They have a very peculiar shape (lower image) with a base or bulb that is prolonged with an elongated neck. However, there are some square-shaped volumetric flasks that have greater weight , better stability, and are easily stored, taking up less space than classic pear-shaped volumetric flasks.
Flasks differ in the material from which they are made, in their shape, in the volume of solution they can hold, and they also differ in the degree of precision or accuracy of the volume they can hold.
Characteristics of a volumetric flask
-The body of the flask is generally pear-shaped, with a flat bottom to facilitate mixing with stir bars.
-The upper part of the flask ends in an elongated and narrow neck that has a white calibration mark engraved on the terminal end, like a ring, which is called the gauge line.
-The volume that can contain “TC” or “IN” is specified on the bulb of the flask, as well as the temperature at which it was calibrated, generally 20 ° C, the degree of precision and the manufacturer.
-It is made of borosilicate glass, transparent, durable, and offers mechanical, thermal resistance, chemical stability and excellent optical quality.
-Each flask has a specific size, therefore it is designed to prepare solutions with a specified volume. That is, a 100 mL flask cannot be used to prepare 50 mL or 25 mL volumes.
-They are reusable, they can be subjected to sterilization processes in autoclaves (heat with pressurized steam).
-It has a mouth with a plastic coating that fits perfectly to the glass stopper that is placed under pressure or by screwing, thus protecting the solution from exposure to the environment, and preventing evaporation.
They are used to make precise dilutions and prepare standard solutions, bringing the level of the solution little by little until the solvent in the solution reaches the top or mark of the gauge.
It is necessary to place the flask at eye level to see the coincidence of the lower edge of the meniscus of the solution with the mark of the gauging. This prevents parallax error and ensures that the exact concentration of the solution is obtained.
Likewise, carefully add the solvent more slowly, as the solution reaches the neck of the flask and the gauging mark is reached, in order to avoid exceeding the gauging mark and altering the desired concentration of the solution. .
The volumetric flask is used to contain exact volumes, not to dispense the volume it contains exactly, since when its contents are emptied, some liquid remains in the flask. In contrast, a pipette is designed to dispense exact volumes.
Types of flasks
Depending on the material of manufacture, there are glass or plastic flasks. In general, those made of glass are more widely used, since they are more resistant to the different chemical substances that could affect the solution when reacting with a plastic one.
Most flasks are transparent, however there are some special amber flasks to hold solutions that are sensitive to light.
Different size flasks are available to prepare different volumes of solution, which can range from 1 milliliter to 20 liters of solution. The 1 or 2 mL ones are not shaped like the larger ones – they look like test tubes.
Color-coded volumetric flasks are available on the market. Likewise, there are flasks that are classified according to their degree of precision or accuracy. There are those of class A or N ° 1, which are the flasks that have the highest precision, and also offer the best information on their quality on the labels.
Type B flasks have less precision and do not specify on their labels the characteristics with which they were calibrated. There are different manufacturers of volumetric flasks, among which are Kimax, Pyrex, Corning, Cole-Parmer, among others.
Uses of the volumetric flask
The use of volumetric flasks is wide in research, quality control, chemistry, biochemistry, routine laboratories and in teaching laboratories, especially when precision is essential as a determining factor in the results of an experimental process.
They are used to mix liquids, or prepare solutions with solids that dissolve in liquids. As the case may be, they are used to prepare solutions of exact concentration, which therefore require a precise final volume.
Likewise, with it, stock solutions can be prepared, from which the dilute solutions required in numerous analytical processes are obtained.
Quantitative and volumetric analysis
They are useful in numerous quantitative analyzes, to prepare solutions that are required in volumetric analysis procedures, such as titration for example. They are not used to heat or boil the liquid at any point in the process.
A type A flask is most commonly used in analytical chemistry laboratories where greater precision is required in measuring the volume of solutions.
Meanwhile, the flasks classified within group B, due to their lower precision, are used for qualitative tests and in educational practice laboratories.
They also serve to store, save the solutions that will be used later.
Indications for use of the flask
To prepare solutions, it can be done directly in the volumetric flask, carefully adding the corresponding solute and proceeding to add the solvent little by little, dissolving the solute well if it is solid, mixing or shaking without sudden movements. However, it is advisable to dissolve the solid beforehand in a beaker .
As the level of the solution approaches the gauge mark, continue to add solvent more carefully, little by little using a pipette, or a small bottle in case the solvent is distilled water.
The end point of solvent addition is when the meniscus of the solution meets the mark or line marked on the flask. The meniscus is the curve formed by the liquid in the neck of the flask, the shape of which will respond to the characteristics of the surface tension of the solvent and the properties of adhesion to the glass or plastic of the flask.
It is properly closed with the respective flask stopper and the solution is finished mixing. It is recommended that the flask be handled by the neck so as not to heat the solution.