Method for comparing the optical densities of standard and test stains
solutions
To determine the concentration of a substance, take part of the test solution, prepare a colored solution from it for photometry, and measure its optical density. Then two or three standard colored solutions of the analyte of known concentration are prepared in the same way and their optical densities are measured at the same layer thickness (in the same cuvettes).
The optical densities of the compared solutions will be equal to:
for the test solution
for standard solution
Dividing one expression by the other, we get:
Because 1 X = l ST, E l= const, then
The comparison method is used for single determinations.
Graduated graph method
To determine the content of a substance using the calibration graph method, prepare a series of 5-8 standard solutions of different concentrations (at least 3 parallel solutions for each point).
When choosing the concentration range of standard solutions, the following principles are used:
It must cover the area possible changes concentrations of the test solution, it is desirable that the optical density of the test solution corresponds approximately to the middle of the calibration curve;
It is desirable that in this concentration range at the selected cuvette thickness I and analytical wavelength l the basic law of light absorption was observed, i.e. the schedule D= /(C) was linear;
Operating range D, corresponding to the range of standard solutions, should ensure maximum reproducibility of measurement results.
Under the combination of the above conditions, the optical densities of standard solutions relative to the solvent are measured and a graph of the dependence D = /(C) is plotted.
The resulting curve is called a calibration curve (calibration graph).
Having determined the optical density of the solution D x, find its values on the ordinate axis, and then on the abscissa axis - the corresponding concentration value C x. This method is used when performing serial photometric analyses.
Additive Method
The additive method is a variation of the comparison method. Determining the concentration of a solution by this method is based on comparing the optical density of the test solution and the same solution with the addition of a known amount of the substance being determined. The additive method is usually used to simplify work, to eliminate the interfering influence of foreign impurities, and in some cases to assess the correctness of the photometric determination method. The additive method requires mandatory compliance with the basic law of light absorption.
The unknown concentration is found by calculation or graphical methods.
Subject to the basic law of light absorption and a constant layer thickness, the ratio of the optical planes of the test solution and the test solution with the additive will be equal to the ratio of their concentrations:
Where Dx- optical density of the test solution;
D x + a- optical density of the test solution with the additive;
C x- unknown concentration of the test substance in the test colored solution;
S a- concentration of the additive in the test solution.
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 a 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.
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 based on 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 expedited 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. 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)
Induction training
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) _________________________________
Determine the analytical signal of the sample ( y x) and the signal of the same sample with the addition of some additive of the determined component of known content ( y x + ext), then the unknown concentration of the component being determined is:
where V add, V sample are the volumes of additive and sample, respectively.
Another goal of analytical chemistry is to lower the detection limit. This is due to the continuously growing requirements for the purity of materials used in the space and military industries.
Under detection limit understand the minimum concentration of a substance that can be determined by the chosen method with a certain acceptable error. Quite often, analytical chemists use the term « sensitivity» , which characterizes the change in the analytical signal with a change in the concentration of the component being determined, i.e. above the detection limit the method is sensitive to the component being determined, below the detection limit it is insensitive,
Exists some ways increasing the sensitivity of reactions , For example:
1) concentration (increase in sample signal):
2) increasing the purity of reagents (reducing the background signal).
Reaction sensitivity is reduced the following factors:
1) heating. As a rule, it leads to an increase in solubility, and, consequently, to a decrease in the magnitude of the analytical signal;
2) excess reagent. May lead to the formation of by-products, for example:
Hg 2+ + 2 I - ® HgI 2 ¯ (red precipitate);
HgI 2 + 2 I - ® 2- (colorless solution);
3) discrepancy between the acidity of the environment. May lead to lack of analytical response. Thus, the oxidation reactions of halides with potassium permanganate in acidic environments significantly depend on the pH of the environment (Table 5.1);
4) interfering components. May lead to the formation of by-products.
Table 5.1
Optimal acidity of the medium during the oxidation of halides with potassium permanganate
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Oxidation reaction |
Optimal acidity of the environment |
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2 I - ® I 2 + 2 e |
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2 Br - ® Br 2 + 2 e |
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2 Cl - ® Cl 2 + 2 e |
The first three factors that reduce the sensitivity of the reaction can be dealt with by careful execution of analytical procedures.
The influence of foreign (interfering) ions is suppressed by the use of complexing substances, oxidizing agents or reducing agents. These substances are called masking agents, and the procedure itself is called masking of interfering ions.
Thus, when detecting Co(II) using a reaction with potassium thiocyanate, the analytical signal is the appearance of a blue color of the solution due to the formation of tetrarodancoboltate(II) ion:
Co 2+ + 4 SCN - = 2- (blue solution).
If Fe(III) ions are present in the solution, the solution will acquire a blood-red color, since the stability constant of complex 3- is much greater than the stability constant of the cobalt(II) thiocyanate complex:
Fe 3+ + 6 SCN - = 3- (dark red solution).
Those. the iron(III) ions present are interfering with cobalt(II) ions. Thus, in order to determine Co(II), it is necessary to first (before adding the KSCN solution) mask Fe(III). For example, “bind” iron(III) ions into a complex that is more stable than 3-. Thus, complexes 3-, 3-, 3- are more stable with respect to 3-. Therefore, solutions of KF, K 2 HPO 4 or (NH 4) 2 C 2 O 4 can be used as masking agents.
Method of standards (standard solutions)
Using the single standard method, the magnitude of the analytical signal (at ST) is first measured for a solution with a known concentration of the substance (Cst). Then the magnitude of the analytical signal (y x) is measured for a solution with an unknown concentration of the substance (C x). The calculation is carried out according to the formula
C x = C st ×y x / y ST (2.6)
This calculation method can be used if the dependence of the analytical signal on concentration is described by an equation that does not contain a free term, i.e. equation (2.2). In addition, the concentration of the substance in the standard solution must be such that the values of the analytical signals obtained using the standard solution and a solution with an unknown concentration of the substance are as close as possible to each other.
Let the optical density and concentration of a certain substance be related by the equation A = 0.200C + 0.100. In the selected standard solution, the concentration of the substance is 5.00 μg/ml, and the optical density of this solution is 1.100. A solution of unknown concentration has an optical density of 0.300. When calculated using the calibration curve method, the unknown concentration of the substance will be equal to 1.00 μg/ml, and when calculated using one standard solution, it will be 1.36 μg/ml. This indicates that the concentration of the substance in the standard solution was chosen incorrectly. To determine the concentration, one should take a standard solution whose optical density is close to 0.3.
If the dependence of the analytical signal on the concentration of a substance is described by equation (2.1), then it is preferable to use not the method of one standard, but the method of two standards (method of limiting solutions). With this method, the values of analytical signals are measured 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. The unknown concentration is calculated using the formulas
Cx = C 2 (y x - y 1) + C 1 (y 2 – y x) / y 2 - y 1
The additive method is usually used in the analysis of complex matrices, when the matrix components influence the magnitude of the analytical signal and it is impossible to accurately copy the matrix composition of the sample.
There are several varieties of this method. When using the calculation method of additives, the analytical signal value for a sample with an unknown concentration of a substance (y x) is first measured. Then a certain exact amount of the analyte (standard) is added to this sample and the value of the analytical signal (ext) is measured again. The concentration of the component being determined in the analyzed sample is calculated using the formula
C x = C to6 y x / y ext – y x (2.8)
When using the graphical method of additives, several identical portions (aliquots) of the analyzed sample are taken, and no additive is added to one of them, and various exact amounts of the component being determined are added to the rest. For each aliquot, the magnitude of the analytical signal is measured. Then a graph is constructed characterizing the linear dependence of the magnitude of the received signal on the concentration of the additive, and it is extrapolated to the intersection with the abscissa axis. The segment cut off by this straight line on the abscissa axis is equal to the unknown concentration of the substance being determined.
It should be noted that formula (2.8) used in the additive method, as well as the considered version of the graphical method, do not take into account the background signal, i.e. it is believed that the dependence is described by equation (2.2). The standard solution method and the additive method can only be used if the calibration function is linear.
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