The method of additives in analytical chemistry is the essence. Analytical Chemistry
IN one standard solution method measure the value of the analytical signal (y st) for a solution with a known concentration of the substance (C st). Then the magnitude of the analytical signal (y x) is measured for a solution with an unknown concentration of the substance (C x).
This calculation method can be used if the dependence of the analytical signal on concentration is described by a linear equation without a free term. The concentration of the substance in the standard solution must be such that the values of the analytical signals obtained when using the standard solution and a solution with an unknown concentration of the substance are as close as possible to each other.
IN method of two standard solutions measure the values of analytical signals for standard solutions with two different concentrations of a substance, one of which (C 1) is less than the expected unknown concentration (C x), and the second (C 2) is greater.
or 
The method of two standard solutions is used if the dependence of the analytical signal on concentration is described by a linear equation that does not pass through the origin.
Example 10.2.To determine the unknown concentration of a substance, two standard solutions were used: the concentration of the substance in the first of them is 0.50 mg/l, and in the second - 1.50 mg/l. The optical densities of these solutions were 0.200 and 0.400, respectively. What is the concentration of the substance in a solution whose optical density is 0.280?
Additive Method
The additive method is usually used in the analysis of complex matrices, when the matrix components affect the magnitude of the analytical signal and it is impossible to accurately copy the matrix composition of the sample. This method can only be used if the calibration graph is linear and passes through the origin.
When using calculation method of additives First, the magnitude of the analytical signal is measured for a sample with an unknown concentration of the substance (y x). Then a certain exact amount of the analyte is added to this sample and the value of the analytical signal (y ext) is measured again.
If it is necessary to take into account dilution of the solution
Example 10.3. The initial solution with an unknown concentration of the substance had an optical density of 0.200. After 5.0 ml of a solution with a concentration of the same substance of 2.0 mg/l was added to 10.0 ml of this solution, the optical density of the solution became equal to 0.400. Determine the concentration of the substance in the original solution.
= 0.50 mg/l
Rice. 10.2. Graphical method of additives
IN graphical method of additives take several portions (aliquots) of the analyzed sample, add no additive to one of them, and add various exact amounts of the component being determined to the rest. For each aliquot, the magnitude of the analytical signal is measured. Then a linear dependence of the magnitude of the received signal on the concentration of the additive is obtained and extrapolated until it intersects with the x-axis (Fig. 10.2). The segment cut off by this straight line on the abscissa axis will be equal to the unknown concentration of the substance being determined.The method is applicable in linear regions of the calibration curve.
2.1. Multiple addition method
Several (at least three) portions of volume Vst are introduced into the test solution, prepared as specified in the private pharmacopoeial monograph. solution with a known concentration of the ion being determined, observing the condition of constant ionic strength in the solution. Measure the potential before and after each addition and calculate the difference ∆E between the measured
potential and potential of the test solution. The resulting value is related to the concentration of the ion being determined by the equation:
where: V – volume of the test solution;
C is the molar concentration of the ion being determined in the test solution;
Build a graph depending on the volume of additive Vst. and extrapolate the resulting straight line until it intersects with the X axis. At the intersection point, the concentration of the test solution of the ion being determined is expressed by the equation:
2.2. Single addition method
To the volume V of the test solution, prepared as described in the private monograph, add the volume Vst. standard solution of known concentration Cst. Prepare a blank solution under the same conditions. Measure the potentials of the test solution and the blank solution before and after adding the standard solution. Calculate the concentration C of the analyte using the following equation and making the necessary corrections for the blank solution:
where: V is the volume of the test or blank solution;
C is the concentration of the ion being determined in the test solution;
Vst. – added volume of standard solution;
Cst. – concentration of the ion being determined in the standard solution;
∆E – potential difference measured before and after the addition;
S is the slope of the electrode function, determined experimentally at a constant temperature by measuring the potential difference of two standard solutions, the concentrations of which differ by a factor of 10 and correspond to the linear region of the calibration curve.
2. PHYSICAL AND PHYSICAL-CHEMICAL METHODS OF ANALYSIS The analytical service of enterprises includes control of technological processes, control of raw materials and finished products. Control of technological processes, as a rule, should be carried out quickly, efficiently, in accordance with the speed of technological processes, but in many cases it is sufficient to carry it out only for individual components. For this purpose, rapid, often continuous methods, preferably fully or partially automated, should be used. Control of raw materials and finished products is often selective, discrete, but requires high accuracy and simultaneous determination of several components (and often several dozen). With a large volume of production, and therefore a large flow of samples, in order to solve the required problems, the analytical service of enterprises must have a modern laboratory for spectral and X-ray analysis, and a sufficient equipment park for carrying out physicochemical methods of analysis. As a result, in the analytical service of metallurgical and mechanical engineering enterprises over the past decades, the role of classical chemical methods of analysis has fundamentally changed: gravimetry and titrimetry, which from the main source of measurement information for all types of control have turned into a means for carrying out precision determinations of large and average amounts of substances, as well as a tool for assessing the accuracy of instrumental determinations and calibration of reference materials (RM). 41 2.1. REFERENCE SAMPLES Standard samples (RM) are specially prepared materials, the composition and properties of which have been reliably established and officially certified by special state metrological institutions. Reference materials (RM) are standards for the chemical composition of materials. They are manufactured and certified in special metrological institutions. Certification of RM is the establishment of the exact content of individual elements or components of RM through analysis using the most reliable methods in several of the largest and most reputable analytical laboratories in the country, certified at the state level. The analysis results obtained there are compared and processed at the head office. Based on the averaged data obtained, a RM passport is compiled, which indicates the certified content of individual elements. In addition to state standard samples, it is possible to produce comparison samples in individual industries, institutions, and laboratories. To assess the correctness of the analysis results when using any method, the RM that is closest in composition to the one being analyzed is selected. 42 2.2. ANALYTICAL SIGNAL. METHODS FOR CALCULATING CONCENTRATIONS Chemical analysis, that is, a set of actions that are aimed at obtaining information about the chemical composition of the analyzed object, regardless of the method of analysis (classical chemical or instrumental methods) includes three main stages: – sampling; – sample preparation for analysis; – chemical analysis to detect a component or determine its quantity. When carrying out the analysis, at the final stage of the analysis, the analytical signal is measured, which is the average of the measurements of any physical quantity S, functionally related to the content of the determined component by the relation S = f (c). The analytical signal, depending on the type of analysis, can be the mass of the sediment in gravimetry, the optical density in absorption spectroscopy, the emission intensity of the spectrum line, the degree of blackening or brightness of the analytical line in emission spectroscopy, the strength of the diffuse current in amperometry, the value of the emf of the system, and etc. When a component is detected, the appearance of an analytical signal is recorded, for example, the appearance of a color, a precipitate in a solution, a line in the spectrum, etc. When determining the amount of a component, the value of the analytical signal is measured, for example, the mass of the sediment, the intensity of the spectrum line, the value of the current strength, etc. are measured. The form of the function S = f (c) is established by calculation or experiment and can be presented in in the form of a formula, table or graph, while the content of the component being determined can be expressed in mass units, in moles or in terms of concentration. 43 Since each analytical determination represents a whole system of complex processes, when measuring the analytical signal, which is a function of the content of the component being determined, the analytical background signal is simultaneously measured, functionally related to the content of accompanying interfering components, as well as “noise” ” arising in measuring equipment. The useful analytical signal, which is actually a function of the content of the analyzed component, is the difference between the measured analytical signal and the analytical background signal. It is theoretically impossible to take into account the influence on the result of the analysis of each of the numerous factors acting simultaneously. To experimentally take into account these influences and isolate a useful analytical signal, certain techniques are used, in particular, standards are used. Standard samples (CO) or, more often, laboratory standards similar to industrial standard samples from current products or in the form of artificial chemical mixtures are used as standards. Their composition in all components exactly corresponds to the composition of the analyzed sample. The measurement technique, regardless of the instrumental method of analysis used, is based on one of three possible methods: – comparison method (method of standards); – method of calibration (calibration) graph; – additive method. Approaches to calculating concentrations based on measuring the values of the physical signal of the standard Set and the analyzed sample San also do not depend on the specific analysis method used. Let us consider in more detail each of these calculation methods. The comparison method is most often used for single determinations. To do this, measure the value of the analytical signal in the comparison sample (in the reference sample) Set with a known concentration of the determined 44 component set, and then measure the value of the analytical signal in the test sample Sx. The measured parameter S is related to the concentration directly proportional to the dependence Set = k · set and Sx = k · сx. Since the proportionality coefficient k is a constant value, then Set / set = Sx / cx and the concentration of the determined component in the analyzed sample cx can be calculated using the formula cx = (set ·Sx) / Set The calibration curve method is used for serial determinations. In this case, a series of 5–8 standards (solutions or solid samples) with different contents of the component being determined is prepared. For the entire series, under the same conditions, the values of the values of the analytical signal are measured, after which a calibration graph is constructed in coordinates S – c, with the values of the independent variables (c) being plotted along the abscissa axis, and their functions (S) along the ordinate axis. The unknown concentration cx is determined graphically from the value of the measured signal Sx. If the resulting dependence S - с is nonlinear, then the graph is constructed in semi-logarithmic or logarithmic coordinates: logS – с, S – logс or logS – logс. The plotting is usually done using the least squares method (OLS). The slope of the line determines the sensitivity of the method. The greater the angle of inclination of the curve to the abscissa axis, the smaller the error of determination. The calibration graph can also be represented as a linear equation S = a + b c. The additive method is used to determine small contents of components at the limit of the instrumental sensitivity of the method, as well as in the case of a difficult to reproduce complex background for the component being determined. In the additive calculation method, the analytical signal of the analyzed sample Sx with an unknown concentration of the analyte component cx is first measured. Then a standard additive with a known content of set is introduced into the same sample and the value of the analytical signal Sx+et is measured again. The unknown concentration cx is found by calculation: Sx = k cx, Sx+et = k (cx + set), from where cx = set · Sx / (Sx+et - Sx) The formula is valid only if, as a result of the introduction of the additive, the total the volume of the solution practically does not change, that is, solutions with a high concentration of the component being determined are used as additives. In addition to the calculation method, the graphical method of additions is also used. Titration methods are based on a series of measurements of analytical signals during titration (see section 1.4.), if a change in concentration is accompanied by a change in any physical property (potential, current, absorption, optical density) . This change is depicted graphically: the values of the volume of added titrant are plotted on the abscissa axis, and the values associated with the concentration (or its logarithm) by a functional dependence are plotted on the ordinate axis. The resulting dependence is called a titration curve. On this curve, a point is determined that corresponds to the equivalent ratio of a certain substance and titrant, that is, the equivalence point or equivalent volume of titrant. The curve can be logarithmic (potentiometric titration) or linear (photometry, amperometric titration). The concentration is calculated in the same way as in a conventional titration (see section 1.4). 46 2.3. OPTICAL METHODS OF ANALYSIS Applied spectroscopy methods (spectral methods) are based on the study of the interaction of electromagnetic radiation with atoms or molecules (ions) of the substance under study. As a result of the interaction, an analytical signal appears containing information about the properties of the substance under study. The frequency (wavelength) of the signal depends on the specific properties of the analyzed compound, that is, it is the basis for qualitative analysis, and the signal intensity is proportional to the amount of the substance and is the basis for quantitative determinations. For analytical purposes, the spectrum region from 106 to 1020 Hz is used. This region includes radio waves, microwaves, infrared (thermal), visible, ultraviolet and x-ray radiation. The optical region includes infrared (IR), visible (V) and ultraviolet (UV) radiation. Analysis methods based on the interaction of electromagnetic radiation in this region with atoms and molecules of matter are called optical spectral methods. Spectrum (from Latin spectrum - representation) is a set of different values that a given physical quantity can take. Optical spectral analysis includes absorption methods using the absorption spectra of molecules (ions) and atoms in the B-, UV- and IR regions, and emission methods using the emission spectra of atoms and ions in the UV- and B-. areas. Using absorption and emission analysis methods in the UV and B regions, problems of establishing the elemental composition of a sample are solved. Absorption methods based on the study of the spectra of molecules or ions are called molecular absorption, and those based on the study of the spectra of atoms are called atomic absorption. 47 2.3.1. Molecular absorption spectroscopy (photoelectrocolorimetry) Quantitative absorption analysis is carried out in the visible, ultraviolet and infrared regions of the spectrum. Quantitative absorption analysis in these spectral regions is based on the use of the Bouguer-Lambert-Beer law. If the intensity of the incident monochromatic radiation passing through the light-absorbing solution is denoted by I0, the intensity of the output radiation by I, then – log (I / I0) = A = ε l s, where A is absorption (the old designation is optical density D) ; c - molar concentration; l is the thickness of the absorbing layer, cm; ε is the molar absorption coefficient, which is equal to the optical density of the solution at the solution concentration c = 1 mol/l and the thickness of the absorbing layer l = 1 cm. Absorbance (optical density) is measured using instruments called photoelectrocolorimeters. Therefore, the method is called photoelectrocolorimetry or simply photometry. Photometric methods have been developed to practically determine all elements in the analysis of a wide variety of objects. Almost always, the measurement of light absorption is preceded by the conversion of the component being determined into a new chemical form characterized by strong absorption, that is, having a high value of the molar absorption coefficient. Most often these are colored complex compounds with inorganic or organic ligands. Since there is a linear relationship between the absorption value (optical density) and concentration, by measuring the optical density value, it is possible to calculate the concentration of the analyzed solution. To do this, you can use the comparison method, the calibration graph method, or the addition method. 48 The methodology for performing elemental analysis in molecular absorption spectroscopy includes: – taking an average sample; – taking a sample of a sample substance or measuring the volume of a solution for a liquid sample; – dissolution of the sample (in water, in mineral acids or their mixtures, in alkali) or decomposition of the sample by fusion with subsequent transfer into solution; – separation of interfering components or their masking; – carrying out an analytical reaction; – measurement of the analytical signal; – calculation of the content of the component being determined. Problem No. 3 considers the use of the calibration graph method, which is usually used for multiple serial determinations. To obtain a series of standard solutions with increasing concentrations, the method of diluting the initial primary standard solution prepared from pure metals, salts, oxides, and standard samples is used. Then the prepared solutions are photometered (their optical density is measured) and, based on the photometric results, a calibration graph is constructed in the coordinates optical density - volume of the standard solution, since recalculation of the volume to concentration inevitably necessitates rounding the data when constructing the graph, and therefore, it reduces the accuracy of the determination. Using the finished graph, the content of the element in the analyzed solution is determined after measuring its optical density. Both standard solutions for constructing a calibration curve and the test solution must be prepared using the same method in volumetric flasks of the same capacity and have approximately the same composition for all components, differing only in the content of the component being determined. 49 The constructed calibration graph can be used for repeated determination of the element content in samples of the same type. Example. Photoelectrocolorimetric determination of the silicon content in steel was carried out on the basis of the formation of a blue silicon-molybdenum complex using the calibration graph method. A sample of steel weighing 0.2530 g was dissolved in acid and, after appropriate treatment, 100 ml of the test solution was obtained. An aliquot (equal part) of this solution with a volume of 10 ml was placed in a volumetric flask with a capacity of 100 ml, all the necessary reagents were added and 100 ml of a colored solution of blue silicon-molybdenum complex was obtained. The optical density (absorption) of this solution is Ax = 0.192. To plot the graph, a standard (reference) solution was prepared with a silicon content of 7.2 μg/ml (T(Si) = 7.2 μg/ml). The volumes V of the standard solution taken to plot the graph are equal to 1.0; 2.0; 3.0; 4.0; 5.0; 6.0 ml. The measured values of the optical densities Aet of these solutions correspond to the following values: 0.060; 0.105; 0.150; 0.195; 0.244; 0.290. Determine the content (mass fraction) of silicon in the steel sample under study. Solution The solution to the problem includes the following steps: 1. Construction of a calibration graph. 2. Determination from the calibration graph of the silicon content corresponding to the measured value of the optical density of the solution under study. 3. Calculation of the content (mass fraction) of silicon in the analyzed steel sample, taking into account the dilution of the analyzed solution. 50
It is necessary to determine the amount of dry matter and the required amount of working solution of the ShchSPK additive to prepare 1 ton of cement-sand mixture.
For the calculation, the following mixture composition (% mass) was adopted:
sand - 90, cement - 10, water - 10 (over 100%), ShchSPK (% of the mass of cement based on dry matter). Sand moisture content is 3%.
For the adopted composition, the preparation of 1 t (1000 kg) of the mixture requires 1000·0.1 = 100 kg (l) of water. The filler (sand) contains 1000·0.9·0.03 = 27 liters of water.
The required amount of water (taking into account its content in the filler) is: 100 - 27 = 73 l.
The amount of anhydrous additive ShchSPK for preparing 1 ton of the mixture with a content of 10% (100 kg) of cement in 1 ton of the mixture will be: 100·0.020 = 2 kg.
Due to the fact that the ShchSPK additive is supplied in the form of a solution of 20 - 45% concentration, it is necessary to determine the dry matter content in it. We take it equal to 30%. Therefore, 1 kg of solution of 30% concentration contains 0.3 kg of anhydrous additive and 0.7 l of water.
We determine the required amount of ShchSPK solution of 30% concentration to prepare 1 ton of the mixture:
The amount of water contained in 6.6 kg of concentrated additive solution is: 6.6 - 2 = 4.6 liters.
Thus, to prepare 1 ton of the mixture, 6.6 kg of additive solution of 30% concentration and 68.4 liters of water for dilution are required.
Depending on the need and capacity of the mixer, a working solution of the required volume is prepared, which is defined as the product of the consumption of the additive solution and water (per 1 ton of mixture), the productivity of this mixer and the operating time (in hours). For example, with a mixing plant capacity of 100 t/h for one shift (8 hours), it is necessary to prepare the following working solution: 0.0066 100 8 = 5.28 (t) of a 30% solution of ShchSPK and 0.684 100 8 = 54.72 (t) water for dilution.
A solution of 30% concentration of ShchSPK is poured into water and mixed well. The prepared working solution can be fed into the mixer using a water dispenser.
Appendix 27
FIELD METHODS FOR QUALITY CONTROL OF SOILS AND SOILS TREATED WITH CEMENT
Determination of the degree of soil crushing
The degree of crushing of clay soils is determined according to GOST 12536-79 on average samples weighing 2 - 3 kg selected and sifted through a sieve with holes of 10 and 5 mm. Soil moisture should not exceed 0.4 soil moisture at the yield limit W t. At higher humidity, the average soil sample is first crushed and dried in air.
The remaining soil on the sieves is weighed and the content of the sample in the mass is determined (%). The content of lumps of the appropriate size P is calculated using the formula
where q 1 - sample mass, g;
q is the mass of the residue in the sieve, g.
Determination of moisture content of soils and mixtures of soils with binders
The moisture content of soils and mixtures of soils with binders is determined by drying an average sample (to constant weight):
in a thermostat at a temperature of 105 - 110 °C;
using alcohol;
radioisotope devices VPGR-1, UR-70, RVPP-1 in accordance with the requirements of GOST 24181-80;
carbide moisture meter VP-2;
moisture meter of the N.P. system Kovalev (the density of wet soils and the density of the soil skeleton are also determined).
Determination of humidity by drying an average sample with alcohol
A sample of 30 - 50 g of sandy fine-grained soils or 100 - 200 g of coarse-grained soils is poured into a porcelain cup (for the latter, the determination is made on particles finer than 10 mm); the sample together with the cup is weighed, moistened with alcohol and set on fire; then the sample cup is cooled and weighed. This operation is repeated (approximately 2 - 3 times) until the difference between subsequent weighings exceeds 0.1 g. The amount of alcohol added the first time is 50%, the second - 40%, the third - 30% of the sample weight soil.
Soil moisture W is determined by the formula
where q 1, q 2 are the mass of wet and dried soils, respectively, g.
The total moisture content for all particles of coarse soils is determined by the formula
W = W 1 (1 - a) + W 2, (2)
where W 1 is the moisture content of the soil containing particles smaller than 10 mm, %;
W 2 - approximate moisture content of soil containing particles larger than 10 mm, % (see table of this appendix).
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Approximate humidity W 2,%, when coarse soil contains particles larger than 10 mm, fractions of one |
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Erupted |
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Sedimentary |
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Mixed |
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Determination of humidity with a carbide moisture meter VP-2
A sample of soil or a mixture of sandy and clayey soils weighing 30 g or coarse soils weighing 70 g is placed inside the device (the moisture content of coarse soil is determined on particles smaller than 10 mm); Ground calcium carbide is poured into the device. After tightly wrapping the lid of the device, shake it vigorously to mix the reagent with the material. After this, you need to check the tightness of the device, for which you bring a burning match to all its connections and make sure that there are no flashes. The mixture is mixed with calcium carbide by shaking the device for 2 minutes. The pressure reading on the pressure gauge is carried out 5 minutes after the start of mixing if its readings are less than 0.3 MPa and after 10 minutes if the pressure gauge readings are more than 0.3 MPa. The measurement is considered complete if the pressure gauge readings are stable. The moisture content of fine-grained soils and the total moisture content for all fractions of coarse-grained soils are determined using formulas (1) and (2).
Determination of natural humidity, density of wet soil and density of the soil skeleton using the N.P. device. Kovaleva
The device (see figure in this appendix) consists of two main parts: a float 7 with a tube 6 and a vessel 9. Four scales are printed on the tube, indicating the density of soils. One scale (Vl) is used to determine the density of wet soils (from 1.20 to 2.20 g/cm 3), the rest - the density of the skeleton of chernozem (Ch), sandy (P) and clayey (G) soils (from 1.00 up to 2.20 g/cm 3).
Device N.P. Kovaleva:
1 - device cover; 2 - device locks; 3 - bucket-case; 4 - device for sampling with a cutting ring; 5 - knife; 6 - tube with scales; 7 - float; 8 - vessel locks; 9 - vessel; 10 - calibration weight (plates);
11 - rubber hose; 12 - bottom cover; 13 - float locks; 14 - cutting ring (cylinder) with bottom cover
The auxiliary accessories of the device include: a cutting steel cylinder (cutting ring) with a volume of 200 cm 3, a nozzle for pressing the cutting ring, a knife for cutting the sample taken by the ring, a bucket-case with a lid and locks.
Checking the device. An empty cutting ring 4 is installed in the lower part of the float 7. A vessel 9 is attached to the float using three locks and immersed in water poured into a bucket-case 3.
A correctly balanced device is immersed in water until the beginning of the “Vl” scale, i.e. readings P (Yo) = 1.20 u/cm3. If the water level deviates in one direction or another, the device must be adjusted with a calibration weight (metal plates) located in the bottom cover 12 of the float.
Sample preparation. A soil sample is taken with a soil carrier - a cutting ring. To do this, level the platform at the test site and, using a nozzle, immerse the cutting ring until the ring with a volume of 200 cm 3 is completely filled. As the cutting cylinder (ring) is immersed, the soil is removed with a knife. After filling the ring with soil with an excess of 3 - 4 mm, it is removed, the lower and upper surfaces are cleaned and cleared of adhering soil.
Work progress. The work is carried out in three steps: determine the density of wet soil on the “Vl” scale; establish the density of the soil skeleton according to one of three scales “H”, “P”, “G” depending on the type of soil; calculate natural humidity.
Determination of the density of wet soil on the "Vl" scale
The cutting ring with soil is installed on the lower cover of the float, securing it with the float with locks. The float is immersed in a bucket-case filled with water. On the scale at the water level in the case, a reading is taken corresponding to the density of wet soil P (Yck). The data is entered into a table.
Determination of the density of the soil skeleton using the “H”, “P” or “G” scales
The soil sample from the soil carrier (cutting ring) is transferred completely into the vessel and filled with water to 3/4 of the vessel’s capacity. The soil is thoroughly ground in water with a wooden knife handle until a homogeneous suspension is obtained. The vessel is connected to a float (without a soil carrier) and immersed in a bucket-case with water. Water through the gap between the float and the vessel will fill the rest of the space of the vessel, and the entire float with the vessel will be immersed in water to a certain level. A reading taken from one of the scales (depending on the type of soil) is taken as the density of the soil skeleton Pck (Yck) and entered into the table.
Calculation of natural humidity
Natural (natural) humidity is calculated from test results using the formulas:
where P (Yo) is the density of wet soil on the “Vl” scale, g/cm 3 ;
Pck (Yck) - density of the soil skeleton according to one of the scales ("H", "P" or "G"), g/cm 3 .
Determination of strength in an accelerated way
To quickly determine the compressive strength of samples from mixtures containing particles smaller than 5 mm, samples weighing about 2 kg are taken from every 250 m 3 of the mixture. Samples are placed in a vessel with a tight-fitting lid to maintain moisture and delivered to the laboratory no later than 1.5 hours later.
Three samples measuring 5 x 5 cm are prepared from the mixture using a standard compaction device or by pressing and inserted into hermetically sealed metal molds. Forms with samples are placed in a thermostat and kept for 5 hours at a temperature of 105 - 110 ° C, after which they are removed from the thermostat and kept for 1 hour at room temperature. The aged samples are removed from the molds and the compressive strength is determined (without water saturation) according to the method of App. 14.
The result of the determination is multiplied by a factor of 0.8, and a strength is obtained corresponding to the strength of the samples after 7 days of hardening in wet conditions and tested in a water-saturated state.
The quality of the mixture is determined by comparing the compressive strength values of samples determined by the accelerated method and 7-day-old laboratory samples from the reference mixture. In this case, the strength of the reference samples must be at least 60% of the standard strength. Deviations in the strength indicators of production and laboratory samples should not exceed when preparing mixtures:
in quarry mixing plants +/- 8%;
single-pass soil mixing machine +/- 15%;
road mill +/- 25%.
For mixtures of soils containing particles larger than 5 mm, the compressive strength is determined on water-saturated samples after 7 days of hardening in wet conditions and compared with the compressive strength of reference samples. The quality of the mixture is assessed similarly to mixtures made from soils containing particles smaller than 5 mm.
Appendix 28
SAFETY INSTRUCTION CHECKLIST
1. Site (working place)
2. Last name, initials
3. What kind of work is it aimed at?
4. Last name, initials of the foreman (mechanic)
Introductory briefing
Introductory safety training in relation to the profession
Conducted ___________
Signature of the person conducting the safety briefing
____________ " " _________ 19__
On-the-job training
Safety briefing at the workplace ___________________
(Name of workplace)
workers comrade ___________________ received and assimilated.
Worker's signature
Signature of the master (mechanic)
Permission
Comrade _____________________ allowed to work independently
___________________________________________________________________________
(Name of workplace)
as ________________________________________________________________
" " ___________ 19__
Head of the section (foreman) _________________________________
Interest in the additive method in ionometry is due to the fact that it plays a more significant role than the additive method in other analytical methods. The ionometric addition method offers two great advantages. Firstly, if the fluctuation in ionic strength in the analyzed samples is unpredictable, then the use of the common calibration curve method gives large determination errors. The use of the additive method radically changes the situation and helps to minimize the determination error. Secondly, there is a category of electrodes whose use is problematic due to potential drift. With moderate potential drift, the addition method significantly reduces the determination error.
The following modifications of the additive method are known to the general public: standard additive method, double standard additive method, Gran method. All these methods can be sorted into two categories according to an explicit mathematical criterion that determines the accuracy of the results obtained. It lies in the fact that some additive methods necessarily use a previously measured value of the slope of the electrode function in the calculations, while others do not. According to this division, the standard addition method and the Gran method fall into one category, and the double standard addition method into another.
1. Standard addition method and Gran method.
Before outlining the individual characteristics of one or another type of additive method, we will describe the analysis procedure in a few words. The procedure consists of adding a solution containing the same analyzed ion to the analyzed sample. For example, to determine the content of sodium ions, additions of a standard sodium solution are made. After each addition, electrode readings are recorded. Depending on how the measurement results are further processed, the method will be called the standard addition method or the Gran method.
The calculation for the standard addition method is as follows:
Cx = D C (10DE/S - 1)-1,
where Cx is the desired concentration;
DC is the amount of additive;
DE is the potential response to the introduction of the DC additive;
S is the slope of the electrode function.
The calculation by Gran's method looks somewhat more complicated. It consists of plotting a graph in coordinates (W+V) 10 E/S from V,
where V is the volume of added additives;
E - potential values corresponding to the introduced additives V;
W is the initial sample volume.
The graph is a straight line intersecting the x-axis. The intersection point corresponds to the volume of added additive (DV), which is equivalent to the desired ion concentration (see Fig. 1). From the law of equivalents it follows that Cx = Cst DV / W, where Cst is the concentration of ions in the solution that is used to introduce additives. There can be several additives, which naturally improves the accuracy of determination compared to the standard additive method.
It is easy to notice that in both cases the slope of the electrode function S appears. From this it follows that the first stage of the additive method is the calibration of the electrodes for the subsequent determination of the slope value. The absolute value of the potential is not involved in the calculations, since to obtain reliable results, only the constancy of the slope of the calibration function from sample to sample is important.
As an addition, you can use not only a solution containing a potential-determining ion, but also a solution of a substance that binds the detected sample ion into a non-dissociating compound. The analysis procedure does not fundamentally change. However, there are some specific features that should be taken into account in this case. The features are that the experimental results graph consists of three parts, as shown in Fig. 2. The first part (A) is obtained under conditions where the concentration of the binding substance is less than the concentration of the potential-determining substance. The next part of graph (B) is obtained with approximately equivalent ratios of the above substances. And finally, the third part of the graph (C) corresponds to conditions under which the amount of binding substance is greater than the potential-determining one. Linear extrapolation of part A of the graph to the x-axis gives the value DV. Region B is not usually used for analytical determinations.
If the titration curve is centrally symmetric, then region C can be used to obtain analytical results. However, in this case, the ordinate should be calculated as follows: (W+V)10 -E/S.
Since the Gran method has greater advantages than the standard additive method, further discussions will primarily concern the Gran method.
The advantages of using the method can be expressed in the following points.
1. Reducing the determination error by 2-3 times due to an increase in the number of measurements in one sample.
2. The additive method does not require careful stabilization of the ionic strength in the analyzed sample, since its fluctuations are reflected in the absolute value of the potential to a greater extent than in the slope of the electrode function. In this regard, the determination error is reduced compared to the calibration graph method.
3. The use of a number of electrodes is problematic, since the presence of an insufficiently stable potential requires frequent calibration procedures. Since in most cases the potential drift has little effect on the slope of the calibration function, obtaining results using the standard addition method and the Gran method significantly increases the accuracy and simplifies the analysis procedure.
4. The standard addition method allows you to control the correctness of each analytical determination. Control is carried out during processing of experimental data. Since several experimental points take part in the mathematical processing, drawing a straight line through them each time confirms that the mathematical form and slope of the calibration function have not changed. Otherwise, the linear appearance of the graph is not guaranteed. Thus, the ability to control the correctness of the analysis in each determination increases the reliability of the results.
As already noted, the standard addition method allows determinations to be 2-3 times more accurate than the calibration curve method. But to obtain such accuracy of definition, one rule should be used. Excessively large or small additions will reduce the accuracy of the determination. The optimal amount of additive should be such that it causes a potential response of 10-20 mV for a singly charged ion. This rule optimizes the random error of the analysis, however, in those conditions in which the additive method is often used, the systematic error associated with changes in the characteristics of ion-selective electrodes becomes significant. The systematic error in this case is completely determined by the error from changing the slope of the electrode function. If the slope changes during the experiment, then under certain conditions the relative error of determination will be approximately equal to the relative error from the change in slope.
