- Characteristics of the volumetric pipette
- Physical
- Flush lines
- Lettering
- Calibration
- In
- Former
- Classification
- Features
- Difference with graduated pipette
- References
The volumetric or volumetric pipette is a volume measuring instrument designed to minimize experimental errors. It guarantees high accuracy in taking an aliquot of liquid, making it extremely useful in quantitative studies.
The error made when using a volumetric pipette is minimal, as indicated in the following example: with a volumetric pipette of 2 mL capacity, a maximum error of 0.006 mL is made; With a 5 mL volumetric pipette the maximum error is 0.01 mL; and with a 10 mL pipette the error is only 0.02 mL.
Source: Zelda F. Scott, from Wikimedia Commons
That is, if a volume of 2mL is taken, the real value is in the range 2,000 ± 0.006 mL.
In the image above you can see a 50 mL volumetric pipette; there are even 100mL or more, depending on the nature of the analysis. These pipettes ensure accurate transfer of a selected volume of standard solutions or samples; therefore, they are essential in quantitative determinations (how much there is of a chemical species).
Characteristics of the volumetric pipette
Physical
Volumetric pipettes have an engraved ring at the top called the flush line. If the pipette is filled to this line, and discharged properly, the volume indicated by the volumetric pipette will be poured.
In addition, they have a bulb-shaped dilation in their central part that contributes to the accuracy of the pipettes, as this allows the pipette tip, which ends in a conical shape, to be of a reduced diameter; as well as the upper region of the pipette, precisely where the scratch line is located.
As the region of the graduated pipette where the leveling line is located is small in diameter, it allows the reading of the meniscus of the liquid to be with a minimum error, reducing what is called parallax error.
There are volumetric pipettes with one or two lines of marking: one at the top, and another at the bottom of the dilation, near the conical end of the volumetric pipette.
Flush lines
When volumetric pipettes have a single make-up line, the volume poured is between the make-up line and the end of the tapered pipette tip; Whereas if they have two make-up lines, the volume to be poured cannot go beyond the second make-up line, since an error would be made due to excess liquid.
Volumetric pipettes with two lines of leveling have the advantage that a deformation or break in the conical tip does not affect the accuracy of the volumes poured by the volumetric pipette.
However, as two readings are required to determine the volume poured out of the pipette, it can increase the error made when using this instrument.
Lettering
The enlarged part of the volumetric pipette has a set of information about the characteristic of the pipette. In it you can read:
-The manufacturer brand of the pipette.
-The nominal volume of the pipette, for example, if it is 25 mL.
-The maximum error that is committed when using the volumetric pipette, in this example is ± 0.03 mL. The reference temperature is generally 20 ºC.
-Other information that appears in the dilated area of the volumetric pipette is the waiting time, usually 5s. That is, after the volume has been transferred, wait 5 seconds before removing the pipette from the receiving container (test tube, beaker, flask, etc.).
-'Ex 'means pour calibration; class A or B is an indicator of the error made when using the volumetric pipette; and 'S' means fast dump.
Calibration
Volumetric materials are calibrated based on the liquid capacity they store (In per inlet) or the volume they discharge (Ex discharge).
In
In calibration is generally used for flasks and beakers, but also for pipettes for measuring viscous liquids (such as syrups, petroleum, suspensions).
The volume reported by the In-calibrated volumetric pipettes is that which is capable of storing; therefore, when measuring with them, they must be emptied completely.
Former
Ex-calibrated materials imply that the volume reported on their labels is the same that they discharge. So when you have an Ex graduated pipette, there will always be a small residue that does not count as a measured volume.
To ensure that there is no confusion between said residue and any hanging drop from the tip of the pipette, it is necessary to discharge all the liquid transferred vertically; then wait a few seconds, and immediately afterwards the tip will slide along the inner wall of the container, to eliminate all traces of drops.
Classification
Pipettes are classified in class A and class B. Class B pipettes have a maximum permissible or tolerated error that is generally twice that of class A pipettes. Therefore, class A pipettes are considered as higher precision.
Features
Its function is unique: to transfer a volume of liquid measured with maximum precision from one container to another, in order to give it the desired or required use. For this, it has a set of characteristics that allows it to fulfill this function.
Difference with graduated pipette
-The graduated pipette has a graduated volume scale with a marking of divisions and subdivisions; while the volumetric pipette has a single gauge, and in some cases, two gauges.
-The use of the graduated pipette allows to measure different volumes of liquid, depending on the marking of the pipette. While in volumetric pipettes only the volume indicated on the labels can be measured.
-Its precision and accuracy are greater than those obtained with the graduated pipette.
-The graduated pipettes are classified into serological pipettes and Mohr-type pipettes. Serological pipettes are graduated up to the tip of the pipette, whereas in the Mohr pipette the graduation of the pipette does not reach the tip.
References
- Vitlab: Competence in Labware. (sf). Competence in volume measurement.. Recovered from: vitlab.com
- Glossaries (September 3, 2018). Volumetric pipette. Recovered from: glosarios.servidor-alicante.com
- Wikipedia. (2018). Pipette. Recovered from: en.wikipedia.org
- Flournoy, Blake. (May 15, 2018). What Is the Purpose of a Pipette? Sciencing. Recovered from: sciencing.com
- Seely O. (April 5, 2018). Use of a volumetric pipet. Chemistry Libretexts. Recovered from: chem.libretexts.org