Goal of the work. Learn how to independently calibrate glass chemical measuring containers taking into account temperature and air pressure.

Theoretical part. Graduation is necessary, since glassware manufactured at the factory does not always meet technical standards and the diameter of pipettes (burettes, volumetric flasks) does not meet the requirements of the standard, which leads to significant errors in chemical analysis.

Chemical glassware is graded as follows: in dry a volumetric flask (pipette, burette) is filled with distilled water to the mark, and then the weight of the liquid is determined by weighing on an analytical balance m V. Using reference data on the density of water at various temperatures, calculate the volume of suspended liquid at a given temperature V V. After this, the calculations do not end, since it is customary to recalculate the volume of liquid to the volume that would be occupied by liquid at a temperature of 20 0 C. This takes into account the fact that chemical glass expands or contracts when the temperature changes.

EQUIPMENT AND MATERIALS. Chemical glassware of 1st and 2nd accuracy classes: burettes for 25 and 50 ml, pipettes for 1, 2, 5, 15, 25, 50 ml, volumetric flasks for 25, 50, 100, 250 ml.

Progress. The calibration procedure involves several stages.

A. Calibration of volumetric flasks

1. Weigh the water poured into a measuring glass container. m V.

2. Calculate the volume of suspended liquid and according to the data in the table. 4 find the volume value W for temperature and atmospheric pressure that were recorded during weighing. The required volume of suspended liquid at temperature and pressure during the experiment will be equal to

V in = W × m in /1000.

Table 4. Volume W 1000.00 g water at different temperatures

3. Determine the volume of water that would be at a temperature of 20 0 C. According to the table. 5 find the total correction D W in the last column on the expansion of glass and the specific gravity of water at the calibration temperature. Next, the final volume of the measuring container at 20 0 C is calculated using the formula:

V in 20 = V V× (1 + D W/1000).

Table 5 . Corrections for glass expansion and specific gravity of water

and total correction depending on temperature.

B. Burette calibration

Fill out the table. 6 and build a dot plot of the volume error from these data D V , ml, from added volume V , ml, from a burette. The volume error can be either positive (Fig. 1) or negative.

D V , ml

V, ml

Fig.1. Burette calibration chart

Table 6. Experienced data on burette calibration

V. Kal adjustment of pipettes

Using a rubber bulb, fill the pipette with water up to the mark and then pour the volume of water for which the pipette is designed into a pre-weighed dry glass, then weigh the mass of the poured water m V. Further actions are carried out in the same way as for volumetric flasks.

Report

Process the results obtained and draw a conclusion using the data in the table. 7, about the possibility of using the measuring utensils you received for work. Ask your teacher about a chemistry glassware class if one is not listed.

Table 7 . Permissible deviations in milliliters

from the capacity of chemical containers at 20 0 C.

Difficult. Medical, pharmaceutical, chemical and food chemists and engineers use measuring vessels every day to quickly and accurately dispense or sample liquid and bulk reagents. Vapers, distillers, magicians, pharmacists, herbalists and other non-laboratory workers also cannot do without measuring glass vessels. Measurement of liquids and bulk solids is carried out using special containers with graduations that show the exact capacity of the container.

Types of measuring laboratory glassware

All glass laboratory glassware or the plastic has marks by which you can dial the exact volume of the solution (measuring flasks) or you can determine how much liquid is in the container (cylinders, graduated test tubes, beakers). The production of this type of cookware is strictly regulated by regulatory documentation, all units of manufactured products are calibrated for pouring or pouring out and the actual error does not exceed the norms of ND (GOST, DSTU, ISO, AOCS, etc.).

For each batch or even each unit of measuring glassware, a quality certificate is given indicating the actual deviation from the calibration standard. So, to calibrate pipettes, burettes or cola, special standard standards of 1st and 2nd digits are used. Standardized verification of measuring laboratory glassware is carried out at 20°C, and measurements are also taken at at least two more points. Based on the results obtained, we distinguish types of measuring laboratory glassware in terms of accuracy - 1 or 2 classes. By default, the error for measuring vessels of the first class does not exceed half the division value, for the second class it is the smallest division value.

Recently, verification has taken place calibration of laboratory measuring glassware. Verification provides information about whether the dishes comply with GOST or not. And calibration gives real numbers - by how many cm³ the actual capacity of a particular vessel differs. These data are used in calculations, especially if it is necessary to validate the methodology. Such accuracy is important for determining trace amounts of certain chemicals, and this is especially important for chromatographic studies.

Laboratory glassware is not intended for heating or cooling, but the deformation rate of glass at different temperatures needs to be known, since it should be insignificant so that the operating temperature range is not only 20°C, but also ±5°C, which is usually found in laboratories. For high-quality measuring utensils, the value of glass expansion during thermal exposure is so insignificant that for some types of work this number can be neglected. So a 1 dm³ volumetric flask when heated by 5 °C will increase its capacity by only 0.0015 dm³.

– measuring chemical laboratory glassware, allowing you to accurately measure the volume of a liquid reagent during titration or other manipulations. This is a tube with marks, open at the top and at the bottom with a locking mechanism, cast from light or dark glass. This type of glassware is calibrated only for pouring.

Burettes are available in a variety of volumes, but the most popular are 10.25 and 50 cm³. The optimal flow rate is 1-2 cm³/sec with the tap or capillary fully open. If more cm³ of reagent is used for titration, reduce the sample size. Or, on the contrary, by analogy. Burettes are often an integral part of various analyzers (coke calcimeter, gas analyzer, chromatograph).

Heat-resistant glass with a minimum number of internal defects is suitable for the manufacture of burettes, since it is necessary that the calibration remains unchanged after repeated use and washing of dishes.

Burettes, their varieties

Main types of burettes:

• With tap – a glass or Teflon tap allows you to adjust the flow rate of liquid without constant manual adjustment.
• Without a tap - straight tubes with an open upper end and a rubber tip at the bottom with a small capillary. The rubber drain tube is clamped with a metal clamp of various designs or a glass bead. This allows you to precisely regulate the volume of dripping solution, but you must constantly keep the hole open.

There are a huge number of varieties of burettes, but the most popular is a straight one with a regular one-stroke stopcock. Burettes with a side outlet are popular, which allows for accuracy and objectivity due to automatic zero setting. Microburettes allow titration taking into account hundredths and tenths of cm³ of titrant.

Like other measuring utensils, burettes come in accuracy classes 1 or 2. The main criteria are a flow rate of 20-35 seconds, an error of ±0.006 cm³ for the first class and 15-35 seconds with an error of 0.015 cm³.

Burettes with autozero

Burettes with the ability to set the zero automatically have gained great popularity. Such burettes are a double tube with a pressure cylinder. An automatic burette is installed on the vessel with the reagent, thus there is practically no access to air, the shelf life of the solution increases, and the quality of the reagent remains unchanged. Automatic burettes are an excellent solution for routine analyzes in production or in a research laboratory.

Using a rubber bulb, the solution is pumped into the burette through the outer tube to the very top, above zero. After the pressure stops building up, the excess solution is returned to the container with the reagent, and the level is set exactly opposite the zero mark.

It is produced in two accuracy classes; the error and the smallest division price depend on the accuracy class and tube volume.

Depending on the purpose, the structure of the burette is divided into the following types:

• Volumetric. The most common ones allow you to measure solutions up to 0.01 cm³. This includes Mohr's burette.
• Gas. The amount of gas during the reaction is recorded, for example, with a Hempel burette.
• Weight. For ultra-precise analysis of liquids, gases, then titrimetry and graphimetry intersect.
• Microburettes. Allows you to study processes by measuring up to 0.005 cm³ (Bang microburette).
• Piston. The piston squeezes out the solution, measurements are taken from bottom to top, and not vice versa, as in conventional burettes.

Burettes are also classified according to the following parameters:

• By waiting time - with a set time (type II) and without it (type I).
• According to the design of the valve (only for type II) - with a side valve, one-way, two-way, without a valve, with an auto-zero and a two-way valve.

Rules for working with a burette

Conventional burettes (without a stopcock or with a one-way stopcock) are filled through the top using a small funnel or glass container with a spout. The tube at the funnel and the spout of the vessel must be narrower than the thickness of the burette tube so that the air displaced by the reagent escapes without obstruction. It is advisable to rinse the burette with the reagent that will be used for titration.

Fill the burette above zero, then drain clearly to zero - transparent solutions along the lower border, dark-colored solutions along the upper border (eyes at the level of the liquid layer). To better see the boundary, you can attach a special screen to the back of the burette - white cardboard with a clear black horizontal stripe. If you move the screen so that the color separation boundary is 1 mm below the zero point, the liquid level will become clearly visible and will appear black. High-quality modern burettes are produced with a white stripe on the back of the burette, along the middle of which there is a clear blue stripe.

There should be no air in the liquid layer. To remove bubbles, you can release the solution with maximum flow, holding the burette at an angle. If this does not work, you can place the tip of the burette in a beaker with the titration solution, then use a bulb to suck it into the burette through the top hole, the bubbles will move from the tip to the top of the burette.

The burette is fixed in a tripod - firmly, strictly vertically. The tap is turned depending on whether the laboratory assistant is left-handed or right-handed. The flasks are held with one hand, rotating during titration, the tap is opened with the other, adjusting the dripping speed, and closed at the end of the reaction.

Under no circumstances should burettes be left with the reagent for a long time; after use, they should be rinsed with distilled water. When using highly alkaline reagents, it is better to use burettes without taps, since all mechanisms seal the lye crystals tightly if you leave the solution for at least a day.

To prevent dust from getting inside the glass tube, a test tube or glass is placed on top of it.

Important! Burettes are calibrated using water, so it is correct to use reagents with a viscosity close to the calibration solution.

Volumetric flasks

Measuring pipettes

They are glass or plastic tubes with printed graduations and are designed to measure precise volumes of liquids during transfer or titration. They produce chemically inert and heat-resistant glass.

A huge number of types of pipettes are available:

• The top edge can be narrow or wide.
• The nose can be long (up to 5 cm) or short.
• Pipettes can be smooth, with extensions (spherical, barrel-shaped).
• Graduated or with one mark (set volume - Mohr pipettes).
• With a scale from top to bottom and vice versa, with markings to the very end or not, with different division scales, the price of a minimum mark.
• From white and dark glass.
• Glass, plastic.

Conventional pipettes from 0.5 to 200 cm³. Micropipettes are also available that allow you to sample up to 0.001 mm³.

Important information is printed on the pipette wall: nominal volume, error, accuracy class, etc. Calibration is carried out on water at 20°C for pouring, so accuracy will be necessary when working with such liquids.

Rules for working with pipettes

Pipettes should always be kept clean and away from pipettes. It is optimal to rinse measuring containers several times with distilled water, and finally with double distillate. Before use, rinse it properly with the solution that will be measured.

Store pipettes with a closed upper end (paper stoppers) vertically in a stand, beaker or cylinder, or horizontally in a tray lined with filter paper.

Fill the pipettes using a bulb (or a syringe), dipping the tip into the reagent. Next, take the pear away and quickly apply a moistened index finger to the top part. By adjusting the pressing force, the reagent is drained to zero. Without releasing your finger, transfer the pipette to the receiving vessel and release your finger until all the liquid has drained. At the end, let it drain for another 25 seconds, touching the tip to the wall of the vessel.

Do not shake! Don't blow! The pipettes are calibrated for natural flow, taking into account those microdroplets that remain on the walls.

Important! If the pipette is not an end pipette, you need to drain to the bottom mark, and not to the end!

Graduated cylinders

They are tall glass vessels with graduations on the walls. Used to measure the volume of liquid reagents. Markings in cm³ are applied with paint or engraved on the outside of the glass. Data on capacity, accuracy class and other information are applied to the upper, outer part of the wall.

Manufactured in 2 accuracy classes, with an error in accordance with ND. There are products from 5 to 2000 cm³. For manufacturing I use thermo- and chemically resistant materials (glass, special polymer plastic). Models are made from dark and light materials.

All cylinders can be divided according to several criteria:

• Spout – there are models with or without spouts, with plugs (polished, rubber, screw).
• Cylinder material: glass, plastic.
• The material of the base, its shape and its removability - removable, plastic, non-removable, glass bases, with round and hexagonal bases.

Cylinder calibration is carried out using distilled water at standard temperature. Depending on the volume of the vessel and the division scale, the division price will be:

Rules for working with cylinders

The cylinder is filled with solution until the liquid reaches the required mark. In this case, it is necessary to hold the dishes at eye level when performing measurements at 20°C or taking into account the change in volume with changes in temperature. You can not hold the cylinder suspended, but put it on a flat surface and lower yourself so that your eyes are at the level of the required marks.

Beakers

This type of measuring glass is used either for measuring volume with low accuracy, or for settling cloudy solutions. Calibration using the distrode is carried out by pouring out. They produce high and low accuracy classes. It is a cylindrical or conical vessel. The marking is contrasting on the outer wall of the vessel, the scale goes from bottom to top. Sometimes it has a base with an extension; models are available with and without handles.

Usually produced with a capacity of 50-1000 cm³. the division price will be 10% of their volume for vessels up to 250 cm³, and 5% for larger volumes.

Very often, beakers are used to separate sediment and liquid in turbid substances. The sediment collects at the bottom of the beaker. Convenient to use for separating immiscible liquids and determining their volume.

Regardless of the material and type of beaker, they must meet the following requirements:

• The boundary between the separation of substances in the beaker is clearly visible.
• Strength.
• Sustainability.
• Reliability of marking – durability, chemical resistance.
• Convenience of washing.

The affordable availability of beakers allows this type of measuring glass to be widely used in all areas of the laboratory.

Measuring tubes

- these are glass or plastic test tubes with a scale printed on the outside of the vessel, used for measuring small volumes of liquid reagents, carrying out reactions, separating substances, settling sediments, centrifugation or other operations.

Typically, 10 cm³ tubes are used, but 5 to 25 cm³ are also available. The marking on the top of the test tube gives information about the capacity, division price and design (1 – polished neck, 2 – smooth edges of the vessel).

They are available with a simple neck, for which you can use rubber plugs, with a polished or screw neck - for glass, plastic, Teflon plugs or screw screws.

For their production, thermo- and chemically resistant materials (plastic and glass) are used. The temperature that such glassware can withstand depends on the purpose and to what temperature the processing will take place.

To separate the sediment, you can use sedimentation or, if you need to speed up the process, centrifugation. Usually, ordinary cylindrical vessels with a sharp end (“carrots”) or pear-shaped ones are used. The marking starts from the very day of the test tube, translated into mm or g/kg of sediment.

Working with measuring laboratory glassware

You can only use perfectly washed dishes - “squeaky clean”. To do this, first clean it of coarse dirt, then wash it thoroughly using a washcloth or soft brush and a non-abrasive cleaning agent. Afterwards, wash with running water to remove any remaining dirt and detergent. Next, at least two rinses in distilled water and a final rinse in bidistillate. Dry the dishes on a vertical dryer or in a ventilated drying cabinet, on a herringbone dryer. It is not advisable to heat the measuring container by more than 10°C.

Store dishes protected from dust. Those dishes that are possible - with stoppers, the rest - with paper lids and caps. Optimally - in a special cabinet, on filtered paper, behind a tightly closed door.

Before use, the dishes are washed several times with the reagent that will be in this container. There should be no air bubbles in the reagent layer, the size of which will result in an inaccurate volume.

Dispensers for liquid reagents

Measuring the exact volume of liquid reagents is an indispensable step in most operations in any laboratory. Therefore, increasing the accuracy of dosing and increasing speed is a direct path to increasing the accuracy of reactions and the productivity of the laboratory assistant. For these functions, solution dispensers are developed, like any measuring glass, they are produced in strict accordance with GOST.

This has led to the emergence of many different types of dispensers, ranging from the simplest, mechanical to fully automated. It is advisable to keep the error in selecting the exact volume of basic substances within about 0.1% (up to 0.2%) of the sampled volume. For indirect reagents, about 1% (maximum 2%) is allowed.

Most dispensers are divided into single- and multi-position. The first allow you to select only a certain volume (by analogy with the Mohr pipette), others allow you to select different volumes, that is, adjustment or full scale, and not just a mark.

To select a constant amount of certain liquid reagents or when selecting hazardous reagents, the use of single-position dispensers is also justified by safety precautions. For example, such tilting dispensers are used for dosing concentrated acids (sulfuric acid, etc.). For such measuring vessels, the error should be within the permissible 2% according to GOST.

Checking the volume of measuring vessels

At least all measuring laboratory glassware GOST 1770-74 corresponds, sometimes you need to check it yourself. This is necessary to find errors during reactions, to calibrate a series of glassware against calibrated or verified by the relevant authorities, to validate and verify methods, and in other cases.

The test consists of measuring the actual capacity of the vessels. You need to find out the exact weight of distilled water under certain conditions (temperature, pressure, etc.). For this purpose, analytical balances of the highest accuracy class are used. For calculations, data is taken from reference tables on water.

Buy measuring laboratory glassware

The use of high-quality measuring glassware is an important condition for the correct operation of any laboratory. The exact volume, correct calculations, purity and completeness of reactions - all this directly depends on the quality of the glass, on the accuracy of marking, on the stability of determining the exact volume. Therefore, you should always strive buy measuring laboratory glassware only from a trusted manufacturer.

Working with experienced suppliers provides a number of advantages:

• Real price for high quality laboratory glassware.
• Guaranteed product quality - accuracy, durability, no defects, etc. Full compliance with GOST.
• All necessary accompanying documents - a quality certificate for each unit of cookware or for the entire batch.
• Measuring utensils certified or calibrated, by agreement.
• Possibility to purchase any types of measuring laboratory glassware of domestic and imported production.

Glassware used in a chemical experiment must meet a number of requirements. The main ones are chemical resistance and heat resistance. Most of it is made of special glass. Such glass is characterized by great chemical resistance; it is very weak or does not break down at all under the influence of acids, alkalis, solutions and molten salts, as well as other aggressive substances. Many types of chemical glass can withstand intense heat - up to red-hot temperatures.

If high heat is required, use quartz glass dishes. Quartz glass can withstand stronger heating than conventional chemical glass; in addition, quartz has a very small coefficient of thermal expansion, so quartz glass dishes can withstand sudden cooling without cracking. Quartz cookware practically does not release its constituent parts into the solution, so it is used when working with especially pure substances.

Chemical containers not intended for heating are also made from ordinary non-heat-resistant glass.

Porcelain dishes are also used in chemical practice. Porcelain products are more chemically and thermally resistant than glass products. Porcelain has greater hardness and therefore mortars and pestles are made from it for grinding crystalline substances. Porcelain is mainly used to make glasses, crucibles, calcination boats, cups and mortars.

Metal utensils are also used for special purposes. Metal beakers and crucibles are used mainly for calcination or carrying out reactions with very aggressive substances, so they are made from chemically inert metals - gold, platinum, silver, nickel, etc.

According to their purpose, chemical glassware is divided into three categories.

1. General laboratory glassware is intended for the widest possible use and is available in almost any laboratory. This includes test tubes, various flasks, beakers, funnels, pipettes, droppers, chemical jars and bottles for storing reagents.

2. Special-purpose utensils include products intended for special purposes: refrigerators, reflux condensers, desiccators, Wulff flasks, gasometers, Kipp apparatus, etc.

3. Measuring glass.

Measuring utensils

Measuring glassware is intended for measuring volumes of liquids or gases. Measuring glassware includes volumetric flasks, measuring cups, burettes, pipettes, and graduated cylinders. Measuring utensils are usually graduated in milliliters.

Measuring utensils require careful and careful handling. Solutions should not be heated in measuring cups, since thermal expansion of the glass may cause residual deformations and the volume of the flask may change. It is also undesirable to store prepared solutions in measuring cups for a long time.

Burettes. Burettes are used to measure volumes of liquid and are calibrated for pouring. Burettes can be macro- or micro-, with a glass stopcock, with a rubber tube and an extended glass tube. In the latter case, either a spring clamp or a glass ball is used to close the burette. The zero division is located at the top of the burette. Macroburette capacity: 10, 25, 50, 100 ml.

Volumetric flasks. Volumetric flasks are designed for preparing standard (with precise concentration) solutions and for diluting test solutions to a certain volume. These are flat-bottomed flasks with a long narrow neck on which a circular mark is applied. They are calibrated to contain a certain volume of liquid (per infusion). Capacity: 25, 50, 100, 200, 250, 500, 1000, 2000 ml. Flasks can be with or without a ground-in stopper.

Volumetric flasks cannot be heated, because... Glass deformation may occur, which entails a change in their capacity. The capacity of the flask indicated on it by the manufacturer is called nominal, and the researcher determines the true capacity.

Pipettes. Pipettes are used to accurately measure a certain volume of solution and transfer it from one vessel to another. They come in 2 types: graduated and simple. Capacity of simple pipettes: 5, 10, 15, 20, 25, 50, 100 ml.

Measurement of liquid volumes is carried out according to the following rules:

1. Measurement is carried out at a temperature of 200C.

2. Pipettes and volumetric flasks should not be handled by their expanded parts, since the heat of the hands causes the glass to expand and the volume of the container can change greatly.

3. The surface of the liquid has the shape of a meniscus, so the flask, pipette or burette is filled in such a way that the liquid touches the division with the lower edge of the meniscus. The measuring cup is kept at eye level.

4. When measuring volumes of opaque or intensely colored liquids, the reading is made along the upper edge of the meniscus.

5. Pipettes and burettes are calibrated for pouring, that is, their nominal volume is equal to the volume of freely flowing liquid. The flasks are calibrated for infusion, that is, the nominal volume of the flask is equal to the volume of liquid poured into the flask.

The results of the analysis depend primarily on the correctness of the readings of the instruments used. Therefore, before taking measurements, it is necessary to ensure that they are correctly calibrated.

At manufacturing plants, measuring containers are marked with a capacity reduced to 20 °C, which is called nominal. But every researcher is obliged to check it.

To check the capacity of measuring vessels - pipettes, burettes, flasks, determine the mass of water that it holds or that pours out of it.

Below (Table 3) are the error limits permissible for first class glassware (GOST 1770-74 “Measured laboratory glassware”).

Table 3 - Permissible deviations in the capacity of measuring containers, ml

When checking the capacity of measuring containers, a number of corrections are introduced. First of all, you should take into account the temperature, which affects the volume occupied by a given mass of water and the volume of the dishes themselves. In addition, the volume occupied by the weighed water is much larger than the volume of the weights, i.e. According to Archimedes' law, they lose less mass than water. Therefore, a correction for suspension in air is necessary (Table 4).

Table 4 - Density of water normalized to 20 °C.

Table 4 shows the density of water normalized to 20 °C if its mass is measured at a certain temperature. This table should be used when calculating the capacity of measuring containers, for which it is necessary to divide the mass of water at a given temperature by the density, which corresponds to this temperature, but normalized to 20 ° C.

Measuring utensilsOFS

In return for the Global FundX, p.849

The requirements of this general pharmacopoeial monograph apply to volumetric glassware used in pharmacopoeial analysis to measure the volume of liquids. Volumetric chemical glassware includes volumetric flasks, pycnometers, pipettes, burettes, as well as graduated cylinders, measuring cups, beakers, and graduated test tubes. Unlike general-purpose chemical glassware, measuring glassware has precise graduations.

Types of measuring utensils

Graduated cylinders(Fig. 1 a) - glass (can be plastic) thick-walled vessels with divisions marked on the outer wall indicating the volume in ml (5 - 2000 ml). There are cylinders equipped with ground-in plugs.

Graduated measuring cups(Fig. 1 b) give the largest error in volume measurement due to the rare divisions indicating the volume.

Beakers(Fig. 1 c) conical-shaped vessels on the wall of which a scale is applied. Beaker capacity 50 – 1000 ml.

Test tubes with divisions- a cylindrical vessel with a semicircular, conical or flat bottom, with a volume of 5 to 25 ml, intended for chemical reactions carried out in small volumes, biological, microbiological procedures, for sampling, measuring a certain volume of poured or poured liquid, or determining the volume of sediment ( centrifugal). A scale corresponding to the capacity of the test tube is printed on the entire side surface. Test tubes can be ground-ground or non-ground, respectively, with or without stoppers.

Glassware for accurate volume measurement includes volumetric flasks, volumetric pipettes and burettes.

Volumetric flasks(Fig. 2 a) are round, flat-bottomed vessels designed for accurate volume measurement (per infusion) when preparing solutions of known concentration. There are narrow-neck and wide-neck volumetric flasks . The diameter of the throat (neck) of the latter is approximately one and a half times larger compared to narrow-necked ones.

There is a ring mark on the neck to which the flask should be filled.

Rice. 2. Volumetric flask (a), pycnometers (b)

In most cases, volumetric flasks have ground glass stoppers. Stoppers made of polyethylene or polypropylene are often used to close volumetric flasks.

Volumetric flasks have a capacity of 1, 2, 5, 10, 25, 50, 100, 200, 250, 500, 1000, 2000 cm3 and are used for preparing solutions with precise concentrations.

Pycnometers– volumetric flasks with a very narrow neck with a capacity of 2 to 50 ml (Fig. 2b). The pycnometer must have a ground stopper. It is used for determination of liquid density.

Pipettes(Fig. 3) are narrow, long glass tubes extended from one end, designed for accurate measurement of volumes of solutions.

The following types of pipettes are distinguished:

Non-graduated with one ring mark - Mohr pipettes (Fig. 3 a) - calibrated for complete drainage. Liquid in them dial to the ring mark And pour to the end;

Ungraded with two ring marks - Mohr pipettes(Fig. 3 b) - liquid in them dial to the top mark And pour to the bottom;

- graduated(Fig. 3 c, d), on which there are divisions along the entire length; These pipettes can measure any volume within its capacity indicated on the label.

The capacity of the pipette - usually from 1 to 100 cm3 - is indicated by the manufacturer in the upper or middle part.

Pipettes with a capacity of less than 1 ml are called micropipettes; with their help, you can select volumes measured in tenths and hundredths of ml. Graduated pipettes, in which only the minimum (or maximum) volume is indicated on the scale, are called full-flow pipettes (Fig. 3d); the maximum volume is taken with these pipettes by pouring liquid from the top division to the end. More convenient and safe to use dispensing pipettes have become widespread, guaranteeing

high accuracy and repeatability of the volume of measured liquids in

range from 2 to 5000 µl.

Unipipettes designed to measure doses of a constant volume (Fig. 3 d).

Varipipettes– These are adjustable-capacity pipettes for measuring doses of any volume within the specified limits (Fig. 3 e). The dispensers in these pipettes can be mechanical or electronic. Draw the liquid into a pipette using a dispenser or a rubber bulb.

Burettes- a cylindrical glass tube with graduations, stopcock or clamp, graduated in milliliters. Burettes are used for precise measurements of small volumes and titrations to determine the quantitative content of a substance.

There are two types of burettes:

Type I - no set waiting time for 1st and 2nd classes;

type II - with a set waiting time for 1st class only.

Volume burettes(Fig. 4, a-d) with a division price of 0.1 ml allow you to count with an accuracy of 0.02 ml. Mohr's tapless burettes (Fig. 4, b) have a rubber tube 1 with a capillary 2 in the lower part. The rubber tube is clamped either with a Mohr clamp (Fig. 4, b), or a glass ball or rod with a spherical thickening is placed inside it. The liquid flows out of such a burette when you press the top of the ball with your fingers.

U burettes with automatic zero(Fig. 4, d) the zero mark is the upper cut of the process.

Fig.4 Burettes:
(a) - with a one-way valve
(b) - rubber tube
(c) – two-way valve
(d) - automatic zero
(e, f) - devices for measuring liquid volumes

Microburettes differ from volumetric burettes in their small volume (2 ml, 5 ml). They have graduations of 0.01 ml, which makes it possible to take readings with an accuracy of 0.005 ml.

Material

Glass measuring utensils must be made of glass that has the necessary chemical properties that ensure resistance to aggressive environments, light, etc.

Borosilicate glass is used to make glassware, which consists of oxides of alkali and alkaline earth metals (calcium, sodium or potassium) added to the silica in the base of ordinary (silicate) glass. When they are replaced with boron oxide, glass acquires special properties - a low coefficient of linear thermal expansion, increased chemical and mechanical stability.

The glass from which the dishes are made must be free of visible defects, and internal stress must be relieved to the required limits.

Capacity measurement accuracymeasuring utensils

In laboratory tests, domestic measuring utensils of accuracy class 1 or 2 (in accordance with GOST) or foreign measuring utensils A or B of accuracy class of the International Standard (ISO) are used. Class 1 or Class A is intended for more precise products used in quantitative determination; Class 2 or class B - for less precise measurements.

Measurement error limits

Error limits mean the maximum permissible error difference between any two points on the scale. The measurement errors of the drained liquid should not exceed the values ​​indicated in the table. 1.

Table 1.

Calibration of laboratory glassware

Volumetric flasks, pycnometers, pipettes and burettes must be checked before use. Before checking, the measuring containers are thoroughly washed and dried. Dried measuring vessels used for “pouring” (pipettes and burettes) are moistened with purified water before testing: they are poured into the vessel being tested and allowed to stand for 1-2 minutes, after which they are poured out, as in normal use. Checking volumetric glassware consists of determining the mass of purified water, free of impurities and dissolved air, poured into the glassware to the mark (measured flasks and pycnometers) or poured out of it (pipettes and burettes) at a given temperature and atmospheric pressure.

When checking pipettes, the water from them is drained into a bottle with a lid and weighed. Without pouring the water out of the beaker, lower the full pipette into it again and weigh it. They do this for the third time. Of the three values ​​of water mass, the average is taken. When checking burettes, measure the mass of its entire volume, and then the mass of water every 10 ml. For accurate calibration, the mass of each milliliter is checked. The temperature at which measuring glassware is calibrated must be 20° C. In practice, when calibrating and checking measuring glassware, tables are used showing how much purified water of a certain temperature must be weighed in air of the same temperature so that its volume corresponds to 1 liter at 20°C .

Table 1. Table of the mass of 1 liter of water suspended in air using brass weights at different temperatures

For second class cookware, the permissible error limits are doubled.

Working with measuring utensils

The volume of liquid can be measured with varying degrees of accuracy, which is determined by the analysis task. Depending on the relative error allowed when measuring volume, measuring utensils are divided into two groups - for approximate and accurate measurement of volume. Vessels for approximate volume measurements include graduated cylinders, graduated beakers, beakers, and graduated test tubes. The relative error when measuring volume using such utensils is 1% or more. This dish is primarily intended for pouring. The term “on pour” means that if you pour the contents of a filled measuring vessel into another vessel, the volume of liquid poured at room temperature will correspond to the capacity indicated on the vessel.

graduated cylinders,graduated measuring cups, beakers,test tubes with divisions. To measure the required volume of liquid, it is poured into a measuring vessel until the lower edge of the meniscus reaches the level of the desired division.

Volumetric flasks. Each volumetric flask is marked with the temperature at which it has a volume precisely marked on it. The term “infusion” means that if a volumetric flask is filled with liquid exactly to the mark, the volume of liquid at room temperature will correspond to the capacity indicated on the flask.

The volume of liquid poured from the flask will be slightly less than the marked volume, since some of it will remain on the walls. Therefore, ordinary volumetric flasks are not suitable for measuring the exact volume of liquid and then pouring it out. Volumetric flasks intended for pouring have two marks. The top mark is intended “for pouring,” that is, if you fill the flask to this mark and pour out the contents, the poured liquid will have the volume indicated on the flask. The solution in the flask is brought to the mark in several steps. First, pour water 0.5 - 1 cm below the mark, then, using a pipette, add liquid drop by drop until the edge of the meniscus of the solution touches the mark.

Fig.6. Monitoring the correct installation of the meniscus in the volumetric flask

For transparent aqueous solutions must touch the mark bottom edge meniscus, for cloudy and brightly colored aqueous solutions – upper(Fig. 5). At the same time, the flask is held in front of you for the top necks so that the mark was at eye level(Fig. 6). In a large volume flask (500 - 2000 ml), the solution should be brought to the mark by placing the flask on a flat horizontal surface. You cannot hold the flask by its lower part, as volume distortion may occur due to the heat imparted by the hand.

The solvent, like the solution in the flask, must be at room temperature. It is impossible to bring hot or cold solutions to the mark, since the density of liquids depends on temperature and, therefore, the determined volume will differ from the volume indicated on the volumetric flask. Alcohol, aqueous-alcohol solutions and solutions of organic solvents are brought to the mark after keeping them for 20 minutes at 20ο C.

After bringing the liquid level to the mark, close the flask with a stopper, and, holding the latter with the thumb or index finger of the right hand or palm, mix the resulting solution well, turning the flask up and down at least 7 - 10 times. Despite the fact that after mixing, the liquid level in the volumetric flask drops below the ring mark, since part of the solution remains on the stopper, it is impossible to bring the liquid level to the ring mark again after mixing.

If necessary, heat the solutions in volumetric flasks in a water bath (to the temperature specified in the regulatory document), then before bringing the solution to the mark, cool the flasks and keep them at a temperature of 20ο C for 20-30 minutes.

Measuring pipettes. Draw the liquid into a pipette using a dispenser or a rubber bulb.

To fill any pipette, the liquid level should be 2-3 cm above the mark. The pipette should be held strictly vertically, raised above the solution so that the mark is at eye level, and the liquid should be released drop by drop until the edge of the solution meniscus coincides with the mark. Next, the pipette is transferred to another vessel, touching its lower end to the inner surface of this vessel, and the liquid is allowed to drain slowly. If you quickly pour out the liquid, a significant part of it will remain on the walls of the pipette. The remaining liquid (for pipettes with one mark or for full drainage) is removed by touching the tip of the pipette to the edge of the tilted vessel for a few seconds, then slightly rotate the pipette around its axis. The remaining liquid cannot be blown out of the pipette, since this volume is not taken into account when calibrating the measuring glass. In case of complete pouring to the spout, it is necessary to wait 15 seconds before removing the pipette from the receiving vessel.

Volumetric burettes. Before starting work, the burette is washed twice with purified water and rinsed twice with the solution that will be in it.

The burette prepared for work is fixed vertically in a stand, then the burette is filled with solution through a funnel with a short end that does not reach the zero division. If the burette has a two-way valve 2 (Fig. 4, c), then filling is carried out by attaching a rubber hose from a bottle with a solution to a curved tube. The burette is filled with liquid a few millimeters above the zero line and the descending meniscus is placed on this line. The solution is then released so that it fills the burette to the end of the spout.

In burettes with a glass tap, liquid is drawn by sucking the bulb through the top hole with the tap open. To remove air bubbles, lift the tip of the burette with a rubber tube at an angle, open the clamp slightly and release the liquid until all the air is removed.

The burette is set to zero only after that how to make sure that the tip of the burette is filled with solution. The funnel used to pour the solution into the burette is removed. Drops remaining on the funnel may increase the volume of liquid in the burette, which may cause an incorrect test result.

During titration, do not touch the burette nose to the walls of the receiving vessel. The drop remaining on the spout after pouring is completed is added to the poured volume by touching the inside of the receiving vessel. If the burette does not have a waiting time set, there is no need to wait for any liquid remaining on the walls to drain.

The pouring time should not exceed 45 s for 1 ml burettes. Some Class 1 (Class A) burettes have a waiting time of 30 seconds. Only after this the solution in the burette is set to zero division, and not a single air bubble should remain in its lower part. If they remain, the volume of liquid used for titration will be determined incorrectly.

When filling large burettes (as well as other measuring vessels) with easily foaming liquids, the waiting time for the foam to settle should be long - until the last bubble disappears, and reaching the meniscus is carried out carefully along the walls of the filled vessel. The lower edge of the meniscus is always chosen as the reference point for the solution level in the burette (Fig. 4e). The burette is calibrated along this edge. Only in the case of opaque solutions (an aqueous solution of KMnO4, a solution of I2 in an aqueous solution of KI, etc.) is it necessary to take a reading along the upper edge of the meniscus.

In a burette with an automatic zero, the solution supplied from below through the tube rises to the upper section of the process, the excess will flow out of the burette through the tube (Fig. 4). After stopping the supply of the solution, its level will be established automatically at the upper cut of the process. The first mark on the scale of such a burette indicates 1 ml. The glass taps of burettes should be very lightly lubricated with petroleum jelly or a lanolin-wax alloy. Excessive lubricant on microburettes is especially dangerous, since it can rise up the burette and, contaminating its internal surface, disrupts the normal wetting of the walls of the burette with the solution.

Solutions of caustic and carbonic alkalis are kept in burettes with clamps, since when storing these solutions in burettes with glass taps, the taps often “stick.” The upper end of the burette is protected from dust and solution evaporation with a small glass or a wide but short test tube.

Installation of the meniscus

Before each titration, be sure to set the liquid level in the burette to zero on the scale. The volume is measured using the burette along the corresponding edge of the meniscus (Fig. 5), while the observer’s eyes should be at the level of the meniscus to avoid measurement errors.

Accurate determination of the lower edge of the meniscus is complicated by the phenomenon of reflection, and errors from parallax (relative displacement of the meniscus due to movement of the observer's eye) are possible if the eyes are not exactly at the height of the meniscus. For volumetric flasks and pipettes, the mark surrounds the entire neck or tube, allowing an accurate reading to be taken. With burettes, the mark occupies only part of the circumference of the tube. Therefore, to correctly measure the level of the solution in the burette, various devices are used. For example, they hold a piece of white cardboard or a frosted glass plate behind the burette, or put a paper frame on the burette (Fig. 4 e, f).

Washing measuring utensils

Washing of volumetric glassware is carried out similarly to conventional laboratory chemical glassware, sequentially performing the following procedures:

P preparatory work; before soaking with a napkin/filter paper, remove grease from the burette taps and connections (if any), other grease stains and inscriptions made during operation;

Z soaking and washing in the washing solution; The shelf life of the solution for soaking dishes is 24 hours, reuse of this solution is not allowed;

- rinsing- carry out with running tap water, and then three times with distilled water;

- control of the cleanliness of dishes carried out visually; glassware is considered clean if water does not leave drops on the inner walls.

To wash measuring utensils, depending on the nature of the contamination, use:

- ultrasonic baths,

- organic solvents (polar and non-polar);

For washing, chemical grade solvents are used, and for rinsing, chemical grade solvents are used; in this case, strict safety measures must be observed (working in a fume hood, etc.), since most organic solvents are toxic and flammable;

- acids and oxidizing agents ( concentrated hydrochloric, sulfuric, nitric or chromic acids, or their solutions);

Note. Work with acids is carried out in a fume hood. An ammonia solution should not be used to rinse containers in which work with organic solvents is carried out.

Use of dichromic acid (“chrompic”):

Dichromic acid is very aggressive, and therefore requires a special set of waste disposal measures. As a replacement, commercial acid-containing solutions or a mixture of acids listed above can be used.

Note. Particular care should be taken when working with dichromic acid. Spent dichromic acid is handed over in accordance with the rules adopted in the laboratory.

Drying dishes

After rinsing, the dishes are turned upside down, for which they use a special board with pegs, on which the washed dishes are placed and left at room temperature until they dry. After washing and drying, clean pipettes are placed in special stands (tripods).

Note. If specified by the manufacturer, it is allowed to dry the measuring glass in a dry-heat oven at the temperature recommended by the manufacturer.

In case of emergency, dry the dishes by rinsing with acetone or reagent grade ethanol. Residues of solvents are collected and handed over in accordance with the rules adopted in the laboratory.

Goal of the work˸calibrate the measuring utensils˸

- option 1– burette;

– option 2– graduated pipette or Mohr pipette;

– option 3- measuring flask.

The essence of the work. In titrimetric methods of analysis, the reproducibility and accuracy of the final result are to a very large extent determined by the accuracy of the preparation of standard solutions and the accuracy of measuring the volumes of the titrant and titrated substance. To accurately measure volumes, burettes, pipettes and volumetric flasks of two accuracy classes of different capacities and modifications are used, which are produced by industry in accordance with GOST requirements and are calibrated at a temperature of 20°C.

The nominal capacity of a measuring cup does not always correspond to its true capacity. This affects the accuracy of titrimetric determinations, so to obtain accurate results it is necessary to calibrate the glassware. If discrepancies are greater than acceptable, such dishes are rejected or corrections to the nominal volume are taken into account when working with them.

Distilled water is used for calibration. The dishes and water intended for filling them are first kept for at least 1 hour in the laboratory so that they reach room temperature. The water temperature is measured with a thermometer with an error of no more than 0.5°C.

Burettes used for measuring precise volumes during titrations and other operations. All of them are intended to measure the liquid poured from them, therefore they are calibrated to pouring out. There are macro- and microburettes. The 50 ml burettes used in macroanalysis are graduated into milliliters and fractions of a milliliter with the smallest division value of 0.1 ml, and the 25 ml burettes are graduated either similarly or with the smallest division value of 0.05 ml. Hundredths of a milliliter are counted by eye with an accuracy of no more than half the division value. Microburettes have a capacity of 1, 2, 5, 10 ml with the smallest division price of 0.01–0.02 ml.

Burettes are manufactured in accordance with GOST 29251-91, ISO 9002-94, ISO 385-84. The error limits for burettes of the 2nd accuracy class with a capacity of 25 and 50 cm 3 at a temperature of 20 ° C should not exceed ± 0.1 cm 3.

Pipettes serve for measuring and transferring the exact volume of solution from one vessel to another, they come in two types, graduated and with one mark (Mohr pipettes) with a capacity from 1 to 100 ml. Graduated pipettes are less accurate than Mohr pipettes. There are micropipettes with a capacity of 0.1–0.2 ml.

Pipettes are calibrated for pouring. The volume of freely flowing liquid with which the pipette is prefilled is the nominal volume. According to GOST 29169-91, ISO 9002-94, ISO 835-81, ISO 648-77, the limits of permissible error of the nominal capacity of pipettes should not exceed the values ​​​​specified in table. 7.