Back to Library

(AI-Powered Suggestions)

Related Topics

A topic from the subject of Calibration in Chemistry.

  • 1 year ago
  • 9 min read
Calibration of Volumetric Glassware

Introduction
Volumetric glassware, such as pipettes, burettes, and volumetric flasks, is used to accurately measure the volume of liquids in chemistry. Accurate measurements are crucial for reliable experimental results. Calibration ensures the glassware meets required accuracy and precision standards. Several methods exist for calibrating volumetric glassware, each with advantages and disadvantages.

Basic Concepts
Calibration relies on the principle of mass-volume relationship of water. A known mass of water (at a specific temperature) is dispensed into the glassware. The volume is then calculated using the density of water at that temperature. The difference between the measured volume and the nominal volume is the calibration error.

Equipment and Techniques
Calibration requires precise equipment and meticulous technique. Essential equipment includes:

  • Analytical balance (capable of high precision weighing)
  • Thermometer (to measure water temperature for density correction)
  • Volumetric glassware to be calibrated (e.g., pipette, burette, volumetric flask)
  • Distilled or deionized water
  • Drying oven (to dry the glassware before weighing)
The primary technique is the gravimetric method. This method involves weighing a known volume of water dispensed from the glassware, which is more precise than direct volume measurement.

Procedure (Gravimetric Method - Example for a Pipette):
1. Clean and dry the pipette thoroughly. 2. Weigh the empty receiving vessel (e.g., weighing bottle). 3. Fill the pipette to the calibration mark with distilled water at a known temperature. 4. Dispense the water into the pre-weighed vessel. 5. Weigh the vessel with the water. 6. Calculate the mass of water dispensed. 7. Using the density of water at the measured temperature, calculate the dispensed volume. 8. Compare the dispensed volume to the pipette's nominal volume to determine the calibration correction.

Data Analysis
The mass of water and its density are used to calculate the actual volume dispensed. The difference between the nominal volume (marked on the glassware) and the actual volume is the calibration error. This error is expressed as a percentage of the nominal volume:
Calibration Error (%) = [(Nominal Volume - Actual Volume) / Nominal Volume] x 100

Applications
Accurate calibration is essential for various applications in chemistry, including:

  • Preparing solutions of precise concentrations
  • Performing accurate titrations
  • Quantitative analysis
  • Ensuring the reliability of experimental results

Conclusion
Calibration of volumetric glassware is crucial for obtaining accurate and precise results in chemical experiments. Regular calibration, using appropriate methods, maintains the reliability and integrity of experimental data.

Calibration of Volumetric Glassware
Key Points
  • Volumetric glassware is used to accurately measure volumes of liquids.
  • Calibration is the process of verifying the accuracy of volumetric glassware.
  • Calibration is performed using a known mass of liquid (usually water) and a calibrated analytical balance.
  • The temperature of the water is crucial as the density of water changes with temperature. The calibration should be done at a specified temperature (often 20°C) and the volume adjusted accordingly using density correction tables.
  • The results of calibration are recorded on a calibration certificate.
  • It is important to have volumetric glassware calibrated regularly to ensure accurate measurements, especially for precise analytical work.
Main Concepts

Calibration is the process of determining the true volume of volumetric glassware. It involves measuring the mass of a known volume of liquid (usually distilled water) of known density at a specific temperature. The volume is then calculated using the density and mass of the water. This calculated volume is then compared to the glassware's nominal volume to determine any deviation.

Volumetric glassware is a type of glassware designed for accurate volume measurements. Common types include:

  • Pipettes (volumetric and graduated)
  • Burettes
  • Volumetric Flasks
  • Graduated Cylinders

Each type has specific procedures for proper use to ensure accurate measurements. For example, with volumetric pipettes, one should allow the liquid to drain completely (without blowing out) and wait a few seconds before removing the pipette from the receiving vessel.

Calibration certificates are essential documents that record the results of the calibration process. These certificates should include:

  • The type and identification number of the volumetric glassware (e.g., serial number).
  • The nominal volume of the glassware.
  • The measured volume (including uncertainty).
  • The date of calibration.
  • The temperature at which the calibration was performed.
  • The name and accreditation details of the calibrating laboratory (if applicable).
  • The signature or electronic signature of the calibrating technician.

Calibration certificates provide traceability and ensure that the glassware is fit for its intended purpose.

Procedure (Example for Volumetric Flask):
  1. Clean and dry the volumetric flask thoroughly.
  2. Weigh the empty, dry flask on an analytical balance.
  3. Fill the flask to the calibration mark with distilled water at a known temperature.
  4. Weigh the flask with the water.
  5. Calculate the mass of the water.
  6. Using the known density of water at the specified temperature, calculate the actual volume of the water.
  7. Compare the actual volume to the nominal volume of the flask.
  8. Record the results on a calibration certificate.
Experiment: Calibration of Volumetric Glassware
Purpose:
  • To determine the exact volume delivered by a buret or pipette.
Materials:
  • Buret or pipette
  • Analytical balance
  • Deionized water
  • Weighing paper or weighing boat
  • Thermometer (to measure water temperature)
Procedure:
  1. Clean the buret or pipette thoroughly and rinse it several times with deionized water.
  2. Dry the buret or pipette by either blotting it with a lint-free tissue or rinsing it with acetone and allowing it to air dry. Avoid touching the interior surfaces.
  3. Weigh a clean, dry weighing paper or weighing boat to the nearest 0.0001 g. Record this weight (W1).
  4. Place the weighing paper or weighing boat under the tip of the buret or pipette.
  5. Rinse the buret or pipette with deionized water and fill it to slightly above the calibration mark.
  6. Carefully adjust the liquid level to the calibration mark using a dropper or pipettor. Ensure there are no air bubbles.
  7. Dispense the liquid into the weighing paper or weighing boat. Allow any clinging drops to drain (using a standardized waiting time).
  8. Record the final weight (W2) of the weighing paper or weighing boat and the water temperature.
  9. Calculate the mass of the water delivered: Mass = W2 - W1
  10. Calculate the volume (V) delivered by the buret or pipette using the formula: V = Mass / d, where 'd' is the density of water at the measured temperature (look up density in a reference table).
  11. Repeat steps 4-9 several times (at least 3) to obtain multiple measurements.
Results:
  • Create a data table with columns for: Trial Number, Initial Weight (W1), Final Weight (W2), Mass of Water (W2 - W1), Water Temperature, Density of Water (d) at measured temperature, and Calculated Volume (V).
  • Calculate the average volume delivered by the buret or pipette. Also calculate the standard deviation to assess precision.
Discussion:
  • Discuss the importance of proper cleaning and drying techniques for accurate calibration.
  • Compare the average calibrated volume to the nominal volume of the glassware. Comment on any significant deviations. What are potential sources of error?
  • Analyze the precision of your measurements (using standard deviation). What factors influence precision?
  • Discuss the significance of using the density of water at the measured temperature for accurate volume calculations.
Conclusion:
  • Summarize the results of the calibration, stating the average calibrated volume and its precision.
  • State whether the glassware is accurately calibrated based on your results.
  • Suggest improvements to the procedure to minimize errors.

Share on: