations in this group form precipitates in the separation processes of group 1-4 cations and thus remain in the final solution.

The flowchart for separating the five groups of cations is given below.

Classification of Cations

The list of cations comprising the seven groups and the group reagent used for precipitating them out from the solution is tabulated below.

Materials Required

1. Test tubes
2. Boiling tubes
3. Test tube holder
4. Test tube stand
5. Corks
6. Filter paper
7. Delivery tube
8. Reagents
9. Measuring cylinder

Procedure

(i) Preliminary Test of Cations

(ii) Charcoal Cavity Test

In this test the cations is converted into the metal carbonate in a charcoal cavity which decomposes on heating to metal oxide or even to the metallic state. The cation present can be detected from the colour of the bead or residue left in the cavity or deposit formed outside the cavity called incrustation.

ZnSO₄ + Na₂CO₃ → ZnCO₃ + Na₂SO₄

ZnCO₃ → ZnO (yellow when hot) + CO₂

CuSO₄ + Na₂CO₃ → CuCO₃ + Na₂SO₄

CuCO₃ → CuO + CO₂

CuO + C → Cu (reddish) + CO

CdCl₂ + Na₂CO₃ → CdCO₃ + 2NaCl

CdCO₃ → CdO (reddish brown) + CO₂

Procedure for Charcoal Cavity Test

Make a small cavity in charcoal block with the help of a borer. Mix the given mixture with sodium carbonate or fusion mixture, moisten it with a drop of water, heat it in the charcoal cavity in the reducing flame with the help of a mouth blowpipe and observe the colour of the residue left.

(iii) Borax Bead Test

This test is used for the detection of cations in coloured mixtures which may contain copper, nickel, iron or manganese. The borax bead reacts with these metal oxides to form metabolites which have characteristic colour. Some metaborates when heated in reducing flame form lower metaborates or even metals thus causing a change in the colour of bead.

Na₂B₄O₇.10H₂O → Na₂B₄O₇(White porous mass) → NaBO₂ + B₂O₃

B₂O₃ + CuSO₄ → Cu(BO₂)₂(blue green cupric metaborate) + SO₃↑

2Cu(BO₂)₂ + C → 2CuBO₂(colourless coprous metaborate) + B₂O₃ + CO↑

2CuBO₂ + C → 2Cu (red) + B₂O₃ + CO↑

Procedure for Borax Bead Test

• Make a small loop at the end of the platinum wire.
• Heat the loop in a Bunsen flame until it is red hot.
• Dip it into powdered borax placed on a watch glass and heat again. The borax will first swell up and will fuse giving a colourless, transparent glass like bead.
• Touch the hot bead with conc.HCl and touch it with the given mixture or salt. Heat it in an oxidising (non-luminous) and reducing (luminous) flame. Observe the colour of the bead and draw inferences.

(iv) Flame Test

Certain cations like fifth group radicals Ba²⁺, Sr²⁺ and Ca²⁺ in the form of their chlorides impart characteristic colours to the non-luminous flame. The chlorides of these cations are thermally ionised. The ions so formed absorb heat energy and get excited. The extra energy is released in the form of light of the characteristic colour in the visible part of spectrum. Since different metal ions emit light energy of different wavelengths, the colours imparted by the different cations to the flame are also different.

Procedure for Flame Test

• Take a platinum wire and dip it in conc.HCl taken on a watch glass and heat it strongly in the flame. Repeat this process until the wire does not impart any colour to the flame.
• Dip platinum wire in con.HCl and then touch it with a given salt and heat it in the flame of the Bunsen burner. Observe the colour of the flame and draw inferences.

Observation and Inference

(i) Charcoal Cavity Test

(ii) Borax Bead Test

(iii) Flame Test

Results

The given salt contains ________ (NH₄⁺, K⁺, Ag⁺, Hg₂²⁺, Pb²⁺, Hg²⁺, Cu²⁺, Bi³⁺, Cd²⁺, As³⁺, Sb³⁺, Sn⁴⁺, Al³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cr³⁺, Zn²⁺, Mn²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mg²⁺, Na⁺) cation.

Precautions

1. Read the label carefully on the bottle before using any reagent or chemicals. Never use a reagent that is unlabeled.
2. Always use an apron, an eye protector and hand gloves in the chemical laboratory.
3. In smelling chemicals or vapours, be careful. Always gently fan the vapours to your nose
4. Do not unnecessarily mix chemicals with reagents. Do not taste chemicals at all.
5. For dilution, always pour acid into water. Never add acid to water.
6. Never add or throw sodium metal into the sink or dustbin.
7. Be careful as the test tube is heated. When heating or adding a reagent, the test tube should never point to yourself or your neighbours.
8. For dilution, always pour acid into water. Never add acid to water.
9. Keep clean your working environment. Never throw in the sink any papers and glass. For this purpose, always use dustbin.
10. Be careful with explosive compounds, flammable substances, toxic gasses, electrical appliances, glass products, flames and hot substances.
11. Always use the minimum quantity of reagents. The use of excessive reagents not only leads to chemicals being wasted, but also causes environmental damage.
12. Always wash your hands after the laboratory work has been completed.