Gravimetric Analysis

Gravimetric analysis is a classical quantitative chemical analysis method that determine the amount of an analyte based on the mass of a solid. it is highly accurate and precise.

Principle: 

Gravimetric analysis is based on the measurement of mass to determine the quantity of an analyte in sample. It relies on converting the analyte into a pure stable and weighable in solid form. 

                                                     

                                                     1.Sample Preparation

                                     (Dissolve analyte in suitable solvent)

                                                                 ↓

                                                      2. Precipitation

                                (Add reagent to form an insoluble compound)

                                                                 ↓

                                                      3. Digestion

                               (Let precipitate stand to improve purity/crystallinity)

                                                                  ↓

                                                      4. Filtration

                                          (Separate precipitate from solution)

                                                                  ↓

                                                       5. Washing

                                     (Remove impurities from the precipitate)

                                                                  ↓

                                                       6. Drying / Ignition

                                    (Dry or heat precipitate to constant mass)

                                                                  ↓

                                                      7. Weighing

                                         (Measure mass of dried compound)

                                                                  ↓

                                                        8. Calculation

                                (Use stoichiometry to determine analyte mass or %)

 Types of Gravimetric Analysis:

1. Precipitation Method – Most common. Analyte is converted into an insoluble precipitate (e.g., forming AgCl from Cl⁻).
2. Volatilization Method – Analyte is volatilized and collected(e.g., H₂O, CO₂).
3. Electrogravimetry – Depositing a metal on an electrode and weighing it.

1. Precipitation Method-

The precipitation method is the most widely used form of gravimetric analysis. It involves converting the analyte into a sparingly soluble compound (precipitate), which can then be filtered, dried, and weighed.

Principle:

Analyte is precipitated as a known, insoluble compound by adding a suitable reagent. The mass of the pure, dry precipitate is then used to calculate the amount of analyte based on stoichiometry.

Steps in Precipitation Method:

1. Sample Preparation

• Dissolve the sample in a suitable solvent.
• Remove any impurities or interfering ions.

2. Precipitation

• Add a precipitating agent slowly with constant stirring.
• Precipitate should form as large, pure crystals to ease filtration.
• Conditions should be adjusted for:

                              Correct pH
                              Temperature
                              Concentration

Example: Cl⁻ + Ag⁺ → AgCl (white precipitate)

3. Digestion

The precipitate is allowed to sit in the solution (often heated) to:

• Grow larger crystals
• Improve filterability
• Reduce impurities (occlusion)

4. Filtration

The precipitate is separated using:

• Filter paper (e.g., Whatman)
• Sintered glass crucible

5. Washing

The precipitate is washed with cold distilled water or special washing solutions to remove:

• Excess reagents
• Soluble impurities

NOTE: Over-washing can lead to loss of precipitate (peptization).

6. Drying or Ignition

• The washed precipitate is dried at a specific temperature.
• Or ignited (burned at high temperatures) to convert to a stable, weighable form.

Example: CaC₂O₄·H₂O → (ignite) → CaO

7. Weighing

• The dried/ignited residue is cooled in a desiccator and weighed.
• Repeat until a constant mass is obtained.

8. Calculation

• Use the known chemical formula of the precipitate and stoichiometric relationships to determine the amount of analyte in the original sample.

Flow chart:

                                                   1. Sample Dissolution
                                          (Dissolve analyte in solvent)
                                                                ↓
                                                   2. Precipitation
                                       (Add reagent to form precipitate)
                                                                ↓
                                                   3. Digestion
                                       (Let stand, often with heat, to purify)
                                                                ↓
                                                   4. Filtration
                                        (Separate precipitate from solution)
                                                                ↓
                                                    5. Washing
                                        (Remove soluble impurities)
                                                                ↓
                                                   6. Drying or Ignition
                                      (Convert to a stable compound)
                                                               ↓
                                                   7. Weighing
                                           (Obtain constant mass)
                                                               ↓
                                                  8. Calculation
                                      (Determine amount of analyte)

Example: Determination of Chloride as Silver Chloride (AgCl)

1. Sample contains Cl⁻ ions.
2. Add AgNO₃ → AgCl precipitates.
3. Filter, wash, dry, and weigh AgCl.
4. Calculate amount of Cl⁻ using:

$$\text{Mass of Cl⁻} = \left(\frac{35.45}{143.32}\right) \times \text{Mass of AgCl}$$

$$\text{2.Volatilization Method-}$$

$$\text{In volatilization method analyte is converted into a volatile compound that is collected and weighed directly or indirectly.}$$

$$\text{Principle-In this method, the analyte is liberated as a gas (e.g., H₂O, CO₂, SO₂, NH₃) by heating or chemical reaction. The gas is either: Collected and weighed (directly), orCaptured by an absorbing medium and weighed as a new compound (indirectly).Steps in Volatilization Gravimetric Analysis: 1. Sample PreparationWeigh and place the sample in a suitable container (e.g., crucible, combustion boat).Dry or dissolve if necessary. 2. Conversion to Volatile FormThe sample is heated or reacted chemically to convert the analyte into a volatile compound.}$$

$$\text{ Example: CaCO₃ → CaO + CO₂ ↑}$$

$$\text{3. Collection of Volatile Product}$$

$$\text{The gas is either:Measured by loss in mass of the sample.Or absorbed in a substance of known reactivity (e.g., CO₂ in NaOH solution).Or collected and weighed directly (rare). 4. WeighingThe change in mass is measured:Loss in mass = mass of volatile substanceOr mass gain in the absorber = mass of volatile substance 5. Calculation Use stoichiometry to calculate the quantity of analyte in the original sample.}$$

$$\text{Flow Chart: Volatilization Gravimetric Method}$$

$$\text{ 1. Sample Weighing and Preparation ↓ 2. Heat or React Sample to Release Volatile Compound ↓ 3. Capture Volatile Product (or Measure Mass Loss) ↓ 4. Weigh Absorber or Measure Mass Difference ↓ 5. Stoichiometric Calculation of Analyte}$$

$$\text{Examples of Volatilization Gravimetric Methods 1. Water Content Determination (Moisture Analysis)Heat hydrated compound (e.g., CuSO₄·5H₂O).Water evaporates.Mass loss = Mass of H₂O. 2. Carbon Dioxide DeterminationHeat carbonate (e.g., CaCO₃).CO₂ is released and absorbed in NaOH or KOH.Mass gain of absorber = Mass of CO₂. CaCO₃ → CaO+CO₂↑}$$

$$\text{ heat}$$

$$\text{Advantages:High accuracy when gases are fully collected. Useful for volatile analytes and decomposition reactions. Limitations:Requires special apparatus for gas collection.Risk of gas escape or incomplete reaction.}$$

$$\text{3.Electrogravimetry (EGM)-}$$

$$\text{Electrogravimetry is a quantitative electroanalytical technique in which a metal ion in solution is electrolyzed and deposited on an electrode, usually the cathode.}$$

$$\text{Principle of Electrogravimetry: An electric current is passed through a solution containing the analyte (metal ion), causing it to deposit on the electrode. The mass gain of the electrode is used to calculate the amount of the analyte based on Faraday’s laws of electrolysis.}$$

$$Apparatus Used in Electrogravimetry:Electrolytic cell (typically glass or plastic)Cathode (e.g., platinum or stainless steel foil) – where the metal depositsAnode (inert, like platinum or graphite)DC Power supply or batteryMagnetic stirrer or heater (optional, to improve deposition)Analytical balance (for weighing the electrode)Step-by-Step Process of Electrogravimetry: 1. Sample PreparationDissolve the metal-containing sample in a suitable acidic electrolyte (e.g., H₂SO₄ or HNO₃).Adjust pH and temperature if necessary. 2. ElectrolysisImmerse clean electrodes into the electrolyte solution.Apply a controlled voltage or current.Metal ions get reduced and deposited on the cathode. 3. End Point DetectionElectrolysis is continued until all metal ions are deposited (often verified by testing a small aliquot or observing constant current drop). 4. Washing and DryingRemove the cathode, rinse with distilled water, then ethanol.Dry in an oven or desiccator. 5. WeighingWeigh the dried cathode before and after electrolysis.Mass increase = mass of deposited metal. 6. CalculationUse the weight of deposited metal to calculate:The concentration of the metal in the original solution.Or use current/time and Faraday’s law to confirm theoretical values.$$

$$\text{Flow Chart: Electrogravimetry Process}$$

$$                                  1. Sample Dissolution & Electrolyte Preparation                                                                    ↓                                  2. Insertion of Electrodes in Electrolytic Cell                                                                    ↓                                  3. Apply DC Current → Metal Ion Reduction Begins                                                                    ↓                                  4. Deposition of Metal on Cathode                                                                    ↓                                  5. Remove, Wash, and Dry Cathode                                                                    ↓                                  6. Weigh Cathode (Before and After)                                                                    ↓                                  7. Use Mass Difference for Analysis (via Faraday’s Law)Advantages of Electrogravimetry:High precision and accuracy.Direct method – no need for precipitating agents.Applicable to very dilute solutions.Clean and efficient – no waste precipitates.Limitations:Not suitable for non-metallic analytes.Requires good control of voltage/current.Co-deposition of impurities can occur.Electrodes must be cleaned carefully. $$