Volumetric pipette: Characteristics, Uses, Calibration and Errors by Capacity Highlights

It is called volumetric pipette to a specific type of pipette for the most accurate and precise measurement of liquids in a laboratory. A pipette is nothing more than a transparent borosilicate glass cylinder, which is an element that is easy to clean, chemically an inert material 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 precise 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 presents 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, volumetric pipettes of class A being the highest quality precision. It is the most used in analytical chemistry for standardized solutions.

Class B volumetric pipettes are allowed twice as much error as class A pipettes. They are used for non-standard solutions for daily laboratory use.

Main characteristics

The volumetric or volumetric pipette is also the rest of the pipettes, a transparent glass cylinder with its conical bottom end. However, it has some specific characteristics:

Ampule in the 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 subsequently 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 in the pipette. That is why there are different sizes according to the need of the technician.

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

You may have one or two gauging

In case of having only one gauge or mark above the ampoule, it means that the pipette must be flushed up to that mark so that, when emptying it, it overturns the volume that indicates the capacity of the pipette on its outside. They should wait 15 seconds since the last drop falls.

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

In the case of pipettes with two gauges or markings, one above the ampule and another below the ampoule, it indicates that the pipette must also be leveled to the upper mark, but that when emptying it, it must be released until the lower mark and no more than that.

In the case of double gauging pipettes, the capacity written on the cylinder refers to the amount of liquid contained between both measurements. These are less used than volumetric pipettes of a capacity.

It must remain clean

It is recommended to wash and purge the pipette 3 times with the liquid to be transferred, in order to ensure that any drop of liquid that may remain adhered 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 should be done by means of a propipette and never by suctioning the mouth.

This type of pipettes are not blow-out or blow-out pipettes, as it is known in English. Therefore, under no circumstances can the remaining liquid remaining at the tip of the pipette be blown 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 in the volume transported.

- 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.

Calibration of the volumetric pipette

Calibration is the process by which the accuracy 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 you need a very clean and thorough technique.

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

2- Distilled water is placed in an Erlenmeyer flask and left at room temperature for 15 minutes. Then the water temperature is measured.

3- Weigh a beaker on a scale and record the weight, with an error of one tenth of a mg.

4- With the use of a propipeta, the pipette is filled with the water contained in the Erlenmeyer and the water is transferred to the beaker. Then the glass is weighed again and the mass of the transferred water is calculated.

5- The process is repeated 3 times.

6- Calculate and determine the average of the four pipette measurements.

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

Types of calibrations

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

Calibrated to pour

They are marked with the words"TD","vert"or"ex". It means that the volume indicated on the pipette corresponds exactly to the volume that the pipette pours. The liquid that remains adhered to the walls was already taken into consideration when performing the calibration.

Calibrated to contain

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

Errors by capacity

The capacity error refers to the known"error limit"according to the volume capacity of the pipette used.

The error data by capacity are tabulated as follows:

Nominal capacity (mL)	Maximum allowed error (mL)
	A class	Class B
0.5	0.005	0.01
1	0.008	0.015
2	0.01	0.02
5	0.015	0.03
10	0.02	0.04
twenty	0.03	0.06
25	0.03	0.06
fifty	0.05	0.1
100	0.08	0.15
200	0.1	0.2

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

References

1. Francisco Rodríguez. Calibration of pipettes. 08/02/2017 Retrieved from: franrzmn.com
2. Oliver Seely. Helpful hints on the use of a Volumetric Pipet. Retrieved from: csudh.edu
3. Ricardo Pinto J. November 13, 2009. Volumetric pipette. Retrieved from: wikiciencias.casadasciencias.org
4. Joi Phelps Walker. Volumetric Glassware. General Chemistry 2 labs using argument-driven inquiry. First Edition. 2011. Retrieved from: webassign.net
5. Kent Koeman 7/4/2014. How to use various types of pipettes. Metrology-driven pipette calibration. Tte laboratories. Retrieved from: ttelaboratories.com