Determination of organic substances in water. Determination of lead in urban vegetation Qualitative determination of lead in biological material

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Course work

Determination of lead in urban vegetation

Introduction

lead titrimetric metal reagent

Lead is a toxic substance whose accumulation affects a number of body systems and is especially harmful to young children.

Childhood exposure to lead is estimated to contribute to approximately 600,000 new cases of intellectual disability in children each year.

Lead exposure is estimated to cause 143,000 deaths per year, with the heaviest burden in developing regions.

In the body, lead enters the brain, liver, kidneys and bones. Over time, lead accumulates in teeth and bones. Human exposure is typically determined using blood lead levels.

There is no known level of lead exposure that is considered safe.

The main sources of lead pollution are motor vehicles using lead - containing gasoline, metallurgical plants, smoke sources such as thermal power plants, etc.

Plants absorb lead from soil and air.

They perform a useful role for humans, acting as adsorbents for lead in the soil and air. Dust containing lead accumulates on plants without spreading.

According to the data on the content of mobile forms of heavy metals in plants, one can judge the contamination of a certain space with them.

This course work examines the lead content in urban vegetation.

1. Leeliterature review

The literature review is based on the book “Analytical Chemistry of Elements. Lead".

1. 1 AboutGeneral information about lead

Svinemts (lat. Plumbum; denoted by the symbol Pb) is an element of the 14th group (outdated classification - the main subgroup of group IV), the sixth in the periodic system of chemical elements of D.I. Mendeleev, with atomic number 82 and thus contains the magic number of protons. The simple substance lead (CAS number: 7439-92-1) is a malleable, relatively fusible metal of a silvery-white color with a bluish tint. Known since ancient times.

The lead atom has the electronic structure 1s 2 2s 2 p 6 3s 2 p 6 d 10 4s 2 p 6 d 10 f 14 5s 2 p 6 d 10 6s 2 p 2 . The atomic mass is assumed to be 207.2, but its fluctuations by 0.03 - 0.04 a.c. are possible.

Lead is a component of more than 200 minerals, but only three of them (galena, anglesite, cerussite) are found in nature in the form of industrial deposits of lead ores. The most important of these is galena PbS (86.5% Pb).

Under the influence of substances dissolved in natural waters and during weathering, it turns into anglesite PbSO 4 (63.3% Pb), which, as a result of double exchange with calcium and magnesium carbonates, forms cerussite PbCO 3 (77.5% Pb).

In terms of industrial production, lead ranks fourth in the group of non-ferrous metals, second only to aluminum, copper and zinc.

For the production of lead, polymetallic sulfide and mixed ores are of greatest importance, since pure lead ores are rare.

It is used for radiation protection purposes, as a structural material in the chemical industry, and for the manufacture of protective coatings for electrical cables and battery electrodes. Large quantities of lead are used to make various alloys: with bismuth (a coolant in nuclear technology), with tin and small additions of gold and copper (solders for the manufacture of printed circuits), with antimony, tin and other metals (solders and alloys for printing and antifriction purposes). The ability to form intermetallic compounds is used to produce lead telluride, from which IR ray detectors and converters of thermal radiation energy into electrical energy are prepared. A large proportion of lead is used in the synthesis of organometallic compounds.

Many lead-containing organic compounds are products of “minor” chemistry, but are of great practical importance. These include lead stearate and phthalate (thermal and light stabilizers for plastics), basic lead fumarate (thermal stabilizer for electrical insulators and vulcanizing agent for chlorosulfopolyethylene), lead diamyldithiocarbamate (multifunctional lubricating oil additive), lead ethylenediaminetetraacetate (radiocontrast agent), lead tetraacetate (oxidizing agent in organic chemistry). Among the practically important inorganic compounds we can name lead oxide (used in the production of glasses with a high refractive index, enamels, batteries and high-temperature lubricants); lead chloride (production of current sources); basic carbonate, lead sulfate and chromate, red lead (paint components); titanate - zirconate. lead (production of piezoelectric ceramics). Lead nitrate is used as a titrant.

The exceptional diversity and importance of the mentioned applications of lead have stimulated the development of numerous methods for the quantitative analysis of various objects. 1.2. Lead content in natural objects

The earth's crust contains 1.6*10 -3% by mass of Pb. The cosmic abundance of this element, according to various authors, varies from 0.47 to 2.9 atoms per 106 silicon atoms. For the Solar System, the corresponding value is 1.3 atoms per 10 6 silicon atoms.

Lead is found in high concentrations in many minerals and ores, in micro- and ultra-microquantities - in almost all objects of the surrounding world.

Other objects contain lead (% by weight); rain water - (6-29) * 10 -27, open source water - 2 * 10 -8, sea water - 1.3 open ocean water on the surface - 1.4 * 10 -9, at a depth of 0.5 and 2 km - 1.2*10 -9 and 2* 10 -10, respectively, granites, black shale, basalts - (1 - 30)*10 -4, sedimentary clay minerals - 2*10 -3, volcanic rocks of the Pacific belt - 0 .9*10 -4, phosphorites - from 5*10 -4 to 3*10 -2.

Brown coal - from 10 -4 to 1.75 * 10 -2 , oil - 0.4 4 * 10 -4 , meteorites - from 1.4 * 10 -4 to 5.15 * 10 -2 .

Plants: average content - 1*10 -4, in areas of lead mineralization - 10 -3, food 16*10 -6, puffball mushrooms collected near the highway - 5.3*10 -4, ash: lichens - 10 - 1, coniferous trees - 5*10 -3, deciduous trees and shrubs - up to 3*10 -3. Total lead content (in tons): in the atmosphere - 1.8 * 10 4 , in soils - 4.8 * 10 9 , in sediments - 48 * 10 12 , in ocean waters - 2.7 * 10 7 , in waters rivers and lakes - 6.1 * 10 -4 , in subsoil waters - 8.2 * 10 4 , in water and land organisms: living - 8.4 * 10 4 , dead - 4.6 * 10 6 .

1.2 Issources of lead pollution

Sources of lead in various areas of human and animal habitats are divided into natural (volcanic eruptions, fires, decomposition of dead organisms, sea and wind dust) and anthropogenic (activities of lead producing and processing enterprises, combustion of fossil fuels and waste from its processing).

In terms of the scale of emissions into the atmosphere, lead ranks first among microelements.

A significant portion of the lead contained in coal is released into the atmosphere when burned along with flue gases. The activity of just one thermal power plant, consuming 5000 tons of coal per day, annually releases 21 tons of lead and comparable amounts of other harmful elements into the air. A significant contribution to air pollution with lead comes from the production of metals, cement, etc.

The atmosphere is polluted not only by stable but also by radioactive isotopes of lead. Their source is radioactive inert gases, of which the longest-living, radon, even reaches the stratosphere. The resulting lead partially returns to the earth with precipitation and aerosols, polluting the soil surface and water bodies.

1.3 Thattoxicity of lead and its compounds

Lead is a poison that affects all living things. It and its compounds are dangerous not only due to their pathogenic effect, but also due to the cumulative therapeutic effect, high accumulation rate in the body, low rate and incomplete excretion with waste products. Lead Hazard Facts:

1. Already at a concentration of 10 -4% in the soil, lead inhibits the activity of enzymes, and highly soluble compounds are especially harmful in this regard.

2. The presence of 2*10 -5% lead in water is harmful to fish.

3. Even low concentrations of lead in water reduce the amount of carotenoid and chlorophyll in algae.

4. Many cases of occupational diseases have been registered among those working with lead.

5. Based on the results of 10 years of statistics, a correlation has been established between the number of deaths from lung cancer and the increased content of lead and other metals in the air of areas of industrial enterprises consuming coal and petroleum products.

The degree of toxicity depends on the concentration, physicochemical state and nature of lead compounds. Lead is especially dangerous in a state of molecular ion dispersion; it penetrates from the lungs into the circulatory system and from there is transported throughout the body. Although lead and its inorganic compounds act qualitatively similarly, their toxicity increases in sync with their solubility in biological fluids of the body. This does not diminish the danger of poorly soluble compounds that change in the intestine with a subsequent increase in their absorption.

Lead inhibits many enzymatic processes in the body. With lead intoxication, serious changes occur in the nervous system, thermoregulation, blood circulation and trophic processes are disrupted, the immunobiological properties of the body and its genetic apparatus change.

1. 4 OSadditive and titrimetric methods

1. Gravimetric method - the formation of weight forms of lead with organic and inorganic reagents is used. Among inorganic ones, preference is given to lead sulfate and chromate. Methods based on their precipitation are comparable in selectivity and conversion factor, but the determination of Pb in the form of chromate requires less time. It is recommended to obtain both sediments using “homogeneous” precipitation methods.

Organic reagents provide weight forms suitable for the determination of smaller quantities of Pb, with more favorable conversion factors than lead chromate or lead sulfate.

Advantages of the method: crystallinity of the precipitate and high accuracy of results in the absence of interfering impurities. Relative error of determination 0.0554-0.2015 Pb< 0,3%. С применением микроаппаратуры выполнены определения 0,125-4,528 мг РЬ с относительной погрешностью < 0,8%. Однако присутствие свободной HN0 3 недопустимо, а содержание солей щелочных металлов и аммония должно быть возможно малым.

2. Precipitation titration with visual indicators. Titration with organic and inorganic reagents is used. In the absence of impurity ions precipitated by chromate, direct titrimetric methods with indication of the titration end point (ETP) by a change in the color of methyl red or adsorption indicators are most convenient. The best option for the titrimetric determination of Pb by the chromate method is the precipitation of PbCr0 4 from an acetic acid solution, followed by dissolving the precipitate in 2 M HC1 or 2 M HC10 4, adding excess potassium iodide and titrating the liberated iodine with Na 2 S 2 0 3.

3. Titration with EDTA solutions. Due to the versatility of EDTA as an analytical reagent for most cations, the question arises of increasing the selectivity of Pb determination. To do this, they resort to preliminary separation of mixtures, the introduction of masking reagents and regulation of the reaction of the medium to pH values > 3. Usually, titration is carried out in a slightly acidic or alkaline medium.

The end point of the titration is most often indicated using metallochromic indicators from the group of azo- and triphenylmethane dyes, derivatives of diatomic phenols and some other substances, the colored Pb complexes of which are less stable than ethylenediaminetetraacetate of lead. In weakly acidic media, titrate against 4 - (2-pyridylazo)-resorcinol, thiazolyl-azo-and-cresol, 2 - (5-bromo-2-pyridylazo) - 5-diethylaminophenol, 1 - (2-pyridylazo) - 2-naphthol , 2 - (2-thiazolylazo) - resorcinol, azo derivatives of 1-naphthol4-sulfonic acid, xylenol orange, pyrocatechol violet, methylxylenol blue, pyrogallol and bromopyrogallol red, methylthymol blue, hematoxylin, sodium rhodizonate, alizarin S and dithizone.

In alkaline environments, eriochrome black T, sulfarsazene, 4 - (4,5 - dimegyl-2-thiazolylazo) - 2-methylresorcinol, a mixture of acid alizarin black SN and eriochrome red B, pyrocatecholphthalein, strong solochrome 2 RS, methylthymol blue and murexide ( titration of total amounts of Pb and Cu).

4. Titration with other complexing substances. The formation of chelates with DCTA, TTGA, and sulfur-containing complexing agents is used.

1.5 Fotometric methods of analysisabout light absorption and scattering

1. Determination as sulfide. The origins of this method and its first critical assessment date back to the beginning of our 20th century. The color and stability of a PbS sol depend on the particle size of the dispersed phase, which is influenced by the nature and concentration of dissolved electrolytes, the reaction of the medium, and the preparation method. Therefore, these conditions must be strictly observed.

The method is not very specific, especially in an alkaline environment, but the convergence of results in alkaline solutions is better. In acidic solutions, the sensitivity of determination is lower, but it can be slightly increased by adding electrolytes, for example NH 4 C1, to the analyzed sample. The selectivity of determination in an alkaline medium can be improved by introducing masking complexing agents.

2. Determination in the form of complex chlorides. It has already been indicated that Pb chlorine complexes absorb light in the UV region, and the molar extinction coefficient depends on the concentration of Cl ions - In a 6 M HCl solution, the absorption maxima of Bi, Pb and Tl are sufficiently distant from each other, which makes it possible to simultaneously determine them by light absorption at 323, 271 and 245 nm, respectively. The optimal concentration range for determining Pb is 4-10*10-4%.

3. Determination of Pb impurities in concentrated sulfuric acid is based on the use of characteristic absorption at 195 nm relative to a standard solution, which is prepared by dissolving lead in H2S04 (special purity).

Determination using organic reagents.

4. In the analysis of various natural and industrial objects, the photometric determination of Pb using dithizone, due to its high sensitivity and selectivity, occupies a leading place. In various variants of existing methods, the photometric determination of Pb is performed at the wavelength of the maximum absorption of dithizone or lead dithizonate. Other variants of the dithizone method are described: photometric titration without phase separation and a non-extraction method for the determination of lead in polymers, in which a solution of dithizone in acetone is used as a reagent, diluted with water before use to a concentration of the organic component of 70%.

5. Determination of lead by reaction with sodium diethyldithiocarbamate. Lead is easily extracted by CCl4 in the form of colorless diethyldithiocarbamate at various pH values. The resulting extract is used in the indirect method for determining Pb, based on the formation of an equivalent amount of yellow-brown copper diethyldithiocarbamate as a result of exchange with CuS04.

6. Determination by reaction with 4 - (2-pyridylazo) - resorcinol (PAR). The high stability of the red Pb complex with PAR and the solubility of the reagent in water are the advantages of the method. For the determination of Pb in some objects, for example in steel, brass and bronze, a method based on the formation of a complex with this azo compound is preferable to the dithizone one. However, it is less selective and therefore, in the presence of interfering cations, requires preliminary separation by the HD method or extraction of lead dibenzyldithiocarbamate with carbon tetrachloride.

7. Determination by reaction with 2 - (5-chloropyridip-2-azo) - 5-diethylaminophenol and 2 - (5-bromopyridyl-2-azo) - 5-diethylaminophenol. Both reagents form 1:1 complexes with Pb with almost identical spectrophotometric characteristics.

8. Determination by reaction with sulfarsazene. The method uses the formation of a reddish-brown water-soluble complex of composition 1: 1 with an absorption maximum at 505-510 nm and a molar extinction coefficient of 7.6 * 103 at this wavelength and pH 9-10.

9. Determination by reaction with arsenazo 3. This reagent, in the pH range 4-8, forms a blue complex with a composition of 1:1 with lead with two absorption maxima - at 605 and 665 nm.

10. Determination by reaction with diphenylcarbazone. In terms of reaction sensitivity, when extracting the chelate in the presence of KCN, and in terms of selectivity, it approaches dithizone.

11. Indirect method for determining Pb using diphenylcarbazide. The method is based on the precipitation of lead chromate, its dissolution in 5% HC1 and the photometric determination of dichromic acid by reaction with diphenylcarbazide using a filter with a maximum transmission at 536 nm. The method is time-consuming and not very accurate.

12. Determination by reaction with xylenol orange. Xylenol orange (KO) forms a 1:1 complex with lead, the optical density of which reaches its limit at pH 4.5-5.5.

13. Determination by reaction with bromopyrogalpol red (BOD) in the presence of sensitizers. Diphenylguanidinium, benzylthiuronium and tetraphenylphosphonium chlorides are used as sensitizers that increase the color intensity but do not affect the position of the absorption maximum at 630 nm, and cetyltrimethylammonium and cetylpyridinium bromides at pH 5.0.

14. Determination by reaction with glycinthymol blue. The complex with glycinthymol blue (GBL) of composition 1:2 has an absorption maximum at 574 nm and a corresponding molar extinction coefficient of 21300 ± 600.

15. Determination with methylthymol blue is performed under conditions similar to those for the formation of a complex with GTS. In terms of sensitivity, both reactions are close to each other. Light absorption is measured at pH 5.8-6.0 and a wavelength of 600 nm, which corresponds to the position of the absorption maximum. The molar extinction coefficient is 19,500. Interference from many metals is eliminated by masking.

16. Determination by reaction with EDTA. EDTA is used as a titrant in indicator-free and indicator photometric titrations (PT). As in visual titrimetry, reliable FT with EDTA solutions is possible at pH > 3 and titrant concentration of at least 10-5 M.

Luminescent analysis

1. Determination of Pb using organic reagents

A method has been proposed in which the intensity of chemiluminescence emission is measured in the presence of Pb due to the catalytic oxidation of luminol with hydrogen peroxide. The method was used to determine from 0.02 to 2 μg Pb in 1 ml of water with an accuracy of 10%. The analysis lasts 20 minutes and does not require preliminary sample preparation. In addition to Pb, the oxidation reaction of luminol is catalyzed by traces of copper. The method, which is much more complex in its hardware design, is based on the use of the fluorescence quenching effect of fluores-132 derivatives and is valuable in the formation of chelates with lead. More selective in the presence of many geochemical satellites of Pb, although less sensitive, is a fairly simple method based on increasing the fluorescence intensity of the water-blue lumogen in a dioxane-water mixture (1: 1) in the presence of Pb.

2. Methods of low-temperature luminescence in frozen solutions. Freezing the solution is most easily solved in the method for determining lead in HC1, based on photoelectric recording of the green fluorescence of chloride complexes at -70°C.

3. Analysis of the luminescence burst during defrosting of samples. The methods of this group are based on a shift in the luminescence spectra when the analyzed sample is thawed and measurement of the observed increase in radiation intensity. The maximum wavelength of the luminescence spectrum at -196 and -70°C is 385 and 490 nm, respectively.

4. A method is proposed based on measuring the analytical signal at 365 nm in the quasi-line luminescence spectrum of crystal phosphorus CaO-Pb cooled to liquid nitrogen temperature. This is the most sensitive of all luminescent methods: if an activator is applied to the surface of tablets (150 mg CaO, diameter 10 mm, pressing pressure 7-8 MN/m2), then the detection limit on the ISP-51 spectrograph is 0.00002 μg. The method is characterized by good selectivity: a 100-fold excess of Co, Cr(III), Fe (III), Mn(II), Ni, Sb (III) and T1 (I) does not interfere with the determination of Pb. Bi can also be determined simultaneously with Pb.

5. Determination of lead by the luminescence of a chloride complex sorbed on paper. In this method, luminescent analysis is combined with the separation of Pb from interfering elements using a ring bath. The determination is carried out at ordinary temperature.

1.6 Alelectrochemical methods

1. Potentiometric methods. Direct and indirect determination of lead is used - titration with acid-base, complexometric and precipitation reagents.

2. Electrogravimetric methods use the deposition of lead on electrodes, followed by weighing or dissolution.

3. Coulometry and coulometric titration. Electrogenerated sulfhydryl reagents are used as titrants.

4. Volt-amperometry. Classical polarography, which combines rapidity with fairly high sensitivity, is considered one of the most convenient methods for determining Pb in the concentration range of 10-s-10 M. In the vast majority of works, lead is determined by the reduction current of Pb2+ to Pb° on a mercury dropping electrode (DRE), usually occurring reversibly and in the diffusion mode. As a rule, cathodic waves are well expressed, and polarographic maxima are especially easily suppressed by gelatin and Triton X-100.

5. Amperometric titration

In amperometric titration (AT), the equivalence point is determined by the dependence of the current value of the electrochemical transformation of Pb and (or) titrant at a certain value of the electrode potential on the volume of the titrant. Amperometric titration is more accurate than the conventional polarographic method, does not require mandatory temperature control of the cell, and is less dependent on the characteristics of the capillary and indifferent electrolyte. It should be noted that the AT method has great potential, since analysis is possible using an electrochemical reaction involving both Pb itself and the titrant. Although the total time spent on AT execution is greater, it is fully compensated by the fact that there is no need for calibration. Titration is used with solutions of potassium dichromate, chloranilic acid, 3,5-dimethyldimercapto-thiopyrone, 1,5-6 is (benzylidene)-thio-carbohydrazone, thiosalicylamide.

1.7 FiPhysical methods for lead determination

Lead is determined by atomic emission spectroscopy, atomic fluorescence spectrometry, atomic absorption spectrometry, X-ray methods, radiometric methods, radiochemical and many others.

2 . ExperimentalPart

2.1 MehDefinition code

This work uses the determination of lead in the form of a dithizonate complex.

Figure 1 - structure of dithizone:

The maximum absorption of lead dithizonate complexes is 520 nm. Photometry is used against a solution of dithizone in CCl 4 .

Double ashing of the test sample is carried out - dry and “wet” method.

Double extraction and reaction with auxiliary reagents serve to separate interfering impurities and ions, and increase the stability of the complex.

The method is highly accurate.

2. 2 Etctests and reagents

Spectrophotometer with cuvettes.

Drying cabinet.

Muffle furnace.

Electric stove.

Electronic balance

Drip funnel 100 ml.

Chemical vessels.

A weighed portion of dry plant material 3 pcs. 10 gr.

0.01% solution of dithizone in CCl 4 .

0.02 N HCl solution.

0.1% hydroxylamine solution.

10% solution of yellow blood salt.

10% solution of ammonium citrate.

10% HCl solution.

Ammonia solution.

Soda solution.

Indicators are thymol blue and phenol red.

Standard solutions of lead, with its content from 1,2,3,4,5,6 µg/ml.

2. 3 Etcpreparation of solutions

1. 0.1% hydroxylamine solution.

W=m water/m solution =0.1%. The mass of the solution is 100 g. Then the weight is 0.1 g. Dissolved in 99.9 ml of double-distilled water.

2.10% solution of yellow blood salt. W=m water/m solution =10%. The mass of the solution is 100 g. Then the weight is 10 g. Dissolved in 90 ml of double-distilled water.

3.10% ammonium citrate solution. W=m water/m solution =10%. The mass of the solution is 100 g. Weight - 10 g. Dissolved in 90 ml of double-distilled water.

4.10% HCl solution. Prepared from concentrated HCl:

You need 100 ml of solution with W=10%. d conc HCl = 1.19 g/ml. Therefore, it is necessary to take 26 g of concentrated HCl, V = 26/ 1.19 = 21.84 ml. 21.84 ml of concentrated HCl was diluted to 100 ml with double-distilled water in a 100 ml volumetric flask to the mark.

5. 0.01% dithizone solution in CCl4. W=m water/m solution =10%. The mass of the solution is 100 g. Then the weight is 0.01 g. Dissolved in 99.9 ml CCl 4.

6. Soda solution. Prepared from dry Na 2 CO 3 .

7. 0.02 N HCl solution. W=m v-va /m r-ra =? Conversion to mass fraction. 1 liter of 0.02 N HCl solution contains 0.02 * 36.5 = 0.73 g of HCl solution. d conc HCl = 1.19 g/ml. Therefore, you need to take 1.92 g of concentrated HCl, volume = 1.61 ml. 1.61 ml of concentrated HCl was diluted to 100 ml with double-distilled water in a 100 ml volumetric flask to the mark.

9. A solution of the thymol blue indicator was prepared from a dry substance by dissolving it in ethyl alcohol.

2. 4 Mehshaking influences

In an alkaline environment containing cyanide, dithizone extracts thallium, bismuth and tin (II) together with lead. Thallium does not interfere with colorimetric determination. Tin and bismuth are removed by extraction in an acidic medium.

Silver, mercury, copper, arsenic, antimony, aluminum, chromium, nickel, cobalt and zinc in concentrations not exceeding twelve times the concentration of lead do not interfere with the determination. The interfering influence of some of these elements, if present in fifty-fold concentrations, is eliminated by double extraction.

Determination is hampered by manganese, which, when extracted in an alkaline medium, catalytically accelerates the oxidation of dithizone with atmospheric oxygen. This interference is eliminated by adding hydroxylamine hydrochloride to the extracted sample.

Strong oxidizing agents interfere with the determination because they oxidize dithizone. Their reduction with hydroxylamine is included in the determination.

2. 5 Thoseexperimental technique

The plant material was dried in a drying oven in a crushed state. Drying was carried out at a temperature of 100 0 C. After drying to an absolutely dry state, the plant material was thoroughly crushed.

Three 10 g portions of dry material were taken. They were placed in a crucible and placed in a muffle furnace, where they were ashed for 4 hours at a temperature of 450 0 C.

Afterwards, the plant ash was soaked in nitric acid while heating and dried (from here on - the operations are repeated for all samples).

Then the ash was again treated with nitric acid, dried on an electric stove and placed in a muffle furnace for 15 minutes at a temperature of 300 0 C.

Afterwards, the clarified ash was dug in with hydrochloric acid, dried, and dug in again. The samples were then dissolved in 10 ml of 10% hydrochloric acid.

Next, the solutions were placed in 100 ml dropping funnels. 10 ml of a 10% solution of ammonium citrate was added, then the solution was neutralized with ammonia until the color of thymol blue turned blue.

After this, extraction was carried out. 5 ml of a 0.01% solution of dithizone in CCl 4 was added. The solution in the dropping funnel was shaken vigorously for 5 minutes. The dithizone layer, after being separated from the main solution, was drained separately. The extraction operation was repeated until the initial color of each new portion of dithizone stopped turning red.

The aqueous phase was placed in a dropping funnel. It was neutralized with a soda solution until the color changed from phenol red to orange. Then 2 ml of a 10% yellow blood salt solution, 2 ml of a 10% ammonium citrate solution, and 2 ml of a 1% hydroxylamine solution were added.

Then the solutions were neutralized with a soda solution until the color of the indicator (phenol red) turned crimson.

Next, 10 ml of a 0.01% solution of dithizone in CCl 4 was added, the sample was vigorously shaken for 30 seconds, then the dithizone layer was poured into a cuvette and spectophotometered against a solution of dithizone in CCl 4 at 520 nm.

The following optical densities were obtained:

The calibration graph was constructed under the same conditions; standard solutions of lead concentrations from 1 to 6 μg/ml were used. They were prepared from a lead solution with a concentration of 1 μg/ml.

2.6 ReExperiment resultsenta and statistical processing

Data for constructing a calibration graph

Calibration chart

According to the calibration graph, the concentration of lead in one kilogram of dry plant mass is equal to

1) 0.71 mg/kg

2) 0.71 mg/kg

3) 0.70 mg/kg

What follows from the determination conditions is that the lead concentration in the standards is measured in μg/ml; for the analysis, the lead content was measured in 10 ml, recalculated for one kilogram of dry plant material.

Average mass value: X av = 0.707 g.

Variance =0.000035

Standard deviation: = 0.005787

Youwater

1. Based on a literature review.

Using a literature review, general information about the element, its determination methods was studied, and the most suitable one was selected according to its accuracy and compliance with those used in everyday practice.

2. Based on the results of the experiment.

The experiment showed that the method can be used to determine low lead contents; the results are highly accurate and repeatable.

3. In accordance with MPC.

List of references used

1. Polyansky N.G. Svinets.-M.: Nauka, 1986. - 357 p. (Analytical chemistry of elements).

2. Vasiliev V.P. Analytical chemistry. At 2 p.m. 2. Physico-chemical methods of analysis: Textbook. For chemical technology Specialist. Vuzov.-M.: Higher. school, 1989. - 384 p.

3. Fundamentals of analytical chemistry. In 2 books. Book 2. Methods of chemical analysis: Textbook. For universities/Yu.A. Zolotov, E.N. Dorokhova, V.I. Fadeeva and others. Ed. Yu.A. Zolotova. - 2nd ed., revised. And additional - M.: Higher. school, 2002. - 494 p.

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Drinking water quality research

For the study, samples of tap water and purified water using household filters Aquaphor (jug), Aquaphor (tap), Barrier (jug) were taken. The following indicators were studied: pH value, content of zinc (II), copper (II), iron (III) ions, water hardness.

pH value

5 ml of the test water is poured into the test tube, the pH is determined using a universal indicator, and the pH value is assessed using a scale:

Pink-orange - pH=5;
Light yellow - pH=6;
Light green - pH=7;
Greenish-blue - pH=8.

Filtered water has a slightly acidic reaction medium, while the medium of unfiltered water is close to neutral.

Determination of iron ions

To 10 ml of the test water, 1-2 drops of HCl (1:2) and 0.2 ml (4 drops) of a 50% solution of potassium thiocyanate KNCS were added. Stir and observe color development. This method is sensitive and can detect up to 0.02 mg/l of iron ions.

Fe3+ + 3NCS- = Fe(NCS)3

Lack of color - less than 0.05;
Barely noticeable yellowish-pink - from 0.05 to 0.1;
Weak yellowish-pink - from 0.1 to 0.5;
Yellowish-pink - from 0.5 to 1.0;
Yellowish-red - from 1.0 to 2.5;
Bright red more than 2.5.

The highest concentration of iron (III) ions is in unfiltered water.

Determination of lead ion (qualitative)

Potassium iodide gives a characteristic PbI2 precipitate in solution with lead ions. A little KI is added to the test solution, after which, by adding CH3COOH, the contents of the test tube are heated until the initially slightly characteristic yellow precipitate of PbI2 is completely dissolved. The resulting solution is cooled under the tap, and PbI2 falls out again, but in the form of beautiful golden crystals Pb2+ +2I- = PbI2. Purified and unfiltered water does not contain lead (II) ions.

Determination of copper ion (qualitative)

5 ml of the water to be tested is placed in a porcelain cup, evaporated to dryness, then 1 drop of concentrated (25%) ammonia solution is added. The appearance of an intense blue color indicates the presence of copper ions. 2Сu2+ +4NH4ОН = 22+ +4H2O

Determination of water hardness

100 ml of test water is added to a 250 ml conical flask, 5 ml of ammonia buffer solution is added, and an indicator (eriochrome black) is added at the tip of a spatula. Then the solution should be mixed and slowly titrated with a 0.05 N solution of Trilon B until the color of the indicator changes from cherry to blue.

Preparation of the eriochrome black (dry) indicator: for this, 0.25 g of the indicator is mixed with 50 g of dry sodium chloride, previously thoroughly ground in a mortar.

Preparation of a buffer solution: 10 g of ammonium chloride (NH4Cl) is dissolved in distilled water, 50 cm3 of 25% ammonia solution is added and adjusted to 500 cm3 with distilled water.

Preparation of a 0.05 N solution of Trilon B: 9.31 g of Trilon B is dissolved in distilled water and adjusted to 1 dm3. The solution is stable for several months.

The total stiffness is calculated using the formula:

F mg-eq/l = (Vml*N g-eq/l*1000 mg-eq/g eq) / V1ml,

where: V is the volume of Trilon “B” solution used for titration, ml.

N - normality of Trilon "B" solution g-eq/l.

V1 is the volume of the test solution taken for titration, ml.

When assessing water hardness, it is characterized as follows:

very soft - up to 1.5 mEq/l;
soft - from 1.5 to 4 mEq/l;
medium hardness - from 4 to 8 mEq/l;
hard - from 8 to 12 mEq/l;
very hard - more than 12 mEq/l.

Tap water is hard, water that has been purified with a Barrier filter has medium hardness, water that has been purified with an Aquaphor filter (jug and tap) is soft and of medium hardness.

Can water be harmful to health? Tap water can contain very dangerous and even toxic substances, water treatment plants are worn out, and water, before entering the house, must travel a long way through old water pipes, where it becomes contaminated with heavy metal salts and inorganic iron (rust). The need for clean water is constantly increasing, and the source water entering treatment plants becomes dirtier from year to year. After purification, the water becomes drinkable, but smells of bleach. The concentration of chlorine is not dangerous for a healthy person, but for some categories of sick people the presence of chlorine, even in small concentrations, greatly worsens their health. All this adversely affects human health. It is necessary to use filters for water purification at home. The quality of purified water at home is better than the quality of tap water. Using household filters, you can purify water that contains not only mechanical particles (sand, rust, etc.), but also various organic and inorganic compounds that are hazardous to health. Water that has been purified through a filter becomes less hard.

Filters completely remove chlorine from water, which kills bacteria and plays the role of a “preservative”. But you need to use purified water as quickly as possible after filtration, because in water devoid of a “preservative”, bacteria begin to multiply especially quickly in a clean and warm environment (water) that is pleasant for them.

So what is water? The question is far from simple... One thing we can definitely say is that water is the most unique substance on earth, on which the state of health depends.

Determination of pH of the test water:

Barrier - pink-orange (pH=5);
Aquaphor (jug) - pink-orange (pH=5);
Aquaphor (tap) - pink-orange (pH=5);
Unfiltered water is light yellow (pH=6).

Results of determination of iron (III) ions:

Barrier - Barely noticeable yellowish-pink from 0.05 to 0.1;
Aquaphor (jug) - absence less than 0.05;
Aquaphor (faucet) - absence less than 0.05;
Unfiltered water - yellowish-pink from 0.5 to 1.0.

Results of determination of lead (II) ions:

Barrier - no sediment. In 3 drops the water became discolored;
Aquaphor (jug) - no sediment. In 2 drops the water became discolored;
Aquaphor (tap) - no sediment. In 2 drops the water became discolored;
Unfiltered water - no sediment. In 10 drops the water became discolored.

Hardness of the tested water:

Barrier - 7 mEq/l;
Aquaphor (jug) - 5 mEq/l;
Aquaphor (tap) - 4 mEq/l;
Unfiltered water - 9 mEq/l.

Lesson - workshop

(project activity of 9th grade students at a general chemistry lesson when studying elements - metals)

“Study of the content of lead ions in soil and plant samples of the village of Slobodchiki and its effect on the human body.”

Prepared and carried out

teacher of biology, chemistry

Sivokha Natalya Gennadievna

The purpose of the lesson:

Show the effect of heavy metals on human health using the example of lead and study the ecological situation of the village of Slobodchiki by determining lead ions in soil and plant samples.

Lesson objectives:

Summarize the knowledge gained about heavy metals. To introduce students in more detail to lead, its biological role and toxic effects on the human body;

To expand students’ knowledge about the relationship between the use of lead metal and the ways it enters the human body;

Show the close relationship between biology, chemistry and ecology, as subjects that complement each other;

Fostering a caring attitude towards one’s health;

Instilling interest in the subject being studied.

Equipment: computer, multimedia projector, presentations of mini-projects completed by students, a stand with test tubes, a glass rod, a funnel with a filter, 50 ml beakers, filter paper, a measuring cylinder, a scale with weights, filter paper, scissors, an alcohol lamp or a laboratory tile.

Reagents: ethyl alcohol, water, 5% sodium sulfide solution, potassium iodide, soil samples, vegetation samples prepared by the teacher.

Why is a group of elements called “heavy metals”? (all these metals have a large mass)
What elements are considered heavy metals? (iron, lead, cobalt, manganese, nickel, mercury, zinc, cadmium, tin, copper, manganese)
What effect do heavy metals have on the human body?

In ancient Rome, noble people used plumbing made from lead pipes. Molten lead was poured into the joints of stone blocks and water supply pipes (it’s not for nothing that the word plumber in English means “plumber”). In addition, slaves used cheap wooden utensils and drank water directly from wells, while slave owners used expensive lead vessels. The life expectancy of rich Romans was much shorter than that of slaves. Scientists have suggested that the cause of early death was lead poisoning from the water used for cooking. However, this story has a continuation. In the state of Virginia (USA), burials of those years were examined. It turned out that in fact the skeletons of slave owners contain significantly more lead than the bones of slaves. Lead was known 6-7 thousand years BC. e. the peoples of Mesopotamia, Egypt and other countries of the ancient world. It was used to make statues, household items, and writing tablets. Alchemists called lead Saturn and designated it with the sign of this planet. Lead compounds - “lead ash” PbO, lead white 2PbCO3 Pb (OH)2 were used in Ancient Greece and Rome as components of medicines and paints. When firearms were invented, lead was used as a material for bullets. The toxicity of lead was noted back in the 1st century. n. e. Greek physician Dioscorides and Pliny the Elder.

The volume of modern lead production is more than 2.5 million tons per year. As a result of industrial activities, more than 500-600 thousand tons of lead enter natural waters annually, and about 400 thousand tons settle through the atmosphere onto the Earth's surface. Up to 90% of the total amount of lead emissions comes from gasoline combustion products containing lead compounds. The main part of it enters the air with the exhaust gases of vehicles, a smaller part - when burning coal. From the air near the soil layer, lead settles into the soil and enters the water. The lead content in rain and snow water ranges from 1.6 µg/l in areas remote from industrial centers to 250-350 µg/l in large cities. It is transported through the root system to the above-ground part of plants. At 23 m from a road with traffic volumes of up to 69 thousand cars per day, bean plants accumulated up to 93 mg of lead per 1 kg of dry weight, and at 53 m – 83 mg. Corn growing 23 m from the road accumulated 2 times more lead than 53 m. Where the road network is very dense, 70 mg of lead per 1 kg of dry matter was found in fodder beet tops, and 90 mg in collected hay. Lead enters the body of animals with plant foods. Lead content in various products (in mcg); pork meat - 15, bread and vegetables - 20, fruits - 15. Lead enters the human body with plant and animal foods, settling up to 80% in the skeleton, as well as in the internal organs. Humans, who represent one of the last links in the food chain, are at greatest risk from the neurotoxic effects of heavy metals.

Determination of lead ions in plant samples.

Purpose of the work: to determine the presence of ions in plant samples.

Equipment: two beakers of 50 ml each, a measuring cylinder, a scale with weights, a glass rod, a funnel, filter paper, scissors, an alcohol lamp or a laboratory hotplate.

Reagents: ethyl alcohol, water, 5% sodium sulfide solution

Research methodology.

1. Weigh 100 g. plants, preferably of the same species, for a more accurate result (plantain), at different distances from each other.

2. Grind thoroughly, add 50 ml to each sample. mixture of ethyl alcohol and water, stir so that the lead compounds go into solution.

3. Filter and evaporate to 10 ml. Add the resulting solution dropwise to a freshly prepared 5% sodium sulfide solution.

4. If there are lead ions in the extract, a black precipitate will appear.

Determination of lead ions in soil.

Purpose of the work: to determine the presence of lead ions in the soil.

Equipment: two beakers of 50 ml each, a measuring cylinder, scales with weights, a glass rod, a funnel, filter paper.

Reagents: potassium iodide, water.

Research methodology:

1. Weigh 2 g of soil and pour it into a beaker. Then add 4 ml of water and stir well with a glass rod.

2.Filter the resulting mixture.

3. Add 1 ml of 5% potassium iodide to the filtrate. When lead ion reacts with potassium iodide, a yellow precipitate is formed.

Pb +2 + 2 I - = P bI 2 (yellow precipitate)

4.Dip the edge of a 1 cm strip of filter paper into the resulting solution. When the substance rises to the middle of the paper, take it out and put it to dry. The dried filter paper will show a clear trace of sediment. Over time (after 3-5 days), the yellow color of lead iodide will appear brighter.

Lead is poisonous and has cumulative properties (the ability to accumulate in the body). As a result, the presence of lead in all types of canned food is not allowed.

The main sources of lead in canned food are semi-deposits, the lead content of which is limited to 0.04%, and solder. The presence of substances in canned products that can dissolve metals can lead to the transition of lead into the contents of the can during long-term storage of canned food. The lead content in the product is determined in the case of long-term storage and the presence of solder deposits on the inside of the can.

The method is based on obtaining a solution of lead chloride after ashing a sample of the product, precipitation of metal sulfides from the solution and determination of lead in a saturated solution of sodium acetate in the presence of potassium dichromate.

Analysis procedure: 15 g of the crushed product is placed in a porcelain cup with a diameter of about 7 cm, dried in a sand bath or in a drying cabinet, and then carefully charred and ashed over low heat or in a muffle furnace with the walls of the muffle glowing slightly red. Add 5 ml of dilute hydrochloric acid (ratio 1:1), 1 drop of hydrogen peroxide to the ash and evaporate to dryness in a water bath. 2 ml of 10% hydrochloric acid and 3 ml of water are added to the dry residue, after which the contents of the cup are filtered through a filter pre-moistened with water into a conical flask with a capacity of 100 ml. The cup and filter are washed with 15 ml of distilled water, collecting the washing water in the same flask. The resulting solution is heated to 40-50 ˚C, passing hydrogen sulfide through it for 40-60 minutes through a narrow tube reaching the bottom of the flask. In this case, lead, tin, and copper sulfides precipitate. The precipitate of sulfides and sulfur is separated by centrifugation in a 10 ml test tube. The liquid is drained, and the precipitate of metal sulfides is washed 1–2 times with a 1% solution of hydrochloric acid saturated with hydrogen sulfide. To the washed sulfide precipitate, immediately add 5 drops of a 10% sodium hydroxide solution (to avoid oxidation of lead sulfide into alkali-soluble sulfate), heat in a boiling water bath, add 10 ml of water and centrifuge. If there is a large sediment, treatment with sodium hydroxide is carried out twice.

To the precipitate of lead and copper sulfides add 5-10 drops of a mixture of strong sulfuric and nitric acids, taken in equal quantities, and carefully heat them on a small burner flame until the nitric acid vapors are completely removed and white thick sulfur trioxide vapors appear. After cooling, add 0.5–1.5 ml of distilled water and the same amount of ethanol to the test tube. If after adding water and alcohol the solution remains clear, then lead salts are considered undetected. When turbidity appears in the solution or a white precipitate forms, lead sulfate is separated with diluted ethanol (ratio 1:1). To the lead sulfate precipitate remaining in the centrifuge tube, add 1 ml of a saturated solution of sodium acetate, previously slightly acidified with acetic acid, and heat in a boiling water bath for 5 - 10 minutes. Then add 1 ml of distilled water, after which the contents of the test tube are filtered through a small filter moistened with distilled water. The filtrate is collected in a 10 ml graduated cylinder. The test tube and filter are washed several times with small portions of distilled water, collecting the wash water in the same cylinder. The volume of the solution is adjusted to the mark with water and mixed. 5 ml of solution from the cylinder is transferred to a centrifuge tube, 3 drops of 5% potassium dichromate solution are added and mixed. If the solution remains clear for 10 minutes, it is considered that no lead has been detected. If lead is present in the solution, a yellow turbidity (PbCrO4) appears. In this case, a quantitative determination of lead is carried out.

To quantify lead, a certain volume of solution (0.5 - 2 ml) is transferred from the cylinder into a flat-bottomed test tube with divisions of 10 ml. A standard solution with a lead content of 0.01 is added to three other similar test tubes; 0.015 and 0.02 mg. In test tubes with a standard solution, add such an amount of saturated sodium acetate solution, slightly acidified with acetic acid, so that its content in the test and standard solutions is the same (if 1 ml of the test solution is taken for the quantitative determination of lead, then 0. 1 ml sodium acetate). Next, distilled water to 10 ml is added to all four test tubes, mixed and 3 drops of a 5% solution of potassium dichromate are added. The contents of the test tube are mixed well and after 10–15 minutes the turbidity of the test solution is compared with the turbidity of standard solutions.

X= (A·10·1000)/ V·15, (6)

Where X - lead content in 1 kg of product, mg;

A– amount of lead in a test tube with a standard solution, mg;

10 – volume of dilution, ml;

V– volume of solution taken for comparison with the standard solution, ml; 15 – weight of product, g.

Preparation of a standard solution of lead nitrate. 160 mg of lead nitrate is dissolved in a small amount of distilled water in a 100 ml volumetric flask, add 1 drop of concentrated nitric acid, mix and adjust the volume to the mark with distilled water; 1 ml of such a solution contains 1 mg of lead, 2 ml of the solution is transferred to a 100 ml volumetric flask, and the volume is adjusted to the mark with distilled water. The last solution is standard. 1 ml contains 0.02 mg of lead.

After mineralization of organs with sulfuric and nitric acids, lead and barium will be present in the sediment in the form of BaSO 4 and PbS0 4 . Optimal conditions for quantitative precipitation

The concentrations of Ba 2 + and Pb 2 + are: the concentration of H 2 SO 4 in the mineralization is ~20% H 2 SO 4, the absence of nitrogen oxides (partial dissolution of PbSO 4 and, to a much lesser extent, BaS0 4 in nitric acid), time precipitation (~24 hours). Due to co-precipitation, the sediment may also contain Ca 2+, Fe 3+, Al 3+, Cr 3+, Zn 2+, Cu 2+, etc. When Co-precipitation of Cr 3+, the sediment is colored dirty green. To avoid loss of Cr 3+, the dirty green precipitate is treated by heating with a solution of ammonium persulfate in a 1% solution of sulfuric acid. The undissolved precipitate is analyzed for Ba 2 + and Pb 2 +, and the filtrate is left for the quantitative determination of chromium. In order to separate Ba 2+ and Pb 2+ (the presence of Pb 2+ interferes with the detection of Ba 2+), the sediment directly on the filter is carefully treated with 0.5-10 ml (depending on the size of the sediment) of a hot solution of ammonium acetate 1, achieving completeness dissolving PbSO 4 ;

Qualitative detection

The filtrate is tested for lead: a) by reaction with dithizone (HrDz)

Dithizone (diphenylthiocarbazone) has found widespread use in inorganic analysis. Depending on the pH of the environment in solutions, dithizone can exist in two forms:

In the enol form, the reagent is slightly soluble in organic solvents (chloroform, carbon tetrachloride). In the ketonnon form, oi dissolves quite well in them, forming solutions colored intensely green. In alkaline solutions it gives the HDz anion, colored orange.

With many metal cations [Mn, Cr, Co, Ni, Zn, Fe(III), Tl, Cu, Cd, Ag, Pb, Bi, Hg], dithizone gives intracomplex salts (dithizonates), usually soluble in nonpolar organic compounds. ski solvents (SHC1 3, CC1 4). Many of the intracomplex compounds are brightly colored.

and secondary dithizonates:

Primary dithizonates are distinguished:

Primary dithizonates are formed with all cations. Secondary dithizonates are formed with only a few metals (HgDz, Ag 2 Dz, CuDz, etc.). Fisher, who introduced dithizone into analytical practice (1957), attributes the following structure to them:

Where a metal can produce both primary and secondary dithizonate, everything depends on the pH reaction of the medium: in an acidic environment, primary dithizonate is formed, in an alkaline environment and in the absence of a reagent, secondary dithizonate is formed.

Both the formation and extraction of dithizonates depend primarily on the pH of the medium.

To detect lead, the solution obtained by treating the sediment of PbS0 4 and BaS0 4 with ammonium acetate is shaken with a solution of dithizone in chloroform (CC1 4): in the presence of Pb 2 +, the appearance of purple-red color

The reaction is highly sensitive - 0.05 μg R 2+ in 1 ml. The detection limit for Pb 2+ by this reaction in organs is 0.02 mg.

Under the described conditions of chemical toxicological analysis, the reaction is almost absolutely specific, since the production of Pb(HDz) 2 is preceded by the conversion of Pb 2+ to PbSO 4, i.e., the separation of Pb 2+ from most other elements. Mainly Fe 3 + and Cr 3 + can coprecipitate with PbSO 4 . At the same time, Fe 3+ has a low affinity for dithizone, and Cr 3+ forms uncolored compounds with dithizone.

One of the advantages of the reaction is the ability to combine qualitative analysis for Pb 2+ with quantitative determination. In this case, if there is a purple-red color of the chloroform layer, first

quantification (see page 302). Then, after measuring the color density of Pb(HDz) 2 on a photoelectrocolorimeter, lead dithizonate for further qualitative reactions is vigorously shaken for 60 seconds with 0.5-2 ml (depending on the volume and color intensity of the extract) 1 N. HNO 3 (or HC1) solution:

Pb(HDz) 2 >- Pb(N0 8) 2 + 2H 2 Dz

(layer of organic- (aqueous (layer of organic-

soluble mortar layer)

creator) creator)

Depending on the volume of the aqueous layer, the solution is further studied by microcrystalline or macrochemical reactions.

I. With a small volume of the aqueous layer (0.5 ml), the entire volume is divided into 2 parts, carefully evaporated and the reactions are carried out: a) a double salt of cesium iodide and lead is obtained - CsPbl 3. Acidify 1/2 of the remainder with 30% acetic acid and mix with several crystals of potassium iodide:

1-2 crystals of cesium chloride are added to the solution - after some time a greenish-yellow precipitate of cesium and lead iodide falls. When viewed under a microscope, needle-shaped crystals can be observed, often collected in bundles and spheroids.

Optimal conditions: 30°/o acetic acid solution, absence of mineral acids, a small amount of CsCl and excess KI.

The sensitivity of the reaction is 0.01 μg. The reaction makes it possible to detect (detection limit) 0.015 mg Pb 2+ per 100 g of the test object;

b) the formation of potassium, copper and lead hexanitrite KrCuPb(NO 2) 6. The second part of the residue is mixed with 1-2 drops of a saturated solution of copper acetate and carefully evaporated to dryness. The residue is dissolved in 2-3 drops of a 30% acetic acid solution and several crystals of potassium nitrite are added. In the presence of Pb 2+, after 5-10 minutes KrCu Pb(NO 2) 6 crystals appear across the entire field of view in the form of black or brown (with small amounts of Pb 2+) cubes. Optimal conditions: 30% CH 3 COOH solution, absence of mineral acids, excess potassium nitrite. The sensitivity of the reaction is 0.03 μg. The detection limit for Pb 2+ in biological material is 0.015 mg per 100 g of organ.

P. If the volume of the aqueous layer is large (2 ml or more), it is neutralized to pH 5.0 using universal indicator paper, divided into 4 parts and examined by reactions:

a) PbS formation:

Pb(N0 3) 2 + H 2 S = PbSJ + 2HN0 3.

The precipitate does not dissolve in dilute sulfuric and hydrochloric acids, but dissolves in dilute nitric acid with the release of nitrogen oxides and elemental sulfur:

3PbS + 8HNO 3 = 3Pb(NO 3) 2 + 2NO + 3S + 4H 2 O;

b) formation of PbS0 4:

Pb(OCOCH 3) 2 + H 2 SO 4 = PbSO 4 | + 2CH 3 COOH

Lead sulfate is slightly soluble in water (1:22,800 at 15°); in dilute sulfuric acid its solubility is even less; it is practically insoluble in alcohol; It dissolves significantly in nitric acid, and even better - in hydrochloric acid, especially when heated:

When water is added, lead sulfate precipitates again.

Lead sulfate precipitate dissolves in solutions of caustic soda, caustic potassium, acetate and ammonium tartrate (difference from barium sulfate and strontium sulfate):

When dissolved in ammonium tartrate, Pb 2 0(C 4 H 4 0 6) 2 is formed.

c) formation of PbCr0 4; insoluble in acetic acid, but
soluble in mineral acids and caustic alkalis:

2Pb(OSOCN 3) 3 + K 2 Cr 2 0 7 + HON - 2CH 3 COOK + 2PbSiu 4 + 2CH 3 COOH.

d) the fourth part is studied by microchemical reactions
obtaining CsPbl 3 and K2CuPb(N0 2)e.

Quantitative determination of Pb 2+ after its isolation in the form of lead sulfate is possible by several methods:

a) dichromate and odometric based on the excess of dichromate that has not reacted with Pb 2+. The definition is based on the following reactions:

The dichromate-iodometric method of determination gives good results (93% with an average relative error of 1.4°/o) with a content of 2 to 100 mg of lead per 100 g of organ. For quantities of lead less than 2 mg (determination limit), the method is unreliable. For example, in the presence of 1 mg Pb 2 + in 100 g of organ, an average of only 37% is determined;

b) e xtraction of the action-photo meter and h e s k and by the diti-zonate of lead. The method is based on the above sensitive and rather specific reaction:

Pb(OSOCN 3) 2 4- 2H a Dz (at pH 7-10) - Pb(HDz) a + 2CH 3 COOH.

The resulting dithizonate is extracted with chloroform at a pH above 7.0 until the extraction of Pb 2+ is complete. The extracts are combined, washed with a KCN solution in the presence of NH 4 OH, settled, the volume is measured, and then the color density of the chloroform extract is determined on FEC at a full length of 520 nm in a cuvette with an absorbing layer thickness of 1 cm. Chloroform serves as a reference solution. Beer's law is observed within the range of 0.0001 - 0.005 mg/ml.

c) complexometric, which is common to many divalent and some trivalent cations.

The principle of complexometric titration comes down to the following: a small amount of the corresponding indicator is added to the test solution containing a certain cation at a strictly defined pH value - a highly water-soluble colored complex compound of the indicator with the cation is formed. When titrated with Trilon B (complex III), the disodium salt of ethylenediaminetetraacetic acid, the complex of the cation with the indicator is destroyed, since Trilon B forms a more stable complex with the cation being determined. At the equivalent point, a free indicator is released, coloring the solution in the color characteristic of the indicator at a given pH value of the medium.

Most cations are determined in an alkaline medium, for which an ammonia buffer (a mixture of ammonia and ammonium chloride) is introduced into the titrated solution.

The determination of Pb 2+ (or other divalent cation) is based on the following reactions:

A. N. Krylova recommends back titration of Trilon B to determine Pb 2+ (used to determine cations that react with NH 4 OH solution). The essence of the method is as follows: the test solution is diluted with water to 100-150 ml and mixed with an excess of 0.01 N. solution of Trilon B. 10 ml of ammonium chloride buffer 2 and 0.1 - 0.2 g of dry zriochrome black T (mixture with NaCl 1:200). The excess current of Trilon B is titrated with 0.01 N. solution of ZnCl 2 until the blue-blue color changes to red-violet. 96% is determined with an average relative error of 6.2% at 1 mg Pb 2 + per 100 g of organ; 97% with an average relative error of 27% at 10 mg. The limit of determination is 0.5 mg Pb 2 + per 100 g of organ.

Toxicological significance. The toxicological significance of lead is determined by the toxic properties of metallic lead, its salts and some derivatives, and their wide and varied use in industry and everyday life.

Particularly dangerous in relation to lead poisoning are the mining of lead ores, lead smelting, production of batteries, lead paints [lead white 2PbCO 3 .Pb(OH) 2 and red lead Pb 3 O 4], the use of which in the USSR is limited only to the painting of ships and bridges , tinning, soldering, use of lead glaze PbSi0 3, etc. With insufficient labor protection, industrial poisoning is possible.

In a number of cases, sources of household poisoning were poor-quality tinned, enameled, porcelain-earthware and earthenware covered with glaze.

Cases of lead poisoning through drinking water (lead pipes), snuff wrapped in lead paper, after a gunshot wound, etc. are also known. Cases of poisoning with lead salts and tetraethyl lead are also known.

Lead is a protoplasmic poison that causes changes mainly in nervous tissue, blood and blood vessels. The toxicity of lead compounds is largely related to their solubility in gastric juice and other body fluids. Chronic lead poisoning gives a characteristic clinical picture. The lethal dose of different lead compounds is not the same. Children are especially sensitive to it. Lead is not a biological element, but is usually present in water and food, from where it enters the body. A person who is not working with lead absorbs 0.05-2 g of lead per day (on average 0.3 mg), as N.V. Lazarev points out. Lead compounds can accumulate in bone tissue, liver, and kidneys. About 10% of it is absorbed by the body, the rest is excreted in the feces. Lead is deposited in the liver and tubular bones, and somewhat less so in flat bones. In other organs it is deposited in small quantities. Hence the possibility of detecting lead in the internal organs of the corpses of people who died from other causes, and the need to quantify it if the results of a qualitative analysis are positive.

The natural content of lead (according to A. O. Voinara, in milligrams per 100 g of organ) in the liver is 0.130; in the kidney 0.027; in tubular bones 1.88; in the stomach and intestines 0.022 and 0.023, respectively.