Gravimetric Analysis
Quantitative Estimation by Mass | Analytical Chemistry
1. Principle
Gravimetric Analysis is a technique where the analyte (the substance being analyzed) is converted into an insoluble precipitate of known chemical composition, which is then separated, dried/ignited, and weighed to determine the amount of analyte.
Core Concept: Mass Relationship (Stoichiometry).
Example: $AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3$.
Mass of $AgCl$ ppt relates directly to mass of $Cl^-$ in the sample.
2. Steps in Gravimetric Analysis
- Preparation of Solution: Dissolving the sample.
- Precipitation: Adding a suitable precipitating agent to form a stable precipitate.
- Digestion (Ostwald Ripening): Heating the precipitate in contact with the mother liquor. This allows small particles to dissolve and redeposit on larger particles, making the precipitate filterable.
- Filtration and Washing: Removing impurities.
- Drying or Ignition:
- Drying: Removing moisture (low temp).
- Ignition: Heating at high temp to convert precipitate into a stable chemical formula (e.g., $Fe(OH)_3 \xrightarrow{\Delta} Fe_2O_3$).
- Weighing: Using a high-precision balance.
3. Common Precipitating Agents
| Analyte | Precipitating Agent | Precipitate Formed |
|---|---|---|
| Chloride ($Cl^-$) | Silver Nitrate ($AgNO_3$) | $AgCl$ (White) |
| Sulphate ($SO_4^{2-}$) | Barium Chloride ($BaCl_2$) | $BaSO_4$ (White) |
| Nickel ($Ni^{2+}$) | Dimethylglyoxime (DMG) | Ni-DMG Complex (Red) |
| Iron ($Fe^{3+}$) | Ammonium Hydroxide ($NH_4OH$) | $Fe_2O_3 \cdot xH_2O$ (Red-Brown) |
4. Calculations (Gravimetric Factor)
The Gravimetric Factor (GF) converts the mass of the precipitate to the mass of the analyte.
$$ GF = \frac{\text{Atomic Wt of Analyte}}{\text{Molecular Wt of Precipitate}} \times \frac{a}{b} $$
Where $a, b$ are stoichiometric coefficients to balance the element of interest.
Final Formula:
$$ \% \text{ Analyte} = \frac{\text{Weight of Ppt} \times GF}{\text{Weight of Sample}} \times 100 $$Example: Determine $Cl$ in $AgCl$.
$GF = \frac{Atomic \ Wt \ Cl}{Molecular \ Wt \ AgCl} = \frac{35.5}{143.5} \approx 0.247$.
Practice Quiz
Test your knowledge on Gravimetry.
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