Photometric determination of lead in aqueous solutions by reaction with xylenol orange. Guidelines for the photometric determination of lead in air Photometric determination of lead
abstract
Coursework contains: ___ pages, 4 tables, 2 figures, 8 references. The object of research in term paper are complex foods chemical composition.
The purpose of the work is to determine the content of lead in food products and compare with the MPC.
Research method - atomic absorption.
Methods of sample preparation are given. Analyzed and summarized data on the content of lead compounds in food objects (objects).
Scope - analytical and toxicological chemistry, laboratories for standardization and quality of food products produced by light industry, pharmaceutical chemistry.
Keywords: LEAD, ATOMIC ABSORPTION SPECTROSCOPY, ABSORPTION, STANDARD SOLUTION, CALIBRATION GRAPH, CONTENT, MPC
Introduction
1. Literature review
1.3 Sample preparation
2. Experimental part
conclusions
Introduction
The use of materials containing lead and its compounds has led to the contamination of many objects environment. Determination of lead in metallurgical products, biological materials, soils, etc. presents difficulties because it is usually accompanied by other divalent metals. To solve such analytical task The atomic absorption method of determination has become widespread due to the availability of equipment, high sensitivity and sufficient accuracy.
Food products may contain not only useful material, but also quite harmful and dangerous to the human body. Therefore, the main task of analytical chemistry is the quality control of food products.
Namely, in this course work, the atomic absorption method for determining lead in coffee is used.
1. Literature review
1.1 Chemical properties lead
In the periodic table D.I. Mendeleev, lead is located in group IV, the main subgroup and has an atomic weight of 207.19. Lead in its compounds can be in the +4 oxidation state, but +2 is most characteristic of it.
In nature, lead occurs in the form of various compounds, the most important of which is the lead luster PbS. The prevalence of lead in the earth's crust is 0.0016 wt. %.
Lead is a bluish-white heavy metal with a density of 11.344 g/cm 3. It is very soft and easy to cut with a knife. Melting point of lead 327.3 about C. In air, lead is quickly covered with a thin layer of oxide, which protects it from further oxidation. In the voltage series, lead comes directly before hydrogen; its normal potential is - 0.126 V.
Water itself does not interact with lead, but in the presence of air, lead is gradually destroyed by water to form lead hydroxide:
Pb+O 2+H2 O=2Pb(OH) 2
However, when in contact with hard water, lead is covered with a protective film of insoluble salts (mainly sulfate and basic lead carbonate), which prevents further action of water and the formation of hydroxide.
Diluted hydrochloric and sulfuric acids do not act on lead due to the low solubility of the corresponding lead salts. Lead readily dissolves in nitric acid. Organic acids, especially acetic, also dissolve lead in the presence of atmospheric oxygen.
Lead also dissolves in alkalis, forming plumbites.
1.2 Physiological role of lead
Lead metabolism in humans and animals has been studied very little. Its biological role is also not completely clear. It is known that lead enters the body with food (0.22 mg), water (0.1 mg) and dust (0.08 mg). Typically, the lead content in the body of a man is about 30 µg%, and for women about 25.5 µg%.
From a physiological point of view, lead and almost all of its compounds are toxic to humans and animals. Lead, even in very small doses, accumulates in the human body, and its toxic effect gradually increases. With lead poisoning, gray spots appear on the gums, functions are impaired nervous system felt pain in internal organs. Acute poisoning leads to severe damage to the esophagus. In people who work with lead, its alloys or compounds (for example, printing workers), lead poisoning is an occupational disease. A dangerous dose for an adult is in the range of 30-60 g of Pb (CH3COO) 2*3H 2O .
1.3 Sample preparation
The selection and preparation of a laboratory sample is carried out in accordance with the NTD for this species products. Two parallel samples are taken from the combined laboratory sample.
Products with a high sugar content (confectionery, jams, compotes) are treated with sulfuric acid (1: 9) at the rate of 5 cm 3 acids per 1 g of dry matter and incubated for 2 days.
Products with a fat content of 20-60% (cheese, oilseeds) are treated with nitric acid (1:
) based on 1.5 cm 3 acid per 10 g of dry matter and incubated for 15 minutes.
The samples are dried in an oven at 150 about C (if there are no aggressive acid fumes) on an electric stove with low heat. Simultaneous irradiation of samples with an IR lamp can be used to speed up the drying of samples.
The dried samples are carefully charred on an electric stove or gas burner until the emission of smoke ceases, preventing ignition and emissions.
The crucibles are placed in a cold electric furnace and, increasing its temperature by 50 about With every half hour, bring the temperature of the oven to 450 about C. Mineralization is continued at this temperature until gray ash is obtained.
The ash cooled to room temperature is moistened drop by drop with nitric acid (1:
) based on 0.5-1 cm 3 acids on a sample, evaporated in a water bath and dried on an electric stove with low heat. Place the ash in an electric furnace, bring its temperature to 300 about C and incubated for 0.5 h. This cycle (acid treatment, drying, ashing) can be repeated several times.
Mineralization is considered complete when the ash becomes white or slightly colored without charred particles.
Wet mineralization. The method is based on the complete decomposition organic matter samples when heated in a mixture of concentrated nitric acid, sulfuric acid and hydrogen peroxide and is intended for all types of products stern butter and animal fats.
A portion of liquid and puree products is introduced into a flat-bottomed flask, wetting the walls of a 10-15 cm glass 3bidistilled water. You can take a sample directly into a flat-bottomed flask.
A portion of solid and pasty products is taken on an ash-free filter, wrapped in it and placed with a glass rod on the bottom of a flat-bottomed flask.
Beverage samples are taken with a pipette, transferred to a Kjeldahl flask and evaporated on an electric stove to 10-15 cm3 .
A portion of dry products (gelatin, egg powder) is placed in a flask and 15 cm 3bidistilled water, mix. Gelatin is left for 1 hour to swell.
Sample mineralization.Mineralization of samples of raw materials and food products except for vegetable oils, margarine, edible fats:
Nitric acid is added to the flask at a rate of 10 cm 3for every 5 g of the product and incubated for at least 15 minutes, then 2-3 pure glass balls, close with a pear-shaped cork and heat on an electric stove at first weakly, then more strongly, evaporating the contents of the flask to a volume of 5 cm3 .
The flask is cooled, 10 cm 3nitric acid, evaporated to 5 cm 3. This cycle is repeated 2-4 times until brown vapors stop.
Add 10 cm to the flask 3nitric acid, 2 cm 3sulfuric acid and 2 cm 3hydrogen peroxide for every 5 g of the product (mineralization of dairy products is carried out without the addition of sulfuric acid).
To remove residual acids, add 10 ml of acid to the cooled flask. 3bidistilled water, heated until white vapor appears and then boiled for another 10 minutes. Cool down. Adding water and heating is repeated 2 more times.
If a precipitate forms, add 10 ml of 3bidistilled water, 2 cm 3sulfuric acid, 5 cm 3 of hydrochloric acid and boil until the precipitate dissolves, supplementing the evaporated water. After the precipitate has dissolved, the solution is evaporated on a water bath to wet salts.
Mineralization of vegetable oils, margarine, edible fats:
lead food product chemistry
The flask with a sample is heated on an electric stove for 7-8 hours until a viscous mass is formed, cooled, 25 cm 3nitric acid and reheat carefully, avoiding vigorous foaming. After foaming stops, 25 cm 3nitric acid and 12 cm 3hydrogen peroxide and heated until a colorless liquid is obtained. If the liquid darkens, 5 cm is periodically added to it. 3nitric acid, continuing heating until mineralization is complete. Mineralization is considered complete if the solution remains colorless after cooling.
acid extraction. The method is based on the extraction of toxic elements with dilute (1:
) by volume with hydrochloric acid or diluted (1: 2) by volume with nitric acid and is intended for vegetable and butter oils, margarine, edible fats and cheeses.
Extraction is carried out in a heat-resistant chamber with a sample of the product. 40 cm3 are introduced into the flask with a cylinder 3hydrochloric acid solution in bidistilled water (1:
) by volume and the same amount of nitric acid (1: 2). A few glass balls are added to the flask, a refrigerator is inserted into it, placed on an electric stove, and boiled for 1.5 hours from the moment of boiling. Then the contents of the flask are slowly cooled to room temperature without removing the refrigerator.
A flask with an extraction mixture of butter, fats or margarine with acid is placed in a cold water bath to solidify the fat. The hardened fat is pierced with a glass rod, the liquid is filtered through a filter moistened with the acid used for extraction, into a quartz or porcelain bowl. The fat remaining in the flask is melted in a water bath, 10 cm 3acids, shake, cool, after cooling, the fat is calcined and the liquid is drained through the same filter into the same bowl, then washed with 5-7 cm 3bidistilled water.
The extraction mixture of vegetable oil with acid is transferred to a separating funnel. The flask is rinsed 10 cm 3acid, which is poured into the same funnel. After phase separation, the lower aqueous layer is drained through a filter moistened with acid into a quartz or porcelain bowl, the filter is washed with 5-7 cm 3bidistilled water.
The extraction mixture of cheese and acid is filtered through an acid-moistened filter into a quartz or porcelain bowl. The flask is rinsed 10 cm 3acid, which is filtered through the same filter, then the filter is washed with 5-7 cm 3bidistilled water.
The filtered extract is carefully evaporated and charred on an electric stove, and then ashed in an electric furnace.
1.4 Methods for the determination of lead
1.4.1 Concentration of trace amounts of lead ion using nanometer particles of titanium dioxide (anatase) for the purpose of their subsequent determination by inductively coupled plasma atomic emission spectrometry with electrothermal sample evaporation
Atomic emission spectrometry with inductively coupled plasma ( ICP-AES) -widely used and very promising method of elemental analysis. However, it has some drawbacks, including relatively low detection sensitivity, low sputtering efficiency, spectral noise, and other matrix effects. Therefore, ICP-AES does not always meet the requirements modern science and technology. The combination of ICP-AES with electrothermal sample evaporation (ETI-ICP-AES) significantly expands the possibilities of the method. By optimizing the pyrolysis and evaporation temperature, the elements to be determined can be sequentially evaporated, separating them from the sample matrix. This method has the advantages of high sample injection efficiency, the ability to analyze small quantities of samples, low absolute detection limits, and the ability to directly analyze solid samples.
Tools and conditions of analysis.We used an ICP generator with a power of 2 kW and a frequency of 27 ± 3 MHz; ISP burner; graphite furnace WF-1А; RO5-2 diffraction spectrometer with a diffraction grating of 1300 lines/mm with a linear dispersion of 0.8 nm/mm; pH meter Mettle Toledo 320-S; model 800 precipitating centrifuge.
Standard solutions and reagents.Stock standard solutions with a concentration of 1 mg/ml are prepared by dissolving the appropriate oxides (spectroscopic purity) in dilute HCl followed by dilution with water to the specified volume. A suspension of polytetrafluoroethylene was added to each standard solution to a concentration of 6% w/o.
Triton X-100 of reagent grade (USA) was used. The rest of the reagents used were of spectroscopic purity; double distilled water. Titanium dioxide nanoparticles less than 30 nm in diameter.
Method of analysis.The required volume of a solution containing metal ions is placed in a 10 ml graduated test tube and the pH value is adjusted to 8.0 with 0.1 M HCl and an aqueous solution of NH 3. Then, 20 mg of titanium dioxide nanoparticles are added to the test tube. The tube is shaken for 10 min. (preliminary experiments have shown that this is sufficient to achieve adsorption equilibrium). The tube is left for 30 minutes, then the liquid phase is removed using a centrifuge. After washing the precipitate with water, 0.1 ml of a 60% suspension of polytetrafluoroethylene, 0.5 ml of a 0.1% agar solution, 0.1 ml are added to it. Triton X-100 and diluted with water to 2.0 ml. The mixture is then dispersed with an ultrasonic vibrator for 20 minutes to achieve homogeneity of the suspension before it is introduced into the evaporator. 20 μl of the suspension is added to the graphite furnace after heating and stabilization of the ICP. After drying, pyrolysis, and evaporation, the sample vapors are transferred to the ICP by a flow of a carrier gas (argon); atomic emission signals are recorded. Before each injection, the graphite furnace is heated to 2700°C to clean it.
Application of the method.The developed method is used to determine Pb 2+in samples of natural lake water and river water. Water samples were filtered through a 0.45 µm membrane filter immediately after sampling and then analyzed.
1.4.2 Determination of lead by real-time combining concentration followed by reverse phase HPLC
Instruments and reagents. A diagram of the HPLC system with real-time concentration ("on-line") is shown in Fig. 1.1 The system consists of a Waters 2690 Alliance pump (in diagram 2), a Waters 515 pump (1), a Waters 996 photodiode array detector (7) , six-way switch valve (4), high volume injector (holds up to 5.0 ml of sample) (3) and columns (5,6). The concentration column was Waters Xterra™ RP 18(5 µm, 20 x 3.9 mm), Waters Xterra™ RP analytical column 18(5 µm, 150 x 3.9 mm). pH was determined with a Beckman F-200 pH meter; optical density was measured with a Shimadzu UV-2401 spectrophotometer.
Figure 1.1Schematic diagram of a real-time concentration system using a switching valve
All solutions were prepared with ultrapure water prepared with a Milli-Q50 Sp Reagent Water System (Millipore Corporation). A standard solution of lead (P) with a concentration of 1.0 mg/ml, working solutions with an ion concentration of 0.2 µg/ml are prepared by diluting standard solutions. Use tetrahydrofuran (THF) for HPLC (Fisher Corporation), pyrrolidine-acetic acid buffer solution concentration of 0.05 mol/l. Glassware before use was soaked for a long time in a 5% nitric acid solution and washed clean water.
Experimental technique. The required volume of the standard solution or sample is added to a 25 ml volumetric flask. 3, add 6 ml of solution T 4CFP with a concentration of 1 x10 -4mol / l in THF and 4 ml of a solution of pyrrolidine-acetic acid buffer solution with a concentration of 1 x 10 -4mol/l and pH 10, dilute to the mark with water and mix thoroughly. The mixture is heated on a boiling water bath for 10 minutes. After cooling, dilute to the mark with THF for further analysis. The solution (5.0 ml) is injected into the dispenser, sent to the concentration column using mobile phase A at a rate of 2 cm3/min. At the end of concentration, by eliminating the six-way valve, metal chelates with T 4The CFP adsorbed in the upper part of the concentration column are eluted with a flow of mobile phases A and B at a rate of 1 ml/min in the opposite direction and sent to the analytical column. A three-dimensional chromatogram was recorded in the wavelength range of the absorption maximum of 465 nm using a detector with a photodiode array.
1.4.3 Stripping voltammetric determination of lead using a glassy carbon electrode system
Instruments and reagents.For research, an electrode system was used, which is an assembly of three identical glassy carbon (GC) electrodes (indicator, auxiliary, comparison) pressed into a common body of tetrafluoroethylene. The length of each electrode protruding from the body is 5 mm. The surface of one of them, chosen as an indicator, was electrochemically treated with an asymmetric current at densities in the range of 0.1–5 kA/m 2recommended for metals. The optimal surface renewal time was found experimentally and was 10–20 s. The indicator electrode served as the anode and the stainless steel electrode as the cathode. We used 0.1 M aqueous solutions of acids, salts, alkalis, as well as 0.1 M solutions of alkalis or salts in a mixture of organic solvents with water in a ratio of 1:19 by volume. The state of the treated surface was observed visually using a microscope "Neophot 21 with an increase of about 3000.
Method of analysis.After processing, the electrode assembly was used to determine 3*10 -6M of lead (II) by stripping voltammetry against a background of 1 * 10 -3M HNO 3. After electrolysis at -1.5 V for 3 min with stirring with a magnetic stirrer, a voltammogram was recorded on a PA-2 polarograph. The potential of the anode peak of lead remained constant and amounted to - 0.7 V. The linear potential sweep rate was 20 mV/s, the sweep amplitude was 1.5 V, the current sensitivity was 2 * 10-7 A/mm.
Aqueous LiNO solutions 3, NaNO 3, KNO 3as a processing electrolyte, they make it possible to obtain stable heights already in the second measurement with satisfactory reproducibility (2.0, 2.9 and 5.4%, respectively). The greatest sensitivity of readings is achieved when using an electrolyte having a smaller cation.
1.4.4 Atomic absorption determination of lead by dosing suspensions of carbonized samples using Pd-containing activated carbon as a modifier
Analytical measurements were performed on a SpectrAA-800 atomic absorption spectrometer with a GTA-100 electrothermal atomizer and a PSD-97 autosampler (Varian, Australia). Pyrocoated graphite tubes with an integrated platform (Varian, Germany), hollow cathode lamps for lead (Hitachi, Japan) and cadmium (C Varian, Australia) were used. Measurements of the integrated absorption with correction of nonselective light absorption (deuterium system) were carried out at a spectral slit width of 0.5 nm and a wavelength of 283.3 nm. The highest grade argon served as the shielding gas. The temperature program of the atomizer operation is given in Table 1.1
Tab. 1.1 Temperature program of operation of electrothermal atomizer GTA-100
StageTemperature, °СDrying 190Drying 2120Pyrolysis1300Cooling50Atomization23ООCleaning2500
Palladium-containing compositions based on activated carbon and carbonized hazelnut shells were studied as modifiers for the atomic absorption determination of Pb in a graphite furnace. The metal content in them was 0.5–4%. To assess the changes that occur with the components of the synthesized modifiers under reducing conditions during the analysis, the materials were treated with hydrogen at room temperature.
A solution with a known concentration of Pb was prepared by diluting GSO No. 7778-2000 and No. 7773-2000 with 3% HNO 3. The range of concentrations of working standard solutions of the element for constructing calibration dependencies was 5.0–100 ng/mL. Deionized water was used to prepare solutions. .
When constructing the curves of pyrolysis and atomization, both the standard solution of the element and the carbonized "Standard sample of the composition of ground wheat grain ZPM-01" were used. In the first case, 1.5 ml of a standard element solution (50 ng/ml Pd in 5% HNO 3) and 10-12 mg of palladium-containing activated carbon; the suspension was homogenized and dosed into a graphite furnace. In the second, the same amount of the modifier was added to the prepared suspension of the carbonized sample (5-10 mg of the sample in 1-2 ml of 5% HNO3 ).
1.4.5 Photometric determination and concentration of lead
Analytical lead acetate was used in the work. Compounds (Fig. 1, which are dibasic acids) were obtained by azo coupling of a solution of 2-hydroxy-4 (5) chloride - nitrophenyldiazonium and the corresponding hydrazone. Solutions of formazans in ethanol were prepared by accurately weighing them.
The absorbance of solutions was measured on a Beckman UV-5270 spectrophotometer in quartz cuvettes (l = 1 cm). The concentration of hydrogen ions was measured on an I-120M ionometer.
Reagents interact with lead ions, forming colored compounds. Bathochromic effect during complexation is 175 - 270 nm. Complex formation is affected by the nature of the solvent and the structure of the reagents (Fig. 1).
The optimal conditions for the determination of lead are a water-ethanol medium (1:
) and pH 5.5-6.0, created by an ammonium-acetate buffer solution. The detection limit for lead is 0.16 µg/mL. The duration of the analysis is 5 min.
The most interesting is the use of formazan as a reagent for preconcentration and subsequent photometric determination of lead. The essence of the concentration and subsequent determination of lead (II) using formazan is that a lead complex is extracted from an aqueous ethanol solution in the presence of Ni, Zn, Hg, Co, Cd, Cr, Fe ions with a formazan chloroform solution.
For comparison, we used the method for determining lead with sulfarsazene (GOST, MU issue 15, No. 2013-79). The results of the analysis of model solutions by two methods are given in Table 1.2 Comparison of dispersions according to the F-criterion showed that Fexp< Fтеор (R= 0.95; f 1=f 2= 5); This means that the dispersions are uniform.
Tab. 1.2 results of lead determination in model solutions (n=6; P=0.95)
Introduced, μg/ml Found Found F exp F theorsulfarsazene, µg/mlS r formazan, µg/mlS r 4.14 2.10 3.994.04 ±0.28 2.06±0.29 3.92 ±0.17 0.29 3.92 ±0.172.8 5.5 1.74.14 ±0.07 2.10 ±0.08 3.99 ± 0.072.1 *10 -2 2.5*10-2 2.1*10-23.97 3.57 3.374.53
2. Experimental part
Measuring instruments, reagents and materials:
When performing according to this technique, the following measuring instruments, devices, reagents and materials are used:
· Atomic absorption spectrometer
· Spectral lamp with hollow cathode
· Compressed air compressor
· Reducer - according to GOST 2405
· Glasses laboratory, with a capacity of 25-50 cm3 - according to GOST 25336
· Volumetric flasks of the second class of accuracy with a capacity of 25-100 cm3
· Funnels laboratory in accordance with GOST 25336
· Distilled water
· Nitric acid concentrated, x. hours, GOST 4461-77
· Lead standard solution (c = 10-1 g/l)
Definition conditions:
§ Wavelength in the determination of lead? =283.3 nm
§ Monochromator slit width 0.1nm
§ Lamp current 10 mA
Method of measurement:
Atomic absorption spectroscopy is based on the absorption of radiation in the optical range by unexcited free lead atoms formed when the analyzed sample is introduced into a flame at a wavelength of ? =283.3 nm.
Safety requirements:
When performing all operations, it is necessary to strictly observe the safety rules when working in a chemical laboratory, corresponding to GOST 126-77 "Basic safety rules in a chemical laboratory", including the rules safe work with electrical devices with voltage up to 1000 volts.
Preparation of lead calibration solutions:
Solutions are prepared using a standard lead solution with a concentration
c= 10-1 g/l.
To build a calibration graph, solutions of the following concentrations are used:
*10-4, 3*10-4, 5*10-4, 7*10-4, 10*10-4g/l
10 ml standard solution 3add to a 100 ml flask, dilute to the mark with distilled water. Add 1, 3, 5, 7, 10 ml of the intermediate solution (solution with a concentration of 10 -2g/l). Make up to the mark with distilled water. A garduirovochny graph is built in coordinates A, y. e from s, g/l
Table 2.1 Measurement results
concentration, g/lSignal, c.u. e. 0.000130.0003150.0005280.0007390.001057
Sample preparation:
I take a sample of coffee weighing 1.9975 g.
I put it in a glass with a capacity of 100 ml.
I dissolve the sample in 20 ml of concentrated nitric acid.
I evaporate the contents of the glass in a water bath to half the original volume, stirring occasionally.
The solution in the glass after evaporation is cloudy, therefore, using a laboratory funnel and a paper filter, I filter the contents of the glass into a glass with a capacity of 25 ml.
I put the filtered solution into a 25 ml flask and make up to the mark with distilled water.
Thoroughly mix the contents of the flask.
I put part of the solution from the flask into a pipette, which serves as a sample for determining the lead content.
To determine an unknown concentration, the solution is injected into the atomizer and after 10-15 seconds the readings of the device are recorded. The average readings of the device are plotted on the y-axis of the calibration graph, and on the abscissa axis, the corresponding concentration value is found, cx g / l
To calculate the concentration in the sample, I use the calculation formula:
C \u003d 0.025 * Cx * 10-4 * 1000 / Mnav (kg)
Table 2.2 Measurement results
ProbaSignal, at. e. AverageC X , g/l 123 coffee15141514.666672.9*10 -4cheese00000apple juice00000grapes juice00000cream3222.333337.8*10 -5water00000shampoo00000
Based on the tabular data, I calculate the concentration of lead in the samples:
Sample MPC, mg/kg coffee 10 cream
С (Pb in coffee sample) = 3.6 mg/kg
С (Pb in cream sample) = 0.98 mg/kg
conclusions
The paper describes methods for the determination of lead by various physicochemical methods.
Methods of sample preparation for a number of food objects are given.
Based on the literature data, the most convenient and optimal method for the determination of lead in various food products and natural objects was chosen.
The method used is characterized by high sensitivity and accuracy, along with the absence of a response to the presence of other elements, which makes it possible to obtain the true values of the content of the desired element with a high degree of reliability.
The chosen method also makes it possible to carry out studies without special difficulties in sample preparation and does not require masking of other elements. In addition, the method makes it possible to determine the content of other elements in the test sample.
According to the experimental part, it can be concluded that the lead content in Black Card coffee does not exceed the maximum permissible concentration, therefore the product is suitable for sale.
List of used literature
1. Glinka N.I. General chemistry. - M.: Nauka, 1978. - 403 p.
Zolotov Yu.A. Fundamentals of analytical chemistry. - M.: Higher. school; 2002. - 494 p.
Remy G. Course of General Chemistry. - M: Ed. foreign lit., 1963. - 587 p.
GOST No. 30178 - 96
Yiping Hang. // Journal. analyte Khim., 2003, V.58, No. 11, p.1172
Liang Wang. // Journal. analyte Khim., 2003, V.58, No. 11, p.1177
Nevostruev V.A. // Journal. analyte Khim., 2000, V.55, No. 1, p.79
Burilin M.Yu. // Journal. analyte Khim., 2004, V.61, No. 1, p.43
Maslakova T.I. // Journal. analyte Khim., 1997, V.52, No. 9, p.931
1. Determination as sulfide. The origins of this method and its first critical appraisal date back to the beginning of our 20th century. The color and stability of the PbS sol depend on the particle size of the dispersed phase, which is affected by the nature and concentration of dissolved electrolytes, the reaction of the medium, and the method of preparation. 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 the determination is less, but it can be somewhat increased by adding electrolytes, such as 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 solution of HC1, 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 from 4-10*10-4%.
3. The determination of Pb impurities in concentrated sulfuric acid is based on the use of the characteristic absorbance at 195 nm with respect to a standard solution prepared by dissolving lead in H2SO4 (high purity).
Determination using organic reagents.
4. In the analysis of various natural and industrial facilities The photometric determination of Pb using dithizone occupies a leading position due to its high sensitivity and selectivity. In various variants existing methods photometric determination of Pb is performed at the maximum absorption wavelength of dithizone or lead dithizonate. Other versions 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 to a concentration of the organic component of 70% before use.
5. Determination of lead by reaction with sodium diethyldithiocarbamate. Lead is well extracted with CCl4 as a colorless diethyldithiocarbamate at various values pH. The resulting extract is used in an indirect method for determining Pb, based on the formation of an equivalent amount of yellow-brown copper diethyldithiocarbamate as a result of exchange with CuSO4.
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, such as steel, brass and bronze, the method based on the formation of a complex with this azo compound is preferable to the dithizone method. However, it is less selective and, therefore, in the presence of interfering cations, it requires preliminary separation by the BC 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 with a composition of 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 of 4-8 forms a blue complex with lead in the composition 1:1 with two absorption maxima - at 605 and 665 nm.
10. Determination by reaction with diphenylcarbazone. According to the sensitivity of the reaction, during the extraction of 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% HCl, and the photometric determination of dichromic acid by reaction with diphenylcarbazide using a filter with a maximum transmission at 536 nm. The method is lengthy 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 a 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 at pH 6.5, and cetyltrimethylammonium and cetylpyridinium bromides at pH 5.0.
14. Determination by reaction with glycinthymol blue. A 1:2 complex with glycinthymol blue (GTS) has an absorption maximum at 574 nm and a corresponding molar extinction coefficient of 21300 ± 600.
15. Determination with methylthymol blue is carried out under conditions as for the formation of a complex with GTS. In sensitivity, both reactions approach 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 ratio is 19,500. Interference from many metals is eliminated by masking.
16. Determination by reaction with EDTA. EDTA is used as a titrant in non-indicator and indicator photometric titration (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 radiation in the presence of Pb is measured due to catalytic oxidation 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, traces of copper catalyze the luminol oxidation reaction. A method based on the use of the fluorescence quenching effect of fluores-132 derivatives is much more difficult in terms of instrumentation 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 water-blue lumogen in a mixture of dioxane-water (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 the determination of lead in HC1, based on the photoelectric detection of green fluorescence of chloride complexes at -70°C.
3. Analysis by luminescence burst during sample defrosting. The methods of this group are based on the shift of the luminescence spectra during the thawing of the analyzed sample and the measurement of the observed increase in the radiation intensity. The wavelength of the maximum of the luminescence spectrum at -196 and - 70 ° C, respectively, is 385 and 490 nm.
4. A method is proposed based on measuring the analytical signal at 365 nm in the quasi-linear luminescence spectrum of CaO-Pb crystal phosphorus cooled to liquid nitrogen temperature. This is the most sensitive of all luminescent methods: if an activator is applied to the tablet surface (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. Simultaneously with Pb, Bi can also be determined.
5. Determination of lead by the luminescence of the chloride complex adsorbed on paper. In this method, luminescence analysis is combined with the separation of Pb from interfering elements using a ring bath. The determination is carried out at ordinary temperature.
Electrochemical methods
1. Potentiometric methods. Used direct and indirect definition lead - by titration with acid - basic, complexometric and precipitation reagents.
2. Electrogravimetric methods use lead deposition on electrodes, followed by weighing or dissolution.
3. Coulometry and coulometric titration. Electrogenerated sulfohydryl 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 10–10 M. proceeding reversibly and in the diffusion mode. As a rule, cathodic waves are well pronounced, 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 electrochemical conversion current 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 to a lesser extent depends on the characteristics of the capillary and the indifferent electrolyte. The great possibilities of the AT method should also be noted, since analysis is possible by an electrochemical reaction involving both Pb itself and the titrant. Although the overall time spent on performing an AT is longer, it is quite 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 uc (benzylidene) - thio - carbohydrazone, thiosalicylamide.
Physical methods for the determination of lead
Lead is determined by atomic emission spectroscopy, atomic fluorescence spectrometry, atomic absorption spectrometry, x-ray methods, radiometric methods, radiochemical methods and many others.
Russian Federation MU (Guidelines)
Guidelines for the photometric determination of lead in air
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METHODOLOGICAL INSTRUCTIONS
FOR PHOTOMETRIC DETERMINATION OF LEAD IN AIR
APPROVED by the Deputy Chief State Sanitary Doctor of the USSR A.I. Zaichenko on June 6, 1979 N 2014-79
I. General part
1. The determination is based on the colorimetric determination of colored solutions formed by the interaction of a lead ion with xylenol orange.
2. The sensitivity of the determination is 1 μg in the analyzed volume of the solution.
3. Iron, aluminum, coal dust, silicate dust containing aluminum and iron, quartz, tin and antimony do not interfere with the determination.
4. The maximum allowable concentration of lead in the air is 0.01 mg/m.
II. Reagents and apparatus
5. Used reagents and solutions.
Stock standard solution containing 100 µg/mL. 0.0183 g Pb (CHCOO). 3 HO is dissolved in acetate buffer pH=6 in a 100 ml volumetric flask and made up to the mark with acetate buffer, shelf life 1 month.
A standard solution of N 2 containing 10 µg/ml of lead is prepared before use by appropriate dilution of the stock solution.
Buffer mixture pH=5.8-6.0; sodium acetate 0.2 M - 9 ... .. * ml, acetic acid 0.2 M - 6 ml.
________________
* Marriage of the original. - Database manufacturer's note.
Xylenol orange, indicator, TU 6-09-1509-72, analytical grade 0.01% solution (stock 100 mg/100 ml). Shelf life 7 days, store in a closed bottle.
The working solution of xylenol orange is prepared by diluting the main (original) 10 times before analysis.
6. Applied utensils and appliances.
aspiration device.
Cartridges for filters.
Chemical test tubes with a height of 150 mm and an internal diameter of 15 mm.
Sulfur
Reducing properties of sulfur
Pour 2-3 ml of concentrated nitric acid into a test tube, add a little sulfur powder and heat the mixture to a boil. To the remaining solution add a solution of barium chloride. The presence of which ion is indicated by the formation of a white precipitate? Write an equation for the oxidation of sulfur with nitric acid.
Reducing properties of thiosulfate
Add 1 ml of starch solution to 2-3 ml of sodium thiosulfate solution and add dropwise iodine solution. Why is blue coloring not obtained? Write the equation for the reaction between iodine thiosulfate and iodine. Add chlorine water to 2 ml of thiosulfate solution. What is observed? Compose the reaction equations. Prove that there are SO 4 2- ions in the solution, which are formed during the oxidation of S 2 O 3 2- ions.
Oxidizing properties of sulfuric acid
Test the effect of dilute sulfuric acid on metallic zinc and concentrated sulfuric acid on metallic copper when heated. How to determine the products of sulfuric acid reduction? Compose the reaction equations.
Charring of organic matter with sulfuric acid
With a glass rod moistened with concentrated H 2 SO 4, write something on paper. Heat the paper slightly, holding it high above the flame. What is observed? Give an explanation. Apply a drop of concentrated H 2 SO 4 to a piece of cloth with a glass rod. After a while, test the fabric for strength.
nitrogen and phosphorus
Turning red phosphorus into white
Place a few grains of red phosphorus into a dry test tube, close with a cotton swab and carefully heat on a low flame. Observe the crystallization of a substance on the cold walls of a test tube and describe the process underlying the experiment.
Equilibrium in ammonia solution
Pipette 2 ml of concentrated ammonia and dilute with water containing 3-4 drops of phenolphthalein solution to 50 ml. Pour the colored solution into three flasks. Put one on the stove to boil the solution, and pour a pinch of solid ammonium chloride into the other and shake. Compare the color in all three flasks and describe the equilibrium and shift of equilibrium in the ammonia solution.
Formation of ammonium salts
Moisten a glass rod with concentrated hydrochloric acid and bring it to a bottle with a concentrated ammonia solution. Describe the observed changes and write an equation for the reaction.
Getting ammonia
In two test tubes, obtain precipitates of zinc and nickel hydroxides and add ammonia solution to them. Describe the changes taking place and write reaction equations.
Oxidizing properties of nitric acid
Dissolve 3-4 crystals of freshly recrystallized iron (II) salt in 5 ml of distilled water. Pour the solution equally into two test tubes. Add 5 drops of concentrated nitric acid to one of them and boil for 2-3 minutes. After cooling, pour a few drops of potassium thiocyanate solution into both test tubes and observe the color change.
The formation of nitric acid and its decomposition
Add dilute sulfuric acid to 3-4 ml of concentrated NaNO 2 solution. Observe the change in color of the solution (against the background of white paper) and write down the reaction equations.
Nitric acid opening reaction
Pour 1 ml of a saturated solution of iron (II) sulfate into a test tube,
3 ml concentrated sulfuric acid, cool the mixture. Having tilted the tube, carefully pour 1-2 ml of nitric acid solution (1:1) along the wall. A brown SO 4 ring forms at the point of contact between the two liquids. Write the reaction equation in two stages: the reduction of nitric acid to nitric oxide (II) and the formation of a complex compound.
2. Metals
Reducing properties of copper:
a) experiment under thrust! Test the effect of dilute and concentrated sulfuric and nitric acids on metallic copper in the cold and when heated. Write down the equations of the reactions carried out;
b) take a piece of copper wire with tongs and ignite it in a muffle furnace. Write down the reaction equation.
Hydrolysis of copper and silver salts:
a) test solutions of copper (II) and silver salts with litmus paper. Mark and write down the equations of hydrolysis reactions;
b) add a saturated solution of soda to a concentrated solution of copper sulfate. Describe the reciprocal hydrolysis reaction, considering that basic copper carbonate precipitates.
Oxidizing properties of the Cu 2+ ion
To a solution of copper sulfate (II) add a solution of potassium iodide. In this case, a white precipitate of Cu 2 J 2 is formed and a yellowing of the solution is observed. Write an equation for the reaction.
Coloring the flame with copper salts
Dip the nichrome wire in a copper (II) chloride solution and bring it into the burner flame. Fix the color of the flame.
Reducing properties of zinc
In three separate test tubes, study the effect of water, hydrochloric acid, and a concentrated alkali solution on metallic zinc. Write the reaction equations and predict the ratio of cadmium to the same reagents.
Obtaining hydroxides
To solutions of salts ZnCl 2, CdCl 2, Hg (NO 3) 2, Hg 2 (NO 3) 2 add a few drops of alkali solution and write down the reaction equations. Then add an excess of alkali to all sediments, fix the differences in the behavior of zinc, cadmium and mercury hydroxides.
Reagents and equipment: 1. FEK - 56. 2. Salt of lead. 3. Acetic acid (CH 3 COOH). 4. Sodium acetate (CH 3 COONa). 5. Volumetric flasks for 100 ml (7 pieces). 6. Burette 25 ml. 7. Nitric acid (1:2).
8. Xylenol orange (indicator).
Progress
Preparation of a buffer solution with pH 4.5.
Weigh 22.57 g of sodium acetate (CH 3 COOHa. H 2 O). Add 5.78 ml of concentrated acetic acid to the salt solution and place the mixture in a 0.5 L volumetric flask, making up to the mark with water, stirring.
Preparation of an aqueous solution of xylenol orange.
A weighed sample of 0.06725 g of xylenol orange is placed in a 0.5-liter volumetric flask, dissolved in 100 ml of water and brought to the mark with water, stirring. The prepared solution has a concentration of 2. 10 - 2 mol / l.
Preparation of lead standard solution.
We dissolve 1 g of metallic lead (os.ch.) in 50 ml of nitric acid, diluted 1:2, and quantitatively transfer the resulting solution into a 1-liter volumetric flask, making up to the mark with water.
To build a calibration graph, we select 20 ml of a standard solution of lead nitrate in a 200 ml volumetric flask, bringing water to the mark, adding 1 ml of nitric acid (1: 2) to the flask. The solution has a concentration of 10 µg/ml.
Construction of a calibration graph
In flasks of 100 ml, we add from a burette 5, 10, 12, 15, 18, 20 ml of a standard solution of lead nitrate, the concentration of which is 10 μg / ml. Add to each flask 10 ml acetate buffer pH 4.5 and 10 ml xylenol orange solution. After 15 minutes, we measure the optical density of the prepared solutions on a photoelectrocalorimeter using a light filter No. 4. We build a calibration graph in the coordinates "С Р b (μg / ml) - optical density D".
Determination of lead concentration in the analyzed solution. We select a volume of 10 ml from the analyzed solution, add 10 ml of a buffer solution with a pH of 4.5 and 10 ml of xylenol orange with a concentration of 2 × 10 - 2 mol / l. We bring to the mark with water and after 15 minutes we measure the optical density on the device. According to the calibration graph, we find the concentration of the solution in a flask of 100 ml and, taking into account the dilution, determine the concentration of lead in the initial solution (0th solution - H 2 O).
METHODOLOGICAL INSTRUCTIONS
FOR PHOTOMETRIC DETERMINATION OF LEAD IN AIR
APPROVED by the Deputy Chief State Sanitary Doctor of the USSR A.I. Zaichenko on June 6, 1979 N 2014-79
I. General part
1. The determination is based on the colorimetric determination of colored solutions formed by the interaction of a lead ion with xylenol orange.
2. The sensitivity of the determination is 1 μg in the analyzed volume of the solution.
3. Iron, aluminum, coal dust, silicate dust containing aluminum and iron, quartz, tin and antimony do not interfere with the determination.
4. The maximum allowable concentration of lead in the air is 0.01 mg/m.
II. Reagents and apparatus
5. Used reagents and solutions.
Stock standard solution containing 100 µg/mL. 0.0183 g Pb (CHCOO). 3 HO is dissolved in acetate buffer pH=6 in a 100 ml volumetric flask and made up to the mark with acetate buffer, shelf life 1 month.
A standard solution of N 2 containing 10 µg/ml of lead is prepared before use by appropriate dilution of the stock solution.
Buffer mixture pH=5.8-6.0; sodium acetate 0.2 M - 9? .. * ml, acetic acid 0.2 M - 6 ml.
________________
Xylenol orange, indicator, TU 6-09-1509-72, analytical grade 0.01% solution (stock 100 mg/100 ml). Shelf life 7 days, store in a closed bottle.
The working solution of xylenol orange is prepared by diluting the main (original) 10 times before analysis.
6. Applied utensils and appliances.
aspiration device.
Cartridges for filters.
Chemical test tubes with a height of 150 mm and an internal diameter of 15 mm.
Volumetric flasks, GOST 1770-59, with a capacity of 25 and 100 ml.
Graduated pipettes, GOST 1770-59, with a capacity of 1, 2 and 5 ml.
Glasses chemical with a capacity of 25 ml.
Sand bath.
Photoelectrocolorimeter FEK-?..*-57 or SF-10.
________________
* Marriage of the original. - Database manufacturer's note.
AFA-VP-20 filters.
III. Air sampling
7. Air at a rate of 20 l/min is aspirated through an AFA-VP-20 filter placed in a plexiglass cartridge. For analysis, 300 liters of air should be taken.
IV. Definition Description
8. Before analysis, it is necessary to check the cleanliness of the glassware as follows: Is the glassware rinsed after washing?..* ml of distilled water with 1 ml of 0.01% xylenol orange. The color should be yellow. When the yellow color of the solution changes to purple, the dishes must be washed again as follows: with water, with acetate buffer, again with water, with distilled water.
________________
* Marriage of the original. - Database manufacturer's note.
The filter with the sample is transferred in an expanded state to a funnel D=7 cm and the sample is treated with 5 ml of glacial acetic acid. In this case, the precipitate dissolves with simultaneous filtration through the same filter. The filter is washed with 5 ml of distilled water, the filtrate is collected in a glass beaker with a capacity of 40-50 ml and evaporated in a sand bath at a temperature of 160-190° to a dry residue. The dry residue is dissolved in 4 ml of acetate buffer, 1 ml of 0.01% xylenol orange is added, and after 10 minutes the optical density of the solutions is measured on a photoelectrocolorimeter at a wavelength of 536 nm in a cuvette with a layer thickness of 10 mm.
For quantitative determination, a calibration graph of the dependence of the optical density of the standard scale on the concentration of lead in solution is built.
Table 10
Standards scale
Standard number |
Standard solution N 2, ml |
Acetate buffer ml |
Xylenol orange, ml |
|