Libra are the most important device in a chemical laboratory, since not a single analysis is complete without determining the mass of the substance and the chemical vessel in which the substance to be weighed is placed.
For convenience of classification, general laboratory or technical scales, and laboratory
Rice. 17.1. Pharmacy scales Rice. 17.2. Laboratory technical scales
analytical balances. Technical scales are intended for technical analyzes and weighing of samples that do not require high precision. Among technical scales, a distinction is made between pharmacy scales VA-4 (Fig. 17.1) and technical scales proper (Fig. 17.2), sometimes called techno-chemical scales. The maximum load of pharmacy scales does not exceed 100 g. They are hung on a tripod or on the finger of the left hand. For unloaded and balanced scales, the pointer should be in a strictly vertical position.
For more accurate weighing, laboratory technical scales are used (see Fig. 17.2). They are more advanced and their maximum load can range from 200 g to several kilograms. They make it possible to weigh with an accuracy of 10 mg.
Laboratory analytical Compared to technical scales, scales have increased sensitivity and accuracy. The most commonly used analytical balances in laboratories are ADV-200 and VLR-200 (Fig. 17.3). A distinctive feature of these scales is that they have special devices (dampers), with the help of which the vibrations of the rocker arm and pointer quickly stop. These scales have a device for hanging small weights (weighing from 10 to 990 mg) on the beam and removing them from it, as well as a light screen for reading the position of the scale needle.
Rice. 17.3. Analytical balances: A - ADV-200; b - VLR-200
The maximum (or maximum) load of these scales is 200 g. The ADV-200 scales allow you to weigh with an accuracy of 0.0001-0.0002 g; VLR-200 scales - accurate to 0.00005 g.
Correct scales always show the same results after several weighings of the same object, or with the same masses (weights) on two cups, the rocker retains its original position.
The sensitivity of the scale is considered sufficient when the needle deflects by 3-5 divisions with an overload of 1 mg. For weighing on damper analytical balances, a special analytical weight is used, which is stored in a special box with slots for each weight (Fig. 17.4). Analytical weights have weights of 50, 20, 10, 10, 10, 5, 2, 1, 1, 1 g or 50, 20, 20,
Rice. 17.4. Analytical weight
Rice. 17.5. Automatic analytical balances
10, 5, 2, 2, 1 g. There are no milligram weights. As mentioned above, they are replaced by special rings that are suspended inside the scales. The box also contains tweezers, with which you take the weights.
For microanalysis, analytical balances are used ( microbalance.) for weighing substances from 0.01 to 0.001 mg (from 0.00001 to 0.000001 g), their maximum load is 20 g. They are equipped with a device for monitoring the deflection of the needle. When weighing, use a special weight designed for such scales.
Currently, chemical laboratories use single-cup automatic damped scales with a digital display (Fig. 17.5), operating on the compensation principle. During the weighing process, the mass of the load is compensated by weight rings until the balance beam reaches equilibrium. The process of establishing equilibrium is accelerated by mechanical or electromagnetic damping.
QUESTIONS
1. What are the objectives and methods of quantitative analysis?
2. What are chemical, physicochemical and physical methods of analysis?
3. How are samples taken for analysis?
4. What types of samples are distinguished in quantitative analysis?
5. What sampling rules exist: a) for liquids; b) for solid and granular substances; c) for plant materials?
6. What types of balances are used in a chemical laboratory?
7. With what accuracy can you weigh on: a) technical scales; b) analytical balances?
8. What mass of weights are in the analytical weight?
9. What modern balances are currently used in the analytical laboratory?
CHAPTER 18. GRAVIMETRIC (GRAVE) ANALYSIS
Laboratory scales vary in purpose, design, weighing range and other characteristics.
Weighting methods are divided into two fundamentally different groups - the method of comparison with a measure and the method of direct assessment. According to the method of comparison with a measure, the mass of the load is taken equal to the mass of the weights compared with it (simple weighing) or is calculated as the sum of the mass values of the weights and the readings of the scales (precise weighing). The direct assessment method consists in determining the mass of the load using the reading device of the scales without the use of weights.
Most modern laboratory scales use a differential weighing method, in which most of the measured body mass (over 99%) is balanced by weights or a counterweight (zero method), and the remaining small difference between the mass of the body being weighed and the mass of the weights is measured by the angle of deflection of the rocker arm from the initial position equilibrium (direct method) using reference scales.
Laboratory scales are characterized by a number of parameters. The main ones are the following.
1. Maximum permissible load, within the range of which the error of indications is within the established limits. Do not exceed the maximum permissible load for which it is designed this model scales Too much load can cause permanent deformation in the rocker arm, which will damage the balance.
2. The permissible error of readings is the maximum difference between the actual value of the mass of the load being weighed and the readings of the scales. The error value characterizes the correctness of the weighing results under standard conditions and cannot be less than the possible errors of the weights used during weighing and certification of scales.
3. Permissible variation (inconstancy) of readings - the maximum permissible difference in readings of scales when repeatedly weighing the same load under standard conditions using the same weights. The variation value characterizes the reproducibility of the weighing result and, to a large extent, the weighing accuracy.
4. Sensitivity is the limiting ratio of the increment in the deviation of the scale pointer to the increment in the measured value. Sensitivity is determined by the number of scale divisions by which the scale arrow deflects when a load weighing 1 mg is placed on one of the scales. Express sensitivity in scale divisions per milligram or its reciprocal value.
As the load on the cups increases, the sensitivity of the scales decreases, i.e., the greater the mass of the object being weighed, the weaker the scales react to changes in mass.
5. Division price - the division value of the reading devices. Often the division price is consistent with the permissible error value or the variation in the scale readings.
6. Performance - the possible performance of work on the scales, i.e. the possible number of weighings per unit of time.
Classification of scales
According to their purpose, laboratory scales are divided into technical (general laboratory), analytical and special, and weights - into general use and special weights.
The largest weighing limits of technical scales are in the range of 20 g - 50 kg. The most common scales have a load of 0.2-5 kg, with a division price of 0.05-0.1 g.
Analytical balances are used for macro- and microchemical analyzes when weighing the highest and highest accuracy. Depending on the maximum permissible load and division price, analytical balances are divided into the following groups:
Special scales are used to determine quantities depending on mass (weight moisture meters, scales for measuring magnetic susceptibility, etc.).
Analytical group scales belong to accuracy classes 1 and 2, technical scales - to classes 3 and 4. The average reduced weighing error for class 1 scales is 0.0001%; 2 classes - 0.0005%; 3 classes - 0.001%; 4 classes - 0.01%.
General laboratory weights are divided into four classes. Weights of classes 1 and 2 are intended mainly for analytical balances, classes 3 and 4 - for technical ones.
Based on the nature of movement of the moving system, scales are divided into leverless and lever scales. In non-lever scales, the moving system moves back and forth vertically, so weights cannot be used to balance the load being weighed. When using leverless scales, only the method of directly assessing the weighing results is suitable.
Lever scales are characterized by rotation of the moving system around a fixed or conditionally fixed axis. They can be weighted (with overhead or built-in weights) and weightless. Scales with built-in weights are more productive and convenient, but they make it difficult to control the actual mass values of the weights.
Lever scales vary in the type of weigh arm supports and hangers. The most common rigid support is a cushion on which a prism rolls with its sharp edge. Scales with such supports are called prismatic. Prismatic scales are divided into equal-arm, double-prism (single-cup) and quadrant.
Equal-arm scales are basically a lever of the first kind, in which the distances from the application of forces to the fulcrum are equal (Fig. 71). If you place a load with mass M1 on the left pan of the scale, then to return the arrow P to its original position, you will need to place a certain amount of weights (with a known mass) on the right pan. When equilibrium is established, the moments of force acting on the left and right parts of the rocker at the points on which the cups rest, at a distance l1 and l2 from these points to the support point, will be equal: F1l1 = F2l2.
Since l1 = l2, then, therefore, when equilibrium is achieved, F1 = F2. The emergence of forces F1 and F2 is associated with the attraction of bodies on the scales by the Earth. Force F1 determines the attraction of a body with mass M1 to the Earth, i.e. its weight. The unit of weight is newton (N). A Newton is equal to the force that imparts an acceleration of 1 m/s2 to a body weighing 1 kg in the direction of the force. The weight of a body G is related to its mass by the following relations: G = Mg, where M is the mass of the body, and g is the acceleration of gravity. The unit of mass is kilogram (kg).
From the above it follows that scales are devices for determining mass, not weight.
Equal-arm rocker scales are shown in Fig. 72. The equilibrium position of unloaded scales is called the zero point, loaded ones - the equilibrium point.
To protect the ribs of the rocker prisms from damage and rapid wear, all moving parts of the scale can be raised and the ribs of the prisms separated from the plates with which they come into contact. The device used to lift the rocker and earrings is called a locker (isolator). When scales are not in use and when objects and weights to be weighed are placed on cups, the scales must be locked.
Until recently, V-shaped depressions of the rater scale were applied to the rocker arm of equal-arm analytical balances at equal distances from each other (Fig. 73), into which a rater weight of 10 or 5 mg was installed using a special device. By moving the rater along the rocker arm, it was possible to determine the mass with an accuracy of tenths of a milligram.
Modern prismatic scales have vibration dampers for the scale pointer - dampers. In damper scales, the zero point and the equilibrium point are taken to be the scale division opposite which the pointer stops. For scales that do not have dampers, these points are determined by the swing method. This method is based on measuring 3-5 consecutive needle deflections. The first 2-3 fluctuations after turning on the scales are not taken into account, and the next 5 deviations of the arrow in one direction or the other are recorded with an accuracy of tenths on the scale. The zero point is calculated, for example, as follows.
Deviations to the left: -3.4 and -2.8; the average is -3.1.
Deviation to the right: +4.0, +3.5 and 3.0, average +3.5.
Let's find the sum of deviations: +3.5 + (-3.1) = 0.4.
Let's find the zero point: +0.4: 2 = +0.2.
The accuracy of damper scales is of the same order as the accuracy of conventional scales.
Double-prism (single-cup) scales are shown in Fig. 74. In the initial position, all built-in weights are loaded on the suspension and the lever is balanced by a counterweight. Having placed a load on the load-receiving cup using a special weighting mechanism, such a number of built-in weights are removed from the rack so that their total mass approximately corresponds to the weight of the load. The difference between the mass of the load and the mass of the removed weights is determined by the readings of the reading device. Double prism single pan balances are used primarily as analytical balances. The advantages of this design of scales are that work is always carried out with a constant load on the rocker arm, and in this case both the sensitivity of the scales and the weighing accuracy are constant.
Quadrant scales or scales with an upper position of the load-receiving bowl (Fig. 75) are a type of double-prism scale.
Lever scales with supports on elastically deformable elements are produced for weighing small masses. These include torsion bar scales and spring lever ultra microbalances.
General laboratory equal-arm scales
General laboratory equal-arm scales - technical scales mainly of 3rd and 4th accuracy classes - are used for weighing relatively large masses. They can be enclosed in a glass display case and equipped with a weight mechanism with built-in weights, or they can be hung on a stand mounted on a stand without a weight mechanism. The simplest type of equal-arm scale with two cups is a manual or pharmacy scale.
Technochemical scales of the VLT-200g (T-200) and VLT-1kg (T-1000) types are shown in Fig. 76. When weighing, by turning the lock handle, the scales are brought into the working position. The permissible error for VLT-200g scales is ±60 mg, for VLT-1kg ±200 mg.
More advanced technochemical scales of the VLR-1kg type consist of an equal-arm rocker with a pointer, a column with a support pad, an insulating device and two load-receiving cups suspended on the end prisms of the rocker. The scales are equipped with an oil vibration damper of the rocker arm and a device for mechanical weighting of built-in weights (from 10 to 990 mg).
Before weighing, make sure the scale is level and installed correctly. If necessary, the scales can be installed strictly horizontally using screw feet. Then you need to check the deflection of the arrow and ensure that it is completely aligned with the control stroke of the scale dial.
In recent years, equal-arm technical two-cup scales of the VLT type have been significantly modernized and serial production of a number of new models of VLR type scales has been carried out, class 2 accuracy (with an error of ±10 mg), with a load capacity of 1, 10, 20 and 50 kg, with a scale division of 10 mg.
The VLR scales are placed in a glass case with doors on two sides. At the upper end of the column there is a cushion on which the middle prism of the rocker rests with its edge. An oil damper is installed at the base of the column. At the ends of the rocker, prisms are fixed in special saddles, onto which earrings with load-receiving cups are hung. Ring weights (from 100 to 900 mg and from 10 to 90 mg) connected to the major and minor limbus are hung and removed from the bar attached to the right earring using a weight mechanism.
There is an arrow in the middle of the rocker, and at the bottom of the column there is a scale on which the balance of the scales is checked. An insulating device (lock) is mounted under the base of the scales. The lock must be opened and closed carefully, by smoothly rotating the handwheel at the moment when the scale arrow passes past the zero mark of the scale.
General laboratory quadrant scales
In recent years, quadrant scales have become widespread due to their speed of operation. These are two-prism scales with an upper cup position. Magnetic vibration damper. There is an optical device and a screen on which the weighing results are read. The application and removal of overhead weights is carried out using a handle located on the metal body of the scale. The scales are connected to the network alternating current through a built-in transformer mounted under the scale display case.
Quadrant scales are intended for determining the mass of various substances and materials during laboratory technical analyzes and preparative work.
The operating principle of the scales is based on balancing the moment of forces created by the measured mass, quadrant deflection and built-in weights.
Currently, six modifications of laboratory quadrant scales of class 4 VLKT and VLK are produced with weighing limits from 160 to 10,000 g.
VLKT scales (Fig. 77) have a tare compensation mechanism, which allows you to increase weighing performance and is designed to set the scale to zero after placing the tare on the scale pan.
The value of the measured body weight on the scale is found by summing the readings on the optical scale and on the counter. The number of hundreds or thousands of grams is counted by a counter, in the window of which the numbers 0, 1, 2, 3 and 4 appear, depending on the mass of the weights removed from the suspension.
Weighing on technical scales
The scales are installed on strong, stable tables in the laboratory working area strictly vertically and plumb. Before weighing, check whether the scales are installed correctly, then lower the rocker arm with a lock and observe the oscillations of the needle on the lower scale. If the arrow deviates from zero by the same number of divisions to the right and left, the scale can be used. Otherwise, the balance of the scales is achieved using the balancing nuts of the rocker arm.
The mass to be weighed is placed on the left platform of the scale, the weights of the gram set are placed on the right, and the weights of the milligram set are hung by the weight mechanism.
It is better to determine the mass of a substance using the double weighing method, which consists of the following: place the object being weighed on the left pan of the scale, and weights on the right pan until the scale needle reaches the zero mark of the scale. After this, the object to be weighed is transferred to the right cup, and the weights to the left. If one of the cups outweighs the other, then by adding or removing weights, the equilibrium point is again established. The actual mass of the object being weighed is equal to the arithmetic average of the results of these two weighings. At the end of weighing, the object being weighed is removed from the scales, the weights and weights are removed, placing them in the prescribed order in the case.
Analytical balances
To an even greater extent than technical and technochemical balances, the balances of the analytical group have undergone modernization in recent years. At the same time, many chemical laboratories still successfully use equal-arm rater scales without dampers - periodic swing scales not equipped with built-in weights. The peculiarity of working on periodic swing scales is to determine their zero point. The rocker arm, freed from the arrester, begins to perform gradually damped oscillations. The zero point and the equilibrium point are determined by the method of repeated deflections of the rocker arrow. Before determining the zero point, the rater must be removed from the rocker arm if the zero is in the center of the rocker arm, or set to zero if the zero is at the left end of the rocker arm.
To determine the sensitivity of the balance, set the equilibrium point under various loads. To do this, after establishing the zero point, place the rater on the rocker arm (with locked scales) so that it shows 1 mg, lower the arrester and determine the equilibrium point.
For example, if the zero point of the scale is +0.2 divisions, and the equilibrium point with a load on the right cup of 1 mg is +3.8 divisions, then the sensitivity of the scale is found by placing 5, 10, 20, 30, 40 on both cups in succession , 50 and 100 g. The results obtained are plotted on a graph.
When using rater damper scales, mass determination using built-in or overhead weights is carried out only up to 10 or 5 mg (i.e., up to the weight of the rater). Further balancing is carried out using a rater, which is set only to the nearest whole milligram to equilibrium. If weighing accuracy greater than 0.1 mg is not required, tenths of a milligram are found by moving the rater along the rocker. If more accurate weighing is necessary, the rater is installed as in the previous case, and tenths and hundredths of a milligram are found by the difference between the zero point and the found equilibrium point based on the previously determined sensitivity of the scales for a given load.
Let, for example, the zero point of the scales be +0.5; the balance point of the scales with a load of 14.3300 g on the right pan and the rater on the 3 mg mark is +2.0; The sensitivity of the scale with a load of 14.5 g is 4 divisions per 1 mg. Obviously, the object being weighed is not completely balanced. If the rater is moved to the 4 mg division, then the equilibrium point will move 4 divisions to the left, i.e. it will be equal to -2.0. In order for the equilibrium point to coincide with the zero point (+0.5), the rater must be moved by (2.0 - 0.5)/4.0 = 0.38 divisions, i.e. by 0.38 mg. Therefore, the mass of the object being weighed will be equal to 14.3300 g (on the scale) + 0.00038 g (reading by the rater) - 14.33038 g.
Many laboratories use two-cup equal-arm laboratory analytical balances VLA-200 g-M (AD-200) with the following basic characteristics: maximum permissible load 200 g; optical scale measurement range ±10 mg; the error due to the unequal arms of the rocker is no more than 2 mg. The weights are controlled using dials. When the small dial rotates, tens of milligrams are added or removed, and when the large dial rotates, hundreds of milligrams are added. The dials rotate independently of each other. The scales are turned on and off using a handle placed on the locking roller placed on the front wall of the base.
Currently, the industry produces mainly two-cup equal-arm scales of the VLR type, for example, scales of the accuracy class VLR-200g and VLR-20g. The VLR-20g scales, replacing the VLM-20g-M semi-microanalytical balances, are distinguished by their high sensitivity and smaller overall dimensions. On the basis of the VLR-200g scales with an electronic attachment, the VLE-200g electronic scales are produced.
Technical data of the VLR-200g scales (Fig. 78) and the VLR-20g scales are given below:
When using scales of the VLR-200g type, first of all, turn on the illuminator in the network, after which, without opening the doors of the scale cabinet, carefully turn the locking disk until it stops. Automatically lights up at the same time electric lamp illuminates on the screen of the weightograph an enlarged image of a microscale attached to the pointer of the scale. If the scales are not loaded, the zero of the scale must exactly coincide with the vertical line on the screen (mark). Otherwise, coincidence is achieved by rotating the adjusting screw located outside on the lower board of the scales above the locking disk. Then the load is placed on the left pan of the scales, and on the right - gram weights from the set of weights for the scales; in this case, find the mass of the number of whole grams. Close the cabinet door; turning the small dial with tenths of a gram, combine the fixed pointer with the various numbers on the disk. Each time you turn the dial, you must first lock the scales. Having established the number of tenths of a gram, find hundredths of a gram using a large dial. Next, the locking disc is turned until it stops and, after the arrow of the rocker arm stops oscillating, the position of the vertical line is measured on the scale on the screen. Large divisions of this scale, corresponding to milligrams, are indicated by numbers with a “+” or “-” sign. The plus shows that the value of the reading made must be added to the mass of the weights placed on the scales, and the minus must be subtracted.
After the weighing is completed, the result is recorded, the weighed object and weights are removed from the scales. To release the rocker from the built-in weights, rotate the disc handles until the fixed pointer aligns with the zero division of both discs.
In addition to equal-arm analytical balances of the VLR type, the industry produces class 2 single-arm balances of the VLDP-100g type (Fig. 79). The principle of weighing on double-prism scales is based on balancing the moment created by the load and the moment obtained when removing the built-in weights from the suspension. The balance beam is an unequal lever; a saddle with a load-receiving prism is attached to the short arm, and a reading scale to the long arm. An earring rests like a cushion on the load-receiving prism of the rocker, to which a bar is rigidly attached for applying built-in weights. A weight mechanism is used to remove and apply built-in weights. Simultaneously with the removal of the weights, the value of their mass (in g) is displayed in the three left windows of the screen. For precise weighing, the rocker arm is calmed using an air damper; with preliminary – oil. The handle for putting the scales into working position is located on the left side of the scales. Preliminary weighing is carried out by turning the handle away from the operator, and precise weighing is carried out by the operator. The zero position of the scale during preliminary weighing is adjusted with the handle located with right side scales, above; for precise weighing - with the handle at the bottom. The pre-weighing mechanism is designed to determine the mass of the built-in weights. To take a reading on the scale, there is a reference mark on the screen in the form of two parallel lines.
The weighing result is determined by the sum of the readings of the reading scale, the counters of the weight mechanism and the dividing device. Weighing range from 0 to 100 mg. The price of the smallest scale division is 0.05 mg. Weighing error ±0.065 mg.
Installation of analytical balances
Installation of analytical balances begins with the selection of premises and organization of the chemist's workplace. The room for installing scales of classes 1 and 2 should consist of a weighing room and a preparation room. One of the conditions imposed on the weighing room is its complete isolation from adjacent laboratory premises.
For the weight room, choose a bright, dry room. It is desirable that it be located on the ground floor, with windows facing north. The weight room should be maintained at a constant temperature of about 20°C. Scales must be protected from exposure to heat and air currents, as well as from dampness, dust, harmful gases and shocks. To reduce the influence of air and heat flows, it is recommended to close windows and doors with thick curtains. Windows should be double glazed and sealed tightly; Windows and vents cannot be opened. It is recommended to ventilate the weighing room with a fan, and only when weighing is not in progress. It is recommended to cover the floor with linoleum, which is easy to clean from dust and is a poor heat conductor.
Scales should be installed in a horizontal position on particularly strong pedestals that protect the scales from any shocks. It is not recommended to move the scale from place to place.
Analytical balances with a maximum load of 100 g or more are recommended to be installed on a console table, consisting of a concrete slab, freely lying on shock-absorbing rubber or foam pads in the table frame, resting on two metal brackets attached to the main wall.
It is advisable to install semi-microanalytical balances on a table with massive legs. The table consists of a massive lid, into the frame of which there is felt, a reinforced concrete mosaic slab and linoleum.
The lamps in the weighing room should sufficiently illuminate the scale scale and, at the same time, not heat up the rocker arms. It is best to install fluorescent lamps.
A table with basic rules for handling scales should be posted in the weighing room.
The cleanliness of the weighing room must be carefully monitored. At the end of weighing, it is recommended to cover the scales with covers.
Do not place anything on the console table or shelf on the brackets where the scales are installed. To the left of the table (shelf) it is advisable to have a mobile table for a desiccator with a weighed substance and for making records.
Rules for using analytical balances
1. The load on the scales should not exceed the maximum for of this type scales They weigh only while sitting against the scales, resting their hands on the table top. The object to be weighed is taken with tweezers, tongs or clean paper and placed in the middle of the left cup. Chemical substances are weighed in a glass container (bunch, ampoule). Do not place chemicals directly on a scale or weigh on a piece of paper.
2. The item being weighed must be at the same temperature as the scale. Therefore, before weighing, the substance should be kept in a desiccator near the scales for 20-30 minutes. If, when weighing, a lamp is turned on over the scales, then this must be done 10-15 minutes before starting work.
3. The substance to be weighed should only be added or subtracted outside the scale cabinet. If the substance to be weighed is spilled on the scale pan or on the bottom of the cabinet, you must immediately sweep it up with a brush.
4. The weights should be placed on the right pan of the scale so that they are in the center of the pan. The weights should be picked up with tweezers with bone (plastic) tips.
5. When the substance to be weighed or the weights are placed on or removed from the scale, the scale must be locked.
6. Before each weighing, their zero point should be checked and, if necessary, set. While observing the deflection of the scale needle, the cabinet doors must be closed.
7. When balancing an object being weighed, start with large weights and then move on to smaller ones.
You should always use the smallest number of weights, for example, take a weight of 2 g, and not two weights of 1 g. On the scale, the weights should lie in a certain order; Small weights should not be stacked on top of each other. Large weights should be placed in the center of the cup so that it does not wobble.
Weighing errors and their elimination
Errors in accurate weighing can occur for various reasons: from the imbalance of the scales; from being suspended in air, and not in emptiness; from changes in body weight during the weighing process due to fluctuations in temperature, humidity and air pressure; from inaccurate mass values of weights; from instrumental errors.
Errors from balance imbalance mostly occur with the method of simple weighing on periodic swing scales. However, corrections for unevenness are not always required. Thus, when determining the percentage composition of a substance (in % (wt.)), when weighing the analyte and its weight form is carried out on the same scales and when the substances being weighed are placed on the same cup, the relative error for both weighings will be approximately the same. But when it is necessary to determine the absolute mass of an object with an accuracy exceeding 0.1 mg, one has to resort to weighing methods that exclude corrections for unequal shoulder, for example, the substitution method.
The Borda substitution method is as follows. The mass to be measured is placed on the right pan of the scale and balanced with any tare mass on the left pan. The equilibrium position E1 is determined. Then the mass to be measured is removed from the right cup without removing the container from the left one, and instead of the removed mass, weights are placed in such an amount that it is possible to read on the scale, and the equilibrium position E2 is determined. The measurement result is equal to the mass of the applied weights plus the reading on the scale and is determined by the formula (E1 - E2)S, where S is the sensitivity of the scales.
The substitution method proposed by D.I. Mendeleev consists in placing weights on one of the cups in an amount corresponding to the maximum load of the scales, and balancing the scales with a tare weight. The body to be weighed is placed on a cup with weights and such a number of weights are removed so that the scales return to the initial equilibrium position. The value of the mass of the body being weighed is determined as the algebraic sum of the mass of the weights removed from the cup and the readings on the scale. This method is the basis for the operating principle of double-prism, single-arm scales.
Errors caused by weighing in air follow from the well-known physical law that every body immersed in a liquid (gas) loses as much weight as the liquid (gas) it displaces weighs. All bodies, therefore, weigh less in air than in empty space. Ordinary weighing in the air would lead to the correct result if the weights lost as much in their mass as the body being weighed loses. However, analytical weights are usually made of stainless steel (p = 8.0 g/cm3) or brass (p = 8.4 g/cm3), and milligram weights are made of aluminum (p = 2.7 g/cm3). If the density of the body being weighed is less than the density of the weights, then the body displaces more air than the weights and, therefore, it weighs less in air than in empty space. The error value usually does not exceed 0.04-0.05%.
Errors caused by changes in the mass of bodies during the weighing process can occur due to the absorption or loss of moisture, evaporation of volatile substances, temperature changes, inattention and carelessness of the experimenter. These errors can be eliminated by weighing the substances by difference in a hermetically sealed small glass container. When weighing by difference, the position of the zero point can be ignored.
Errors in the mass of weights depend on the degree of accuracy in adjusting their mass to the nominal value, the certification error and on irreversible changes in mass during the testing period, mainly due to corrosion. Errors associated with the inaccuracy of the masses of the weights used can be eliminated by comparing them with the mass of standard weights on the scales on which they will be used.
Micro and ultra micro balances
For particularly accurate measurements of small masses when conducting physicochemical studies and microanalyses, precise lever and leverless scales of various designs are used.
Lever spring scales are produced with a maximum load from 20 to 100 mg, with a division price of 10 minus 7 – 10 minus 5 mg (VLU-20 mg and VLU-100 mg). The principle of operation of these scales is based on balancing the moment created by the mass being measured by twisting a quartz stretcher. By design, these are scales on braces with an equal-arm beam and a zero weighing method. The rocker arm is placed in a special container that protects it from the action of air currents and at the same time serves as a heat distributor. The cup with the load to be weighed is carried out from the rocker column into the side compartment of the display case by a manipulator, which is interlocked with the mechanism for opening and closing the columns. The measurement results are counted on the scale of the measuring dial, the division value of which is 0.00032 mg (VLU-20mg) and 0.0005 mg (VLU-100mg). The time to calm down the vibrations of the rocker arm is about 1.5 minutes.
Microanalytical balances VLM-1g are intended for weighing precious stones and metals, as well as for various substances during microchemical analyses. increased accuracy. The scales have equal-arm rockers with two pendants and cups. Full mechanical weight bearing is carried out by two kettlebell mechanisms. The scales are equipped with a mechanism for removing the left cup. Measurement range on optical scale ±1 mg. The optical scale division value is 0.01 mg. Weighing error ±0.07 mg.
To quickly determine the mass of very small quantities of substances, torsion (spring) balances are often used (Fig. 80). They differ from quadrant ones in that the load-receiving cup in them is enclosed in a display case and equipped with a locking device. Torsion bar scales are available with different weighing limits. In laboratory practice, the VT-500 model is often used. The maximum permissible load of the scale is 500 mg, and the smallest is 10 mg. The absolute error of readings at any scale mark is no more than ± 1 mg.
The measuring element in a torsion balance is a spring, the tension of which, when twisted, balances the sample being weighed. The angle of twist of the spring is proportional to the mass of the sample being weighed, therefore the scale scale is graduated in mass units.
When using torsion bar scales VT-500, they are installed at level 1 using support screws 2, after which the rocker arm 3 is released by moving the securing lever 4 to the right. The mass indicator 5 is set to zero using the tension lever 6. With this position of the scales, the balance indicator 7 overlaps the balance line or it is brought to this position by the calibration head located on the back of the scale in the center. Then secure the rocker by moving the securing lever to the left as far as it will go, and begin weighing. To do this, open the safety cover 8, hang the load to be weighed on the hook of the rocker arm 9 and close the cover again. The rocker arm is released by moving lever 4 to the right. By turning lever 6 to the left, move pointer 5 until pointer 7 is positioned exactly on the balance line. In this position, pointer 5 shows on the scale the mass of the load being measured. After weighing, secure the rocker by moving lever 4 to the left, open cover 8, remove the weight from the hook and close the cover. Lever 6 is moved to the right, pointer 5 is set to zero - and the scales are ready for the next weighing.
12.09.2011
In analytical balances (analytical balance), in the international classification, the ratio of the greatest load to the division value (resolution) exceeds 100,000, while maximum value weighing on such equipment cannot exceed 50 kg.
In the domestic classification, there is no concept of “analytical balances” as such. There is equipment of analytical accuracy class. This class includes laboratory, jewelry, and technical scales. Although, the classification we provide is also quite arbitrary and is not officially recognized.
Laboratory scales can be of two accuracy classes - special (division value from 0.1 mcg) and the highest (division value 0.001-0.05 mg). It should be noted that the laboratories also have weighing equipment of an average accuracy class, to which we, within the framework of this classification, classify the “technical” class of this equipment.
Jewelry scales have a special and high accuracy class. They are designed for weighing precious metals and stones.
Technical weighing equipment belongs to the middle accuracy class. The division price in them is 10 mg and above. They are used in production shops, warehouses, and where a very high class of accuracy is not needed.
Analytical group scales can be either mechanical or electronic. Recently, both regulatory requirements and time requirements oblige users to “switch” to electronic measuring instruments. So, for example, regulatory requirements for pawnshops require the mandatory use of a certain type of control and measuring instruments of -I, -II accuracy classes, preferably electronic ones.
Analytical balances have undergone significant changes in recent years. Their assortment has changed completely several times already.
The results of tests or other work carried out with their help directly depend on the technical condition of the scales. Their role is so important that a special room is set up for their storage and use - a weighing room.
Analytical balances are a type that are used when performing physical and chemical analyzes in which the results obtained in the process of measuring the mass of an object must be obtained with particularly high accuracy. That is why the discreteness of such scales cannot exceed 0.1 mg.
According to the accuracy class of the measurements performed, analytical balances belong to the 2nd and 1st classes.
Based on their inherent design features, these scales are divided into two main groups:
- (single-armed and equal-armed); have a contact pair as a sensitive element, consisting of a pad and a prism;
- scales equipped with an elastically deformable measuring device, which is a torsion thread.
The scales of the first group give an accuracy in determining mass no higher than 1 * 10 -6 g. The second are much more accurate. There the limit value is 5*10-8 g.
The group of analytical balances, in addition to the actual analytical ones of the 1st and 2nd accuracy classes, includes microanalytical, semi-microanalytical, torsion and assay scales.
Another type of classification of analytical balances for laboratories according to their design is the division into periodic swing balances and the so-called aperiodic or damper balances.
The main disadvantage of scales with periodic swinging of the pointer is the extremely slow attenuation of the oscillations made by the rocker arm.This leads to increased time for weighing. The use of aperiodic swing made it possible to significantly reduce the time required to obtain a weighing result. The vibrations of the rocker arm in them are damped using a special device - a damper, which performs magnetic or air braking.
Scales with aperiodic swing, in turn, are divided into:
- semi-automatic scales;
- ordinary;
- with lower or upper placement of dampers, which are cylinders attached to the scale pan or its bow; these cylinders move freely inside cylinders of larger diameter.
The third classification option is to classify scales as mechanical (electromechanical), or.
With this division in scales belonging to the first group, the essence of the weighing process is explained by comparing the mass of the object being weighed with a system of springs and weights built into the structure, or external weights, which is performed using an indicator that determines the equilibrium position.
Analytical electronic scales have a different operating principle. It is based on electromagnetic balancing of the object being weighed, and the subsequent implementation of a measurement generated as a result of this action of an electrical signal, which is converted into digital form and displayed on the appropriate displays or indicators.
Analytical balances, which belong to the mechanical group, have an attractive price, but have a number of disadvantages. Among them, it should be noted: relatively rapid mechanical wear of the materials from which the weights and levers are made; poor resistance to corrosion; high complexity of weighing, excessive sensitivity of such scales to external influences (drafts, temperature changes, shocks, etc.).
Electronic scales, on the contrary, are quite simple for weighing. And their requirements for the workplace and external influencing factors are significantly lower. Latest models get a lot additional functions, not typical for mechanical ones. For example: determination of the density of solids and liquids (hydrostatic weighing); dynamic weighing; provided possibility of connecting to a PC or other peripheral device, choice of unit of measurement; automatic zero setting; weighting as a percentage, etc.
Microanalytical and ultra microanalytical balances, which are also scales of the specified analytical group, are used to perform weighing, which requires extremely high accuracy. They allow you to guarantee accuracy expressed to the seventh decimal place
Purpose of analytical balances and scope of application
Analytical balances of the 2nd class are used for precise weighing and determining the mass of material during chemical analyzes using macromethods. The scope of application of such scales is industrial laboratories, laboratories of educational institutions and research institutes.
Electronic scales are used when performing laboratory tests that are particularly complex: assay analysis; weighing cardiac implants; working with toxic, rapidly decomposing or explosive substances; weighing special filters designed to control dust levels in the air;
For calibration of pipettes, syringes, weights and pipettes. Scales of these types are used as control equipment in QMS (quality management) systems.
Design and operating principle
Let's consider the typical design of damper electronic scales, which have recently made up most of the range of similar scales offered on the Russian market.
The most important design element of analytical balances is the rocker arm, on which three agate prisms are mounted. The middle one acts as a support for the rocker at the time of weighing. The remaining two are located at the ends of the rocker, and are supports for the cups of our scales. Connected to the rocker arm is a long pointer (positioned vertically) and having a microscale at its upper end.
A special optical device included in the design of the scales, called a weightograph, allows the laboratory assistant to observe on the screen of this device the movements made by an enlarged image of the measuring scale near the counting line, which is made motionless.
Typically the scale is graduated in increments of 1 mg (0.001 g). In turn, this distance is also divided into 5 or 10 (depending on the brand of scales) divisions.
Dampers are attached to the scale cup or its handle (see above), which speed up weighing.
All analytical balances are equipped with arresters to improve weighing accuracy. This is a special device, with the help of which the beam of the scale is raised and installed in a position in which none of the 3 prisms of the support pads touches.
The arrester is operated by a special handle, which is attached to the bottom of the supporting board of the scale. It should be turned smoothly and very carefully.
Mandatory requirement. At the moment of placing a weighed object or a control load on the scale pan. And also at a time when you plan not to use these scales for a long time, they must be pre-locked.
To protect the scale mechanism from drafts, temperature fluctuations and dust, it is placed in a special glass case.
If you do not have the opportunity to use weighing equipment, then there is another way out - use.
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Rules for working with analytical scales
Laboratory assistants study the rules for working with scales in a course on the fundamentals of theoretical chemistry and refresh their knowledge by studying the installation and operating instructions included in the documentation set supplied with each scale. Everything is spelled out here, what requirements a table for analytical balances must meet, how analytical balances are calibrated, etc.
Today, no pharmaceutical, jewelry, or nanotechnology-based production can operate effectively without analytical balances. And the results of the work carried out directly depend on the quality of the installed scales and their functionality.
According to the accepted international classification, the group of analytical balances includes measuring instruments whose resolution (in other words, the relative value of the maximum load to the minimum division limit) corresponds to a value not less than 10 thousand. Moreover, the weighing limit is not higher than the fifty-kilogram mark.
And according to the classification adopted in the CIS, there is no nomenclature of measuring instruments “ analytical balances" There is equipment with an “analytical” accuracy class. But unofficially, equipment falling into this classification is usually called analytical balances.
Scope of application of analytical balances
The analytical group includes measuring equipment that corresponds to a special (1) accuracy class, determined by a measuring scale with a division value corresponding to 0.0001 grams. The main area of application of analytical class balances is various kinds of laboratory analysis (for example, chromatography, titration), starting with a mandatory weighing procedure. Also, analytical balances today are an absolutely indispensable tool used in medicine in the process of diagnosing all kinds of diseases (especially of an infectious nature) and pharmacology for the precise dosage of various ingredients medicines. The use of analytical class measuring instruments of various kinds by regulatory organizations studying the quality of complex food products has become widespread.
Types of analytical balances
Modern models analytical electronic balances classified according to the following criteria:
- NPV (highest weighing limit). For most popular modifications of analytical balances, this parameter ranges from 60-320 grams;
- platform size. The diameter of the working surface of analytical class measuring instruments is in the range of 75-91 millimeters;
- discreteness of measurements. There is a special category of analytical measuring equipment capable of taking measurements with a measurement limit of 0.00001 g. The majority of analytical balances allow measurements to be made with an accuracy of up to 0.0001 g.
Factors influencing the accuracy of measurements using scales of analytical accuracy class
Few buy analytical balances proper accuracy class, it is necessary to take into account many external factors to carry out correct measurements. The accuracy of data obtained as a result of mass measurements using instruments of analytical accuracy class is influenced by the following factors:
- correct placement of the device (horizontal). The slightest deviation from the ideal horizontal position will certainly lead to significant measurement errors;
- airspace density. This parameter is the combined value of the influence of atmospheric pressure, humidity and temperature. May lead to some increase in measurement error. The most significant factor of the above is the external temperature. With its change by 5 degrees, the accuracy of the readings can change by 0.1 grams;
- latitude at which measurements are taken. Accuracy class of analytical balances, on which measurements are taken in Murmansk and Sochi, may be the same, but the readings may differ significantly. Due to the decrease in the value of the gravitational acceleration with a decrease in the latitude, the scale readings for the same measurements in in this case may vary by 1 gram;
- height of the measurements. Even moving analytical balances between floors entails changes in measurement results, not to mention taking measurements in areas with a significant difference in altitude above sea level;
- air currents. Various climate equipment, open windows, doors, moving employees can cause air currents with an intensity that can distort the results of mass measurements. To eliminate the influence of this factor, wind screens should be used;
- Also, the accuracy of measurements can be influenced by factors such as the electrostatic charge of the object being weighed or its magnetization, the presence of nearby equipment capable of producing a strong electromagnetic field.
To make accurate measurements with analytical balances, they must be calibrated regularly.
Calibration of analytical balances
- with external calibration. To implement it, a calibrated load is required;
- with internal calibration. When checking the measurement accuracy of the device, a separate calibration weight is not required, since the equipment of this group is initially equipped with its own standard - a calibration weight.