Introduction to Gravimetry
Gravimetric analysis is a classical quantitative analytical method based on the measurement of mass. It involves converting an analyte (element, ion, or radical) into a pure, stable compound that can be weighed accurately to determine its concentration.
Gravimetric, electro-gravimetric, and thermal gravimetric analyses are employed for producing pure compounds and measuring their weights for quantitative evaluation.
Advantages of Gravimetric Analysis
- High accuracy and precision with modern analytical balances.
- No need for calibration — it’s an absolute method.
- Allows verification of reaction completion by filtrate examination.
- Cost-effective and simple compared to instrumental methods.
The conversion factor used to relate the weight of the precipitate to that of the analyte is called the Gravimetric Factor.
Precipitation Techniques
Precipitation involves the conversion of dissolved ions into an insoluble solid that can be separated, dried, and weighed. The quality of the precipitate directly affects analytical accuracy.
Ideal Properties of a Gravimetric Precipitate
- Pure and stable composition.
- Free from soluble impurities.
- Insoluble in the mother liquor.
- Easy to filter and wash.
Example: Gravimetric estimation of sulphates or halides.
Colloidal Precipitates
In some gravimetric reactions, colloidal precipitates are formed — these are fine, non-settling particles that can adsorb impurities and resist filtration.
Colloidal Behavior
- Colloidal particles form a primary adsorption layer (charged surface) and a secondary oppositely charged layer, forming an electrical double layer.
- These double layers stabilize the colloid and prevent aggregation.
Coagulation (conversion into filterable solid) can be achieved by heating, stirring, or adding electrolytes that neutralize surface charges.
Examples of Colloids
- Aerosol (gas + liquid/solid, e.g., fog, smoke)
- Emulsion (liquid + liquid, e.g., milk)
- Foam (liquid + gas, e.g., whipped cream)
- Sol (liquid + solid, e.g., paint)
- Solid sol (solid + solid, e.g., pearl)
Properties of Colloids
- Brownian Motion: Zigzag motion of particles due to collisions with solvent molecules.
- Tyndall Effect: Scattering of light beam through a colloid.
Supersaturation
Supersaturation is a thermodynamically unstable state achieved when more solute is present in a solution than normally soluble at a given temperature. It leads to nucleation and crystal growth.
Zones of Supersaturation
- Zone 1 – Metastable Zone: No spontaneous nucleation; crystal growth occurs only with seeding.
- Zone 2 – Nucleation Zone: Crystals nucleate and grow simultaneously.
- Zone 3 – Precipitation Zone: Rapid precipitation of solids from solution.
Slow cooling (low supersaturation) yields larger, purer crystals; rapid cooling (high supersaturation) produces small crystals.
Co-precipitation
Co-precipitation occurs when impurities precipitate along with the desired compound, causing errors in analysis. It can be minimized by digestion, re-precipitation, or slow precipitation.
Mechanisms of Co-precipitation
- Inclusion: Impurity replaces an ion in the crystal lattice (e.g., Pb²⁺ in BaSO₄ crystals).
- Occlusion: Foreign ions trapped inside growing crystals.
- Adsorption: Surface adherence of impurities, common in colloids.
- Mechanical Entrapment: Solvent pockets trapped between growing crystals.
Gravimetric Methodology
In gravimetry, the analyte is converted into an insoluble compound that can be filtered, washed, dried, and weighed accurately. The method involves the following steps:
1. Preparation of Solution
Adjust pH and dilute the sample appropriately. Ensure complete dissolution of analyte and suitable conditions for precipitation.
2. Precipitation
The precipitating reagent is added slowly to form the insoluble product. To ensure large particle formation and reduce nucleation, control relative supersaturation (Q–S)/S by using dilute solutions and slow addition.
3. Digestion of Precipitate
Heating the precipitate below boiling for 30–60 minutes helps larger crystals form through Ostwald ripening, improving purity and filterability.
4. Washing and Filtering
Wash the precipitate to remove adsorbed impurities using minimal liquid. Avoid peptization in colloids by using dilute acid or salts instead of pure water.
5. Drying and Ignition
The precipitate is dried at 120–150°C or ignited at 600–1200°C to obtain a stable compound of known composition suitable for weighing.
Impurities in Precipitates
- Occlusion: Entrapped medium in the crystal structure.
- Inclusion: Substitution of similar ions within the crystal lattice.
- Surface Adsorption: Leads to positive error due to retained ions on surface.
- Post-precipitation: Secondary precipitation of foreign ions if filtrations are delayed.
Apparatus Used in Gravimetry
Filter Paper:
A semi-permeable medium used for separating solids from liquids. Ashless filter papers are preferred for gravimetric work as they leave minimal residue after ignition.
Crucibles:
Cup-shaped containers made from heat-resistant materials like porcelain, alumina, or platinum, used for ignition and weighing of precipitates.
Thermogravimetric Analysis (TGA)
TGA measures weight changes in a sample as temperature increases. It provides information about composition, thermal stability, and decomposition kinetics.
Example:
Decomposition of Calcium Oxalate Monohydrate (CaC₂O₄·H₂O) shows three stages of weight loss in an inert atmosphere (e.g., N₂). The evolved gases (CO₂, CO, H₂O) are identified using mass spectrometry.
Organic Precipitants
Organic precipitants form chelates with metal ions and are used for selective gravimetric analysis of trace metals.
Examples of Organic Precipitants
- Dimethylglyoxime
- Oxalic acid
- α-nitroso-β-naphthol
- Cupferron
- Cupron
- Sodium tetraphenylborate
- Nitron
Estimation of Barium as Barium Sulphate
Principle:
When dilute sulphuric acid reacts with barium chloride, insoluble barium sulphate is formed:
BaCl₂ + H₂SO₄ → BaSO₄ ↓ + 2HCl
Procedure:
- Pipette 25 ml of BaCl₂ solution into a beaker and add 0.5 ml conc. H₂SO₄ and 100 ml water.
- Boil and add H₂SO₄ dropwise until precipitation completes.
- Filter the precipitate and wash with hot distilled water.
- Dry and ignite in a weighed crucible until carbonaceous matter burns off.
- Cool in a desiccator and reweigh to constant mass.
Calculation: Weight of BaSO₄ obtained is used to determine the amount of barium present using stoichiometric relations.
Detailed Notes:
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