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Graduated pipette: characteristics and uses

It has a wide neck, which makes it less precise than the volumetric pipette. Consequently, they are used when taking a volume of solutions in which the precision does not have to be very high. They are used in laboratories to measure volume or transfer a quantity of liquid from one container to another.

Mohr pipette (top) and serological (bottom). Source: Paweena.S / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Graduated pipettes are divided into two types: Mohr or subterminal pipette and serological or terminal pipette. The difference between the two is that, while in the Mohr pipette the graduation is along the tube ending before the tip, in the serological one it reaches the tip.

Characteristics of the graduated pipette

Among the most relevant characteristics of graduated pipettes are the following:

– They are made of plastic or borosilicate glass (pyrex).

– Along the body of the tube there are lines that indicate the total volume. These have numbers that indicate the volume of liquid in the line.

– On the neck of the pipette are printed the specifications that indicate: its maximum volume; the size of its divisions, represented as 1/10, 1/100; the calibration temperature; and a legend identified as TD or TC, by the abbreviations in English of To delivery (ex) or To contain (in), that mean to pour or to empty, respectively.

– Very small volume pipettes allow fairly accurate measurement of fluids, while larger volume measurement pipettes allow less critical measurement.

Applications

Way of reading the measurements of the pipette. Source: Deluxecheese / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The use of graduated pipettes is generally found in chemistry, biology or medicine laboratories. Thanks to its graduated scale, this pipette is used to measure different volumes of liquids.

– You must know the correct way to hold the pipette. The proper way is to take it by the upper third, between the thumb and the middle finger.

– They have a graduation to determine the volume but it must be considered that, for an effective measurement, the final measurement (or total capacity of the graduated pipette) is more accurate than the intermediate measurements. Therefore, the recommendation is to choose the pipette according to the exact volume to be measured.

– The pipette should be placed approximately 6 mm from the bottom of the container, in order to collect the liquid to be measured.

– It is not advisable to suck the liquid with your mouth to avoid risks. For this purpose, use is made of the propipette or pump, closing the nozzle with the tip of the index finger when reaching the required measure.

– Filling can be done by other means, such as ascension or injection.

– Once the liquid is in the pipette, it should be placed at an angle of 10 to 20 °.

– To release the liquid it is only necessary to lift the index finger.

Understanding the pipette specifications is of great importance, as they indicate the calibration. For example: the inscription “1ml in 1/100 TD 20 ° C” on a pipette indicates that the pipette is calibrated in divisions of 1/100, pouring up to 1 ml with liquids no higher than 20 ° C.

Additionally, it is common for graduated pipettes to also have the acronym “AS” inscribed on the tube along with the specifications. This acronym is usually found below the volume of the pipette and indicates the accuracy of the classification: “A” means the highest level precision and “S” means fast delivery.

Differences between graduated pipette and volumetric pipette

– The graduated pipette has a graduated scale, while the volumetric one has a gauge.

– The use of the graduated pipette allows to measure the volume of different liquids according to the range that has carved in the body of the same. In the case of the volumetric pipette, only a single value can be measured.

– The precision of a volumetric pipette is higher than that of a graduated pipette.

Differences between graduated pipette and burette

Illustration of a burette

The burette is a measuring instrument for volume of liquids. It is composed of a long, open glass cylinder at the top with a stopcock at the bottom, to prevent the liquid from escaping.

It has a series of volumetric markings that allow the user to take only the amount of liquid or gas that is desired in a particular laboratory process.

The differences between the graduated pipette and the burette lie in the following main aspects:

– Graduated pipettes are only used to measure liquids, while burettes measure liquids or gases.

– As it has a stopcock, the structure of the burette is different from that of the graduated pipette. This key allows a less precise release than that generated by the volumetric pipette.

– The burettes can hold liquids from 10 to 40 ml. On the other hand, graduated pipettes admit smaller quantities.

– In the case of the burette, measurements are made from top to bottom. Consequently, the difference between the initial and final volume is equal to the total amount of the liquid or solution.

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