How the characteristics of a volumetric flask contribute to precise measurements.
Cole-Parmer Essentials Class A Volumetric Flask with Glass Stopper, 250 mL
This pear-shaped piece of labware distinguishes itself from other beakers and flasks in more than just it’s appearance. The volumetric flask is designed for precise measurements and its characteristics ensure it will perform accurately every time. So, when you use this type of flask, you can be sure you are measuring with the utmost precision for accurate results.
Key characteristics of a volumetric flask influencing precision
Many features ensure the precision of a volumetric flask. It’s helpful to understand all of the characteristics before selecting and using these flasks to ensure reliability of your analytical results.
Design: Ensuring volumetric flask precision
Cole-Parmer Essentials Class B Transparent PMP Volumetric Flask showing the marking point.
The volumetric flask has a long neck with a marking point (usually an etched line or a small constriction) on the neck that indicates the exact volume they are designed to contain. The bulbous base ensures that the flask can be accurately filled to the specified volume.
When you pour a liquid into the flask, the narrow neck minimizes any potential for errors in the volume. It ensures that you’re filling it up to the right mark. When you place the flask on a flat surface, the flat bottom ensures that the liquid is distributed evenly, so you get a consistent measurement every time.
Calibration: Ensuring flasks have minimal errors
Volumetric flasks undergo rigorous individual calibration, guaranteeing a minimal margin of error in their volume capacity. With a single marking point on the neck of the flask corresponding to the intended volume, they simplify the process of filling and measuring accurately.
Classification of volumetric flasks
Volumetric flasks are classified as Class A or Class B. The difference is due to mainly the materials used to make the glass. Class A volumetric flasks meet stringent standards and are typically calibrated to ensure highly accurate results. They are manufactured from borocilicate glass that provided higher precision with a lower tolerance for error in terms of volume measurement. Class B volumetric flasks are made from soda lime glass that provides a slightly higher tolerance for error and are less precise than Class A flasks. While both classes of flasks are suitable for most general laboratory applications, Class A volumetric flasks are preferred when utmost precision is essential, while Class B flasks are more commonly used for general qualitative work where the highest precision is not required.
Material: Choosing between glass and plastic flasks
Your choice between glass and plastic volumetric flasks depends on the specific needs of your laboratory work. Glass offers higher precision and greater chemical resistance. Plastic flasks are less prone to breakage and cost less but may have limitations in terms of chemical resistance.
Glass flasks
Glass flasks are more durable and can withstand a wider range of chemicals and temperature variations without degradation. These are typically made of borosilicate glass, which is highly resistant to chemical reactions and temperature changes. Borosilicate glass is known for its durability and transparency. These are commonly used in applications requiring high precision and accuracy, such as with analytical chemistry and quality control.
Plastic flasks
Plastic volumetric flasks are usually made of polymers like polypropylene or polymethylpentene. These are classified as Class B. These plastics are lightweight and less resistant to chemical and thermal stress compared to glass. Plastic flasks are less durable and are less chemical resistant and can warp when exposed to certain solvents and high temperatures. Plastic may not be as transparent and can become cloudy or scratched, potentially affecting the accuracy of volume measurements. Plastic flasks are often used when the risk of breakage is a concern, and when a high degree of precision is not necessary, such as in educational or less critical applications.
Volumetric flask sizes and capacity
Volumetric flask sizes come in many sizes, but they are designed to contain a single, precise volume, such as 25 mL, 50 mL, 100 mL, 250 mL, 500 mL, or 1,000 mL (1 liter), and so on. The volume indicated on the flask is the maximum capacity it can hold accurately.
How to use a volumetric flask
Meniscus shown on a volumetric flask.
- Add your solute to your solution.
- Add enough solvent to dissolve the solute.
- Keep adding your solvent until it gets close to the line marked on the volumetric flask.
- Use a pipette to fill the flask.
- Use the meniscus* of the solution and the point line on the flask to determine your endpoint.
- Place the volumetric flask stopper into the volumetric flask to seal it.
- Invert the flask to thoroughly mix the solution
*The meniscus, is a concave or convex liquid curve seen in the container. Its shape depends on the surface tension of the liquid and adhesion to the wall of the container.
Volumetric flask uses
Volumetric flasks is a versatile piece of labware ideal for preparing standard solutions, diluting solutions, measuring precise volumes, calibrating other instruments, and titration. They are often seen in chemistry and analytical science.
Comparing volumetric flasks for optimal use
Volumetric flasks are designed for a single specific volume and should not be used for measuring different volumes. They are meant for accuracy and precision in specific applications, where even a slight variation in volume can have a significant impact on the results. Volumetric flasks are used to measure volumes much more precisely than other labware. Let’s look at some comparisons.
Volumetric flask vs graduated flask
Where a volumetric flask is used for precise measuring, a graduated flask is not to be used for precise measuring, but they still provide reliable measurements, typically within a 1 percent margin of error. Their cylindrical shape with a pour spout and graduated hash marks along its sides differentiates them from a volumetric flask. They can be used for applications where utmost precision is not a primary concern. The selection between a volumetric flask and a graduated cylinder depends upon the required level of precision for your specific measurement.
Volumetric flask vs. Erlenmeyer flask
An Erlenmeyer flask, or Erlenmeyer, differs from a volumetric flask in that it is not used for precise measuring at all. It is a versatile laboratory glassware with a conical shape and a wide neck for efficient liquid mixing. It has multiple calibration marks, which provide a general idea of volume but are less accurate than those on volumetric flasks. Erlenmeyer flasks are versatile and commonly used for tasks such as mixing, titrations, general laboratory work, and culturing microorganisms.
Example of a glass volumetric flask
Cole-Parmer Essentials Volumetric Flask, Glass, with Glass Stopper, 2 mL
This volumetric flask is constructed of quality borosilicate 3.3 glass and includes easy-to-read white markers. This flask is calibrated and can withstand a wide temperature range from –50 to 550°C. It meets the ASTM E 438 Type I Class A standard and is autoclavable.
See the complete flask assortment at coleparmer.com
Sources
Meniscus image: E. Generalic, https://glossary.periodni.com/glossary.php?en=volumetric+flask
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