VOLUMETRIC PIPETTE: CHARACTERISTICS, USES, CALIBRATION AND ERRORS - CHEMISTRY - 2024

The volumetric pipette or volumetric pipette is a type of specific pipette for the most accurate and precise measurement of liquids in a laboratory. A pipette is nothing more than a cylinder of transparent borosilicate glass, which is an element that is easy to clean, is chemically inert and undergoes little deformation.

In few cases, the volumetric pipette can also be made of plastic. It has a conical shape at its lower end and is marked with different types of graduations with which the volumes of liquids contained in it are measured.

The volumetric pipette is used primarily to transfer liquids from one container to another in exact and precise quantities, but only allows the transfer of small amounts of liquid; they are usually used for volumes between 1 and 100 milliliters.

The volumetric pipette is also called a volumetric pipette, since it has gauges or measurements that establish the amount of liquid contained and its use is recommended when accuracy and reproducibility are crucial.

They can be classified according to their degree of precision, with Class A volumetric pipettes being the highest quality in terms of precision. It is the most widely used in analytical chemistry for standardized solutions.

Class B volumetric pipettes are allowed twice the maximum error than class A. They are used for non-standardized solutions for daily laboratory use.

Main features

The volumetric or volumetric pipette is, like the rest of the pipettes, a transparent glass cylinder with a conical lower end. However, it does have some specific characteristics:

Blister in cylinder

The characteristic that differentiates it from the rest is that in its central part it usually presents a bulge or blister in the cylinder, and later a narrowing in the lower part.

This ampoule is calibrated for a specific volume; that is, it can only be used to transfer the indicated and calibrated volume into the pipette. That is why there are different sizes according to the need of the technician.

This feature is an advantage in terms of precision and accuracy of the transferred liquid. However, the disadvantage is that its use is determined solely by the transfer of that specific amount of liquid.

It can have one or two gauges

In case of having only one capacity or mark above the ampoule, it means that the pipette must be flush up to that mark so that, when emptying it, it dumps the volume that indicates the capacity of the pipette on the outside. Wait 15 seconds after the last drop falls.

For example, if a volumetric pipette is used that indicates written on the glass that it has a capacity of 20 ml and with a single capacity above the bulge, it means that the pipette must be filled up to that mark so that, once it has been completely emptied into the container to be transferred, the volume transferred is exactly 20 ml.

In the case of volumetric pipettes with two gauges or marks, one above the ampoule and the other below the ampoule, it indicates that the pipette must also be flush to the upper mark, but when emptying it must be released to the lower mark and no more than that.

In the case of double-gauge pipettes, the capacity written on the cylinder refers to the amount of liquid contained between the two measurements. These are less widely used than one-gauge volumetric pipettes.

Must stay clean

It is recommended to wash and purge the pipette 3 times with the liquid to be transferred, to ensure that any drop of liquid that may adhere to the walls of the pipette corresponds to the liquid to be measured.

Does not require the use of the mouth

The filling of the pipette must be done by means of a propipette and never by suction from the mouth.

These types of pipettes are not blow-out pipettes, as they are known in English. Therefore, under no circumstances can the remaining liquid left on the pipette tip be blown out after emptying.

Uses of the volumetric pipette

- It is used in analytical chemistry, mainly in volumetry, because its main characteristic is the accuracy and precision of the transported volume.

- In the preparation of solutions whose concentrations are rigorously known from more concentrated solutions or in which pure liquids are used.

- In determining the acidity of a solution.

Volumetric Pipette Calibration

Calibration is the process by which the precision of the pipette is established. It is the degree of correspondence between the value that the instrument indicates that it is transferring, with the value actually transferred.

The volumetric material is calibrated to transfer a certain volume at a certain temperature, which is normally standardized at 20 ° C.

How to calibrate the volumetric pipette?

To calibrate a pipette requires a very clean and thorough technique.

1- The pipette is cleaned and dried thoroughly to avoid errors in the readings.

2- Place distilled water in an Erlenmeyer flask and leave it at room temperature for 15 minutes. Then the temperature of the water is measured.

3- A beaker is weighed on a scale and the weight is noted, with an error of one tenth of a mg.

4- Using a propipette, fill the pipette with the water contained in the Erlenmeyer flask and transfer the water to the beaker. Subsequently, the glass is weighed again and the mass of the transferred water is calculated.

5- The process is repeated 3 times.

6- The average of the four pipette measurements is calculated and determined.

7- The density of the water is calculated at the temperature of the first measurements, as well as the average of the volume of the water.

Types of calibrations

The volumetric material can be found with two types of calibrations:

Calibrated for pouring

They are marked with the words "TD", "vert" or "ex". This means that the volume indicated on the pipette corresponds exactly to the volume poured by the pipette. The liquid that adheres to the walls has already been taken into consideration when performing the calibration.

Calibrated to contain

They are marked with the words "TC", "cont" or "in". This means that the volume indicated on the pipette corresponds exactly to the volume of liquid contained in the pipette; that is, before being emptied.

Errors by capacity

Capacity error refers to the known “limit error” based on the volume capacity of the pipette being used.

The capacity error data is tabulated as follows:

This means that in a pipette with a volumetric capacity of 2 ml, the expected limit of error is 0.01 ml in class A pipettes and 0.02 ml in class B volumetric pipettes.

Articles of interest

Graduated pipette.

Serological pipette.

Beral pipette.

References

1. Francisco Rodríguez. Pipette calibration. 08/02/2017 Recovered from: franrzmn.com
2. Oliver Seely. Helpful hints on the use of a Volumetric Pipet. Recovered from: csudh.edu
3. Ricardo Pinto J. November 13, 2009. Volumetric Pipette. Recovered from: wikiciencias.casadasciencias.org
4. Joi Phelps Walker. Volumetric Glassware. General Chemistry 2 labs using argument-driven inquiry. First Edition. 2011. Recovered from: webassign.net
5. Kent Koeman. 07/4/2014. How to use various types of pipettes. Metrology-driven pipette calibration. Tte laboratories. Recovered from: ttelaboratories.com