Qualitative Analysis of Inorganic Anions

Comprehensive vocabulary revision flashcards covering the qualitative identification, reactions, confirmations, and separation schemes of carbonates, bicarbonates, sulphur-containing anions, and nitrogen-containing anions.

Qualitative Analysis

The branch of analytical chemistry focused on determining the chemical quality or identity of substances in a sample (answering the question: "WHO are you?").

Quantitative Analysis

The branch of analytical chemistry focused on determining the absolute quantity or concentration of substances in a sample (answering the question: "HOW MUCH are you?").

Anion

The negatively charged component of a salt (such as $\text{Cl}^-$ in $\text{NaCl}$), identified experimentally by reactions that generate characteristic gases or precipitates.

Parent Acid

The neutral acid molecule from which an anion is derived, identified by adding a number of protons ($\text{H}^+$) equal to the magnitude of the anion's negative charge (e.g., $\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{H}_2\text{CO}_3$).

Strong Acids (Analytical List)

Acids that are completely ionized in aqueous solution to yield protons ($\text{H}^+$) and displace weaker or volatile parent acids from their salts: $\text{HCl}$, $\text{HBr}$, $\text{HI}$, $\text{HNO}_3$, $\text{H}_2\text{SO}_4$, and $\text{HSCN}$.

Dry Test

A preliminary identification test performed by adding acid (such as dilute $\text{HCl}$ or concentrated $\text{H}_2\text{SO}_4$) directly to a solid salt, where the primary diagnostic clue is gas evolution or effervescence.

Wet Test

An analytical identification test performed by adding a specific aqueous reagent (such as $\text{BaCl}_2$, $\text{CaCl}_2$, $\text{AgNO}_3$, or lead acetate) to a salt solution to observe the formation or absence of a characteristic precipitate.

Carbonic Acid ($\text{H}_2\text{CO}_3$)

The diprotic weak parent acid of carbonate and bicarbonate that ionizes in two equilibrium steps ($\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$ and $\text{HCO}_3^- \rightleftharpoons \text{H}^+ + \text{CO}_3^{2-}$) and is chemically unstable, decomposing readily into $\text{H}_2\text{O}$ and $\text{CO}_2\uparrow$.

Carbonate Anion ($\text{CO}_3^{2-}$)

An anion whose salts are insoluble except those of $\text{Na}^+$, $\text{K}^+$, and $\text{NH}_4^+$, reacting with dilute acid to evolve $\text{CO}_2$ gas and forming immediate white precipitates with cold $\text{BaCl}_2$, $\text{CaCl}_2$, and $\text{AgNO}_3$.

Bicarbonate Anion ($\text{HCO}_3^-$)

A completely water-soluble mono-hydrogen carbonate ion that yields effervescence with acid but gives no precipitate in the cold with $\text{BaCl}_2$, $\text{CaCl}_2$, or $\text{AgNO}_3$ until heated or treated with $\text{NH}_3$.

Lime-Water Test

A confirmatory test where passing $\text{CO}_2$ (or $\text{SO}_2$) into clear $\text{Ca(OH)}_2$ forms an insoluble white turbidity ($\text{CaCO}_3\downarrow$ or $\text{CaSO}_3\downarrow$) on short passage, which completely clears upon prolonged passage due to the formation of soluble bisalts ($\text{Ca(HCO}_3)_2$ or $\text{Ca(HSO}_3)_2$).

Fixed Acid

An acid characterized by a high boiling point—specifically concentrated sulfuric acid ($\text{H}_2\text{SO}_4$) with a boiling point of $330^\circ\text{C}$—allowing it to displace volatile acids ($\text{HCl}$, $\text{HNO}_2$, $\text{HNO}_3$) from their salts upon heating.

Sulphuric Acid Superpowers

The three primary chemical properties of concentrated $\text{H}_2\text{SO}_4$: functioning as a strong fixed acid, acting as a powerful dehydrating agent capable of charring organic material, and serving as a moderate oxidizing agent.

Sulphide Anion ($\text{S}^{2-}$)

A reducing sulphur anion derived from weak $\text{H}_2\text{S}$ gas that evolves a rotten-egg odor with dilute $\text{HCl}$, forms black precipitates with $\text{Pb}^{2+}$ ($\text{PbS}\downarrow$) and $\text{Ag}^+$ ($\text{Ag}_2\text{S}\downarrow$), and forms a canary-yellow precipitate ($\text{CdS}\downarrow$) with $\text{CdCO}_3$.

Sulphite Anion ($\text{SO}_3^{2-}$)

A reducing sulphur anion derived from moderate-strength $\text{H}_2\text{SO}_3$ that releases suffocating $\text{SO}_2$ gas upon treatment with dilute $\text{HCl}$ and forms a white $\text{BaSO}_3$ precipitate that dissolves in dilute $\text{HCl}$ with the liberation of $\text{SO}_2$.

Thiosulphate Anion ($\text{S}_2\text{O}_3^{2-}$)

A reducing sulphur anion derived from $\text{H}_2\text{S}_2\text{O}_3$ that reacts with dilute $\text{HCl}$ to yield both suffocating $\text{SO}_2$ gas and a distinct yellow precipitate of elemental sulphur ($\text{S}^0\downarrow$).

Sulphate Anion ($\text{SO}_4^{2-}$)

A stable sulphur anion that is neither an oxidizing nor a reducing agent, exhibits no dry reaction with dilute $\text{HCl}$, and precipitates white $\text{BaSO}_4$ with $\text{BaCl}_2$ that remains completely insoluble in dilute $\text{HCl}$.

Hepar Test

A special dry-fusion test for sulphate ($\text{SO}_4^{2-}$) where the salt is heated with $\text{Na}_2\text{CO}_3$ and charcoal ($\text{C}$) to yield $\text{Na}_2\text{S}$, which upon moistening liberates $\text{H}_2\text{S}$ that stains metallic silver black ($\text{Ag}_2\text{S}\downarrow$).

KCN Special Test (Thiosulphate)

A confirmatory test where $\text{S}_2\text{O}_3^{2-}$ is boiled with $\text{KCN}$ in alkaline conditions to generate thiocyanate ($\text{SCN}^-$), which produces an intense blood-red coloration ($[\text{Fe(SCN)}]^{2+}$) after cooling, acidifying, and adding $\text{Fe}^{3+}$.

Nitrite Anion ($\text{NO}_2^-$)

The reactive anion of weak nitrous acid ($\text{HNO}_2$) containing nitrogen in the $+3$ oxidation state, which releases brown $\text{NO}_2$ gas upon treatment with dilute $\text{HCl}$ and exhibits dual oxidizing and reducing properties.

Nitrate Anion ($\text{NO}_3^-$)

The stable anion of strong nitric acid ($\text{HNO}_3$) containing nitrogen in the fully oxidized $+5$ state, which gives a negative dry test with dilute $\text{HCl}$, does not decolorize acidified $\text{KMnO}_4$, and functions solely as an oxidizing agent.

Dual Redox Nature of Nitrite

The capacity of $\text{NO}_2^-$ ($\text{N}$ in $+3$ state) to act as an oxidizing agent by gaining $1\,e^-$ to form $\text{NO}$ ($+3 \rightarrow +2$) or as a reducing agent by losing $2\,e^-$ to form $\text{NO}_3^-$ ($+3 \rightarrow +5$).

Brown-Ring Test

A wet confirmatory test for nitrogen anions where reduction generates $\text{NO}$, which complexes with excess $\text{Fe}^{2+}$ to form the brown coordination species $[\text{Fe(NO)}]^{2+}$ as a diffused ring with nitrite in dilute acid or a sharp ring with nitrate in concentrated acid.

Ammonia Test (Nitrate Reduction)

A special confirmatory test for nitrate ($\text{NO}_3^-$) where boiling the sample with aluminum ($\text{Al}^0$) or zinc ($\text{Zn}^0$) powder in aqueous alkaline hydroxide ($\text{OH}^-$) reduces the anion to volatile ammonia gas ($\text{NH}_3\uparrow$).

Iodide/Starch Paper Test

A gas-phase redox confirmation test where brown oxidizing $\text{NO}_2$ gas oxidizes iodide ($\text{I}^-$) to free iodine ($\text{I}_2$), which subsequently reacts with starch on the test paper to produce an intense blue coloration.

Acidified Potassium Permanganate ($\text{KMnO}_4$) Test

A redox diagnostic test where reducing agents ($\text{S}^{2-}$, $\text{SO}_3^{2-}$, $\text{S}_2\text{O}_3^{2-}$, $\text{SO}_2$, $\text{NO}_2^-$) bleach purple permanganate ($\text{MnO}_4^-$) to colorless manganese(II) ions ($\text{Mn}^{2+}$).

Acidified Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) Test

A redox diagnostic test where reducing agents reduce orange dichromate ($\text{Cr}_2\text{O}_7^{2-}$) in acid solution to green chromium(III) ions ($\text{Cr}^{3+}$).