Introduction date 01.01.93
1. PURPOSE AND SCOPE
This International Standard specifies the design principles and specifications for glass volumetric utensils.
The requirements of this standard are recommendations.
2. LINKS
The unit of volume is a cubic centimeter (cm 3), in some cases - a cubic decimeter (dm 3) or a cubic millimeter (mm 3).
Note f. In accordance with the International System of Units (SI), the term "milliliter" (ml) is widely used instead of "cubic centimeter" (cm 3), "liter" (l) - instead of "cubic decimeter" (dm 3), "microliter "(µl) - instead of "cubic millimeter" (mm 3).
3.2. Standard temperature
For standard temperature, i.e. the temperature at which the product contains or drains the nominal volume of liquid (nominal capacity) is taken as 20 °C.
Note: If countries with tropical climates need to operate at ambient temperatures well above 20°C and these countries do not accept 20°C as their standard temperature, they are advised to adopt 27°C as their standard temperature.
4. VOLUME MEASUREMENT ACCURACY
4.1. In normative and technical documentation (hereinafter referred to as NTD), where two accuracy classes are required, a higher degree of accuracy should be designated as class 1, a lower one as class 2.
4.2. Volume error limits should be established for each type of product, depending on the method and purpose of use and accuracy class.
1 This ten-digit series was adopted because tenths of decimals, such as 31.5, would indicate a precision that is not required and is practically impossible to determine.
All technical documentation for measuring utensils should include nomograms made on a logarithmic scale, as shown in the appendix.
6.3.2. Numerical values of the lowest division value of products with a scale must be selected from the range: 1; 2; 5 or decimal multiples of these values.
6.3.3. For glass measuring utensils for special purposes, graduated for direct reading of the volume of a special liquid, the corresponding volume of pure water should be indicated in the NTD, this is done so that the product can be verified using water.
Products with a flat base must be stable and on a flat surface must stand without swinging, the scale axis must be vertical, unless otherwise specified.
When installing an empty product on an inclined plane, the product must not tip over. The angle of inclination is specified for each type of product.
Products with a non-flat base must also meet all these requirements.
6.5. Drain tips
1 The requirement prohibiting the presence of sharp narrowing of the internal channel is aimed at ensuring that broken drain tips are not soldered to the product again, since after soldering the error limits for the drained volume of liquid can change significantly for no apparent reason.
6.5.2. The spout of the drain tip must be treated in one of the following ways, listed in order of preference:
a) smooth grinding at right angles to the axis, small external chamfer, melted;
b) smooth grinding at right angles to the axis and a small external chamfer;
c) cut at a right angle to the axis and melted.
When reflowing, the drain tip breaks off less, but there should not be a narrowing of the internal channel (p.) or a large internal stress.
6.5.3. The drain tip should be manufactured together with products of classes 1 and 2.
6.6. Traffic jams
a) convenience and reliability in operation;
b) the same shape and proportions for products of several standard sizes;
c) limiting the value of the maximum internal diameter in the plane of the mark or marks (clause and appendix); such restriction may be direct, indicating the diameter, or indirect - indicating the minimum length of the marks;
d) the required distance between the marks, determined by p.;
e) stability requirements (p. ) 1 .
1 Stability is checked by the angle of deviation of the center of gravity relative to the edge of the base. The height of the center of gravity depends not only on the size, but also on the density of the glass in various parts of the product. The specified dimensions must be such that the stability requirements are met.
Linear dimensions should be set in millimeters.
7.2. The requirements for linear dimensions should not be set more stringent than indicated in paragraph .
7.3. In order to provide maximum freedom in the manufacture of products in accordance with the requirements of paragraph , sizes can be divided into two categories: basic and recommended.
7.4. In the NTD, where both categories of sizes are indicated, the requirements of paragraphs c, d should be included as the main sizes.
a) mean value ± tolerance;
b) maximum and minimum value;
c) maximum or minimum value.
2 When choosing a method of expressing dimensions (item a or b), one should be guided by the principles of economy and simplicity, and also in order to avoid the use of higher accuracy than provided.
7.7. A double restriction on the tolerances of linear dimensions should be avoided, for example, if the overall height is limited in accordance with paragraph a or b and two or more additional sizes are given within the overall height of the product, the overall height tolerance should be given such that the total tolerances for the remaining dimensions do not exceed the tolerance for the overall height or a smaller part of the product should not be set to a size that may vary depending on the overall height of the product and the dimensions of other parts of the product.
7.8. Additional dimensions shall be expressed as mean values without tolerances, minimum or maximum values. If it is necessary to specify both limits of one or another size, such size should be categorized as basic sizes.
8. MARKINGS
8.1. Marks should be clear, indelible, of uniform thickness.
8.4. The planes of all marks must be perpendicular to the longitudinal axis of the scale. For products with a horizontal base, the marks must be parallel to the plane of the base.
8.5. Marks should be applied on the cylindrical part of the product. The beginning and end of the scale should be applied at a distance of at least 10 mm from the place where the size of the section changes. In some cases (only for measuring utensils of class 2), marks can be applied on a parallel part of the wall of the product with a non-circular cross section, on the conical or conical part of the product.
9. SCALE
9.1. Distance between scale marks
9.1.1. There should be no visible fluctuations in the distance between the marks (except for special cases when the scale is applied to the conical or narrowing part of the product and the division value changes).
(0,8 + 0,02 D), mm,
Where D- the maximum allowable value of the inner diameter, mm, (Appendix ).
9.2. Length of marks (hell)
Location of marks
Crap. 1
9.2.1. For products with a circular cross-section and with a scale, the length of the marks shall vary so that the marks are clearly different. The length of the marks must meet the requirements of paragraphs. ; or .
9.3.2. On products with the lowest division value of 2 cm 3 (or decimal multiples of this value):
a) every fifth mark is long;
b) between two long marks - four short ones (Fig. b).
9.3.3. On products with the lowest division price of 5 cm 3 (or decimal multiples of this value):
a) every tenth mark is long;
b) between two long marks - four evenly spaced middle marks;
c) between two middle marks or middle and long marks - one short mark (Fig. c).
9.4. Location of marks(crap. )
9.4.1. The ends of the short marks on the vertical scales of products, graduated in accordance with scheme I and the provisions of p., should be on an imaginary vertical line located in the center of the product, the marks themselves are located to the left of this imaginary straight line if the product is located frontally to the observer.
9.4.2. The centers of short and medium marks on the vertical scales of products graduated in accordance with schemes II and III and the provisions of paragraphs. and , must be located on an imaginary vertical line located in the center of the product, if the product is placed frontally to the observer.
Length and location of marks
Crap. 2
10. DIGITATION OF MARKS
10.2. On products with two or three marks, the numbers corresponding to the nominal volume should be applied near the corresponding marks, unless another designation method is used (for example, indicated in the note to item d).
10.3. On products with one main mark and a small number of additional marks, the figure corresponding to the main volume may be included in the inscriptions (p.), while additional marks should be indicated accordingly.
10.4. On products with a scale:
a) the scale must be digitized so that the volume corresponding to the scale marks can be freely determined;
b) the numbers must be the same set;
c) every tenth mark should be digitized;
d) the numbers should be applied at the long marks, immediately above the mark, to the right of adjacent short marks.
Note e. If the scale applied to the product is carried out in accordance with clause (i.e., long marks do not run along the entire circumference of the product), then another digitization option is allowed, in which the numbers are located to the right of the end of the long marks in such a way that they crossed the imaginary continuation of the mark;
e) if in some cases it becomes necessary to digitize the middle marks, then the numbers are located to the right of the end of the corresponding mark in such a way that they are crossed by the imaginary continuation of the mark.
11. SIGNS
a) a figure corresponding to the nominal volume (except for products with digitized marks indicating the volume);
b) designation of the unit of measurement (cm 3, ml), in which the product was graduated (p.);
c) designation of the standard temperature (20 °C).
Note f. If 27 °C is taken as the standard temperature, then 20 °C should be replaced by 27 °C.
d) the symbol "H" - to indicate that the product was measured for the content of the specified volume, or the symbol "O" - to indicate that the product was measured for the discharge of the specified volume;
NOTE If on the product some marks correspond to the drained and others to the contained volume, then the letters should be located next to the corresponding marks.
e) designation of the accuracy class (1 or 2) to which the product belongs;
e) waiting time on the products for which it is set (for example 0 + 15 s);
g) designation or brand of the manufacturer or supplier.
a) identification number. This number shall be marked on the handle of the taps, if necessary, and on the plugs, if they are not interchangeable. If the plugs are interchangeable, then on them and on the neck of the product, the size number of the section should be applied in accordance with GOST 8682 ;
b) the time of free draining of clean water (s) for products intended for draining liquid through a drain tip;
c) the chemical formula of the liquid for measuring products intended for direct reading of indications of the volume of a special liquid;
d) margin of error for the volume of this product (for example, ± 0.01 cm 3).
11.3. The following inscriptions should also be applied to the products:
a) if the product is made of glass with a coefficient of thermal (volumetric) expansion that is not included in the range from 25 10 -6 K -1 to 30 10 -6 K -1 (i.e. not included in the range of ordinary types of lime- soda glass), this must be noted so that the appropriate correction table can be selected during verification. This requirement is met by indicating the manufacturer or brand of glass, if the values of the coefficient of thermal expansion are in the corresponding catalogue;
b) if the pipette on the drain is intended to blow the last drop from the drain tip, then the word “blow”, and (or) a white enamel (or etched or sandblasted) strip 3 - 5 mm wide, which is located on distance of 15 - 20 mm from the top of the suction tube.
Note e. In the NTD, the inscription can be made in equivalent terms in other languages.
12. CLARITY OF MARKINGS, NUMBERS AND SIGNS
12.1. The numbers and inscriptions shall be of such size and shape as to be clearly legible under normal conditions of use.
12.2. Marks, numbers and inscriptions must be clear and indelible.
13. COLOR CODING
If color coding is used in the manufacture of pipettes, then such pipettes must comply with the NTD.
APPENDIX A
VOLUME ERROR LIMIT DEPENDING ON VOLUME
Crap. 3
Logarithmic digits on this graph can be applied in tenths or ten times, depending on the number of volumes of the products under consideration and their margins of error in volume.
The bold grid lines of the graph correspond to the error values specified in item , and the volumes specified in item . The graph also shows the error values for products of a different volume, designed for special purposes.
As an example, three curves are considered, which are characterized by the following:
A.1. Curve 1
For this series of sizes, the error limits are directly proportional to the volume, i.e. errors increase with volume. This ratio is intended for a range of product sizes where the volume and diameter are variable and the length is constant over the entire size range, such as graduated pipettes.
Curve slope 1 to the horizontal axis is 45° and for the given example, the margin of error by volume will always be 2% (or 0.2%, or 0.02 % depending on the size of the divisions of the horizontal and vertical axes) on the volume for the entire range of sizes.
Dotted curves 1 A and 1bwith the same slope express a similar proportionality between error and volume, but a proportionality of a different order, corresponding to 1% (or 0.1%, etc.) and 5% (or 0.5%, etc.), respectively.
Points marked with "*" near the curve 1, correspond to less satisfactory error limits that could be obtained if the same error limits were set for sizes 2 and 2.5 (in any part of the graph).
A.2. Curve 2
For a number of sizes, an increase in the margin of error by one digit corresponds to two digits of an increase in volume. A proportion of this order is more suitable for products with one mark, in which all three linear dimensions change proportionally to the increase in volume, for example, for pipettes or flasks with one mark.
Curve slope 2 to the horizontal axis is 26°30". Series of products to which curves with an inclination of less than 45° are applied provide an increase in accuracy with increasing volume. In such cases, many of the plotted points will not be in a straight line. You should choose a curve of such parameters that would best correspond to the points plotted on the graph.After that, it should be checked that for any volume of products the most preferable margin of error was chosen.In the example given, two error values are chosen for volume 5 in both digits, the preferred value is circled in both cases.
A.3. Curve 3
This curve illustrates the relationship between volume and error for a range of products with very small volumes. The upper part of this line is a straight line with an angle of inclination between 26°30" and 45°, which is characterized in the previous paragraph, and the lower part of the line is a curve with a decreasing angle of inclination, which in extreme cases can be equal to 0 at the very end of the curve.
There are two potential reasons for reducing the angle of inclination for products with very small volume:
a) sometimes it is impractical, for practical reasons, to reduce the diameter on the mark line in order to obtain a smaller margin of error, determined in accordance with paragraph . For example, flasks with one mark and a volume of less than 10 cm 3 become inconvenient to use, because. the small diameter of the neck of the flask does not provide quick filling or draining and introduction of the required volume into the pipette neck;
b) for small-sized products calibrated for draining (for example, for pipettes with a volume of less than 0.05 cm 3), the requirements of paragraph on standard deviation may be more stringent than the requirements of paragraph on diameter sizes and error limits (the value should not be less than established values).
The chart shown in Fig. , is illustrative and includes two full logarithmic series on each axis. Values given within these two digits are only logarithmic and do not indicate the order of absolute value.
This graph is included in the relevant NTD and it must be fully digitized so that you can directly read the volume values and the margins of error.
The volumes and limits of error are set in specific NTD for certain types of products. The graph must have dimensions up to 150 mm.
When two classes of accuracy are stipulated in the NTD, it will be enough to include a schedule for class 1 error limits if the accepted ratio of error limits does not differ from the requirements of paragraph.
APPENDIX B
VOLUME ERROR LIMIT RELATED TO MENISCA DIAMETER
The curve on the nomogram was obtained by the formulaL= (0,4 + 0,01 D). Thus, the straight lines corresponding to the volume error limits end at the points of the curve that correspond to the maximum diameters given in the table.
An example of the use of nomograms is given on two selected sections of straight lines.
Along the line A the following values are given:
D from 17 to 20 mm;
V\u003d ± 0.2 cm 3.
In this example, which may refer to a volumetric flask, the upper limitDcomes very close to the limit defined by the curved line.
Along the line IN the following values are given:
D from 3 to 4 mm;
V =±0.02 cm3.
In this example, which may refer to a pipette, either a large diameter or a smaller margin of error is possible. In this case, the margin of error is governed by the standard deviation requirement rather than the size requirement.
Crap. 4
The clause of this standard contains a requirement to include a nomogram of such a sample as an annex to any NTD related to volumetric utensils. It's necessary:
a) for the preparation of NTD;
b) for the regulation of indications for the purpose of subsequent revision of this standard or the preparation of new standards for similar products, facilitating work on their revision, preparation and comparison;
c) to facilitate the work in the preparation of standards, in particular in those cases where additional dimensions are required that are not included in this standard.
The nomogram given in the standard should be drawn up only for those ranges and error limits that are established for a particular product. On the nomogram, a curve of error limits should also be drawn.
APPENDIX C
RELATIONSHIP BETWEEN THE STANDARD DEVIATION OF THE LIMIT OF ERROR
BY THE VOLUME AND THICKNESS OF THE MARK (AND ALSO THE DISTANCE BETWEEN THE MARKS -
FOR PRODUCTS WITH SCALE)
Several requirements are logically related in this International Standard. This is done so that the specified degree of accuracy is achieved when working with products.
The appendix explains the formula for the ratio of the internal diameter of the product to the linear equivalentLand thus to the margin of error in terms of volumev.
The paragraph sets the limit for the thickness of the mark of products without a scale, this limit does not exceed 0.5 linear equivalentLvolume errors.
The paragraph establishes that the linear equivalent should not exceed one division of the scale. For products with two accuracy classes, this requirement defines a volume error for class 1 products of 0.5 scale divisions.
In paragraph, the minimum distance between two marks is set, corresponding to the smallest division of the scale (0.8 + 0.02D) mm, i.e. twice as much asL.
The clause defines a maximum mark thickness of 0.25 of the distance between two marks, and the clause states that the volume error margin must be at least four standard deviations.
An example of a symbol for the relationship between these factors in linear units:
standard deviation - 1;
mark thickness - 2 max ;
L for class 1 - 4 max ;
distance between marks - 8 min .
INFORMATION DATA
1. PREPARED AND INTRODUCED by the Klin independent design and technology bureau for the design of glass instruments and apparatus
2. APPROVED AND INTRODUCED BY Decree of the USSR State Committee for Product Quality Management and Standards dated June 26, 1991 No. 1038
This standard has been prepared by direct application of the international standard ISO 384-78, 1980, “Laboratory glassware. Principles of arrangement and design of measuring utensils "and fully complies with it
3. REFERENCE REGULATIONS AND TECHNICAL DOCUMENTS
4. REPUBLICATION. March 2011
Goal of the work˸calibrate volumetric glassware˸
- option 1- buret;
– option 2– graduated pipette or Mohr pipette;
– option 3- measuring flask.
Essence of work. In titrimetric methods of analysis, the reproducibility and correctness of the final result are to a very large extent determined by the accuracy of preparing standard solutions and the accuracy of measuring the volumes of the titrant and titrated substance. For accurate measurement of volumes, burettes, pipettes and volumetric flasks of two accuracy classes of various capacities and modifications are used, which are produced by the industry in accordance with the requirements of GOST and calibrated at a temperature of 20°C.
The nominal capacity of measuring utensils does not always correspond to its true capacity. This is reflected in the accuracy of titrimetric determinations, therefore, in order to obtain accurate results, it is necessary to calibrate the glassware. In case of discrepancies that are more than permissible, such dishes are rejected or amendments 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 preliminarily kept for at least 1 hour in the laboratory so that they reach room temperature. Water temperature is measured with a thermometer with an error of not more than 0.5 ° C.
Burettes are used to measure precise volumes in titration and other operations. All of them are designed to measure the liquid poured out of them, therefore they are calibrated to pouring out. There are macro- and microburettes. The 50 ml burettes used in macroanalysis are graduated in milliliters and fractions of a milliliter with the value of the smallest division of 0.1 ml, and the 25 ml burettes are graduated either similarly or with a division value of 0.05 ml. Hundredths of a milliliter are counted by eye with an accuracy not greater than half the division value. Microburettes have a capacity of 1, 2, 5, 10 ml with the price of the smallest division 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 to measure and transfer the exact volume of a solution from one vessel to another, they are of two types - graduated and with one label (Mohr's pipettes) with a capacity of 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 calibrate pouring out. The volume of freely flowing liquid with which the pipette is pre-filled is the nominal volume. According to GOST 29169-91, ISO 9002-94, ISO 835-81, ISO 648-77, the limits of the permissible error of the nominal capacity of pipettes should not exceed the values \u200b\u200bspecified in Table. 7.
Goal of the work. Learn how to independently calibrate glass volumetric chemical glassware, taking into account temperature and air pressure.
Theoretical part. Graduation is necessary, because glassware made 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 graduated as follows: 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 that, the calculations do not end, since it is customary to recalculate the volume of the liquid to the volume that the liquid would have occupied at a temperature of 20 0 C. This takes into account the fact that chemical glass expands or contracts when the temperature changes.
Appliances and materials. Chemical glassware of the 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 steps.
A. Volumetric flask calibration
1. Weigh the water poured into the measuring glassware m V.
2. Calculate the volume of the suspended liquid and according to the table. 4 find volume value W for the temperature and atmospheric pressure that were recorded at the time of weighing. The desired 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 of water at various temperatures
| Temperature t, 0 C | Specific gravity of water, r in, g / cm 3 | Volume at atmospheric pressure | ||
| 740 mm. rt. Art. W 740 ml | 760 mm. rt. Art. W 760 ml | 780 mm. rt. Art. W 780 ml | ||
| 0,99913 | 1001,92 | 1001,95 | 1001,98 | |
| 0,99897 | 1002,08 | 1002,11 | 1002,13 | |
| 0,99880 | 1002,24 | 1002,27 | 1002,30 | |
| 0,99862 | 1002,42 | 1002,45 | 1002,48 | |
| 0,99843 | 1002,61 | 1002,64 | 1002,66 | |
| 0,99823 | 1002,80 | 1002,83 | 1002,86 | |
| 0,99802 | 1003,01 | 1003,04 | 1003,07 | |
| 0,99780 | 1003,23 | 1003,26 | 1003,29 |
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 the glass and the specific gravity of water at the calibration temperature. Further, according to the formula, the final volume of volumetric utensils is calculated at 20 0 C:
V in 20 = V V× (1 + D W/1000).
T a b l e 5 . Corrections for glass expansion and specific gravity of water
and total correction depending on temperature.
B. Burette calibration
Fill in the table. 6 and build on these data a dot plot of the dependence of the volume error D V , ml, from the added volume V , ml, from a buret. The volume error can be either positive (Fig. 1) or negative.
D V , ml
V, ml
Fig.1. Burette calibration chart
Table 6. Experimental data on buret calibration
| The volume of water shown on the burette V, ml | Mass of water m c, d | Desired volume of suspended liquid V , ml | Volume error, D V, ml, D V= V -V |
W. Kal pipette alignment
Using a rubber bulb, draw water into the pipette 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 poured water m V. Further actions are carried out in the same way as for volumetric flasks.
Report
Process the results and draw a conclusion using the data in Table. 7, about the possibility of using the measuring utensils you received for work. Ask the instructor about the chemical glassware class if it is not listed.
Table 7 . Tolerances in milliliters
from the capacity of chemical glassware at 20 0 C.
Lab #1
CHEMICAL EXPERIMENT TECHNIQUE
Goal of the work: Familiarize yourself with the main types of chemical glassware. To master the technique of weighing and measuring volumes of liquids.
Theoretical part
Chemical vessels
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 high chemical resistance, it is very weak or does not break down at all under the action of acids, alkalis, solutions and molten salts, as well as other aggressive substances. This property is very important, since chemical glassware should not release its constituents into the substance or solution that are in it, as this will lead to contamination of the substance. Many grades of chemical glass withstand strong heating - up to a red heat temperature. However, abrupt cooling of hot glass almost always leads to its cracking, and this must be kept in mind when conducting experiments. Glass cracking can also occur when glassware or devices are heated unevenly, so the test tube or flask must be evenly heated before heating.
If strong heating is required, quartz glassware is used. Quartz glass withstands stronger heating than ordinary chemical glass, in addition, quartz has a very small coefficient of thermal expansion, so quartz glassware withstands sudden cooling and does not crack. Quartz utensils practically do not release their constituents into the solution, so they are used when working with highly pure substances.
Chemical utensils not intended for heating are also made from ordinary non-heat-resistant glass. It is possible to distinguish non-heat-resistant dishes from heat-resistant ones by the following features: heat-resistant glass has a thickness of about 2 - 3 mm, which, as a rule, is the same in all parts of the product. Non-heat-resistant glass is usually thicker and may have irregular thickenings in various parts of the cookware or appliance.
Porcelain dishes are also used in chemical practice. Porcelain products are more chemically and thermally resistant than glass products. Porcelain has a greater hardness and therefore mortars and pestles are made from it for grinding crystalline substances. However, porcelain products are more expensive than glass products and have one common drawback - they are opaque. Therefore, the list of porcelain products is rather limited. Porcelain is mainly used for making glasses, crucibles, calcining boats, cups and mortars.
Metal utensils are also used for special purposes. Metal beakers and crucibles are mainly used for calcining or carrying out reactions with very aggressive substances, so they are made of chemically inert metals - gold, platinum, silver, nickel, etc.
According to their purpose, chemical utensils are divided into two categories.
1. Glassware for general laboratory purposes is designed for the widest range of applications and is available in almost any laboratory. This includes test tubes, various flasks, beakers, funnels, pipettes, droppers, chemical jars, and reagent storage bottles.
2. Special-purpose utensils include products designed for special purposes: refrigerators, reflux condensers, desiccators, Wulff flasks, gasometers, Kipp apparatus, etc.
A special class is measured utensils. Volumetric utensils are designed to measure volumes of liquids or gases. Volumetric utensils include volumetric flasks, measuring cups, burettes, pipettes, graduated cylinders. Measuring utensils are usually graduated in milliliters. Measurement of volumes of liquids is carried out according to the following rules.
1. The measurement is made at a temperature of 20 0 С.
2. Pipettes and volumetric flasks should not be taken by the expanded parts, since the glass expands from the heat of the hands and the volume of the dishes can change greatly.
3. The surface of the liquid has the shape of a meniscus, so filling the flask, pipette or burette is done in such a way that the liquid touches the division with the lower edge of the meniscus. Measuring utensils are 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.
Measuring utensils require careful and careful handling. Solutions must not be heated in volumetric utensils, since thermal expansion of the glass may cause permanent deformations and the volume of the flask may change. It is also undesirable to store prepared solutions in volumetric dishes for a long time.
The actual capacity of even new measuring utensils may differ significantly from that indicated on the label. Therefore, before use, measuring utensils must be calibrated - to establish its real capacity. Calibration of volumetric glassware is based on the weighing of the volume of distilled water contained in the volumetric glassware.
measuring utensilsOFS
Instead of GFX, p.849
The requirements of this General Pharmacopoeia Monograph apply to volumetric utensils used in pharmacopoeial analysis to measure the volume of liquids. Measuring chemical utensils include volumetric flasks, pycnometers, pipettes, burettes, as well as volumetric cylinders, measuring cups, beakers, test tubes with divisions. Unlike general-purpose chemical glassware, measured glassware has precise graduations.
Types of measuring utensils
Measuring cylinders(Fig. 1 a) - glass (may be plastic) thick-walled vessels with divisions printed on the outer wall indicating the volume in ml (5 - 2000 ml). There are cylinders equipped with ground plugs.
Graduated measuring cups(Fig. 1 b) give the largest error in measuring volume due to rare divisions indicating volume.
Beakers(Fig. 1 c) cone-shaped vessels on the wall of which a scale is applied. The capacity of the beakers is 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, designed 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 ( centrifuge). The scale corresponding to the capacity of the test tube is printed on the entire side surface. Test tubes can be with a thin section, without a thin section, respectively with stoppers and without stoppers.
Volumetric flasks, volumetric pipettes, and burettes are used to accurately measure volumes.
Volumetric flasks(Fig. 2 a) are round flat-bottomed vessels designed to accurately measure the volume (per infusion) when preparing solutions of a known concentration. Distinguish between narrow-necked and wide-mouthed volumetric flasks . The diameter of the throat (neck) of the latter is approximately one and a half times larger than that of the narrow-necked ones.
There is a ring mark on the neck, up to which the flask should be filled.
Rice. 2. Volumetric flask (a), pycnometers (b)
In most cases, volumetric flasks have ground glass stoppers. Often stoppers made of polyethylene or polypropylene are 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 preparation of solutions with precise concentration.
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 liquid density determination.
Pipettes(Fig. 3) are narrow long glass tubes drawn from one end, designed to accurately measure the volumes of solutions.
Rice. 3. Volumetric pipettes: ungraded (a, b): graduated (c, d); pipettes - dispensers (d, e) |
There are the following types of pipettes:
Ungraded with one ring mark - Mohr's pipettes (Fig. 3 a) - calibrated for full drain. Liquid in them dial up to the ring mark And pour to the end;
Ungraded with two ring marks - Mohr pipettes(Fig. 3 b) - liquid in them dial up to the top mark And poured 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 stamp.
The capacity of the pipette - usually between 1 and 100 cm3 - is indicated by the manufacturer at the top or middle of the pipette.
Pipettes with a capacity of less than 1 ml are called micropipettes; they can be used to select volumes measured in tenths and hundredths of a ml. Graduated pipettes, in which only the minimum (or maximum) volume is indicated on the scale, are called full drain pipettes (Fig. 3d), the maximum volume is taken with these pipettes, pouring out the liquid from the top division to the end. More convenient and safer pipettes-dispensers, which guarantee
high accuracy and repeatability of the volume of measured liquids in
range from 2 to 5000 µl.
Unipipettes designed to measure constant volume doses (Fig. 3e).
varipipettes– these are pipettes of adjustable capacity for measuring doses of any volume within the specified limits (Fig. 3 f). The dispensers in these pipettes can be mechanical or electronic. Draw liquid into a pipette using a dispenser or a rubber bulb.
Burettes- a cylindrical glass tube with graduations, a tap or a clamp, graduated in milliliters. Burettes are used for accurate measurement of small volumes and titration when determining the quantitative content of a substance.
Burettes are of two types:
type I - without fixed waiting time of the 1st and 2nd classes;
type II - with a set waiting time of the 1st class only.
Volumetric burettes(Fig. 4, a-d) with a division value of 0.1 ml allow you to count with an accuracy of 0.02 ml. Mohr's stopless 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 a stick with a spherical thickening is placed inside it. Liquid from such a burette flows out when you press your fingers on the top of the ball.
At burettes with automatic zero(Fig. 4, d) the zero mark is the upper section of the process.
Fig.4 Burettes:
(a) - with a one-way valve
(b) - rubber tube
(c) - two-way valve
(d) - automatic zero
(d, f) - devices for reading liquid volumes
Microburettes differ from volumetric burettes in a small volume (2 ml, 5 ml). They have a graduation of 0.01 ml, which makes it possible to make readings with an accuracy of 0.005 ml.
Material
Glass volumetric utensils must be made of glass that has the necessary chemical properties to ensure resistance to aggressive media, light, etc.
For the production of glassware, borosilicate glass is used, which includes 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 by boron oxide, the 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 the internal stress must be relieved to the required limits.
Capacity Measurement Accuracymeasuring utensils
In laboratory tests, domestic measuring utensils of 1 or 2 accuracy classes (in accordance with GOST) or foreign volumetric utensils A or B of the International Standard (ISO) accuracy class are used. Class 1 or Class A is for more accurate items used in quantification; 2nd class or class B - for less accurate measurements.
Limits of measurement error
Error limits mean the maximum allowable error difference between any two points on the scale. Measurement errors of the drained liquid should not exceed the values specified in Table. 1.
Table 1.
Calibration of laboratory volumetric glassware
Volumetric flasks, pycnometers, pipettes and burettes must be checked before use. Before checking, the measuring utensils are thoroughly washed and dried. Dried measuring utensils used for “pouring” (pipettes and burettes) are moistened with purified water before testing: pour it into the utensils to be checked and let stand for 1-2 minutes, after which they are poured out, as in normal use. Checking volumetric utensils consists in determining the mass of purified water, free of impurities and dissolved air, poured into the utensil up to the mark (volumetric 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 lowered into a bottle with a lid and weighed. Without pouring out the water from the bottle, a full pipette is lowered into it again and weighed. They do this for the third time. Of the three values of the mass of water, the average is taken. When checking burettes, the mass of its entire volume is measured, and then the mass of water every 10 ml. For accurate calibration, check the mass of each milliliter. The temperature at which measured glassware is calibrated should be equal to 20 ° C. In practice, when calibrating and checking measured 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
Temperature of water and air in οС | Weight of 1 liter of water, g | ||||
For second-class cookware, the margins of error are doubled.
Working with measuring utensils
The volume of a liquid can be measured with varying degrees of accuracy, which is determined by the task of the analysis. Depending on the relative error allowed when measuring volume, volumetric utensils are divided into two groups - for approximate and accurate measurement of volume. Vessels for approximate measurement of volume include graduated cylinders, graduated beakers, beakers, test tubes with divisions. The relative error in measuring volume using such dishes is 1% or more. This dish is intended mainly for pouring. The term "for pouring" means that if the contents of a filled measuring vessel are poured into another vessel, then the volume of liquid poured out at room temperature will correspond to the capacity indicated on the vessel.
measuring cylinders,graduated measuring cups, beakers,test tubes with divisions. To measure the desired 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 labeled with the temperature at which it has the exact volume indicated on it. The term "infusion" means that if you fill a volumetric flask with liquid exactly to the mark, then the volume of liquid at room temperature will correspond to the capacity indicated on the flask.
The volume of the liquid poured out of the flask will be slightly less than the marked one, since part 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 upper mark is intended for “pouring out”, i.e. 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, water is poured 0.5 - 1 cm below the mark, then, using a pipette, the liquid is poured drop by drop until the edge of the meniscus of the solution touches the mark.
Fig.6. Monitoring the correct setting 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). In this case, the flask is held in front of you for the top necks so that 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. Do not hold the flask by its lower part, as distortion of volume 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, because the density of liquids depends on temperature and, therefore, the determined volume will differ from the volume indicated on the volumetric flask. Alcohol, water-alcohol solutions and solutions of organic solvents are brought to the mark after keeping them for 20 minutes at 20°C.
After bringing the liquid level to the mark, the flask is closed with a stopper, and, holding the latter with the thumb or forefinger 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 falls 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, the solutions are heated in volumetric flasks in a water bath (to the temperature specified in the regulatory document), then, before bringing the solution to the mark, the flasks are cooled and kept at a temperature of 20ο C for 20-30 minutes.
Measuring pipettes. Draw liquid into a pipette using a dispenser or a rubber bulb.
To fill any pipette, the liquid level must 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, the liquid is released drop by drop until the edge of the meniscus of the solution 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. When the liquid is quickly poured out, a significant part of it will remain on the walls of the pipette. The remaining liquid (for pipettes with one mark or for complete draining) is removed by touching the pipette tip to the edge of the inclined vessel for several seconds, then slightly rotate the pipette around the axis. The rest of the liquid from the pipette must not be blown out, since this volume is not taken into account when grading the volumetric utensils. In case of complete pouring to the spout, it is necessary to wait 15 s 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 a solution through a funnel with a short end that does not reach 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 a descending meniscus is placed on this line. Then the solution is lowered so that it fills the burette to the end of the spout.
In burettes with a glass stopcock, liquid is taken by sucking a pear through the top hole with the stopcock open. To remove air bubbles, the tip of the burette with rubber tube is raised at an angle, the clamp is slightly opened and liquid is released until all air has been removed.
The burette is set to zero. only after to ensure that the tip of the burette is filled with the solution. The funnel with which the solution is poured into the burette is removed. Drops remaining on the funnel may increase the volume of liquid in the burette, which may lead to an incorrect analysis result.
During the titration, do not touch the walls of the receiving vessel with the tip of the burette. The drop remaining on the spout after pouring is completed is added to the poured out volume by touching the inside of the receiving vessel. If the buret does not have a waiting time, it is not necessary to wait for the 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 that, the solution in the burette is set to zero division, while 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 volumetric burettes (as well as other volumetric utensils) with easily foaming liquids, the waiting time for the foam to settle should be long - until the last bubble disappears, and bringing to the meniscus is carried out carefully along the walls of the filled vessel. The lower edge of the meniscus is always chosen as the place for reading the level of the solution in the burette (Fig. 4, e). 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 make a reading along the upper edge of the meniscus.
In a burette with automatic zero, the solution supplied from below through the tube rises to the upper cut of the process, its excess will drain from the burette through the tube (Fig. 4). After stopping the supply of the solution, its level will be set automatically on the upper cut of the process. The first mark on the scale of such a burette is 1 ml. The glass stoppers of the burettes should be very lightly lubricated with petroleum jelly or a lanolin-wax alloy. Abundant lubrication of microburettes is especially dangerous, since it can rise up the burette and, contaminating its inner surface, disrupts the normal wetting of the buret walls with a solution.
Solutions of caustic and carbonic alkalis are kept in burettes with clamps, since when these solutions are stored in burettes with glass taps, the taps often “jam”. The upper end of the burette is closed from dust and evaporation of the solution with a small glass or a wide, but short test tube.
Setting the meniscus
Before each titration, be sure to set the liquid level in the buret to zero on the scale. The buret volume reading is carried out along the corresponding edge of the meniscus (Fig. 5), while the observer's eyes should be at the level of the meniscus in order to avoid measurement errors.
The exact definition of the lower edge of the meniscus is difficult due to the phenomenon of reflection, errors are also possible from parallax (relative displacement of the meniscus due to the movement of the observer's eye), if the eyes are not exactly at the height of the meniscus. For volumetric flasks and pipettes, the mark surrounds the throat or the entire tube, which allows you to take an accurate reading. For burettes, the mark occupies only part of the circumference of the tube. Therefore, to correctly read the level of the solution in the burette, different 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 measured glassware is carried out similarly to ordinary laboratory chemical glassware by sequentially performing the following procedures:
P preliminary work; before soaking with a napkin / filter paper, remove grease from burette taps and connections (if any), other grease stains and inscriptions made during operation;
W soaking and washing in a cleaning solution; the shelf life of the solution for soaking dishes is 24 hours, reuse of this solution is not allowed;
- rinsing- carried 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 the water does not leave drops on the inner walls.
To wash measuring utensils, depending on the nature of the pollution, use:
- ultrasonic baths,
- organic solvents (polar and non-polar);
For washing, solvents of the analytical category are used, and for rinsing, solvents of the chemically pure category are used; at the same time, strict safety measures must be observed (work 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. Ammonia solution should not be used for rinsing dishes in which organic solvents are used.
The use of dichromic acid ("chromic"):
Dichromic acid is very aggressive, and therefore, a special set of measures is required for the destruction of waste. As a substitute, it is possible to use commercial acid-containing solutions or mixtures of the acids mentioned above.
Note. When working with dichromic acid, special care should be taken. Waste 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 a special board with pegs is used, on which the washed dishes are put on and left at room temperature until it dries. Clean pipettes after washing and drying are placed in special stands (tripods).
Note. When specified by the manufacturer, it is allowed to dry the volumetric utensils in a dry oven at a temperature recommended by the manufacturer.
In case of emergency, the dishes are dried by rinsing with acetone or ethanol of chemical pure grade. Residues of solvents are collected and handed over in accordance with the rules adopted in the laboratory.
