Gravimetric Analysis – Comprehensive Bullet-Point Notes

Course Outline & Learning Outcomes

• Course outline lists 5 macro-themes
  • Principle of Gravimetric Analysis
  • Types of Gravimetric Analysis
  • Steps in Gravimetric Analysis (identification: cation & anion)
  • Gravimetric Calculations
  • Applications of Gravimetric Analysis
• Learning outcomes – by end of chapter the learner can
  • Explain principle, types & steps
  • Calculate weight of precipitate / analyte
  • List real-world applications

Principles of Gravimetric Analysis

• Essence: convert analyte → new chemical form whose MASS is measurable
  • e.g. convert $\text{Cl}^-$ to $\text{AgCl(s)}$
• Relies exclusively on precise mass measurement (analytical balance)
• Considered most accurate classical analytical method

Requirements for Success

• Identify insoluble form of analyte (soluble → insoluble)
• Separate analyte from interferents
• Wash precipitate free of impurities / co-precipitants and dry
• Convert to stable weighing form and record mass

Advantages vs. Disadvantages

• Advantages
  • No instrument calibration or solution standardisation
  • Results come directly from experimental data
  • Cleaner (little wet chemistry) yet highly accurate
  • Works with macro-scale samples
• Disadvantages
  • Generally limited to one element/ionic species per run
  • Time-consuming; many heating/drying steps
  • Any missed step → impure sample
  • Requires higher concentration solutions

Types of Gravimetric Analysis

• Four classical branches
  • Precipitation Method
  • Volatilisation Method
  • Electrogravimetry
  • Thermogravimetry

1. Precipitation Method

• Definition: convert analyte into sparingly soluble precipitate → filter, wash, heat to stable known composition → weigh
• “Precipitation” = formation of solid from solution via T°, concentration or chemical change
• Worked example (Cl⁻ determination)
  1. Dissolve $\text{NaCl}$ sample in water
  2. Add excess $\text{AgNO}_3$: $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl(s)}$
  3. Filter
  4. Dry & weigh (subtract filter mass)
  5. Back-calculate $\text{Cl}^-$ from $m_{\text{AgCl}}$
• Sample problem: excess NaCl added to $\text{Ag}^+$ → collect $\text{AgCl}$, dry at $130!–150\,^{\circ}\text{C}$, then compute silver by stoichiometry

2. Volatilisation Method

• Analyte or decomposition product volatilised at suitable T°
• Volatile product collected & weighed OR deduced by mass loss
• Example: quantify $\text{NaHCO}_3$ in antacid tablets
  • Sample + $\text{H}<em>2\text{SO}</em>4$ → $\text{CO}<em>2$ + $\text{H}</em>2\text{O}$ + $\text{NaHSO}_4$
  • $\text{CO}_2$ trapped in pre-weighed absorption tube containing selective absorbent
  • Mass gain of tube → moles $\text{NaHCO}_3$ (1:1 stoichiometry)

(3) Electrogravimetry & (4) Thermogravimetry (mentioned only)

• Electrogravimetry: electro-deposit analyte on electrode & weigh
• Thermogravimetry: monitor continuous mass change vs. T°

Step-by-Step Procedure in Gravimetric Analysis

1. Preliminary Treatment (prepare & condition solution)
   • Eliminate interferents; adjust variables to ensure LOW solubility & filterable form
   • Critical factors
     • Influence of pH (affects solubility, e.g. $\text{CaC}<em>2\text{O}</em>4$ dissolves at low pH)
     • Volume (too large → excessive dilution, coagulation problems)
     • Temperature (higher T° ↑ solubility)
     • Concentration range
     • Other constituents (complexation, masking, common-ion)
2. Precipitation
   • Crystalline vs. colloidal suspension
     • Crystalline: particle >10^{-4}\,\text{cm}, settles spontaneously, easy filtration, LOW RSS
     • Colloidal: $10^{-7}!–10^{-4}\,\text{cm}$, stays dispersed, difficult filtration, HIGH RSS
   • Relative Supersaturation $\displaystyle RSS = \frac{Q-S}{S}$
     • $Q$ = instantaneous solute conc.; $S$ = equilibrium solubility
     • Small $RSS$ → crystal growth dominates → large crystals (desired)
   • To lower $RSS$
     • Heat solution (↑$S$)
     • pH adjust (if $S$ pH-dependent)
     • Add complexing agent (↑$S$)
     • Use dilute precipitating reagent, add slowly with stirring (↓$Q$)
     • Homogeneous precipitation techniques
   • Properties of good precipitates
     • Large crystals, easily filterable, extremely insoluble, stable, known stoichiometry after drying/ignition
   • Favour large particles by
     • Dilute solutions, slow addition, vigorous stirring, hot solution, low pH, slow cooling, miscible organics
3. Digestion
   • Heat mother liquor to promote Ostwald ripening
   • Small crystals dissolve → redeposit on larger ones → improved filterability & purity; colloids convert to crystals
4. Filtration & Washing
   • Filter types (Whatman grades): No.42 fine, 40 medium, 41 coarse/gelatinous
   • Washing removes excess reagents/soluble impurities; prevents peptisation
   • Ideal wash liquid criteria
     • Does not dissolve precipitate
     • Rapidly dissolves impurities
     • Volatile at drying T°
     • Non-interfering in subsequent tests
     • Water is often ideal
   • Demonstration: washing 1×50 mL vs. 5×10 mL – multiple small washings reduce contaminant from $1.0\,\text{g}$ → $0.00001\,\text{g}$
5. Drying or Igniting
   • Drying: $100!–150^{\circ}\text{C}$ to constant mass, removes moisture
   • Ignition: $400!–1000^{\circ}\text{C}$ converts to stable oxide/sulfate etc.
6. Weighing
   • Record mass of dry/ignited precipitate; stoichiometrically relate to analyte

Mechanisms, Contaminants & Purification

• Precipitate formation pathway
  • Nucleation (spontaneous or seed-induced) → Particle growth
  • Rapid cooling → many nuclei → small colloids; slow cooling → few nuclei → big crystals
• Peptisation
  • Coagulated colloid re-disperses; avoid via electrolyte wash (e.g. $\text{HNO}_3$) & minimal washing
• Contaminant introduction
  1. Co-precipitation
  • Inclusion & Occlusion (impurities trapped inside lattice/pockets)
  • Surface adsorption (ions adhered to surface; promoted by large surface area, specific precipitate types, pH/T°)
  • Isomorphous replacement (foreign ion of similar size/charge substitutes in lattice; e.g. $\text{SrSO}<em>4$ in $\text{BaSO}</em>4$)
  1. Post-precipitation (secondary precipitate forms later & deposits)
• Minimisation strategies (summary table)
  • Digestion, dilute solutions, slow reagent addition, washing with volatile electrolytes, reprecipitation
• Reprecipitation protocol
  1. Isolate precipitate
  2. Dissolve in minimal hot solvent
  3. Cool to reform precipitate
  4. Repeat until impurity mass negligible

Precipitating Agents

• Must be selective & yield insoluble product of known stoichiometry
• Inorganic precipitants
  • $\text{AgNO}_3$ → halides
  • $\text{BaCl}<em>2$ or $\text{NaSO}</em>4$ → sulfates
  • $\text{NH}<em>4\text{OH}$ → $\text{Al(OH)}</em>3$, $\text{Fe(OH)}<em>3$ (convert to $\text{Al}</em>2\text{O}<em>3$, $\text{Fe}</em>2\text{O}_3$)
• Organic precipitants (chelating reagents)
  • 8-Hydroxyquinoline (oxine): precipitates many metals; selectivity tuned by pH
  • Dimethylglyoxime: bright red $\text{Ni}$ chelate; specific for $\text{Ni}^{2+}$
  • Sodium tetraphenylborate $(\text{C}<em>6\text{H}</em>5)<em>4\text{BNa}$: precipitates $\text{K}^+$, $\text{NH}</em>4^+$

Gravimetric Calculations

• Gravimetric Factor (GF)
  • $\displaystyle GF = \frac{FW<em>{\text{analyte}}}{FW</em>{\text{precipitate}}} \times \frac{a}{p}$
    • $FW$ = formula weight; $a$ = moles analyte per stoichiometric unit; $p$ = moles precipitate per unit weighed
  • Weight of analyte $= GF \times$ (weight of precipitate)
• Percent analyte in sample
  • $\%A = \frac{m<em>A}{m</em>{sample}} \times 100\%$
• Common GFs
  • $\text{Cl}^-$ via $\text{AgCl}$: $GF = \frac{FW<em>{Cl}}{FW</em>{AgCl}} = 0.2474$
  • $\text{Fe}$ via $\text{Fe}<em>2\text{O}</em>3$: $GF = \frac{FW<em>{Fe}}{2}\big/ FW</em>{Fe<em>2O</em>3} = 0.6994$
  • $\text{P}$ via $\text{Mg}<em>2\text{P}</em>2\text{O}<em>7$: $GF = 0.2783$ (for P) or $0.6378$ (for $\text{P}</em>2\text{O}_5$)
• Worked examples
  1. $1.0\,\text{g}$ Fe-compound → $0.1565\,\text{g}$ $\text{Fe}<em>2\text{O}</em>3$ ⇒ $10.95\%$ Fe
  2. $354\,\text{mg}$ sample → $187\,\text{mg}$ $\text{AgBr}$ ⇒ $22.5\%$ Br⁻ (GF $=0.4255$)
  3. $1.2\,\text{g}$ $\text{NH}<em>4\text{Al(SO}</em>4)<em>2$ gives $0.1798\,\text{g}$ $\text{Al}</em>2\text{O}_3$ ⇒ $7.93\%$ Al

Applications of Gravimetric Analysis

• Inorganic species: $\text{Cl}^-$, $\text{SO}_4^{2-}$, $\text{Fe}^{2+}$, $\text{Na}^+$
• Neutral molecules: $\text{H}<em>2\text{O}$, $\text{SO}</em>2$, $\text{CO}<em>2$, $\text{I}</em>2$
• Organic analyses: lactose in milk, cholesterol in cereal, amino acids in food
• Techniques
  • Inorganic precipitating agents (Table 12.2)
  • Electrolytic reduction for $\text{Co, Ni, Cu, Zn, Ag, In, Sn, Sb, Cd, Re, Bi}$
  • Volatilisation gravimetry for water & $\text{CO}_2$

Example Application: Sulphate Determination

• Acidify hot solution with HCl → slowly add dilute $\text{BaCl}<em>2$: $\text{Ba}^{2+}+\text{SO}</em>4^{2-} \rightarrow \text{BaSO}_4$
• Filter, wash, ignite at red heat, weigh $\text{BaSO}<em>4$ ⇒ compute $\text{SO}</em>4^{2-}$

Ethical & Practical Implications

• High accuracy favours gravimetry in standardisation of reference materials
• Requires careful waste handling (heavy metals like $\text{Ag}^+$, $\text{Ba}^{2+}$)
• Time & resource intensive; modern instrumental methods often preferred unless highest precision is mandatory

Connections & Broader Context

• Builds on solubility product ($K_{sp}$) and equilibria concepts
• RSS, nucleation & crystal growth linked to physical chemistry and materials science (Ostwald ripening)
• Peptisation & colloidal stability relate to surface chemistry & wastewater treatment
• Gravimetric factors echo stoichiometry and molar mass bridging to general chemistry fundamentals

Quick Reference Formulae

• $RSS = \frac{Q-S}{S}$
• $K_{sp} = [\text{M}^{n+}][\text{X}^{m-}]$ (for $\text{MX}$ precipitate)
• $GF = \frac{FW<em>{analyte}}{FW</em>{ppt}}\times\frac{a}{p}$
• $\%\text{Analyte} = \frac{GF\; m<em>{ppt}}{m</em>{sample}}\times 100\%$