What is Qualitative Inorganic Analysis?
Qualitative inorganic analysis is the branch of analytical chemistry that deals with identifying the chemical composition of inorganic substances (salts, minerals, solutions, alloys, etc.) without measuring their exact quantity.
Its goal is to detect the presence of cations (positively charged ions) and anions (negatively charged ions) in a given sample through systematic tests such as precipitation, color changes, flame tests, and solubility behavior.
Unlike quantitative analysis (which measures “how much”), qualitative analysis answers “what is present?”
How is Qualitative Inorganic Analysis Performed?
The process usually follows a systematic stepwise scheme:
1. Preliminary Examination
- Physical inspection – Observe color, odor, and texture (e.g., green crystals may suggest Ni²⁺, blue may suggest Cu²⁺).
- Flame test – Certain metal ions give characteristic flame colors (e.g., Na⁺ = yellow, K⁺ = violet, Ba²⁺ = green).
- Solubility test – Some salts dissolve in water, others don’t.
2. Detection of Anions (Acid Radicals)
Tests are performed to detect negative ions in the sample:
- Carbonates (CO₃²⁻) → Effervescence with dilute HCl, releasing CO₂.
- Chlorides (Cl⁻) → White precipitate with AgNO₃.
- Sulfates (SO₄²⁻) → White precipitate with BaCl₂ (insoluble in HCl).
- Nitrates (NO₃⁻) → Brown ring test with FeSO₄ and conc. H₂SO₄.
3. Detection of Cations (Basic Radicals)
Cations are identified using a group separation scheme based on selective precipitation by common reagents. This is often called the “Systematic Qualitative Analysis of Cations”:
- Group I (Ag⁺, Pb²⁺, Hg₂²⁺) → Precipitate with dilute HCl.
- Group II (Cu²⁺, Cd²⁺, Bi³⁺, Sn²⁺, Sn⁴⁺, Hg²⁺, etc.) → Precipitate with H₂S in acidic solution.
- Group III (Fe³⁺, Al³⁺, Cr³⁺) → Precipitate with NH₄OH in presence of NH₄Cl.
- Group IV (Zn²⁺, Mn²⁺, Co²⁺, Ni²⁺) → Precipitate with H₂S in alkaline solution.
- Group V (Ba²⁺, Sr²⁺, Ca²⁺) → Precipitate with (NH₄)₂CO₃.
- Group VI (Mg²⁺, Na⁺, K⁺, NH₄⁺) → Identified with special tests (e.g., flame test, Nessler’s reagent for NH₄⁺).
4. Confirmatory Tests
Once a group is identified, specific confirmatory tests are done:
- Fe³⁺ → Blood-red color with KSCN.
- Cu²⁺ → Deep blue solution with excess NH₄OH.
- Ba²⁺ → Apple-green flame.
- NH₄⁺ → Ammonia gas released with NaOH (smell/test with moist red litmus).
Instruments Used
Traditionally, qualitative inorganic analysis is done using wet chemistry methods (test tubes, reagents, flame tests).
But modern labs also use instruments like:
- X-ray fluorescence (XRF)
- Inductively Coupled Plasma (ICP)
- Atomic Absorption Spectroscopy (AAS)
In short:
Qualitative inorganic analysis is the systematic identification of ions in an unknown sample using physical observation, group precipitation, and confirmatory tests. It is essential in chemistry labs for detecting unknown salts, testing water/soil, and educational training in analytical chemistry.