Identification of Ions: Systematic Analysis of Anions and Cations

Identification of ions means the detection of cations and anions present in a given sample of salt. The identification is made through the qualitative analysis of inorganic salts. In an inorganic salt, the part contributed by the acid is called anion, and the part contributed by the base is called a cation. In qualitative analysis, the reactions are carried out that is easily perceptible to our senses, such as sight and smell. Such reactions involve precipitate formation, colour change, the evolution of gas etc. The qualitative analysis of an inorganic salt involves the preliminary examination of solid salt and its solution, wet tests (reactions carried out in solution) and confirmatory tests for anions, wet tests and confirmatory tests for cations.

Systematic Analysis of Anions

Step – I: Preliminary Test with Dilute Sulphuric Acid

This test is used to detect Carbonate \(\left({{\text{CO}}_3^{2 – }} \right),\) sulphide \(\left({{{\text{S}}^{2 – }}} \right),\) sulphite \(\left({{\text{SO}}_3^{2 – }} \right),\) nitrite \(\left( {{\text{NO}}_2^ – } \right),\left( {{\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ – }} \right)\) acetate ions as these ions react with dilute sulphuric acid to evolve different gases. In this test, the action of dilute sulphuric acid on the salt is noted at room temperature, and on warming.

To \(0.1\,{\text{g}}\) of the sample salt, \(1 – 2\,{\text{mL}}\) of dilute sulphuric acid is added. Changes, if any, is observed at room temperature. If no gas is evolved, the content of the test tube is warmed.

Confirmatory tests for \(\left({{\text{C}}{{\text{O}}_3}^{2 – }} \right),\left( {{{\text{S}}^{2 – }}} \right),\left({{\text{S}}{{\text{O}}_3}^{2 – }} \right),\left( {{\text{N}}{{\text{O}}_2}^ – } \right),\) and \(\left({{\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ – }} \right)\)

Confirmatory or wet tests for anions is done by using-
1. Water extract for a water-soluble salt. It is made by dissolving salt in water.
2. Sodium carbonate extract for water-insoluble salt. It is prepared by mixing \({\text{1}}\,{\text{g}}\) of salt sample with \({\text{3}}\,{\text{g}}\) of solid sodium carbonate. To this mixture, \(15\,{\text{ml}}\) of distilled water is added and boiled for \(10\) minutes. The solution is cooled and filtered, the filtrate known as the sodium carbonate extract is collected in a test tube.

Like \({\text{C}}{{\text{O}}_2},\) sulphur dioxide also turns lime water milky. But \({\text{C}}{{\text{O}}_2}\) is an odourless gas, and \({\text{S}}{{\text{O}}_2}\) has a characteristic smell.

Step-II: Preliminary Test with Concentrated Sulphuric Acid

Suppose no positive result is obtained from \({\text{dil}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\) test, then the salt is tested with \({\text{conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}.\) To \({\text{0}}{\text{.1 g}}\) of sample salt, \(3 – 4\) drops of \({\text{conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\) is added. Changes, if any, is observed at room temperature. If no gas is evolved, the content of the test tube is warmed.

Confirmatory tests for \({\text{C}}{{\text{l}}^ – },{\text{B}}{{\text{r}}^ – },{{\text{I}}^ – },{\text{NO}}_3^ – ,\) and \({{\text{C}}_2}{{\text{O}}_4}^{2 – }\)

Step-III: Test for Sulphate and Phosphate

If a negative result is obtained in Steps-I and II, then tests for the detection of sulphate and phosphate ions are performed.

Systematic Analysis of Cations

The tests for cations is carried out as follows.

Step – I: Preliminary Examination of the Salt

1. Colour Test

The colour of the salt may provide useful information about the cations.

Characteristic colours of some metal ions:

2. Dry Heating Test

(i) About \({\text{0}}{\text{.1 g}}\) of the dry salt is heated in a clean and dry test tube.

(ii) The colour of the residue is observed when it is hot and also when it becomes cold.

However, these observations cannot be taken as conclusive evidence.

3. Flame Test

The chlorides of several metals are volatile and impart a characteristic colour to the non-luminous flame. This test is conducted with the help of a platinum wire loop.

A tiny loop is made at one end of a platinum wire.

(ii) The loop is cleaned by dipping it into concentrated hydrochloric acid and held against a non-luminous flame. This is done to make sure that the wire imparts no colour to the flame.

\(2 – 3\) drops of conc. \({\text{HCl}}\) is taken on a clean watch glass, and a paste is made by adding a small quantity of salt to it.

The platinum wire loop is dipped in this paste and introduced to the non-luminous (oxidising) flame.

The colour of the flame is observed first with the naked eye and then through blue glass.

The metal is identified as per the following table-

Borax Bead Test

This test is employed only for coloured salts. This is because borax reacts with metal salts to form colourful metal borates or metals.

(i) To perform this test, a platinum wire loop is heated in a flame till it is red hot.

(ii) The hot loop is dipped into borax powder and is heated again until borax forms a colourless transparent bead on the loop.

(iii) The bead is dipped in a small quantity of the dry salt and held against non – luminous flame as well as in the luminous flame.

(iv) The colour of the bead is observed in both hot and cold conditions.

Borax, on heating, loses its water of crystallisation and decomposes to give sodium metaborate and boric anhydride.

\(\underset{{{\text{Borax}}}}{\mathop {{\text{N}}{{\text{a}}_2}{{\text{B}}_4}{{\text{O}}_7}.}} 10{{\text{H}}_2}{\text{O}} \to {\text{N}}{{\text{a}}_2}{{\text{B}}_4}{{\text{O}}_7} + 10{{\text{H}}_2}{\text{O}}\)

\({\text{N}}{{\text{a}}_2}{{\text{B}}_4}{{\text{O}}_7} \to \underset{{{\text{Sodium}}\,{\text{metaborate}}}}{\mathop {2{\text{NaB}}{{\text{O}}_2}}} + \underset{{{\text{Boric}}\,{\text{anhydride}}}}{\mathop {{{\text{B}}_2}{{\text{O}}_3}}} \)

Boric anhydride reacts with a metal salt to form metal metaborate, giving different colours to oxidising and reducing flame.

5. Charcoal Cavity Test

In a charcoal cavity test, metallic carbonates decompose to give the corresponding oxide that appears as a coloured residue in the cavity.

The test is performed by making a small cavity in a charcoal block and filling the cavity with about \(0.2\,{\text{g}}\) of salt and about \(0.5\,{\text{g}}\) of anhydrous sodium carbonate. The salt in the cavity is moistened with one or two drops of water. A blowpipe is used to heat the salt in a luminous (reducing) flame, and the colour of oxide/ metallic bead formed in the cavity is observed both when hot and cold. Also, obtain oxidising and reducing flame as shown:

6. Cobalt Nitrate Test

The cobalt nitrate test is performed if the residue in the charcoal cavity is white. The residue is treated with two or three drops of cobalt nitrate solution. The residue is heated strongly in a non-luminous flame with the help of a blowpipe. The colour of the residue is observed. Cobalt nitrate, on heating, decomposes into cobalt (II) oxide, which gives a characteristic colour to the metal oxide present in the cavity.

Thus, with \({\text{ZnO}},{\text{A}}{{\text{l}}_2}{{\text{O}}_3}\) and \({\text{MgO,}}\) the following reactions occur.

\(2{\text{Co}}{\left({{\text{N}}{{\text{O}}_3}} \right)_2} \to 2{\text{CoO}} + 4{\text{N}}{{\text{O}}_2} + {{\text{O}}_2}\)

\({\text{CoO}} + {\text{ZnO}} \to{\text{CoO}}.{\text{ZnO}}\left({{\text{green}}} \right)\)

\({\text{CoO}} + {\text{MgO}} \to {\text{CoO}} \cdot {\text{MgO}}\left({{\text{Pink}}} \right)\)

\({\text{CoO}} +{\text{A}}{{\text{l}}_2}{{\text{O}}_3} \to{\text{CoO}} \cdot {\text{A}}{{\text{l}}_2}{{\text{O}}_3}\left({{\text{Blue}}} \right)\)

Step-II: Wet Tests for Identification of Cations

The first step essential for the identification of cations is to prepare a clear and transparent solution of the salt called the original solution. It is prepared as follows:

Preparation of Original Solution (O.S.)

To prepare the original solution, the following steps are followed-

1. To a small amount of the salt, a few \({\text{mL}}\) of distilled water is added in a clean boiling tube. If the salt does not dissolve, the content of the boiling tube is heated till the salt completely dissolves.
2. If the salt is insoluble in step \(1,\) then a few \({\text{mL}}\) of \({\text{dil}}.{\text{HCl}}\) is added to the salt. If the salt does not dissolve, the content of the boiling tube is heated till the salt completely dissolves.
3. If the salt does not dissolve in step \(1\) and step \(2,\) even on heating, a few \({\text{mL}}\) of \({\text{Conc}}.{\text{HCl}}\) is added to the salt.
4. If salt does not dissolve in \({\text{Conc}}.{\text{HCl,}}\) then add dilute nitric acid to it.
5. If salt does not dissolve even in nitric acid, then the salt is dissolved in aqua regia (a mixture of \({\text{Conc}}.{\text{HCl}}\) and \({\text{Conc}}.{\text{HN}}{{\text{O}}_3}\) in the ratio \(3:1\)).
6. If the salt does not dissolve in aqua regia, it is considered an insoluble salt.

The flow chart given below is used in the determination of the cation present in the inorganic salt. However, this flow chart is only applicable to salt having only one cation.

(I) Analysis of Zero Group Cation (\({\text{NH}}_4^ + \) ion)

(a) To \(0.1\,{\text{g}}\) of salt, \(1 – 2\,{\text{mL}}\) of \({\text{NaOH}}\) solution is added to a test tube and heated. If there is a smell of ammonia, the presence of ammonium ions is confirmed. If white fumes are observed on introducing a glass rod dipped in hydrochloric near the mouth of the test tube, ammonium ions are confirmed. Ammonia gas evolved by the action of sodium hydroxide on ammonium salts reacts with hydrochloric acid to give dense white fumes of ammonium chloride.

(b) A brown precipitate of basic mercury(II) amido-iodine is obtained by passing the gas through Nessler’s reagent.

(II) Analysis of Group-I Cations \(\left({{\text{p}}{{\text{b}}^{2 + }}} \right)\)

A small amount of original solution prepared in hot \({\text{Conc}}.{\text{HCl}}\) is taken in a test tube, and cold water is added to it. The test tube is cooled under tap water. The formation of a white precipitate indicates the presence of  \({\text{p}}{{\text{b}}^{2 + }}\) ions.

If the original solution is prepared in water, then \({\text{dil}}.{\text{HCl}}\) is added to it. The formation of a white precipitate indicates the presence of \({\text{p}}{{\text{b}}^{2 + }}\) ions.

(III) Analysis of Group–II Cations

If group-I is absent, then the excess of water is added to the same test tube. The solution is warmed, and \({{\text{H}}_2}{\text{S}}\) gas is passed for \(1 – 2\) minutes. The test tube is shaken. The appearance of a precipitate indicates the presence of group-II cations. Excess of \({{\text{H}}_2}{\text{S}}\) gas is passed through the solution to ensure complete precipitation.

If a black colour precipitate is obtained, the presence of \({\text{C}}{{\text{u}}^{2 + }}\) or \({\text{p}}{{\text{b}}^{2 + }}\) ions is indicated

If a yellow colour precipitate is obtained, the presence of \({\text{A}}{{\text{s}}^{3 + }}\) ions is indicated.

To the precipitate of group-II, an excess of yellow ammonium sulphide solution is added, and the test tube is shaken. If the precipitate is insoluble, group II-A (copper group) is present. If the precipitate is soluble, this indicates the presence of group-II B (arsenic group).

(IV) Analysis of Group–III cations

If group-II is absent, then to the original solution \(2 – 3\) drops of \({\text{conc}}.{\text{HN}}{{\text{O}}_3}\) is added to oxidise \({\text{F}}{{\text{e}}^{2 + }}\) ions to \({\text{F}}{{\text{e}}^{3 + }}\) ions. The solution is heated for a few minutes. After cooling, a small amount of solid ammonium chloride \(\left({{\text{N}}{{\text{H}}_4}{\text{Cl}}} \right)\) and an excess of ammonium hydroxide \(\left({{\text{N}}{{\text{H}}_4}{\text{OH}}} \right)\) solution is added till it smells of ammonia. The test tube is shaken.

A gelatinous white precipitate indicates the presence of aluminium ion \(\left({{\text{A}}{{\text{l}}^{3 + }}} \right)\).

If the precipitate is brown in colour, the presence of ferric ions \(\left({{\text{F}}{{\text{e}}^{3 + }}} \right)\) is indicated.

(V) Analysis of Group-IV cations

If group-III is absent, then \({{\text{H}}_2}{\text{S}}\) gas is passed through the solution of group III. If white, black or flesh coloured precipitate appears, the presence of group-IV cations is indicated.

(VI) Analysis of Group–V Cations

If group-IV is absent, then a small amount of solid \({\text{N}}{{\text{H}}_4}{\text{Cl,}}\) an excess of \({\text{N}}{{\text{H}}_4}{\text{OH}}\) solution, and solid ammonium carbonate \({\left({{\text{N}}{{\text{H}}_4}} \right)_2}{\text{C}}{{\text{O}}_3}\) is added to the original solution. The formation of a white precipitate indicates the presence of group–V cations.

The white precipitate is dissolved with dilute acetic acid and divided into three parts, each for \({\text{B}}{{\text{a}}^{2 + }},{\text{S}}{{\text{r}}^{2 + }}\)  and \({\text{C}}{{\text{a}}^{2 + }}\) ions. Preserve a small amount of the precipitate for the flame test.

(VII) Analysis of Group–VI Cations

If group–V is absent, then test for \({\text{M}}{{\text{g}}^{2 + }}\) ions is done.

Disodium hydrogen phosphate solution is added to the original solution, and the inner walls of the test tube are scratched with a glass rod. A white crystalline precipitate of magnesium ammonium phosphate is formed, which indicates the presence of \({\text{M}}{{\text{g}}^{2 + }}\) ions.

Summary

Preliminary examination of salt often furnishes important information regarding the identity of the inorganic salt. These examinations involve observing the salt’s general appearance and physical properties, such as colour, smell, solubility, etc., of the salt. Dry tests such as heating of dry salt, flame tests, borax bead test, charcoal cavity test etc., are done to identify the cationic part of the salt. Solubility of salt in water also imparts important information about the nature of ions present in the salt. Gases or vapours evolved in the preliminary tests with \({\text{dil}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}/{\text{dil}}.{\text{HCl}}\) and \({\text{conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\)  also, provide important information about the presence of acid radicals. Preliminary tests should always be performed before starting the confirmatory tests for the ions. This page explains the different methods and procedures to identify the cationic and anionic parts of an inorganic salt.

Frequently Asked Questions (FAQs)

Q.1. What is the qualitative analysis of salts?
Ans: Qualitative analysis of salts or salt Analysis is a chemical technique used to identify the ions present in salt. It is done by analysing its physical and chemical properties and hence determining the identity of the salt.

Q.2. What is the preliminary test in qualitative analysis?
Ans: The preliminary tests in the qualitative analysis involve a physical examination of the salt. It includes colour, smell, texture and density.

Q.3. How can you test the presence of ammonium ions?
Ans: 1. To \(0.1\,{\text{g}}\) of salt, \(1 – 2\,{\text{mL}}\) of \({\text{NaOH}}\) solution is added to a test tube and heated. If there is a smell of ammonia, the presence of ammonium ions is confirmed. If white fumes are observed on introducing a glass rod dipped in hydrochloric near the mouth of the test tube, ammonium ions are confirmed. Ammonia gas evolved by the action of sodium hydroxide on ammonium salts reacts with hydrochloric acid to give dense white fumes of ammonium chloride.
2. A brown precipitate of basic mercury(II) amido-iodine is obtained by passing the gas through Nessler’s reagent.

Q.4. Name the anions detected by \( {\text{conc}} {\text{.}}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}.\)
Ans: The anions detected by \( {\text{conc}} {\text{.}}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\) are  \({\text{C}}{{\text{l}}^ – },{\text{B}}{{\text{r}}^ – },{{\text{I}}^ – },{\text{NO}}_3^ – ,{{\text{C}}_2}{\text{O}}_4^{2 – }.\)

Q.5. Why is platinum metal preferred to other metals for the flame test?
Ans: A platinum wire is preferred to other metals for the flame test because it is unreactive and does not impart any characteristic colour to the flame, which will mask the presence of other metals.