RU2302628C1 - Electrochemical mode of determination of selenium and arsenic in natural objects - Google Patents

Abstract

FIELD: the invention refers to analytical chemistry.

SUBSTANCE: for conducting impressive background monitoring of water ecosystems, identification of natural and antropogenic sources of ingressing in them of arsenic it is necessary to determinate small concentrations in an interval from 0,1 to 3 maximum permissible concentration. The invention proposes an electrochemical mode of determination of selenium and arsenic in natural objects in which for recovery of all forms of elements including hybrids it is necessary to use a solution consisting out of water, 1% of sodium hydroxide and 3% of sodium borohydride. For extraction of selenium and of arsenic microadditions of gold comparable on content with selenium and arsenic are added into the solution. The analyst is accumulated on the surface of the graphite electrode at constant potential 0,0 V during 90 seconds using gold as a collector of hydrogen solenide and arsine.

EFFECT: after the stage of accumulation an analytical signal in the shape of a pick of the cathode current is registered at potential from -0,5 to -0,6 V of selenium and from -0,8 to 0,9 V of arsenic in the mode of differential-impulse current-voltage, linear dependent from the concentration of selenium and arsenic in the volume of the analytical probe.

2 tbl, 3 dwg

Description

The invention relates to the field of analytical chemistry of environmental objects (natural waters, suspended matter of rivers and lakes, soil, bottom sediments and other solid samples) for the quantitative determination of selenium and arsenic by voltammetric method. To carry out representative background monitoring of aquatic ecosystems, identify natural and anthropogenic sources of selenium and arsenic input into them, it is necessary to determine low concentrations in the range from 0.1 to 3 maximum permissible concentrations. (MPCv.r. = 1 μg / L for Se, MPCv.r. = 10 μg / L for As). The upper limit corresponds to the sanitary-toxicological control indicator of maximum permissible concentration. Moreover, the maximum allowable level of exposure to humans through water for selenium and arsenic is 50 μg / L. The normalized indicators for mineral waters, for example, are 5 times lower and amount to 10 μg / L for Se and 6-10 μg / L for As. Thus, the content of elements in most natural objects in the absence of anthropogenesis is from 1 to 30 μg / l.

A known method for the electrochemical determination of arsenic in a quantitative chemical analysis of food products [Quantitative chemical analysis of samples of food products. Methodology for measuring mass concentrations of arsenic and iron by the IV method (MU 08-47 / 078). Tomsk, TPU. 1997. 35 S.] in the range of mass concentrations of arsenic from 0.01 to 1.0 mg / DM ³ . The technique is based on inversion-voltammetric measurements with linear potential sweep in constant current mode of recording the anode current in the form of a peak with a maximum in the range of -0.05 ... + 0.05 V relative to the silver chloride electrode. The preparation of an analytical sample is proposed in multistage, as complexone - Trilon B with a concentration of 0.1 mol / l, and arsenic (V) - hydrazine sulfate as a reducing agent. The restoration of pentavalent arsenic to trivalent is necessary to obtain an analytical signal of the anodic oxidation of arsenic accumulated on the surface of a working ash photographic electrode (ZGE). Inconvenient in this embodiment is a laborious, multi-stage sample preparation, including the stage of calcination for 5-10 minutes at a temperature of 550 ° C. Despite the addition of magnesium salts, which forms a sparingly soluble compound with arsenate [Yu. Yu. Lurie Handbook of analytical chemistry. M.: Chemistry, 1989. P.76], analyte losses in the proposed method are inevitable and thermodynamically caused due to the high pressure of arsenic vapors [Chemical analysis / A series of monographs on analytical chemistry // Selected methods of trace metal analysis: Biological and Eviron-mental Samples / Jon C. Van Loon. 1985 / NY / Toronto / Vol.80. - 351 pp.]. In preliminary experiments on standard solutions, we were not able to construct a calibration graph in the region of 10 μg / L of a standard solution of trivalent arsenic, and the gold-photography electrode gave a reproducible analytical signal a limited number of times.

The known method of quantitative chemical analysis of samples of drinking, mineral, natural and purified wastewater for the content of selenium by voltammetry [MU 08-47 / 082, Tomsk, 1998. Developers: E. A. Zakharova, O. G. Filichkina, N. P. Pikula]. As in the case of arsenic, the technique is based on the electroconcentration of selenium on the GGE followed by the recording of the anode peak of selenium oxidation against a background of perchloric acid in constant current mode.

The service life of the ZG electrode is limited, special conditions for its storage are required, a special program for the formation of gold on the surface of graphite. In this technique, electrolyte change is used to eliminate the interfering effect of ions on the analytical signal.

Of the known technical solutions, the closest in purpose and technical essence to the claimed object is a method for determining arsenic (ISO 6595) and selenium (ISO 9965) by the hydride method with atomic absorption termination of the analytical procedure (prototype), which consists in the thermal decomposition of arsine (hydrogen selenide) into elemental arsenic (selenium) and hydrogen in a quartz cuvette at 900 (800) ° C [G.S. Fomin. Water. Chemical, bacterial and radiation safety control according to international standards. Encyclopedic reference book. M .: "Protector", 2000. S.295-301, S.360-363]. Common to the prototype and the claimed invention is the use of an alkaline solution of sodium borohydride, which is added to an acidified to pH less than 2 sample to form arsenic and selenium hydrides, which are formed after the restoration of all forms of metalloids in solution.

The disadvantages of the prototype include:

- the use of an expensive atomic absorption spectrometer with a deuterium background corrector and an automatic hydride attachment connected to a quartz furnace, which simultaneously serves as an optical cuvette;

- the presence in one device of hydrogen and a furnace heated to 900 (800) ° C, which is a source of increased danger.

The proposed method for the determination of selenium and arsenic, which consists in the chemical reduction of all forms of elements to their hydrides with an alkaline solution of sodium borohydride, followed by extraction of selenium and arsenic from the analyzed solution by electrosorption on the surface of a graphite electrode and gold addition as a collector of hydrogen selenide and arsine, followed by registration of the analytical signal in as cathode current peaks at a potential of from -0.5 to -0.6 V of selenium and from -0.8 to -0.9 V of arsenic in the mode of differential-pulse voltammer Rhee, completely eliminates these shortcomings by providing for wanted objects representative of environmental protection monitoring the concentration ranges from 0.1 to 30 g of arsenic (selenium) per liter of analytical sample, i.e. from 0.01 to 3 PDKv.r. (Maximum permissible concentration of harmful substances for fishery reservoirs). For selenium - from 0.1 to 30 MAC; At the same time, 10-fold excess of selenium does not interfere with the determination of 1 μg of arsenic per liter of sample, and 10 μg of arsenic introduced into a standard solution of selenium with a concentration of 1 μg Se / L allows quantitative determination of its concentration.

The proposed method differs from the prototype in that it reduces the cost of the analysis due to the use of much simpler and cheaper equipment that does not have sources of increased danger for the analyst, eliminates the stage of high-temperature decomposition of the analyte with the aim of atomization. Electrochemical analyzers are easier to automate, much smaller in size and mass, and electrochemical methods make it possible to control an object for the content of two elements in the same sample in one voltammetric cycle. Field-based voltammetric analyzers allow you to control harmful substances directly at the site of water sampling.

The proposed method for the determination of arsenic differs from the known IV-determination by the method of preparing liquid samples, by the technology of the hydride method of reduction with an alkaline (1% sodium hydroxide) solution of sodium borohydride (3%). Moreover, all forms of water-soluble selenium and arsenic are reduced by 99.9% to its hydride. In contrast to the well-known IV-method for determining arsenic, which uses multi-stage preliminary sample preparation, such heating, despite safety measures (magnesium salt additives), leads to loss of arsenic.

In the case of selenium, the drawbacks of the IV method are the special preparation of the ZGE and the stage of changing the electrolyte to a solution (0.10-0.15 mol / L) of perchloric acid (an expensive and dangerous reagent). To exclude analyte losses, the proposed method uses the addition of 0.2 ml of an alkaline 3% solution of sodium borohydride to 10 ml of a water sample previously acidified to a pH of less than 2, and the optimal acidity of the analytical sample corresponds to a 0.1 M solution of hydrochloric acid. In the case of standard solutions of arsenic (III) and selenium (IV), there was a chemical reduction of metalloids to arsine and hydrogen selenide:

4AsO ₂ ^- + 3NaBH ₄ + 4H ⁺ → 4AsН ₃ ↑ + 3NaBO ₂ + 2Н ₂ О

4SeO ₃ ^2- + 3NaBH ₄ + 8H ⁺ → 4Н ₂ Se ↑ + 3NaBO ₂ + 6Н ₂ О

The neutralization reaction proceeding in parallel with the formation (arsine) of selenium hydrogen leads to the formation of sodium chloride and water in amounts equivalent to the amount of sodium hydroxide. The electrolyte - sodium chloride formed during the neutralization reaction is additionally a complexing agent for the auxiliary element - gold, which is introduced into the solution of the analytical sample in amounts comparable to the volume concentration of arsenic and selenium. In verification experiments, gold was introduced into the analyzed sample with a volume of 10 ml in the form of hydrochloric acid in an amount of 0.5 ml of a gold solution with a concentration of 10 μg / L.

To expand the determined concentrations of the elements, a microadditive of gold was introduced in amounts comparable to the volume concentrations of arsenic and selenium.

The adsorption accumulation of arsenic and selenium was carried out at 0 V, respectively, relative to the silver chloride electrode for 90 seconds. The elements were electrosorbed with a graphite electrode in the form of arsine and hydrogen selenide, which were then cathodically reduced at potentials of -0.9 and -0.6 V in the cathode cycle of scanning the electrode potential. The following reactions proceeded:

(-0,9 В)AsH ₃ → As ⁰ + 3H ⁺ +3

(-0.9 V)

(-0,6 В)H ₂ Se → Se ⁰ + 2H ⁺ +2

(-0.6 V)

The electrons released as a result received gold cations (+3), which, discharging on the surface of the working electrode, play the role of a collector. Voltammograms of the cathodic electroreduction of the adsorbed concentrate were recorded in the differential-pulse mode, polarizing the working electrode from 0.0 to -1.2 V at a speed of 120 mV / s. On the voltammogram recorded two cathodic peaks, differing approximately 200 mV (figure 1). These signals correspond to the recovery of selenium and arsenic, which is confirmed by published data.

To register voltammograms, we used the TA-2 voltammetric analyzer (NPP Tekhnoanalit, Tomsk, TU 4215-000-36304081-95) in the differential-pulse potential sweep mode. The following calibration dependences of the maximum cathode current on the concentration of the standard were obtained:

I [nA] = - 0.10-0.05 ° C _As (r = 0.982) of FIG. 2

I [nA] = 0.29-0.19 ° C _Se (r = 0.990) FIG. 3

In parallel, the standards were analyzed by atomic absorption spectrometry with hydride atomization. The results of determining the content of selenium in snow, river water, pore water of bottom sediments, acid extraction from soils are presented in table 1.

The upper limit of the determined concentrations of metalloids is limited by the solubility of their hydrides in a hydrochloric acid background and is about 70 μg (AsH ₃ ) and 40 mg (H ₂ Se) 10 ml of water [Properties of inorganic compounds. Directory. L .: Chemistry, 1983. P.163, 191]. In the case of real objects, the loss of arsenic in the form of organic forms in the proposed method was eliminated using "wet" ashing of solid samples (suspended matter, bottom sediment), for example, 0.100 grams in a mixture of mineral acids: sulfuric + nitric + hydrochloric = 1 ml + 2 ml + 1 ml A sample of the solid sample was placed in a 100 ml heat-resistant flask with a thin section equipped with a reflux condenser, to which acids were added successively and carefully heated until the evolution of nitrogen oxides ceased. Samples wetted by the wet method were quantitatively washed with double-distilled water into a 25 ml volumetric flask and the volume was adjusted to the mark. An aliquot of 1 to 5 ml was taken for analysis, diluting to 10 ml with a background solution of 0.1 mol / L hydrochloric acid containing no arsenic (selenium). The results of the analysis of samples of snow particles, river suspended matter, bottom sediments are presented in table 1. This table shows the results of the analysis of the same samples of natural objects by the proposed method and atomic absorption method. Results converge within random errors. Table 2 illustrates the distribution of arsenic in the same objects. The proposed method is incomparably cheaper and safer than an atomic absorption prototype. The following abbreviations are adopted in tables 1 and 2 below:

AAS - atomic absorption spectrometry;

IVA - inversion voltammetry;

ES - solid particles of snow mass;

BB - suspended substance;

PV - pore water (bottom sediments);

DO - bottom sediments;

GDP - water extraction of soils;

KVP - acid extract of soils;

The results of the IVA presented in tables 1 and 2 were obtained under optimal conditions (claims): 0 V storage potential, 90 s storage time, registration of cathode signals of selenium - (0.5 ... 0.6 V) and arsenic - ( 0.8 ... 0.9 V) in the differential-pulse mode at a cathode sweep potential speed of 120 mV / s ranging from 0 V to -1.2 V. The potentials for the appearance of cathode peaks of selenium and arsenic are determined by the potential-determining reduction reactions presented above hydrides to elements in the zero oxidation state. The electrochemical analytical technology proposed in the method provides the limits for determining the concentration of selenium and arsenic necessary for the implementation of monitoring studies, differs from existing analogues in its simplicity and safety.

Claims (1)

The electrochemical method for determining selenium and arsenic in natural objects, which consists in the fact that to restore all forms of arsenic to arsenic hydride, a solution consisting of water, 1% sodium hydroxide and 3% sodium borohydride is used, which is added in an amount of 0.2 ml to 10 ml of a liquid analytical sample, previously acidified with hydrochloric acid to a pH of less than 2, characterized in that, to extract selenium and arsenic, microadditives of gold comparable in content with selenium and arsenic are introduced into the analyzed solution and accumulate anal it on the surface of the graphite electrode at a constant potential of 0.0 V for 90 s, using gold as a collector of hydrogen selenide and arsine, and after the accumulation stage, an analytical signal is recorded in the form of a cathode current peak at a potential from -0.5 to -0.6 V selenium and from -0.8 to -0.9 V arsenic in the mode of differential-pulse voltammetry, linearly dependent on the concentration of selenium and arsenic in the volume of the analytical sample.

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