Preliminary Dry Tests: Flame Test and Borax Bead Test
When an unknown inorganic salt is presented in an examination setting, the initial investigative steps almost invariably involve dry tests. These do not require dissolving the salt into a solution immediately. Instead, they provide rapid, highly indicative clues about the presence of specific metallic cations.
The Flame Test relies on the excitation of valence electrons when a metallic salt is introduced into the non-luminous Bunsen burner flame. The salt is typically converted into a volatile chloride by mixing with concentrated hydrochloric acid. As the electrons return to their ground state, they emit energy characteristic of specific atomic energy level transitions, manifesting as distinct visible colors.
For instance, sodium salts impart an intense golden yellow color to the flame. Potassium gives a lilac or pale violet color that is often best viewed through cobalt glass to filter out interfering yellow sodium impurities.
Alkaline earth metals like calcium yield a brick red flame, strontium displays a crimson red color, and barium produces an apple green hue. Copper salts typically impart a bluish-green flame with a bright green border. Examiners frequently test your ability to match these flame colors with their corresponding metal cations, making memorization vital for JEE and NEET aspirants.
Complementing the flame test is the Borax Bead Test, which is specifically utilized for the detection of transition metal cations that form colored metaborates. When sodium tetraborate decahydrate, commonly known as borax ($\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}$), is heated on a platinum loop, it swells and then melts. It forms a clear, glassy, colorless bead composed of sodium metaborate and boric anhydride.
When this hot bead is brought into contact with a transition metal salt and reheated in either the oxidizing or reducing flame, it forms distinctive metal metaborates.
“The borax bead changes color depending on the oxidation state of the metal ion and whether the bead is heated in an oxidizing or reducing flame, serving as a classic dry test for copper, iron, chromium, manganese, cobalt, and nickel.”
Key color indicators in the borax bead test include copper, which yields a blue bead in the oxidizing flame and a red opaque (reduced copper(I) metaborate) bead in the reducing flame. Chromium consistently produces a green bead in both oxidizing and reducing flames, while cobalt yields an intense, deep blue bead under both conditions.
Manganese forms an amethyst or violet bead in the oxidizing flame, which becomes colorless in the reducing flame due to reduction to the lower oxidation state $\text{Mn}^{2+}$.
Systematic Identification of Anions Using Acids
Once dry tests provide preliminary guidance, the analytical procedure shifts to wet tests, beginning with the systematic identification of acid radicals, or anions. Anions are broadly classified into three main groups based on their behavior toward specific reagent acids. These include those decomposed by dilute acids (like dilute $\text{HCl}$ or dilute $\text{H}_2\text{SO}_4$), those decomposed by concentrated acids (like concentrated $\text{H}_2\text{SO}_4$), and those identified through special confirmatory tests in solution.
The dilute acid group comprises anions that evolve gases upon treatment with dilute hydrochloric or sulfuric acid at room temperature. These include carbonate ($\text{CO}_3^{2-}$), sulfite ($\text{SO}_3^{2-}$), sulfide ($\text{S}^{2-}$), thiosulfate ($\text{S}_2\text{O}_3^{2-}$), and nitrite ($\text{NO}_2^{-}$).
When a dilute acid is added to a carbonate, colorless, odorless carbon dioxide gas is evolved, which turns lime water milky due to the formation of insoluble calcium carbonate. Sulfides evolve hydrogen sulfide gas ($\text{H}_2\text{S}$), recognizable by its characteristic rotten-egg smell, which turns lead acetate paper shiny black.
Anions that do not react with dilute acids are tested using concentrated sulfuric acid. This group includes chloride ($\text{Cl}^{-}$), bromide ($\text{Br}^{-}$), iodide ($\text{I}^{-}$), nitrate ($\text{NO}_3^{-}$), and acetate ($\text{CH}_3\text{COO}^{-}$).
Concentrated $\text{H}_2\text{SO}_4$ reacts with chlorides to evolve pungent hydrogen chloride gas that forms dense white fumes of ammonium chloride when a glass rod dipped in concentrated ammonia is brought near the test tube mouth. Bromides and iodides yield reddish-brown bromine vapors and violet iodine vapors respectively when treated with concentrated $\text{H}_2\text{SO}_4$, often accelerated by the addition of solid manganese dioxide ($\text{MnO}_2$).
- Nitrate Test: The classic Brown Ring Test is used for $\text{NO}_3^{-}$. Freshly prepared iron(II) sulfate is added to the aqueous salt solution, followed by careful dropwise addition of concentrated sulfuric acid down the side of the test tube. A brown ring at the junction of the two layers confirms the formation of the nitroso-iron(II) complex, $\mathbf{[\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]SO_4}$.
- Sulfate Test: Sulfates ($\text{SO}_4^{2-}$ ) are identified by adding barium chloride solution to an acidified aqueous extract, yielding a dense, white precipitate of barium sulfate ($\text{BaSO}_4$) that is completely insoluble in concentrated hydrochloric acid.
- Phosphate Test: Phosphates ($\text{PO}_4^{3-}$) react with ammonium molybdate in the presence of concentrated nitric acid to produce a characteristic canary yellow precipitate of ammonium phosphomolybdate.
Systematic Separation and Identification of Cations
The identification of basic radicals, or cations, requires a rigorous, systematic separation scheme. Unlike anions, which can often be identified individually from original solutions, cations interfere with one another and must be separated sequentially into analytical groups based on their selective precipitation behavior with specific group reagents. There are traditionally six analytical groups of cations, organized according to their solubility product principles and common ion effects.
Group I Cations consist of lead ($\text{Pb}^{2+}$), mercury(I) ($\text{Hg}_2^{2+}$), and silver ($\text{Ag}^{+}$). The group reagent is dilute hydrochloric acid. These cations are precipitated as insoluble chlorides because their solubility products are exceptionally small compared to other metal chlorides.
To confirm lead separately from silver and mercury(I), the mixed chloride precipitate is treated with hot water. Lead chloride dissolves readily in hot water and precipitates as yellow lead chromate upon adding potassium chromate solution.
Group II Cations include mercury(II) ($\text{Hg}^{2+}$), copper ($\text{Cu}^{2+}$), bismuth ($\text{Bi}^{3+}$), cadmium ($\text{Cd}^{2+}$), arsenic ($\text{As}^{3+}$), antimony ($\text{Sb}^{3+}$), and tin ($\text{Sn}^{2+}$ / $\text{Sn}^{4+}$). They are precipitated as insoluble sulfides by passing hydrogen sulfide gas ($\text{H}_2\text{S}$) in the presence of dilute hydrochloric acid.
The role of dilute $\text{HCl}$ is crucial. It suppresses the dissociation of $\text{H}_2\text{S}$ via the common ion effect, keeping the sulfide ion concentration low enough to precipitate only the highly insoluble sulfides of Group II while keeping more soluble metal sulfides in solution.
Group III Cations comprise iron ($\text{Fe}^{3+}$), aluminum ($\text{Al}^{3+}$), and chromium ($\text{Cr}^{3+}$). The group reagent is ammonium hydroxide ($\text{NH}_4\text{OH}$) in the presence of solid ammonium chloride ($\text{NH}_4\text{Cl}$).
The addition of $\text{NH}_4\text{Cl}$ suppresses the excessive ionization of $\text{NH}_4\text{OH}$ through the common ion effect, preventing the premature precipitation of magnesium hydroxide. Under these controlled alkaline conditions, iron(III), aluminum, and chromium precipitate as their respective hydrated oxides or hydroxides: $\text{Fe}(\text{OH})_3$ (reddish-brown), $\text{Al}(\text{OH})_3$ (gelatinous white), and $\text{Cr}(\text{OH})_3$ (green).
Group IV Cations encompass cobalt ($\text{Co}^{2+}$), nickel ($\text{Ni}^{2+}$), manganese ($\text{Mn}^{2+}$), and zinc ($\text{Zn}^{2+}$). These are precipitated as metal sulfides by passing $\text{H}_2\text{S}$ gas through an ammoniacal alkaline solution (filtrate from Group III). Because these metal sulfides have higher solubility products than Group II sulfides, a higher concentration of sulfide ions is required, which is achieved by neutralizing the acid with ammonium hydroxide.
Group V Cations include barium ($\text{Ba}^{2+}$), strontium ($\text{Sr}^{2+}$), and calcium ($\text{Ca}^{2+}$). The group reagent is ammonium carbonate ($\mathbf{(\text{NH}_4)_2\text{CO}_3}$) in the presence of ammonium chloride and ammonium hydroxide. These alkaline earth metals precipitate as insoluble white carbonates.
They are subsequently separated from one another by selective dissolution and fractional precipitation. Reagents used include potassium chromate (for barium) and ammonium sulfate or ammonium oxalate (for calcium).
Group VI Cations consist of magnesium ($\text{Mg}^{2+}$), sodium ($\text{Na}^{+}$), potassium ($\text{K}^{+}$), and ammonium ($\text{NH}_4^{+}$). Ammonium is tested independently using the original salt solution by heating with sodium hydroxide to evolve pungent ammonia gas, which turns moist red litmus paper blue.
Magnesium is identified by adding disodium hydrogen phosphate in the presence of ammonium chloride and ammonium hydroxide. This yields a white crystalline precipitate of magnesium ammonium phosphate.
Key Points to Remember
- Group I Reagent: Dilute $\text{HCl}$ precipitates $\text{Ag}^{+}$, $\text{Hg}_2^{2+}$, and $\text{Pb}^{2+}$ as chlorides. Lead chloride is soluble in hot water.
- Group II Reagent: $\text{H}_2\text{S}$ in dilute $\text{HCl}$ precipitates copper, mercury, arsenic, antimony, tin, bismuth, and cadmium as sulfides.
- Group III Reagent: $\text{NH}_4\text{OH}$ in the presence of $\text{NH}_4\text{Cl}$ precipitates $\text{Fe}^{3+}$, $\text{Al}^{3+}$, and $\text{Cr}^{3+}$ as hydroxides.
- Group IV Reagent: $\text{H}_2\text{S}$ in ammoniacal medium precipitates $\text{Co}^{2+}$, $\text{Ni}^{2+}$, $\text{Mn}^{2+}$, and $\text{Zn}^{2+}$ as sulfides.
- Group V Reagent: $(\text{NH}_4)_2\text{CO}_3$ in neutral or ammoniacal medium precipitates $\text{Ba}^{2+}$, $\text{Sr}^{2+}$, and $\text{Ca}^{2+}$ as carbonates.
- Brown Ring Test: Confirms nitrate ($\text{NO}_3^{-}$) through the formation of $\mathbf{[\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]SO_4}$.
- Flame Colors: Sodium gives golden yellow, potassium gives lilac, calcium gives brick red, and barium gives apple green.
Important Facts / Formulas
| Analytical Group | Cations Included | Group Reagent | Form of Precipitate |
|---|---|---|---|
| Group I | $\text{Ag}^{+}, \text{Hg}_2^{2+}, \text{Pb}^{2+}$ | Dilute $\text{HCl}$ | Chlorides |
| Group II | $\text{Hg}^{2+}, \text{Cu}^{2+}, \text{Bi}^{3+}, \text{Cd}^{2+}, \text{As}^{3+}, \text{Sb}^{3+}, \text{Sn}^{2/4+}$ | $\text{H}_2\text{S}$ gas in dilute $\text{HCl}$ | Sulfides |
| Group III | $\text{Fe}^{3+}, \text{Al}^{3+}, \text{Cr}^{3+}$ | $\text{NH}_4\text{OH}$ in presence of $\text{NH}_4\text{Cl}$ | Hydroxides |
| Group IV | $\text{Co}^{2+}, \text{Ni}^{2+}, \text{Mn}^{2+}, \text{Zn}^{2+}$ | $\text{H}_2\text{S}$ gas in ammoniacal medium | Sulfides |
| Group V | $\text{Ba}^{2+}, \text{Sr}^{2+}, \text{Ca}^{2+}$ | $(\text{NH}_4)_2\text{CO}_3$ with $\text{NH}_4\text{Cl}$ and $\text{NH}_4\text{OH}$ | Carbonates |
| Group VI | $\text{Mg}^{2+}, \text{Na}^{+}, \text{K}^{+}, \text{NH}_4^{+}$ | No specific group reagent (individual tests) | Various / Soluble |
Previous Year Question Hints
- Question Type 1: Expect questions on why $\text{NH}_4\text{Cl}$ is added before $\text{NH}_4\text{OH}$ in Group III precipitation. Hint: Focus on the suppression of $\text{OH}^{-}$ concentration via the common ion effect to prevent the precipitation of magnesium hydroxide.
- Question Type 2: Identification of colored complexes and precipitates, such as the exact chemical formula of the brown ring complex formed during nitrate ion testing.
- Question Type 3: Borax bead test color changes in oxidizing versus reducing flames for transition metals like copper and manganese.
Quick Revision Summary
- Qualitative analysis systematically determines the composition of unknown salts using dry and wet chemical tests.
- Dry tests include flame tests (identifying metal ions by characteristic emission colors) and borax bead tests (identifying transition metal metaborates).
- Anions are categorized into dilute acid groups, concentrated acid groups, and special reagent groups.
- The brown ring test is the hallmark confirmatory test for nitrate ions, producing a nitroso-iron(II) complex.
- Cations are separated into six analytical groups utilizing specific group reagents based on solubility product principles.
