RU2529660C1 - Sorption-spectrophotometric method of determining lead (ii) - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: metal is concentrated from a sample at a fixed pH value, for which an acetate buffer with pH 3.5-4.5 is added to the analysed solution, an indicator film is immersed in the obtained solution for 30-60 minutes and optical density is measured on a spectrophotometer at wavelength of 610 nm after removing said film from the solution. Concentration of lead is determined by a standard addition method or calibration curve method. The indicator film used is a transparent polymer substrate coated with a gelatine layer with thickness of less than 20 mcm, immobilised with aqueous solution of bromopyrogallol red.

EFFECT: high sensitivity and selectivity of determining lead, wider range of determined concentrations and enabling combination of separation, concentration and direct determination of lead.

3 cl, 4 dwg, 6 tbl

Description

The invention relates to analytical chemistry, in particular to sorption spectrophotometric methods of analysis.

One of the most sensitive methods for determining lead in aqueous solutions is the atomic absorption method of analysis. The disadvantage of this method is the need to use complex instrumental equipment, attract highly qualified personnel for analysis, the relatively high cost of the analysis itself [Britske M.E., Savelyeva A.N. - Journal. analyte. chemistry. - 1972.V.31. - S.2042], [Weiz. B / Atomic Absorption Spectroscopy. Weinheim. - N.Y .: Verlay Chemie, 1976].

In analytical practice, the spectrophotometric method of analysis is the most common and affordable.

A known spectrophotometric method for determining lead by reaction with 4- (2-pyridylazo) -resorcinol in some objects, for example, steel, brass and bronze [Dagnall R.M., West T.S., Young P. - Talanta, 1965. Vol.12. P.583-588]. The method is based on the formation of a complex with an azo compound with its further detection. In the presence of interfering cations, this method introduces an additional stage of preliminary separation by paper chromatography or extraction of lead dibenzidyldithiocarbamate with hydrocarbon tetrachloride [Pollard F.H., McOmie J.F.M., Nickless G.-J.Chomatogr. 1959. Vol. 2. P.284-295]. The method is not very sensitive.

A known method for the determination of lead in natural waters by conducting an indicator reaction with xylenol orange [Gurkina T.V., Igoshin A.M. // Journal. analyte. chemistry. - 1965. - T.20. - S.778-781]. The method is based on the reaction of complexation of xylenol orange with lead at a pH of 4.5-5.8 with preliminary extraction separation and subsequent photometric determination. The disadvantages of this method are the low sensitivity and selectivity, as well as the need for preliminary separation of the components.

Closest to the claimed method is a spectrophotometric method for determining lead by indicator reaction with bromopyrogallol red in the presence of cetyltrimethylammonium or cetylpyridinium bromide [T. Prasada Raoand, T.V. Ramakrishna // Talanta. 1980. V.27. - P.439-441]. It is based on a complexation reaction in the presence of sensitizers that increase the color intensity, but do not affect the absorption maximum. The disadvantage of the prototype is the low sensitivity and selectivity, the need for preliminary ion-exchange separation, as well as the need for the preparation and use of toxic reagents.

The technical result of the claimed invention is the development of a more sensitive and selective method for the determination of lead, characterized by a wider range of detectable concentrations and allowing to combine separation, concentration and direct determination of lead.

To achieve a technical result, it is proposed to concentrate the metal from the sample at a fixed pH value, for which acetate buffer with a pH of 3.5-4.5 is added to the analyzed solution, the indicator film is immersed for 30-60 minutes, after its extraction, the optical density on a spectrophotometer at a wavelength of 610 nm. The concentration of lead is determined by the standard addition method or by the calibration curve method. As an indicator film, a transparent polymer substrate is used, on which a gelatin layer with a thickness of up to 20 μm is applied, immobilized with an aqueous solution of bromopyrogallol red with a concentration of 6.0 · 10 ^-4 to 8.5 · 10 ^-4 M for at least 20 minutes.

Figure 1 shows the absorption spectrum of a gelatin film with an immobilized complex of bromopyrogallol red with lead, characterized by a maximum absorption at a wavelength of 610 nm; figure 2 - graphs of the dependence of the optical density of the indicator films on the exposure time in lead solutions: at 1 · 10 ^-5 M - graph a; at 2 · 10 ^-5 M - schedule b; at 5 · 10 ^-5 M - graph c; at 10 · 10 ^-5 M - graph g; Figure 3 - graphs of the absorbance of the films at the pH of the solution lead concentration of 5 × 10 ^-5 M - schedule and a concentration of 2.5 × 10 ^-5 M - graph b; figure 4 presents the calibration dependence of the optical density of the films on the concentration of lead.

The lead lead solution was prepared by dissolving a sample of lead nitrate crystalline hydrate in distilled water, standardization was performed titrimetrically according to the standard method [Umland, F. Complex compounds in analytical chemistry / F. Umland, A. Jansen, D. Tirig.- M .: Mir, 1975. - 468 p.]. Solutions with a lower concentration of lead were prepared on the day of work by sequential dilution of the stock solution.

As an indicator of the film took a transparent photographic film AGFA company for offset printing with a thickness of the gelatin layer to 20 microns immobilized aqueous Bromopyrogallol Red in a concentration of 6.0 × 10 ^-4 to 8.5 × 10 ^-4 M, for at least 20 minutes, as described in the article by Temerdashev Z.A., Pochinok TB, Tarasova P.V., Guest M.A. Investigation of the immobilization of bromopyrogallol red into a gelatin matrix and evaluation of the possibility of creating an optically transparent sensor for detecting metals on its basis (Analytics and cont ol -. 2012. T. - 16. - №1 -. S.39-45). When using solutions of bromopyrogallol red with concentrations less than 6.0 · 10 ^-4 M, the color was not intense enough; in the case of reagent concentrations greater than 8.5 · 10 ^-4 M, a precipitate gradually precipitates. The time of immobilization of the films should be at least 20 minutes, otherwise the films are painted unevenly.

To study the complexation process that occurs in the gelatin layer over time, the gelatin films previously immobilized by the indicator mixture were lowered into a solution of lead nitrate with a concentration of Pb ^{2+ ions} : (1 · 10 ^-5 M; 2 · 10 ^-5 M; 5 · 10 ^-5 M; 10 · 10 ^-5 M) and withstood different times. The optical density of the films was measured at a wavelength of the maximum absorption of the complex 610 nm (figure 1). At the indicated wavelength, the maximum difference in the absorption of the complex and the reagent is observed. Table 1 presents the optical density values of the indicator films after aging in lead solutions for various times. According to the data in Table 1, graphical dependences of the optical density of the films on the holding time in lead solutions were constructed (figure 2).

Table 1 The dependence of the optical density of the indicator films on the exposure time in lead solutions Lead time, min Optical density of films 1 · 10 ^-5 , M 2 · 10 ^-5 , M 5 · 10 ^-5 , M 10 · 10 ^-5 , M 5 0.01 0.02 0.02 0.13 fifteen 0.08 0.10 0.11 0.27 thirty 0.12 0.15 0.29 0.58 60 0.13 0.19 0.40 0.68 90 0.14 0.19 0.39 0.68 120 0.14 0.19 0.40 0.69

As follows from the graphs in figure 2, the optical density of the films reaches the maximum constant value after 60 minutes of contact with the solution, which indicates the maximum extraction of the metal to be determined from the solution. In the range from 30 to 60 minutes of contact, their optical density also allows analysis. When the contact time is less than 30 minutes, low optical density of the films leads to a loss of sensitivity analysis.

To select the optimal pH value of the medium during the reaction, indicator films were immersed in solutions with a constant content of Pb ^{2+ ions} and a different pH of the solution; after keeping for 60 min, their optical density was measured. The pH value was determined using a pH meter ion meter "Expert-001". Acidity varied in the range of 1.2–11.0 by adding different amounts of potassium hydroxide or hydrochloric acid to the initial solution in the presence of acetate buffer with pH = 4.45. The experiment was carried out for two series of solutions with a constant lead concentration of 5 · 10 ^-5 M (series 1) and 2.5 · 10 ^-5 M (series 2). The experimental data are shown in tables 2, 3.

In accordance with the data obtained, we plotted the dependence of the optical density of the indicator film on the pH of the solution (Figure 3), which shows that the optical density is maximum in the pH range 3.5-4.5.

At a pH of less than 2.5, the optical density of the indicator films is noticeably lower, which is apparently explained by the possible destruction of the complex due to protonation of the acid residue of bromopyrogallol red and protonation of nitrogen atoms of the polypeptide bonds and terminal amino groups of gelatin. A decrease in the optical density of the indicator film at pH greater than 7 can be explained by a decrease in the sorption ability of gelatin as a result of the deployment of triple helices due to repulsion of the same charges of macromolecules.

A significant effect of pH on the optical density of the films leads to stabilization of its value by adding buffer solutions, for example, acetate. It is difficult to carry out an indicator reaction in a solution with a pH of about 4 because the optical densities of the films were greater than 3. High optical densities were also observed for a background film immersed in a buffer solution with a pH of 3.5–4.0, which does not contain lead, which led to a decrease in the ratio of analytical signal: background. It was noted that at this pH value, the reagent was practically not washed out of the film; therefore, the reaction with Pb (II) ions proceeded directly in a thin gelatin layer. At a pH of the solution from 4.5 to 5.5, the reaction proceeded in the solution layer near the surface of the gelatin, the optical density of the background film (comparison film) due to leaching of the reagent turned out to be noticeably lower, therefore, the solution pH of 4.5 was optimal.

table 2 The optical density of the films for different pH (series 1) pH 1.76 2.44 2.70 2.96 3.14 3.43 3,58 3.73 3.79 3.89 4.09 4,51 4.98 6.15 7.22 11.10 A 0.60 0.79 0.88 1.01 1.21 1.38 1.37 1.35 1.28 1.22 1.05 0.66 0.46 0.28 0.22 0.09

Table 3 The optical density of the films for different pH (series 2) pH 1.27 1,50 2.44 2.71 3.15 3.60 3.88 4.22 4.25 4.28 4.30 4.34 A 0.13 0.15 0.24 0.30 0.72 0.78 0.71 0.60 0.34 0.28 0.41 0.30

To build a calibration graph, we prepared a series of solutions with different concentrations of lead ions (1 · 10 ^-6 ; 2 · 10 ^-6 ; 4 · 10 ^-6 ; 6 · 10 ^-6 ; 8 · 10 ^-6 ; 1 · 10 ^-5 ; 2 · 10 ^-5 ; 4 · 10 ^-5 ; 6 · 10 ^-5 ; 8 · 10 ^-5 ; 1 · 10 ^-4 M), the pH of the solutions was maintained by constant addition of acetate buffer with pH = 4.5. Indicator films were lowered into the solutions, held for 60 minutes, after which they were measured on a UV-1800 spectrophotometer (SHIMADZU, Japan) at a wavelength of 610 nm, corresponding to the maximum absorption of the complex. The results are presented in table 4, the calibration dependence in figure 4.

Table 4 The dependence of the optical density of the indicator film on the concentration of lead in solution C (Pb ²⁺ ) · 10 ⁶ , M one 2 four 6 8 10 twenty 40 60 80 one hundred A 0.04 0.06 0,07 0.08 0.11 0.12 0.17 0.31 0.43 0.58 0.65

The linearity range of the optical density of the films from the concentration of lead is in the range of 1 · 10 ^-6 to 1 · 10 ^-4 M.

The detection limit was calculated by the generally accepted formula:

, $$C_{\min }=\frac{3S}{tg\alpha }$$

,

where S is the standard deviation of the background value

$$S=\sqrt{\frac{{\displaystyle \sum \left({\Delta }_i^2\right)}}{n-\mathrm{one}}}$$

tgα is the slope of the calibration curve.

The calculated value of C _min for aqueous solutions of lead by the claimed method is 3.8 · 10 ^-7 M.

To study the interfering effect of extraneous ions on the determination of lead by reaction with bromopyrogallol red immobilized in the gelatin layer, substances were selected that can interact with lead ions and a reagent or are common in environmental objects.

By sequential dilution, 5 · 10 ^-2 , 5 · 10 ^-3 , 5 · 10 ^-4 , 5 · 10 ^-5 , 5 · 10 ^-6 M solutions of the components were prepared, the interfering effect of which was studied. Prepared working solutions containing lead and interfering ion in the ratios of 1: 0.01, 1: 0.1; 1: 1, 1:10, 1: 100. For this, 2.5 cm ^{3 of} 5 · 10 ^-4 M solution of lead, 5 cm ^{3 of} acetate buffer with pH 4.5 and a corresponding amount of a solution of an extraneous ion were introduced into a 25 cm ³ volumetric flask. Indicator films were lowered into the solution and held for 60 minutes; optical density was measured. The optical density of the films after aging in a solution of lead that does not contain foreign matter was 0.44 ± 0.03. The experimental data are presented in table 5.

Table 5 The optical density of the films in the presence of an extraneous component Foreign ion The optical density of the films with a ratio of Pb: foreign ion: 1: 0.01 1: 0.1 1: 1 1:10 1: 100 Na ⁺ 0.42 0.42 0.41 0.41 0.40 ^{K +} 0.43 0.44 0.44 0.43 0.43 Ca ²⁺ 0.46 0.46 0.46 0.44 0.42 Mg ²⁺ 0.44 0.47 0.47 0.45 0.40 Ni (II) 0.44 0.45 0.48 0.52 0.95 Cu (II) 0.44 0.53 0.91 1,66 2.27 Mn (II) 0.43 0.47 0.45 0.44 0.42 Cd ²⁺ 0.45 0.52 0.53 0.54 1.29 Zn ²⁺ 0.43 0.44 0.47 1,69 2.16 Fe (III) 0.46 0.77 1.34 1.81 1.98 Al ³⁺ 0.44 0.45 0.45 0.45 0.45

$$N{O}_3^{-}$$

0.41 0.45 0.44 0.43 0.44 Cl ^- 0.45 0.49 0.48 0.44 0.49 Br ^- 0.45 0.46 0.43 0.45 0.45 $$S{O}_{\mathrm{four}}^{2-}$$

0.44 0.45 0.44 0.41 0.41 $$P{O}_{\mathrm{four}}^{3-}$$

0.45 0.46 0.30 0.15 0.02

, Cl^-, Br^-, $$S{O}_4^{2-}$$

не оказывают влияние на оптическую плотность пленок при определении свинца по реакции с бромпирогаллоловым красным. Мешающее влияние Ni²⁺ и Zn²⁺ проявляется при соотношении со свинцом 10:1. Мешающее влияние фосфат-ионов проявляется при эквимолярном соотношении; Cu(II), Fe(III) и Cd начинают оказывать мешающее влияние на аналитический сигнал свинца при соотношении 1:0,1. На основании изложенного можно сделать вывод, что предлагаемый способ обладает большей селективностью.It was found that Na ⁺ , K ⁺ , Ca ²⁺ , Mg ²⁺ , Al ³⁺ , Mn ²⁺ , $$N{O}_3^{-}$$

, Cl ^- , Br ^- , $$S{O}_{\mathrm{four}}^{2-}$$

do not affect the optical density of the films when determining lead by reaction with bromopyrogallol red. The interfering effect of Ni ²⁺ and Zn ²⁺ appears when the ratio with lead is 10: 1. The interfering effect of phosphate ions appears at an equimolar ratio; Cu (II), Fe (III) and Cd begin to interfere with the analytical signal of lead at a ratio of 1: 0.1. Based on the foregoing, we can conclude that the proposed method has greater selectivity.

The correct determination of lead by means of indicator films was carried out on a real object — in the air of the working area of the Biotech-Yug LLC printing plant (Krasnodar) and the Kuban Tsvetmet plant (Afipsky settlement, Krasnodar Territory). Samples were taken at various points in the working area during various process conditions.

Air samples of each point of the working area were simultaneously aspirated using two PU-ZE electric aspirators at a speed of 40 dm ³ / min through AFA filters for 20 minutes. Electric aspirators provide for the possibility of simultaneously pumping air through two identical filters. Each sample filter was transferred to a porcelain cup, 2 cm3 of a mixture of sulfuric and nitric acids were wetted. Heated in a sand bath until a solid precipitate formed. The sample was then transferred to a muffle furnace and ashed at 500 ° C for 60 minutes. After cooling, the ash was treated with 5 cm ³ acetate buffer with a pH of 4.5, carefully grinding the precipitate, adjusted to 10 cm. The solution was centrifuged, the resulting centrifugate was divided into two parts. The first immersed indicator film for 60 minutes. As an indicator film of comparison, we took a film obtained by similar processing of clean filters. The second part of the sample was analyzed by atomic absorption spectrometry (AAC). For each point, the lead concentration was measured in triplicate (n = 3).

The content of the substance was determined both according to a previously constructed calibration schedule, and by the method of standard additives. The experimental data are presented in table 6.

Table 6 Validation of the methodology An object Sample number

в воздухе, мг/м³ $$C_{Pb}^{2+}$$

in air, mg / m ³ MPC, mg / m ³ Indicator Films (SF) AAS NPP KubanCvetMet CJSC one 0.035 ± 0.003 0.035 ± 0.002 2 0.034 ± 0.002 0.033 ± 0.002 3 0.022 ± 0.003 0.021 ± 0.002 0.01 four 0.011 ± 0.002 0.011 ± 0.001 Printing house of Biotech-Yug LLC one 0.021 ± 0.003 0.021 ± 0.003 2 0.013 ± 0.003 0.011 ± 0.003 3 0.009 ± 0.002 0.009 ± 0.001 four 0.008 ± 0.002 0.008 ± 0.002

Using the proposed method, the detection limit of lead in air is 0.001 mg / m ³ . According to regulatory documents (GN 2.2.5.1313-03 "Maximum permissible concentrations (MPC) of harmful substances in the air of the working area". M., 2006. 268 S.). MPC lead in the working area is 0.01 mg / ^m3. This concentration is an order of magnitude higher than the detection limit of lead by the proposed method.

Compared with the prototype, the proposed method for determining lead in aqueous media is characterized by a wider range of detectable concentrations and greater sensitivity due to the concentration of the reagent in a thin layer of gelatin, as a result of which the concentration of bromopyrogallol red in a thin layer exceeds the concentration in the solution by about 100 times. The method allows you to combine separation, concentration and direct determination of lead.

The resulting technical result provides the distinctive features of the proposed method, i.e. the proposed method has an inventive step, novelty and industrial applicability.

Claims (3)

1. A sorption-spectrophotometric method for determining lead (II), including the indicator reaction of bromopyrogallol red, characterized in that acetate buffer with a pH of 3.5-4.5 is added to the analyzed solution, the indicator film is immersed for 30-60 minutes, the optical density of the indicator film is removed and measured on a spectrophotometer at a wavelength of 610 nm, the concentration of lead is determined by the standard addition method or by the calibration curve method, and the transparent film is used as an indicator film w polymer substrate coated with a layer of gelatin to 20 microns thick immobilized aqueous bromopyrogallol red. 2. The method according to claim 1, characterized in that the indicator film is made of a transparent photographic film for offset printing by AGFA with a gelatin layer thickness of up to 20 μm, immobilized with an aqueous solution of bromopyrogallol red with a concentration of 8.5 · 10 ^-4 M, for not less than 20 minutes. 3. The method according to claim 2, characterized in that the immersion time of the indicator film is 60 minutes.

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