The test tube is a container for liquids in the shape of an elongated tube, made of glass or plastic, which is used to measure the volume of said liquids. This allows faster measurements and volumes greater than those that can be measured with pipettes, although they have less precision.
The test tubes can be used in laboratories to measure liquids or gases. These include a graduation, that is, a millimeter that allows you to read the volume that the content occupies inside.
The test tubes generally have a closed bottom end that will contain the liquid without spilling and an opening at the top. They have a base to lean on. They can be of different sizes and therefore can hold different volumes according to their type.
Cylinders are generally used for chemical analysis, measuring the desired volume within the cylinder and having the exact measurement of a chemical. However, they are not the most accurate instrument to measure volumes, in cases of extreme precision pipettes are used.
Characteristics of a test piece
The test tube is a measuring container, composed of glass or resistant plastic materials with units of volume marked along its length. The marks are calibrated according to the actual volume measurements, by the manufacturers.
Test tubes are a key piece of equipment in a standard scientific laboratory. They are necessary to measure volumes of liquids in routine experiments.
It is basically a volume measuring container with a long thin cylindrical body and necessarily transparent, to be able to use it for measurements. They have a stable base underneath, which allows them to stand on surfaces, without the need for a support.
These cylinders are generally available in various volume capacities ranging from 5ml, 25ml, 50ml, 100ml, 250ml, 500ml to even more than one liter.
The marks on the specimen are classified according to small increments in volume and are marked with horizontal lines, painted on the surface. Most test tubes are made of borosilicate or polypropylene glass that can withstand high temperatures and is not affected by the corrosive nature of the chemicals that are measured in them.
Glass and clear plastic are common materials.
Glass specimens have a high tolerance to heat and chemicals, but it is important to consider the meniscus when making the measurement. Plastic is less prone to breakage but can be less accurate.
How to use the test tube?
- Select a cylinder that is large enough to hold the volume of liquid being measured.
- Confirm that the cylinder is clean and dry. Unwanted particles or liquid droplets on the cylinder could alter the measurement
- Hold the cylinder with one hand while pouring the liquid you are measuring into another container. Graduated cylinders are thin and can easily tip over, so be especially careful when working with harmful or volatile liquids.
- Hold the cylinder at eye level to take a reading. Avoid bending over to read the cylinder while it rests on the table; if pushed, the container could tip over and spill liquid on your face or torso.
- Take the measurement of the liquid at the immersion of the liquid surface. This dip is called the meniscus; It is formed because liquid molecules are attracted more to glass than to each other.
- Look at the horizontal lines on the side of the cylinder. Find out which line the closest meniscus is on.
- Determine the measurement increments in the tube. For example, if the area between the 40 ml mark and the 50 ml mark is divided into ten segments, each segment represents 1 ml.
- Locate the closest complete measurement below the surface of the liquid.
- Count the number of segments to the line closest to the meniscus. Calculate the volume of the liquid by adding the entire measurement to the sum of the segments.
What is the graduated cylinder for?
Graduated cylinders are tall, narrow cylinders that are open at the top and are marked with regular lines that serve to determine the volume of their contents. These are also known as measuring cylinders. This type of laboratory equipment is used to accurately measure the volume of a fluid or object.
When measuring with graduated cylinders, the surface of the water distorts into a concave depression in the surface due to the effects of surface tension. The adhesive forces between the fluid molecules and the cylinder walls cause this distortion.
When looking at the level of the liquid in a cylinder at eye level, the ideal would be to take a measurement by matching the level of the liquid to the nearest graduated mark on the cylinder. However, since the surface is distorted into a concave shape (known as a ‘meniscus’), you should choose the reading that matches the bottom of the meniscus. Although measurement error cannot be completely eliminated, you can reduce it by taking this precaution.
What is the test tube used for in the laboratory?
Most chemicals used in a laboratory are liquid solutions and to ensure that they are used in the correct amounts or proportions, they must be measured. The function of the cylinder is to accurately measure the volumes of liquid.
They are widely used in chemistry and biology laboratories, where it is necessary to use precisely measured amounts of liquids. They are deliberately designed to be long, with a shorter diameter compared to beakers, to facilitate accurate measurements. Measurements made with the naked eye must be done with care.
A series of test tubes with different measurement values are necessary within the laboratory. As mentioned above, it is essential that you measure the volume of the liquid being poured, according to the line that coincides with the bottom of the meniscus. This ensures that your readings are not distorted by the effect of surface tension, which distorts the surface of the liquid.
Test tube types
There are two classes of specimens:
- Class A specimen : Class A specimens are used in testing laboratories and for quality control as validation of analytical methods. Its production is usually more careful.
- Class B specimen: Class B specimens are used for procedures that do not require as much precision. They are used in schools and for academic purposes. They are usually made of plastic so their price is lower than those of class A.
Precautions when using a test tube
When handling a test tube and taking measurements on it, a series of precautions should be observed, especially if it is being used in a laboratory. Here is a list of things to consider before and when using a test tube:
- It cannot be heated. Although the test tube will probably withstand certain degrees of heat, it is not the appropriate instrument to heat what we are measuring. Initially, because it is a very small container, and also because reactions can occur that the cylinder will not withstand and can be dangerous.
- It is not for preparing solutions nor is it suitable for reactions. As we have already mentioned, the test tube is not the appropriate instrument for mixing or making reactions, it should simply be used to measure.
- Concentrated acid cannot be diluted as it will damage the container.
- It cannot be cleaned with detergent, as some scratches or scratches may damage the cylinder and the measurement on it.