SPECIMEN: CHARACTERISTICS, TYPES, EXAMPLES OF USES - CHEMISTRY - 2025

The specimen or graduated cylinder is a volume measuring instrument having the precision necessary for use in many of the activities of the teaching laboratories, research or industry. The cylinder has a wide range of use that is between 5 mL and 2,000 mL.

The test tubes can be made of glass or plastic, depending on the choice of the use to be given. For example, if the cylinder is to be used with hydrofluoric acid that attacks glass, it is recommended to use a plastic cylinder.

Graduated cylinders or test tubes. Source: Pleple2000

Compared to pipettes and burettes, cylinders are lower precision volume measuring instruments. But when compared to Ernlermeyer's beakers and flasks, the volume measurements made with the cylinders have much less error.

The test tubes are always available when dissolution or reaction media, buffer solutions, indicator solutions, among others, must be prepared. Although they are not suitable for dissolving solids, as in the case of beakers, they are still one of the most useful glass materials in the laboratory.

characteristics

Design

It is a graduated cylindrical tube, hence the other name by which it is known. The cylinder can be made of glass or transparent plastic. Its upper end is open to allow the liquid to enter, and it usually ends in the shape of a spout to facilitate the pouring of the contained liquid.

In the mixing cylinder, the upper end is made of ground glass to fit a cap with the same characteristics that guarantees a hermetic closure of its interior. This allows the liquid in the cylinder to be vigorously shaken without spillage.

Of course, the lower end is closed and ends in a base that guarantees the verticality of the specimen. The base is usually made of glass, when the specimen is made of this material. The cylinders have a graduation from 5 ml to 2,000 ml.

materials

The test tubes, in addition to being glass, can be mainly made of two types of plastic: polypropylene and polymethylpentene. The polypropylene test tubes withstand temperatures of 120 ºC in the autoclave, without producing a structural alteration of them; however, these specimens melt at 177 ºC.

Polymethylpropylene specimens are characterized by being very light, have great transparency and greater resistance to impacts than glass specimens.

Plastic material test tubes are used for the measurement of large volumes of liquid; for example 1,000 mL or 2,000 mL.

It should be borne in mind that the test tubes are not highly accurate instruments, so for a volume measurement of a liquid that requires greater precision, whenever possible, it is recommended to use pipettes, burettes or volumetric balloons..

Measurement

It is important to select the test tube to use based on the volume to be measured. If you want to measure a volume of 40 mL, you should not use a 1,000 mL cylinder because you make a very large error in the measurement. When using a 50 mL cylinder the error will be much smaller.

The specimens have an indication of their capacity, that is, the maximum volume they can measure. In addition, their appreciation is indicated, that is, the minimum volume that can be accurately measured.

Appreciation

If we want to measure a volume of 60 mL with a 100 mL cylinder, we can see that it indicates that it can measure up to a volume of 100 mL, and that its appreciation is 1/100 of this capacity (1 mL).

A more detailed observation allows to see that there are 10 large lines in this cylinder, between which there is a difference of 10 mL (100 mL / 10), that is, 1 dL. The thick lines are identified, from bottom to top, as 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mL.

There are 10 small lines between two successive large lines, so in this cylinder, there is a difference of 1 ml (10 ml / 10) between each small line. This corresponds to the appreciation of the specimen.

Reading

To read the volume measured in a test tube, proceed in the same way as with burettes: observe the bottom of the meniscus. Since most solutions are aqueous, the meniscus is concave, and a tangent line is imagined at its bottom that coincides with the nearest mark.

Types

There are two types of specimens: type A and type B.

TO

They are highly accurate, so the error made when using these specimens is very low. These specimens are used in quality control laboratories, as well as in those where the validation of analytical methods is carried out.

The volume tolerance is said to be within the limits of error set by DIN and ISO standards.

B

They are less expensive than type A test tubes and are used in teaching laboratories where high precision is not necessary. The volume tolerance is within twice the error limits for class or type A / As.

Meaning of the acronyms In and Ex

The abbreviation "In" indicates that the amount of volume contained in the cylinder corresponds to the volume printed on it. The acronym “In” is equivalent to the acronym “TC”. It also indicates that the amount collected corresponds exactly to the volume indication printed on the cylinder.

The abbreviation "Ex" means that the amount of liquid poured from the test tube corresponds to the volume printed on it. The acronym "Ex" is equivalent to the acronym "TD".

Examples of uses

Media for dissolvers

One of the stability analyzes to verify the quality of a batch of drugs consists in analyzing how much of its active agent is released, after a preselected time of stirring, in a given medium, in such a way that it mimics how quickly it dissolves inside of the organism.

To do this, solvents are used. Their containers are filled with volumes of up to one liter of solution, which can be previously measured with large test tubes; 500 mL, 250 mL, or 1000 mL inclusive, and then mix the solvents and reagents in a large flask.

Generally, measurements of these volumes do not require much accuracy or precision, which is why test tubes are very useful in these cases.

Mobile phases

In high-performance liquid chromatography (HPLC), large volumes of mobile phases must be constantly prepared, which consist of mixtures of alcohols or nonpolar organic solvents, depending on the compound to be analyzed..

Here again the test tubes are useful, because with them we can measure the volumes of the liquid components separately. Once this is done, they are mixed in a large jar, labeled and identified.

Means for degrees

There are titrations or volumetric titrations that require an acid pH, a buffer solution, or a specific and measurable volume of indicator. For this purpose, before starting to titrate or evaluate, these media are ready in their respective test tubes, which are added to the flask; the order and time depends on the method and the analyte.

Synthesis

Similar as has just been explained with the titrations, the same happens with the syntheses, inorganic or organic, where it is necessary to add reaction media whose volume quantities do not put in doubt the performance of the reaction; that is, they must not be exact or precise.

For example, suppose that 100 mL of glacial acetic acid is to be added to the reaction medium. If you have a 200 or 250 mL cylinder, you can measure this volume with it; however, the beaker is also a good option here, as long as you don't measure much more than the 100 mL required.

Extraction media

Likewise, with the test tubes, the extraction media in which, say, the oil from the peels of some vegetables will be dissolved. For example, once some seeds of a hypothetical fruit have been crushed and pressed, this mass is bathed in n-hexane to extract what remains of its oil; since it is an excellent grease solvent.

Here again the cylinders are used to measure the necessary volumes of n-hexane to pour into the extractor balloon.

Indicator solutions

And although it may already be said, with the cylinders the necessary volumes (usually less than 10 mL) for the indicator solutions can also be measured. These are either added to the titrations to determine the end point of the reaction, or for qualitative analysis or even to verify pH ranges of a sample.

Determination of volumes of solids

Suppose you have a 50-mL cylinder with a volume of water that is equal to 10 mL. If a coin is immersed in this, it will be observed that the water meniscus rises to a new mark; for example, 12.5 mL. This means that the displaced volume of water was 2.5 mL, which corresponds to the volume of the coin.

This simple method has been used to determine the volume of small bodies or objects. The same could be done with a marble, a doll, a chain, a pencil, etc., as long as it can slide between the edges of the test tube.

References

1. John Williams. (2019). What Is a Graduated Cylinder? - Definition, Uses & Function. Study. Recovered from: study.com
2. Wikipedia. (2019). Graduated cylinder. Recovered from: en.wikipedia.org
3. Lab Pro. (2019). Beakers vs. Graduated Cylinders: The Pros and Cons of Common Lab Glassware. Recovered from: labproinc.com
4. Admin. (2017). Test tube. Recovered from: instrumentsdelaboratorio.org
5. Inorganic chemistry. (sf). Test tube. Recovered from: fullquimica.com