VOLUMETRIC CLINICAL LABORATORY EQUIPMENT: CLASSIFICATION, CALIBRATION - SCIENCE - 2024

The volumetric material of a clinical laboratory comprises a set of glass utensils (mostly) that have the function of measuring volumes, for this they have a printed measurement scale. Each measuring instrument has a specific utility within the laboratory.

Some make grotesque measurements without much accuracy, while others are special for measuring more exact volumes. The choice of volumetric material for the execution of a procedure or the preparation of solutions will depend on what the professional needs to do.

Volumetric balloon, Erlenméyer flask, Graduated cylinder, Beaker, Serological pipettes and Dropper. Colash of photos taken by the author MSc. Marielsa Gil.

There are laboratory procedures that do not require volumes to be exact, but in others, accuracy is essential. Therefore, there are them in various forms, details and capacities.

The measurement scale of the different volumetric instruments is expressed in ml or cm ³, however they may vary in their appreciation. The appreciation of an instrument refers to the distance between two measurements, which allows defining the minimum measurable quantity when using that scale.

That is, some allow volumes to be measured taking microliters (µl) into account, such as 1.3 ml. This means that the instrument is capable of measuring 1 ml with 3 µl, therefore its appreciation is good and the minimum measurable quantity is 0.1 ml or what is equal to 1 µl.

On the other hand, there are others in which their measurement scale can only measure specific volumes, that is, the measurement jumps from 1 ml to another without intermediate divisions. For example 1 ml, 2 ml, 3 ml, 4 ml etc. In this case the appreciation is not so good and the minimum measurable quantity is 1 ml.

Another important parameter is the capacity or range of a volumetric instrument. This refers to the maximum volume that can be measured. Eg 0.1 ml, 0.2 ml, 1 ml, 5 ml, 10 ml pipettes, or 100 ml, 250 ml, 500 ml, 1000 ml volumetric flasks.

Classification of volumetric material

Measurement materials are classified into two groups: those that offer an approximate measurement volume and those that offer a more accurate measurement volume.

- Material with approximate measurement volume: graduated cylinder or cylinder, flasks or Erlenmeyer flasks and beakers, graduated conical glasses, Pasteur pipettes and droppers.

- Higher precision volumetric material: terminal or single gauge serological pipettes, subterminal or double gauge serological pipettes, single gauge volumetric pipettes, double gauge volumetric pipettes, burettes, volumetric flasks, automatic micropipettes.

The materials of greater precision in turn are classified in class A and class B. The A are of better quality and have a higher cost, and the B of a lower quality but are cheaper.

Calibration

It is the process by which the difference between the value that the volumetric instrument claims to measure with which it actually measures is analyzed. That difference is the uncertainty value of the instrument and must be considered in your measurements.

In this process, it must be taken into account that the volume measurements vary with changes in temperature, since the heat expands the liquid and the cold contracts it. Therefore, a measurement correction table is used according to the measurement temperature.

The procedure consists of weighing the instrument empty, then weighing the instrument filled with water to its maximum capacity for which it was designed. Then the mass of water must be measured by subtracting the weight of the filled instrument minus the vacuum.

The value obtained is multiplied by the correction factor according to the temperature (the correction table is used).

Then the uncorrected measured value is subtracted from the corrected one. That difference represents the uncertainty value. Subsequently, this procedure is repeated several times to obtain various uncertainty measures. The standard deviation is taken from the total uncertainty. This represents absolute uncertainty.

In order to perform this procedure, it is necessary to confirm that the instruments are clean and physically intact.

Correction table for volumetric measurements according to temperature. Source: Dosal M, Pasos A, Sandoval R and Villanueva M. Experimental analytical chemistry. Calibration of volumetric material. 2007. Available at: depa.fquim.unam.mx

check

The verification step complements the calibration step, since once the absolute uncertainty value has been obtained, the relative uncertainty is also searched and it is verified if the percentage (%) of measurement error is within the permissible ranges established by ISO standards. for each instrument or if it gets out of them.

If it goes outside the permitted value, the material must be discontinued.

Main volumetric materials

-Approximate volume measurement material or low precision

Graduated cylinders or test tubes

As its name indicates, its body is a thin cylinder, it has a base that gives it stability and a spout at the top to help the transfer of liquids. On the body is the scale printed in ml.

The graduated cylinder is used to measure volumes when precision is not very important, they also serve to transfer liquids. There are plastic and glass. Various capacities are available on the market, for example: 25 ml, 50 ml, 100 ml, 200 ml, 500 ml and 1000 ml.

1000 ml cylinders are commonly used to measure 24 hour urine.

Graduated cylinders. Source: Photos taken by the author MSc. Marielsa gil

Beaker

The beaker is cylinder-shaped but wider than the test tube, it has a spout in the mouth that facilitates the transfer of liquids.

Its uses are very diverse. With them you can weigh substances, mix and heat solutions. Available capacities range from 50 ml to 5000 ml.

Regarding quality, they are type C. Therefore, their measurements are not precise at all, and therefore they are not recommended for preparing solutions.

There are several types or designs: Griffin glass, Berzelius glass and flat glass.

Griffin glass

They are glasses with a wide mouth, flat base, straight body, and not very tall. They have a peak on the edge. They are the most frequently used. They have a small printed scale.

Berzelius glass

This glass has a wide mouth, flat base, and a straight body, but its height is higher than that of the Griffin glass.

Flat glass

Wide-mouthed glass, has a spout to help the transfer of substances and is low in height. It does not have a printed measurement scale. It is commonly used for the crystallization of substances and for incubating solutions in water baths.

Hasty vases. Source: Photo taken by the author MSc. Marielsa Gil.

Erlenmeyer flask

The Erlenmeyer flask was designed by Richard August Emil Erlenmeyer, hence its name.

It has a wide base and a narrow neck at the top. In this way it is ideal for mixing solutions, especially for liquids that tend to evaporate, since it can be easily covered with parafilm paper or with a cap made of gauze or cotton.

Between the base and the neck is a printed graduated scale, but its measurement is not precise.

It can also be used to heat solutions. It is frequently used to prepare and sterilize culture media or to preserve non-photosensitive solutions, both at room temperature and in the refrigerator.

It is useful in substance titration or titration procedures and as a receiving vessel in distillation or filtration equipment.

There are several capacities, eg: 50 ml, 125 ml, 225 ml, 500 ml, 1000 ml, and even 6000 ml.

Erlenméyer flasks. Source: Photos taken by the author MSc. Marielsa Gil.

Conical vessels

As the name suggests, they are shaped like an inverted cone. They have a measuring scale and a support base. They are not very precise instruments, therefore they should not be used to prepare solutions that require accuracy.

-Volumetric material of greater precision

Pipettes

There are two types: serological and volumetric.

Serological pipettes

Serological pipettes are thin cylinders that are used to accurately measure volumes. There are two types, terminals and sub-terminals.

The terminals have only one capacity, which is at the top where the measurement scale begins. The measured liquid is released until the last drop comes out.

The sub-terminals have a more precise measurement because they have double gauging, one at the beginning or top of the pipette and another before the end of the pipette. Therefore, the operator must take care of the leveling in the two gauges.

There are 0.1 ml, 0.2 ml, 1 ml, 2 ml, 5 ml, 10 ml and 25 ml. The quality of a pipette is evaluated based on the precision of its measurements. In this sense, the market offers pipettes type A (better quality) and type B (lower quality).

The maximum quantity that can be measured is stated on the top of the pipette. For example, 10 ml. The volume between two measurement lines is described below. For example, 1/10 ml. This means that the volume that you measure from one line to another is 0.1 ml. This is called instrument appreciation.

Serological pipettes and dropper. Source: Photos taken by the author MSc. Marielsa Gil.

Volumetric pipettes

These pipettes are a cylinder like the previous ones, but in the upper part they have a safety bulb, especially to prevent accidents in case of dangerous liquids. In the center they have a more pronounced dilation. After the expansion the thin cylinder continues.

Like serological pipettes there are terminal and subterminal, class A and class B. Volumetric pipettes are more accurate than serological.

Volumetric flasks

The volumetric flask or volumetric flask consists of two parts, the lower part is balloon-shaped and the upper part has a narrow, cylindrical, moderately long neck. In the part of the neck it has a mark called capacity.

It does not have a measurement scale, it only has the maximum capacity that is achieved when the liquid reaches the capacity (level).

To make up this instrument, it must be taken into account that the liquid level will generally be observed in a convex way, so the lower part of the curve must be above the gauging line.

With some liquids having an adhesion force greater than the cohesion force, the liquid-air interface takes on the concave shape. In this case, the upper part of the meniscus should be touching the gauge line.

For this it is necessary that the view of the observer is perpendicular to the line of the gauging. It will not be flush properly if the observer is looking from above or below. These tightening recommendations are also valid for the rest of the volumetric measuring utensils that have capacity.

The volumetric flask is a high precision instrument, used when it is necessary to prepare solutions with an exact concentration. It is ideal for preparing stock solutions, standard solutions, dilutions, etc.

The existing capacities are 25ml, 50ml, 200ml, 250ml, 500ml, 1000ml and 2000ml. Usually the flask expresses its capacity and the temperature at which the liquids should be measured.

Volumetric bottles or flasks. Source: Photos taken by the author MSc. Marielsa Gil.

Burettes

They are graduated glass tubes similar to pipettes, but they have a kind of key or valve (spigot and tap) at the bottom that opens and closes, managing to control the output of the liquid. They are ideal for the solution titration process. There are 10 ml, 20 ml, 25 ml and 50 ml.

Calibrated dripper

This little instrument is a finer graduated cylinder towards the lower end. It usually provides 20 drops for each ml of liquid, that is, one drop is equal to 0.05 ml. To measure the necessary drops, take care that the cylinder does not contain air bubbles. It is sucked with a pacifier.

Cleaning volumetric material

It is very important that the laboratory equipment is washed properly. It is recommended that it be cleaned as quickly as possible after use to avoid deterioration of the material.

After washing, one way to check if it was clean is to see if the wet material has drops of water stuck on its surface. If that happens, the glass is greasy and not very clean. In optimal conditions the surface should be left with a smooth film of water.

Classic wash with soap and water

Before anything else, it should be washed with soap and tap water. Brushes or sponges can sometimes be used to aid cleaning. Subsequently, rinse very well and then pass several times through distilled or deionized water.

Washing with special soaps

Special soaps are available on the market for cleaning laboratory glassware. These soaps come in two forms, a powder and a soap solution.

This type of soap is highly recommended, since it guarantees a more effective cleaning, does not leave any type of residue and does not require scrubbing, that is, it is enough to immerse the material in a tray with soap and water and then rinse very well with water. tap and then deionized.

Acid wash

Sometimes the material can be immersed in 10% nitric acid for a reasonable time and subsequently immersed in deionized water several times.

Chromic mix wash

This type of washing is not done routinely. It is usually indicated when the glassware is very stained or greasy. This mixture is highly corrosive, so it has to be handled with care, and its frequent use damages glassware.

The chromic mixture is prepared by weighing 100 g of potassium dichromate (K ₂ Cr ₂ O ₂) and dissolving in 1000 ml of water, then to this mixture 100 ml of concentrated sulfuric acid (H ₂ SO ₄) is added little by little. In that order.

The glassware is immersed in this solution and left overnight. The next day the chromic mixture is collected and saved to be used on another occasion. This mixture is reusable, as many times as possible, and will only be discarded when it turns green.

The material will require several rinses with plenty of water, as the mixture leaves residue adhering to the glass.

Volumetric material drying

The material can be left to air dry on an absorbent surface, preferably upside down, in the case of instruments that allow it. Another option is oven drying, but this has the drawback that only approximate volume measuring materials can be dried in this way.

High precision measuring materials should never be dried in an oven, as the heat causes them to lose their calibration.

In this case, if it is necessary to dry them more quickly, a little ethanol or acetone is placed inside the instrument and passed over the entire internal surface, and then cleaned. As these substances are volatile the remainder will evaporate quickly, leaving the instrument completely dry.

References

1. Material frequently used in the laboratory. University of Valencia. Analytical Chemistry Department. GAMM Multimedia Guides. Available at: uv.es/gamm
2. Dosal M, Pasos A, Sandoval R and Villanueva M. Experimental analytical chemistry. Calibration of volumetric material. 2007. Available at: depa.fquim.unam.mx
3. Erlenmeyer flask. » Wikipedia, The Free Encyclopedia. 30 May 2019, 19:50 UTC. 4 Jun 2019, 19:58 en.wikipedia.org
4. Volumetric flask. Wikipedia, The Free Encyclopedia. 14 Apr 2019, 19:44 UTC. 4 Jun 2019, 20:54 en.wikipedia.org
5. Cashabam V. Instruction for Verification of Volumetric Material. Available at: academia.edu