Gravimetric Analysis – Chloride Ion Determination & Thermometric Analysis

Gravimetric Analysis

Chloride Ion Determination & Thermometric Analysis

What is Gravimetric Analysis?

Gravimetric analysis is a quantitative analytical method where the amount of analyte is determined by measuring the mass of a pure, solid compound containing the analyte. It is one of the most accurate and precise analytical techniques available.

Key Principles

  • Precipitation: The analyte is converted to a sparingly soluble precipitate.
  • Filtration: The precipitate is separated from the solution.
  • Washing: Impurities are removed by washing the precipitate.
  • Drying/Ignition: The precipitate is converted to a stable compound of known composition.
  • Weighing: The mass of the precipitate is accurately measured.
  • Calculation: The analyte amount is calculated using stoichiometry.

Activity 17: Determination of Chloride Ion (Cl⁻) in a Sample

This experiment demonstrates the gravimetric determination of chloride ions by precipitation as silver chloride (AgCl).

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

The white precipitate of silver chloride is filtered, dried, and weighed. The mass of chloride ions in the original sample is calculated from the mass of AgCl precipitate.

Materials Required

Chemicals

  • Sample containing chloride ions
  • Silver nitrate (AgNO₃) solution
  • Distilled water
  • Nitric acid (for washing)

Glassware

  • Beakers (250 mL, 400 mL)
  • Filter paper (Whatman No. 42)
  • Funnel
  • Watch glass
  • Crucible (porcelain)
  • Measuring cylinders

Equipment

  • Analytical balance (±0.0001 g)
  • Oven (105-110°C)
  • Desiccator
  • Hot plate/stirrer
  • Bunsen burner (optional)

Safety Equipment

  • Lab coat and safety goggles
  • Chemical-resistant gloves
  • Fume hood (AgNO₃ is light-sensitive)
  • First aid kit

General Steps in Gravimetric Analysis

  1. Sample Preparation: Accurately weigh and dissolve the sample.
  2. Precipitation: Add precipitating agent to form an insoluble compound.
  3. Digestion: Heat the precipitate to improve purity and filterability.
  4. Filtration: Separate precipitate from solution using filter paper.
  5. Washing: Remove soluble impurities with appropriate wash solution.
  6. Drying/Ignition: Convert precipitate to constant-weight form.
  7. Weighing: Accurately measure mass of dried precipitate.
  8. Calculation: Determine analyte amount using stoichiometry.

Procedure: Chloride Ion Determination

Analytical Balance
Sample Solution
Filtration Setup
Crucible

Detailed Procedure

  1. Sample Preparation:
    • Weigh approximately 0.5 g of sample accurately (±0.0001 g) using an analytical balance.
    • Transfer to a 250 mL beaker and dissolve in 100 mL distilled water.
    • Add 1 mL of 6M HNO₃ to acidify the solution (prevents precipitation of other silver salts).
  2. Precipitation:
    • Heat the solution to near boiling (70-80°C).
    • Slowly add 0.1M AgNO₃ solution with constant stirring until precipitation is complete (slight excess).
    • Continue stirring for 1-2 minutes after precipitation appears complete.
  3. Digestion:
    • Cover the beaker with a watch glass and heat on a steam bath or hot plate at 70-80°C for 30-60 minutes.
    • Digestion allows small particles to coalesce into larger, more filterable crystals.
  4. Filtration and Washing:
    • Filter through pre-weighed filtering crucible or filter paper (Whatman No. 42).
    • Wash precipitate with 0.01M HNO₃ (5-6 times with 10 mL portions) to remove Ag⁺ ions.
    • Finally wash with 10 mL distilled water to remove HNO₃.
    • Test washings with HCl to ensure complete removal of Ag⁺ ions.
  5. Drying:
    • Transfer filter paper with precipitate to a pre-weighed crucible.
    • Dry at 105-110°C for 1-2 hours until constant weight is achieved.
    • Cool in a desiccator for 30 minutes before weighing.
  6. Weighing:
    • Weigh the cooled crucible with precipitate to ±0.0001 g.
    • Repeat drying-cooling-weighing until constant mass (±0.0002 g between weighings).

Observations and Calculations

Measurement Trial 1 Trial 2 Trial 3
Mass of empty crucible (g) 25.4321 25.4318 25.4320
Mass of crucible + sample (g) 25.9325 25.9322 25.9323
Mass of sample (g) 0.5004 0.5004 0.5003
Mass of crucible + AgCl (g) 26.1268 26.1265 26.1266
Mass of AgCl precipitate (g) 0.6947 0.6947 0.6946
Mass of Cl⁻ in sample (g) 0.1721 0.1721 0.1720
% Cl⁻ in sample 34.40% 34.40% 34.38%

Sample Calculation

Given:

  • Mass of sample = 0.5004 g
  • Mass of AgCl precipitate = 0.6947 g
  • Molar mass of AgCl = 143.32 g/mol
  • Molar mass of Cl = 35.45 g/mol

Calculation:

Mass of Cl⁻ in AgCl = (35.45 / 143.32) × 0.6947 = 0.1721 g

Percentage of Cl⁻ in sample = (0.1721 / 0.5004) × 100 = 34.40%

Average % Cl⁻ = 34.39%

Standard Deviation = 0.01%

Relative Standard Deviation = 0.03%

Sources of Error and Precautions

Common Errors

  • Coprecipitation: Impurities trapped in precipitate matrix (minimize by digestion and washing).
  • Post-precipitation: Additional precipitation during standing (filter immediately after digestion).
  • Incomplete precipitation: Insufficient precipitating agent (add slight excess).
  • Loss during transfer/filtration: Use quantitative transfer techniques.
  • Incomplete drying: Ensure constant weight before final weighing.
  • Photodecomposition of AgCl: Work in dim light or use amber glassware.

Precautions

  • Use distilled, CO₂-free water for washing to prevent Ag₂CO₃ formation.
  • Acidify with HNO₃ to prevent precipitation of Ag₂O or Ag₂CO₃.
  • Wash with dilute HNO₃ followed by water to remove adsorbed ions.
  • Handle AgNO₃ with care – it stains skin and clothing.
  • Store AgCl precipitate in dark to prevent photodecomposition to metallic silver.
  • Use desiccator to prevent moisture absorption during cooling.

Thermometric Analysis

Thermometric analysis involves measuring temperature changes during chemical or physical processes to study reaction kinetics, enthalpy changes, and phase transitions.

100°C
75°C
50°C
25°C
0°C
25.0°C

Equipment for Thermometric Analysis

Temperature Measurement

  • Mercury or alcohol thermometer
  • Digital thermometer with probe
  • Thermocouple with data logger
  • Infrared thermometer (non-contact)

Reaction Vessels

  • Insulated calorimeter
  • Double-walled beaker (Dewar flask)
  • Test tubes with stoppers
  • Reaction flasks with ports

Mixing Equipment

  • Magnetic stirrer with hot plate
  • Stirring rods (glass/Teflon)
  • Overhead mechanical stirrer
  • Vortex mixer

Monitoring & Recording

  • Data logger with temperature sensor
  • Computer with interface software
  • Stopwatch or timer
  • Chart recorder (analog)

Procedure: Thermometric Analysis Setup

Step-by-Step Procedure

  1. Equipment Calibration:
    • Calibrate thermometer using ice point (0°C) and boiling point (100°C).
    • Check calibration of balance and other measuring devices.
  2. Sample Preparation:
    • For solids: Weigh accurately and grind to fine powder if necessary.
    • For liquids: Measure volume accurately using pipette or burette.
    • Allow samples to equilibrate to room temperature.
  3. Experimental Setup:
    • Place reaction vessel in insulated container or use double-walled calorimeter.
    • Insert thermometer/thermocouple, ensuring it doesn’t touch vessel walls.
    • Add magnetic stir bar if using stirrer.
    • Record initial temperature (T₁).
  4. Mixing and Reaction:
    • Start data recording/timer.
    • Add reactant(s) quickly and completely.
    • Begin stirring immediately at constant rate.
    • Record temperature at regular intervals (e.g., every 15-30 seconds).
  5. Monitoring:
    • Continue monitoring until temperature stabilizes or returns to baseline.
    • Record maximum/minimum temperature reached (T₂).
    • Note time taken to reach temperature extremes.
  6. Data Analysis:
    • Plot temperature vs. time graph.
    • Calculate temperature change: ΔT = T₂ – T₁.
    • Determine reaction enthalpy if calorimetry constant is known.
    • Analyze rate of temperature change for kinetic information.

Thermometer Reading Guidelines

Correct Technique

  • Immersion Depth: Ensure thermometer bulb is fully immersed in the substance being measured.
  • Avoid Contact: Don’t let thermometer touch container walls or bottom.
  • Reading Position: Keep eyes level with the meniscus to avoid parallax error.
  • Meniscus Reading: Read at the bottom of the meniscus for mercury, top for colored liquids.
  • Stabilization: Allow sufficient time for thermometer to reach equilibrium temperature.
  • Recording: Record to the smallest division possible (estimate between marks if necessary).

Common Errors in Temperature Measurement

Error Type Cause Prevention
Parallax Error Reading from wrong angle Keep eyes level with meniscus
Immersion Error Bulb not fully immersed Ensure proper immersion depth
Lag Error Insufficient equilibration time Wait for stable reading
Calibration Error Inaccurate thermometer Regular calibration checks
Conduction Error Contact with container Avoid touching sides/bottom
Radiation Error Heat exchange with surroundings Use insulation/shielding

Temperature Change Calculations

For a reaction in solution, the heat evolved/absorbed can be calculated using:

q = m × c × ΔT

Where:

  • q = heat change (J)
  • m = mass of solution (g)
  • c = specific heat capacity of solution (J/g°C, typically 4.18 for aqueous solutions)
  • ΔT = temperature change (°C)

Example: 100 g solution with ΔT = 5.2°C

q = 100 × 4.18 × 5.2 = 2173.6 J = 2.17 kJ

Gravimetric Analysis Simulation

Adjust the sample parameters and observe how they affect the final results. Click “Start Analysis” to begin the simulation.

Initial Mass: 0.5000 g sample
Sample Mass 0.5000 g
0.100 g 1.000 g
Cl⁻ Concentration 34.0%
10% 60%

Simulation Results

Calculated Values:

  • Mass of Cl⁻ in sample: 0.1700 g
  • Expected AgCl precipitate: 0.6867 g
  • Percentage yield: 100.0%
  • Theoretical % Cl⁻: 34.00%

Explanation: As sample mass or chloride concentration increases, the amount of AgCl precipitate increases proportionally. The simulation assumes 100% yield with no experimental losses.

Temperature Change Simulation

Simulate an exothermic or endothermic reaction and observe the temperature change.

100°C
75°C
50°C
25°C
0°C
25.0°C
Reaction Type Exothermic
Endothermic Exothermic
Reaction Intensity Medium
Weak Strong

Short Answer Questions

1. What is the main principle behind gravimetric analysis?
The analyte is converted to an insoluble compound of known composition, which is then filtered, dried, and weighed to determine the amount of analyte.
2. Why is silver nitrate used in chloride determination?
Silver nitrate reacts with chloride ions to form insoluble silver chloride precipitate (AgCl), which has a known composition and is easy to filter and weigh.
3. What is the purpose of digestion in gravimetric analysis?
Digestion allows small precipitate particles to coalesce into larger, more filterable crystals, reducing surface area and improving purity.
4. Why is the precipitate washed with dilute nitric acid instead of water?
Dilute nitric acid prevents peptization (re-dissolution) of AgCl and helps remove adsorbed ions without dissolving the precipitate.
5. What precautions should be taken when working with silver nitrate?
Work in dim light (AgNO₃ and AgCl are light-sensitive), wear gloves (stains skin), and avoid contact with clothing.
6. Why is constant weight important in gravimetric analysis?
Constant weight ensures complete removal of moisture and confirms that the precipitate composition is stable and reproducible.
7. What is thermometric analysis used for?
To measure temperature changes during chemical reactions or physical processes to study kinetics, enthalpy changes, and phase transitions.
8. How do you avoid parallax error when reading a thermometer?
Keep your eyes level with the meniscus of the liquid column in the thermometer.
9. What is the purpose of insulation in calorimetry experiments?
To minimize heat exchange with the surroundings, ensuring accurate measurement of temperature changes due to the reaction alone.
10. Why should a thermometer not touch the sides or bottom of the container?
The container may be at a different temperature than the solution, leading to inaccurate temperature readings.
11. What is the formula to calculate chloride percentage from AgCl mass?
% Cl = (Mass of AgCl × (35.45/143.32) / Mass of sample) × 100
12. What is coprecipitation and how can it be minimized?
Coprecipitation is the inclusion of impurities within the precipitate matrix. It can be minimized by digestion, washing, and controlled precipitation conditions.
13. What safety equipment is essential for gravimetric analysis?
Safety goggles, lab coat, chemical-resistant gloves, and working in a well-ventilated area or fume hood when using hazardous chemicals.
14. How does temperature affect precipitation in gravimetric analysis?
Higher temperatures generally increase solubility but also promote particle growth during digestion, improving filterability.
15. What is the difference between end point and equivalence point in analysis?
Equivalence point is when stoichiometrically equivalent amounts have reacted, while end point is when the indicator changes color (may not exactly match equivalence point).

Interactive Quiz: Gravimetric & Thermometric Analysis

Test your knowledge with this 10-question multiple-choice quiz. Select your answer to see immediate feedback.

Quiz Results

You scored 0 out of 10 (0%)

Applications of Gravimetric Analysis

Environmental Analysis

  • Determination of sulphate in water (as BaSO₄)
  • Analysis of suspended solids in wastewater
  • Measurement of particulate matter in air
  • Soil composition analysis

Industrial Quality Control

  • Purity determination of chemicals
  • Ash content in fuels and food products
  • Moisture content in materials
  • Filler content in polymers and composites

Metallurgical Analysis

  • Determination of nickel as dimethylglyoximate
  • Analysis of phosphorus as magnesium pyrophosphate
  • Silicon content in steels and alloys
  • Precious metal analysis (gold, silver)

Pharmaceutical Analysis

  • Ash content of drugs
  • Loss on drying determinations
  • Heavy metal limits testing
  • Residue on ignition tests

Advanced Techniques

Thermogravimetric Analysis (TGA)

TGA measures changes in physical and chemical properties of materials as a function of increasing temperature. Applications include:

  • Determination of moisture, volatile, and ash content
  • Study of decomposition kinetics
  • Analysis of polymer degradation
  • Characterization of inorganic compounds

Differential Thermal Analysis (DTA)

DTA measures temperature differences between a sample and reference material as they are heated. Used for:

  • Phase transition studies
  • Melting point determination
  • Glass transition temperature measurement
  • Reaction enthalpy determination

Modern Instrumentation

  • Microbalances: Capable of measuring mass changes as small as 0.1 μg
  • Automated Systems: Robotic sample handling and analysis
  • Coupled Techniques: TGA-MS (mass spectrometry), TGA-FTIR (infrared spectroscopy)
  • High-Temperature Analysis: Up to 2000°C for ceramic and refractory materials