← Chemistry — Std 11
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Basic Analytical Techniques

Ch. 3Std 11

Easy Overview

How do you separate, identify, and quantify unknown substances? This chapter covers chromatography (paper, TLC, column, GC, HPLC), spectroscopy (UV-Vis, IR, NMR, MS), and classical techniques (distillation, extraction). Each method has its niche — the skill is choosing the right tool. Chromatography separates based on differential distribution between a stationary phase and mobile phase. Spectroscopy studies light-matter interactions to identify and measure compounds. Classical techniques like distillation and extraction remain essential in every lab.

Paper Chromatography

Stationary phase: filter paper (cellulose + water). Mobile phase: solvent climbs by capillary action. Components partition between phases. Rf = distance by solute / distance by solvent front (0<Rf<1). Depends on solvent, temperature, paper type — run standards alongside. Visualization: UV, iodine, ninhydrin (purple for amino acids). Simple, cheap, used in pharmaceutical QC.

Thin Layer Chromatography (TLC)

Stationary phase: silica gel or alumina on glass/plastic. More polar compounds are retained more on silica. Detection: UV fluorescence (dark spots on bright green), iodine, charring, chemical sprays. Faster (5-30 min) than paper, requires μg samples, better resolution. Used for reaction monitoring, purity checks, identification.

Column Chromatography

Preparative version of TLC. Column packed with silica/alumina. Sample loaded at top, solvent elutes components at different rates. Gradient elution (increasing polarity) improves separation. Fractions collected and monitored by TLC. Separates grams to kilograms. Essential for organic synthesis and natural product isolation.

Principle of Spectroscopy — Light and Matter

Spectroscopy studies how light interacts with matter. E = hν = hc/λ. Electronic transitions (UV-Vis, 200-800 nm) involve valence electrons. Vibrational transitions (IR, 2.5-25 μm) involve bond stretching/bending. Beer-Lambert: A = εcl (absorbance = molar absorptivity × concentration × path length). Quantitative by measuring A at λmax.

UV-Visible Spectroscopy

For compounds with conjugated π systems. Chromophores: ethene (170 nm), benzene (255 nm), β-carotene (450 nm). λmax is characteristic. A ∝ c at λmax. Applications: drug concentrations, metal complexes, kinetics, protein detection (280 nm). Instrument: deuterium lamp (UV), tungsten (Vis), monochromator, detector.

Infrared (IR) Spectroscopy

Identifies functional groups via bond vibrations (4000-400 cm⁻¹). Key: 3700-3200 (O-H, N-H, broad), 3100-2800 (C-H), 2250-2100 (C≡C, C≡N), 1750-1650 (C=O), 1650-1450 (C=C), 1300-1000 (C-O). Fingerprint region (1500-400) is molecule-specific. Essential for confirming organic compounds.

Simple Distillation

Separates liquids with BP difference > 25°C. Apparatus: flask, thermometer, condenser, receiver. Lower BP component vaporizes first, condenses. Temperature constant for pure liquid. Used for solvent purification, desalination. Inefficient for close BPs — vapor contains significant higher-BP component.

Fractional Distillation

Uses fractionating column (packed with glass beads/Rashig rings) for close BPs (~5°C). Multiple vaporization-condensation cycles = theoretical plates. More plates = better separation. Industrial: petroleum refining separates crude into gases, gasoline, kerosene, diesel, residue.

Solvent Extraction

Solute distributes between immiscible solvents: K = C₁/C₂. Mass remaining after n extractions = initial × [V₁/(V₁+KV₂)]ⁿ. Multiple small extractions > one large. Example: three 10 mL extractions more efficient than one 30 mL. Applications: caffeine extraction, natural product isolation.

Electrophoresis

Separates charged molecules in electric field. (+) → cathode, (-) → anode. Speed ∝ charge/size ratio. Gel electrophoresis: agarose or polyacrylamide acts as molecular sieve. DNA fragments by size (fingerprinting), proteins by SDS-PAGE, amino acids by paper electrophoresis.

Gas Chromatography (GC)

For volatile compounds. Sample vaporized (250-300°C), carried by He through capillary column. Lower BP → faster elution. Retention time characteristic. Detectors: FID (organics), MS (identification). GC-MS gives both separation and ID. Applications: petroleum, drug testing, pesticides, environmental.

High Performance Liquid Chromatography (HPLC)

For non-volatile/thermally unstable compounds. Mobile phase pumped at high pressure through packed column (3-5 μm particles). Reverse phase (C₁₈, water-methanol) most common. UV-Vis or MS detection. Workhorse of pharmaceutical analysis. Gradient elution changes solvent composition during run.

Atomic Absorption Spectroscopy (AAS)

Measures metals. Sample atomized in flame or graphite tube. Hollow cathode lamp emits element-specific wavelength. Absorption ∝ concentration (Beer-Lambert). Each element needs its own lamp. Flame AAS: ppm sensitivity. Graphite furnace: ppb. Applications: heavy metals in water, minerals in food, clinical.

Mass Spectrometry (MS)

Determines molecular mass and fragments. Sample ionized (EI, ESI, MALDI). Ions separated by m/z. M⁺ or [M+H]⁺ peak gives molecular mass. Fragments = structural fingerprint. High-resolution MS distinguishes C₂H₆O (46.0419) from CH₂O₂ (46.0055). Coupled with GC/LC. Applications: identification, proteomics, doping detection.

NMR Spectroscopy

Most powerful for structure determination. ¹H NMR: chemical shift δ (0.9 CH₃, 1.2 CH₂, 3.7 CH₂-O, 7.2 aromatic, 10-12 COOH). Integration = proton count. Splitting n+1 = neighbor count. ¹³C NMR: one peak per unique C. Combined = complete structure determination.

Conductometry

Measures ionic conductivity. Conductance G = 1/R (siemens). Conductivity κ = G × cell constant. Conductometric titration: plot conductivity vs titrant volume. Endpoint = slope change. HCl + NaOH: conductivity drops (H⁺ replaced by Na⁺), then rises (OH⁻ added). For colored/turbid solutions.

pH Metry and Potentiometry

Measures potential for ion concentration. pH electrode: E = constant - 0.059×pH at 25°C. Potentiometric titration endpoint = inflection point (max slope, found from derivative). Works for colored/turbid/non-aqueous solutions. Ion-selective electrodes: F⁻, Na⁺, K⁺, Ca²⁺.

Thermal Methods (TGA, DTA, DSC)

TGA: mass change vs temperature — detects dehydration, decomposition. DTA: temperature difference vs reference — endothermic (melting), exothermic (crystallization). DSC: heat flow — quantifies ΔH for transitions. Used in materials science, polymers, pharmaceuticals.

Refractometry

Refractive index n = speed of light in vacuum / speed in medium. Depends on λ (Na D-line, 589 nm), temperature, composition. For solutions, n ∝ concentration. Abbe refractometer: one drop, instant reading. Applications: sugar (Brix), purity of essential oils, liquid identification.

Key Points

  • •Chromatography: differential distribution between stationary and mobile phases
  • •Rf = distance by solute / distance by solvent front (0<Rf<1)
  • •TLC: faster, sharper than paper; used for reaction monitoring
  • •Column chromatography: preparative; gradient elution improves resolution
  • •UV-Vis: electronic transitions; Beer-Lambert A = εcl
  • •IR: bond vibrations; key: 3300 (O-H), 1700 (C=O), 1600 (C=C), 2200 (C≡C)
  • •Simple distillation: BP diff > 25°C; fractional: column for close BPs
  • •More theoretical plates = better separation in fractional distillation
  • •Solvent extraction: K = C₁/Câ‚‚; multiple small extractions more efficient
  • •GC: volatiles, retention time; GC-MS provides separation + ID
  • •HPLC: non-volatiles; reverse phase (C₁₈) most common
  • •AAS: element-specific lamp; flame (ppm), graphite furnace (ppb)
  • •MS: M⁺ peak for molecular mass; fragments for structure
  • •NMR: δ, integration, splitting (n+1 rule) determine structure
  • •Conductometry: for colored/turbid solutions; endpoint = slope change
  • •Potentiometry: Nernst equation; inflection point = endpoint
  • •TGA (mass), DTA/DSC (heat flow) — materials characterization

Practice Questions

  • Explain paper chromatography. How is Rf calculated and what affects it?
  • Two dyes on TLC: A=4.2 cm, B=6.8 cm, solvent=8.5 cm. Calculate Rf. Which is more polar?
  • Differentiate between simple and fractional distillation.
  • State Beer-Lambert law: A=0.45, ε=1.5×10⁴ L mol⁻¹cm⁻¹, l=1 cm. Find concentration.
  • Assign IR absorptions for ethanol, acetone, and benzoic acid.
  • 100 mL aqueous has 5 g of compound, K(org/aq)=4. Compare one 50 mL vs two 25 mL extractions.
  • Compare GC and HPLC with applications.
  • Predict ¹H NMR of ethanol (CH₃CHâ‚‚OH).
  • Explain AAS and why hollow cathode lamp is element-specific.