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DrX Whiz Niraj
Gravimetric Analysis: Principles & Purity
Master the steps of Gravimetry, understand the critical difference between Co-precipitation and Post-precipitation, and learn the estimation of Barium Sulphate.
Gravimetric Analysis is a quantitative analytical method based on the measurement of mass (weight). The principle involves converting the analyte (substance to be analyzed) into an insoluble precipitate of known chemical composition. This precipitate is then separated, washed, dried/ignited, and accurately weighed.
Ideal Conditions for a Precipitate:
- It should be highly insoluble to prevent loss during filtration and washing.
- It should be easily filterable (large crystal size).
- It must possess a known, stable chemical composition after drying or ignition.
A successful gravimetric analysis requires strict adherence to a sequence of steps to ensure the purity and accuracy of the final weight.
The 7 Steps in Detail:
- 1. Preparation of Solution: Adjusting the pH and volume to ensure complete precipitation.
- 2. Precipitation: Adding the precipitating agent slowly with continuous stirring. Dilute and hot solutions are preferred to get large, pure crystals.
- 3. Digestion (Ostwald Ripening): The precipitate is allowed to stand in contact with the mother liquor, usually upon heating. This allows small, impure crystals to dissolve and re-precipitate onto larger crystals, improving purity and filterability.
- 4. Filtration: Separating the precipitate from the mother liquor using Whatman filter paper or sintered glass crucibles.
- 5. Washing: Removing adhering impurities using a dilute electrolyte solution (to prevent peptization/colloid formation).
- 6. Drying and Ignition: Heating the precipitate in a crucible to remove moisture and convert it into a strictly defined, stable chemical form.
- 7. Weighing & Calculation: Using an analytical balance to find the constant weight and calculating the analyte concentration.
A major challenge in gravimetry is that the precipitate can carry down normally soluble impurities from the solution. This contamination happens via two distinct mechanisms.
| Parameter | Co-Precipitation | Post-Precipitation |
|---|---|---|
| Occurrence | Happens DURING the formation of the primary precipitate. | Happens AFTER the primary precipitate has formed. |
| Contamination Site | Impurities are trapped inside the crystal lattice (Occlusion / Inclusion). | Impurities are deposited on the outer surface of the crystals. |
| Effect of Time (Digestion) | Amount of impurity decreases if digested (left in mother liquor). | Amount of impurity increases if left in the mother liquor for a long time. |
| Example | Barium sulphate co-precipitating with Nitrate ions. | Calcium oxalate precipitating on top of Magnesium oxalate. |
Barium Sulphate estimation is the classic example of Gravimetric Analysis. It is used to determine the amount of Sulphate (SO₄²⁻) in a given sample.
Procedure Summary
- The sample containing sulphate is acidified with Dilute HCl.
- It is heated to boiling.
- A hot, dilute solution of Barium Chloride (BaCl₂) is added dropwise with constant stirring.
- The mixture is digested on a water bath (Ostwald ripening).
- Filtered through ashless filter paper, washed with hot water to remove chlorides, and finally ignited and weighed.
Why Acidify with HCl?
Dilute HCl is added before precipitation to prevent the co-precipitation of other barium salts (like Barium Carbonate or Barium Phosphate) which are insoluble in neutral media but soluble in acidic media. This ensures that only Barium Sulphate precipitates out purely.