RU2102736C1 - Method of inversion-volt-amperometric determination of heterovalent arsenic forms in aqueous solutions - Google Patents

Abstract

FIELD: analytical chemistry. SUBSTANCE: method is based on electroaccumulation of As (III) on stationary mercury electrode in the presence of ions Cu²⁺ followed by recording curve of cathode reduction of concentrated copper arsenide. Method involves determination of As (III) content at background of 0.6 M HCl + 0.04 M N₂H₄ HCl + 50 mg/l Cu²⁺ by height of inversion cathode peak at potential (-0.72) V, chemical reduction of As (V) to As (III), measurement of total water-soluble content of arsenic and determination of As (V) content by concentration difference of total and trivalent arsenic. Method involves additional addition of HCl, KJ and Cu²⁺ to solution where As (III) content has been determined, Chemical reduction of As (V) to As (III) is carried out in background electrolyte which has the following components: 5.5 M HCl + 0.1 M KJ + 0.02 M N₂H₄ 2 HCl + 100 mg/l Cu²⁺. Electroaccumulation of arsenic is carried out at potential (-0.55 ± 0.01) V, cathode volt-ampere curve is recorded at the range of voltage from (-0.55) V to (-1.0) V and total arsenic content in solution is determined by height of inversion peak at potential (-0.76 ± 0.01) V. EFFECT: improved method of determination. 8 dwg

Description

 The present invention relates to analytical chemistry and can be used for automatic or rapid analysis of waste and natural waters, as well as technological solutions of non-ferrous metallurgy.

 There is a method of inversion-volt-ampereometric determination of arsenic in treated wastewater of the copper industry (see article Kaplin A.A. Weitz N.A. Mordvinova N.M. Determination of arsenic in treated wastewater of the copper industry. Factory Laboratory, 1977. T. 43, issue 9, p. 1051 1052).

The essence of the known method consists in the preliminary electroconcentration of arsenic on a gold electrode and the subsequent removal of the anode volt-ampereogram. The method is as follows. The analyzed volume (0.1 10 ml) of water is evaporated from 0.5 ml of H ₂ SO ₄ (pl. 1.84) to SO ₃ vapor in a quartz polarographic glass. A few crystals of hydrazine sulfate were added and heated for 5 min to convert the polarographically inactive As (V) to As (III). After cooling, distilled water is added to a volume of 2 ml. Purge the solution with nitrogen, conduct electroconcentration of arsenic on a gold electrode at -0.7 V (NCE) for 1 10 min and remove the anode volt-ampereogram. The potential of the anode peak of arsenic is +0.14 V (US ke). The concentration of arsenic is determined by the method of standard additives. The detection limit of arsenic at an electrolysis time of 5 min, a background solution volume of 3 ml, an electrode surface of 0.15 cm ² is 5 • 10 ^-10 g / ml. Relative determination error + 15%
Thus, the known method does not ensure the determination in wastewater of multivalent forms of arsenic As (V) and As (III), significantly differing in their toxicity. In addition, its significant drawback is sample preparation difficult for automation, including, in particular, operations such as evaporation, heating, cooling, and nitrogen purging.

 There is also a method for determining arsenic in polluted waters by differential impulse anode inversion voltammetry (see Leung PC Subramanian KS Meranger S. Determination of arsenic in polluted waters by differential pulse anodic stripping voltammetry "Talanta", 1982, 29, No. 6, 515 518).

The method is as follows. 5 ml of a sample containing arsenic 1 5 ng / ml is placed in a 25 ml temperature-controlled tube. At higher concentrations of arsenic, the analyzed solution is diluted to a content of 20 to 50 ng / ml As. 2 ml of a mixture (24: 24: 1) of HNO ₃ , HClO ₄ , H ₂ SO ₄ are added to the test tube. At 100 ^° C., the volume in the test tube was adjusted to 1 ml and then evaporation was carried out at 200 ^° C. The residues were transferred to a distillation apparatus and As (V) was reduced with a 1% solution of CuCl in concentrated HCl. AsCl _{3 is} distilled off and collected in a receiver containing 4 ml of distilled water. Distillation is carried out for 16 minutes in a stream of N ₂ at a rate of 140 ml / min. Then concentrated HCl was added to the receiver to a total volume of 5 ml. The accumulation is carried out at a potential of -0.05 V on a pyrolytic graphite electrode for 150 s, the electric dissolution is carried out at a potential sweep speed of 5 mV / s in the region of -0.05 0.3V. The amplitude of the pulse is 100 mV, the frequency is 1 Hz. Total analysis time (without sample preparation) 250 s. The detection limit is 0.15 ng / ml.

 The known method allows you to control the total content of arsenic (regardless of its valence form). The method provides high sensitivity and accuracy of analysis of contaminated wastewater, but at the same time it has a number of significant disadvantages, which include: technically difficult and time-consuming sample preparation, the need for accurate observance of temperature conditions by temperature control, long-term distillation by distillation in a stream of nitrogen. Thus, the known method cannot be used for operational automatic control of heterovalent forms of arsenic in wastewater, and its use in the express analysis mode is limited due to the complex hardware design of sample preparation.

 There is a method of inverse volt-amperometric determination of arsenic in solutions of potassium iodide and nitric acid mercury (see the article by H.Z. Brainin, A. V. Chernyshov, R. P. Lesunov, N. I. Stenin and other Inverse volt-amperometry of arsenic in solutions of potassium iodide and nitric acid mercury (Factory laboratory, N 12, 1980, p. 1076 1079).

The method is based on preliminary electron accumulation of arsenic on a mercury-film graphite electrode at a potential of -0.4 V in 1.5 2 M HCl; 0.4M KI; 3.5 • 10 ^-4 5.5 • 10 ^-4 Hg (NO ₃ ) ₂ for 1 10 min followed by registration of the cathodic polarization curve with a potential linearly varying in time in the interval (-0.4) (-0.9 )IN. The arsenic content is determined by the calibration curve.

The method is as follows. Place 10 100 ml of the solution (depending on the arsenic content) in a beaker for evaporation, previously alkalizing the solution with sodium or potassium hydroxide to pH 9 10 by phenolphthalein. The solution is evaporated to 3 5 cm ³ . Pour 3 5 cm ³ HCl (1: 1) and transfer to a volumetric flask with a capacity of 25 cm ³ , add to the mark with water and mix. An aliquot of 1 5 cm ^{3 is} placed in a volumetric flask with a capacity of 25 cm ³ , add 2.5% HCl to 5 cm ³ , 5 cm ^{3 of} water, 0.9 cm ³ Hg (NO ₃ ) ₂ (5 g / cm ³ ), 1 cm ³ saturated KI solution and 2 drops of ascorbic acid (20 g / cm ³ ). After 1 2 min, 7 cm ^{3 of} HCl (1: 1) are added and the volume is made up to the mark with water. After adding each reagent, the solutions are mixed, and the order of reagents must be observed. The contents of the flask after 20 to 30 minutes are transferred to the electrolyzer. A preliminary polarization of the graphite electrode is carried out at a potential of -0.9 V for a minute. Then, arsenic is concentrated in the same solution at a potential of -0.4 V for 1 2 min. After 15 s, the cathodic polarization curve is recorded with a potential linearly varying in time in the range of (-0.4) (-0.9) V. Record 3 4 polarization curves; the first curve is not taken into account. The arsenic content is determined by the calibration curve.

The method provides high sensitivity measurement of arsenic (up to 0.002 μg / cm ³ ). However, its significant drawback is a complex, lengthy, practically undetectable sample preparation. In addition, the method does not provide determination of heterovalent forms of arsenic.

 The closest in technical essence and the achieved result to the claimed one is a method for determining arsenic by differential pulse cathodic voltammetry with preliminary accumulation on the electrode "hanging mercury drop" (see article: Determination of arsenic in the presence of copper by differential pulse cathodic stripping voltammetry at a hanging mercury drop electrode. Sadana Ram S. "Anal. Chem.", 1983, 55, N 2, 304 307).

The method provides the ability to measure heterovalent forms of arsenic. In this case, the As (III) content is initially determined, then after the chemical reduction of As (V) to As (III), the total arsenic content is controlled and the concentration of As (V) is calculated by the difference in the results of the analysis. Determination of As (III) in drinking water is carried out in a medium of 0.75M HCl in the presence of 0.25% N ₂ H ₄ • HCl and 5 μg / ml Cu ²⁺ . The method is as follows. 10 ml of a water sample preserved by hydrochloric acid hydrazine (at the rate of 0.25 g of hydrazine per 100 ml of water) is transferred to a polarographic cell, acidified with hydrochloric acid to 0.75 M HCl, 50 μl of a Cu (II) solution of 1000 mg / l is added, after which register the current-voltage curve in the following optimal mode:
initial potential: -0.6V
final potential: -0.875V
nitrogen purge time: 240 s
accumulation time: 120 s
calm time: 30 s
potential sweep speed: 8 mV / s
pulse amplitude: 0.075 V.

 At the end of the registration, the height of the peak is measured and the concentration of As (III) is determined according to a calibration curve linear in the range of 0 to 20 mg / L.

The determination of the total arsenic content in water is as follows. An aliquot of the 10 ml sample is placed in a 50 ml beaker containing 1.46 g of sodium chloride and 0.25 g of hydrazine hydrochloride. Arsenic (V) is reduced to arsenic (III) by adding 3.15 ml of concentrated hydrochloric acid and 1 ml of 48% bromic acid, cover the glass with a watch glass and heat the mixture for 45 minutes in a steam bath at a temperature of 95-100 ^o C. Give the solution was cooled to room temperature and the contents of the beaker were carefully transferred to a 50 ml volumetric flask, and then the volume was adjusted to the mark with a 0.25% solution of hydrazine hydrochloride. An aliquot of a 10 ml solution prepared for analysis was poured into an electrochemical cell, 50 μl of a basic copper solution (1000 mg / l) was added and polarized, recording the cathode curve. The concentration of arsenic is determined by the calibration graph.

 One of the necessary conditions for the implementation of the known method is the introduction into a controlled solution of copper ions Cu (II).

При последующей катодной развертке арсениды меди превращаются в арсин (мышьяковистый водород) примерно при потенциале -0,72B. При этом наблюдается выделение водорода, как показано в уравнении:

Максимальная чувствительность и селективность измерения достигается при проведении накопления при потенциале -0,6B. Способ обеспечивает высокую чувствительность и избирательность анализа мышьяка, что позволяет использовать его для контроля сточных и природных вод.In the absence of copper, trivalent arsenic is not determined by voltammetry with accumulation on a stationary mercury-droplet electrode due to the insolubility of arsenic in mercury. However, arsenic, coprecipitating with copper, forms mercury soluble compounds. In this case, the formation of copper amalgam occurs and arsenic reacts, forming copper arsenides during deposition according to the equation:

In a subsequent cathodic scan, copper arsenides are converted to arsine (arsenic hydrogen) at a potential of -0.72B. In this case, hydrogen evolution is observed, as shown in the equation:

The maximum sensitivity and selectivity of the measurement is achieved during the accumulation at a potential of -0.6V. The method provides high sensitivity and selectivity analysis of arsenic, which allows you to use it to control wastewater and natural waters.

 At the same time, a significant drawback of the known method is the difficult in hardware design and time-consuming sample preparation of the analyzed solution, which ensures the recovery of As (V) ions to As (III).

Heating and cooling the solution, repeatedly treating it with chemical reagents, selecting aliquots and bringing them to given volumes practically exclude the possibility of analysis automation and reduce its efficiency. In addition, during the chemical reduction of As (V) to As (III) in HCl, HBr, and N ₂ H ₄ • 2HCl at a temperature of 95 -100 ^o C, toxic fumes of these reagents are possible, which have a harmful effect on the analytical staff.

 The aim of the invention is to increase expressivity, simplify sample preparation, improve environmental friendliness and automation of analysis.

This goal is achieved by the fact that according to the method of inversion-volt-amperometric determination of arsenic different forms, based on the electron accumulation of As (III) on a stationary mercury electrode in the presence of Cu ^{2+ ions} and the subsequent registration of the cathodic reduction curve of concentrated copper arsenide, including determination of the content of As ( III) against the background of 0.6 M HCl + 0.04 M N ₂ H ₄ • ₂ HCl + 50 mg / L Cu ²⁺ in height of the inversion cathode peak at a potential of (-0.72) B, chemical reduction of As (V) to As ( Iii) measuring the total water solution direct determination of arsenic As (V) from the difference between the total and the trivalent arsenic concentration, characterized in that the solution analyzed for the content of As (III), is additionally introduced HCl, KI and Cu ^2+, chemical reduction of As (V) As to (III) is carried out in the background electrolyte composition:
5.5 M HCl + 0.1 M KI + 0.02 M N ₂ H ₄ • ₂ HCl + 100 mg / L Cu ²⁺ , arsenic is electronically accumulated at a potential of (-0.55 ± 0.01) B, cathode current-voltage curve recorded in the voltage range from (-0.55) to (-1.0) B, and the total content of arsenic in the solution is determined by the height of the inversion peak at a potential of (-0.76 ± 0.01) B.

Проведенные нами исследования показали, что при добавлении в концентрированный солянокислый фон иодистого калия обеспечивается эффективное восстановление пятивалентного мышьяка даже при комнатной температуре 20 - 25^oC. Это в существенной мере упрощает инструментализацию пробоподготовки и делает ее доступной для автоматизации.As a result of patent research, no known methods were found that involved the chemical reduction of As (V) to As (III) in a background electrolyte of the composition 5.5M HCl + 0.1M KI + 0.02M N ₂ H ₄ • 2HCl + 100 mg / l Cu ²⁺ , arsenic electron accumulation at a potential of (0.55 ± 0.01) B on a stationary mercury electrode, registration of the cathode current-voltage curve in the voltage range from (-0.55) to (-1.0 ) B and determination of the total arsenic content in the solution by the height of the inversion peak at a potential of (-0.76 ± 0.01) B, i.e. having features similar to those distinguishing the claimed method from the prototype. The conversion of electrochemically inactive As (V) to well polarized As (III) is ensured by the presence of free iodide ions in the composition of the hydrochloric acid electrolyte. The reduction reaction of As (V) with iodide ions is used for analytical purposes. In an acidic environment, pentavalent arsenic is present as AsO ions. $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ whose iodometric determination is widely practiced in analytical chemistry (see, for example, Kreshkov AP, Fundamentals of Analytical Chemistry. Volume 1. M. Chemistry, 1976, p. 473). The method is based on the reaction of reduction of As (V) with iodide ions I ^- to As (III):

Our studies showed that when potassium iodide is added to the concentrated hydrochloric acid background, pentavalent arsenic is effectively restored even at room temperature of 20 - 25 ^o C. This greatly simplifies the instrumentalization of sample preparation and makes it available for automation.

Полученные экспериментальные данные хорошо согласуются с результатами ранее проведенных исследований (см. Белова Т.Я. Беренгард И.Б. Каплан Б.Я. Инверсионная переменнотоковая полярография мышьяка (III) с катодной разверткой потенциала. Заводская лаборатория. 1975. Т. 41, N 11, c. 1318).The presence of iodide ions in the background provides, in addition, a significant increase in the sensitivity of the voltammetric determination of arsenic. Iodide ions are oxidized by atmospheric oxygen, with the formation of triiodide J $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ catalyzed by Cu ^{2+ ions} and photochemically. The increase in the analytical signal of As (III) in the presence of KI and N ₂ H ₄ • 2HCl in the background is due to the contribution of the catalytic current of the reduction of triiodide by arsine on the amalgam surface:

The obtained experimental data are in good agreement with the results of previous studies (see Belova, T.Ya. Berengard, IB, Kaplan, B.Ya. Inverted reverse-current polarography of arsenic (III) with a cathode potential scan. Factory Laboratory. 1975. V. 41, N 11, p. 1318).

It was found that in the analysis of solutions containing As (III) and As (V) against the background of potassium iodide and hydrochloric acid, the inversion peak of arsenic increases in time, while in the absence of As (V), the analytical signal remains unchanged. This indicates the restoration of pentavalent arsenic in the background electrolyte, and the process rate substantially depends on the concentration of HCl. For reliable selective determination of trivalent arsenic, a background electrolyte of the composition was used:
0.6M HCl + 0.04M N ₂ H ₄ • 2HCl + 50 mg / L Cu ²⁺
Against the background of hydrochloric acid, As (III) gives well reproducible acute inversion peaks, which is associated with a high charge transfer rate in the HCl medium (see Otter F. Shtulik N. Yulakova E. Inverse voltammetry. M. Mir, 1980, p. 209). The introduction of hydrazine hydrochloric acid into the background eliminates the possibility of oxidation of As (III) with dissolved oxygen, which increases the reliability and accuracy of the analysis results. The method is not critical to the concentration of N ₂ H ₄ • 2HCl in the analyzed solution over a wide range. It was found that a change in the concentration of hydrazine hydrochloride in the range from 0.01 to 0.1 mol / L practically does not affect the height of the inversion cathode peak of arsenic, but the presence of N ₂ H ₄ • 2HCl in the solution increases the reproducibility of the measurement results.

In FIG. 1 presents curves inversion DIP arsenic background 0,6M HCl + 0,04M N ₂ H ₄ • 2HCl + 50 mg / L Cu ²⁺ _(. U _nak = -0,55B; τ _{nak nak} = 30 _s.). ;
1 background; 2 C _{As (III)} = 0; C _{As (V)} = 0.5 mg / L;
3 C _{As (III)} = 0.5 mg / L; C _{As (V)} = 0;
4 C _{As (III)} = C _{As (V)} = 0.5 mg / L.

 In FIG. Figure 2 shows the dependence of the height of the inverse DIP peak of arsenic on the concentration of HCl.

In FIG. Figure 3 shows the dependence of the height of the inverse DIP peak of arsenic on the concentration of Cu ^{2+ ions} .

 In FIG. Figure 4 shows the dependence of the height of the inverse DIP peak of arsenic on the concentration of KI.

 In FIG. Figure 5 shows the dependence of the height of the inverse DIP peak of arsenic on the electron accumulation potential.

In FIG. Figure 6 shows the inverse DIP curves of arsenic against
5.5M HCl + 0.1M KI + 0.02M N ₂ H ₄ • 2HCl + 100 mg / L Cu ²⁺
(U _inc. = -0.55B; τ _inc. = 15c)
1 background;
2 C _{As (III)} = 0.5 mg / L; C _{As (V)} = 0;
3 C _{As (III)} = 0; C _{As (V)} = 0.5 mg / L;
4 C _{As (III)} = C _{As (V)} = 0.5 mg / L.

In FIG. 7 is a calibration graph for the determination of As (III) and As (V) against a background of 5.5 M HCl + 0.1 M KI + 0.02 M N ₂ H ₄ • ₂ HCl + 100 mg / L Cu ²⁺ .

На фиг. 8 представлена структурная схема вольт-амперометрического анализатора мышьяка.

In FIG. 8 is a structural diagram of a voltammetric arsenic analyzer.

). Измерения производились на вольт-амперометрическом анализаторе АЖЭ-11, изготовленном на АО "Югцветметавтоматика" и предназначенном для работы в режимах прямой и инверсионной дифференциальной импульсной полярографии (ДИП) на датчике со стационарным ртутным электродом (см. Боровков Г.А. Зарецкий Л.С. Бабицкий Л. Б. Внедрение анализатора АЖЭ-11 на предприятиях свинцово-цинковой подотрасли. Цветная металлургия, 1987, N 1, с. 49 51). Схема электрохимической ячейки 3-электродная (рабочий электрод типа "висящая" ртутная капля, электрод сравнения хлор-серебряный, вспомогательный электрод стеклоуглеродный). Длительность наложения прямоугольных импульсов 40 мс с периодом 80 мс. В течение всего времени электронакопления раствор подвергался перемешиванию. Время успокоения перед включением развертки потенциала 10 с. Остальные режимные параметры (амплитуда импульсного поляризующего напряжения, потенциал и длительность электронакопления, скорость развертки потенциала) выбирались в ходе экспериментальных исследований. Анализатор АЖЭ-11 обеспечивает полную автоматизацию всех стадий измерительного процесса, включая пробоподготовку и подачу анализируемого раствора в электрохимическую ячейку; формирование "висящей" ртутной капли; электронакопление в течение заданного промежутка времени; регистрацию вольт-амперной кривой и обсчет ее показателей с выдачей выходного сигнала в единицах концентрации контролируемого вещества; сброс и складирование отработанных ртутных капель и удаление проанализированного раствора в дренаж.From those shown in FIG. 1 inverse differential pulsed polarograms of arsenic against the background of 0.6M HCl + 0.04M N ₂ H ₄ • 2HCl + 50 mg / l Cu ²⁺ shows that, unlike As (III), which forms a clear cathode peak (E _p = - 0.72B), As (V) does not appear in this medium and does not give an analytical signal. Calibration curves for As (III) (the relationship between the height of the inversion peaks and the concentration of arsenic in the solution) are linear in the measurement ranges, mg / L: 0-0.01 (accumulation time

) The measurements were carried out on the AZhE-11 volt-ampere analyzer manufactured at Yugtsvetmetavtomatika JSC and designed to operate in direct and inverse differential pulse polarography (DIP) modes on a sensor with a stationary mercury electrode (see Borovkov G.A. Zaretsky L.S. Babitsky LB Introduction of the analyzer АЖЭ-11 at the enterprises of the lead-zinc sub-industry. Non-ferrous metallurgy, 1987, N 1, p. 49 51). The scheme of the electrochemical cell is 3-electrode (the working electrode is of the type "hanging" mercury drop, the reference electrode is chlorine-silver, and the auxiliary electrode is glassy carbon). Duration of overlapping rectangular pulses 40 ms with a period of 80 ms. During the entire time of electron accumulation, the solution was mixed. The settling time before turning on the potential sweep is 10 s. The remaining operational parameters (amplitude of the pulsed polarizing voltage, potential and duration of electron accumulation, potential sweep speed) were chosen during experimental studies. The AZhE-11 analyzer provides full automation of all stages of the measuring process, including sample preparation and the supply of the analyzed solution to the electrochemical cell; the formation of a "hanging" mercury drop; electron accumulation for a given period of time; registration of the current-voltage curve and calculation of its indicators with the output signal in units of the concentration of the controlled substance; discharge and storage of spent mercury drops and removal of the analyzed solution into the drainage.

Последний растет с увеличением концентрации водорода

что усиливает прямое направление реакции (1). Дальнейшее повышение концентрации соляной кислоты после достижения 5,5 М/л не приводит к существенному увеличению содержания свободных ионов водорода. Максимум удельной электропроводности HCl приходится на 20%-ный раствор (см. Добош Д. Электрохимические константы. М. Мир, 1980, 365 с.).When optimizing the background composition for determining the total arsenic content, we proceeded from the fact that when processing a sample containing As (V) with a mixture of hydrochloric acid and potassium iodide solutions, the reaction rate (1) significantly increases with an increase in HCl concentration. With an increase in the content of hydrochloric acid to 5.5 6.0 M / L, the recovery of pentavalent arsenic occurs almost instantly. The height of the inversion peak reaches its maximum value, remaining stable over time, i.e. Under these conditions, the highest sensitivity and reproducibility of the measurement results is ensured. The increase in the rate of the forward direction of reaction (1), i.e. shift of equilibrium towards the formation of AsO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ in a strong hydrochloric acid solution is determined by the ratio of redox potentials

The latter increases with increasing hydrogen concentration.

which enhances the direct direction of reaction (1). A further increase in the concentration of hydrochloric acid after reaching 5.5 M / L does not lead to a significant increase in the content of free hydrogen ions. The maximum conductivity of HCl is in a 20% solution (see Dobosh D. Electrochemical Constants. M. Mir, 1980, 365 pp.).

 The effect of HCl on the determination of As (V) by the inverse DIP method is shown in FIG. 2, from which it follows that the maximum measurement sensitivity is observed at a concentration of hydrochloric acid 5.5 to 6.0 M / L.

The optimal copper concentration for the inverse voltammetric determination of arsenic is selected against the background of 5.5M HCl + -0.1M KI + -0.02M N ₂ H ₄ • 2HCl. The concentration of Cu ^{2+ ions} ranged from 10 to 140 mg / L. From FIG. Figure 3 shows that the maximum height of the inverse DIP peak of 0.5 mg / L As (V) corresponds to a concentration of 100 110 mg / L Cu ²⁺ .

The study of the effect of potassium iodide content on the determination of arsenic showed that when the KI concentration changes from 0.02 to 0.1 mg / l in the background electrolyte of the composition 5.5M HCl + 0.02M N ₂ H ₄ • 2HCl + 100 mg / l Cu ^{2+ the} height of the inversion peak of arsenic increases sharply; a further increase in the content of iodide ions in the background practically does not change the height of the cathode DIP peak (Fig. 4).

Экспериментально установлено, что изменение концентрации N₂H₄•2HCl в пределах от 0,005 до 0,1 моль/л не оказывает существенного влияния на чувствительность инверсионного вольт-амперометрического определения мышьяка. Вместе с тем присутствие в анализируемом растворе солянокислого гидразина позволяет повысить надежность анализа As.The introduction of hydrazine hydrochloric acid into the background electrolyte does not significantly change the character of the current – voltage curve of arsenic, but it makes it possible to increase the reproducibility of measurement results. This may be due to the high reducing ability of hydrazine, which makes it possible to stabilize the content of iodide ions in the solution by restoring the resulting molecular (triiodide) iodine (Korovin N.V. Gidrazin. M. Chemistry, 1980, 272 pp.):

It was experimentally established that a change in the concentration of N ₂ H ₄ • 2HCl in the range from 0.005 to 0.1 mol / L does not significantly affect the sensitivity of the inverse voltammetric determination of arsenic. At the same time, the presence of hydrazine hydrochloride in the analyzed solution makes it possible to increase the reliability of As analysis.

The dependence of the height of the inversion peak of arsenic on the electron storage potential U _n is shown in FIG. 5. The highest sensitivity of determination of As (V) is achieved by maintaining U _n in the range from -0.525 to -0.55 B.

Thus, on the basis of experimental data, the composition of the background electrolyte and the conditions of electroconcentration for inverse volt-ampereometric monitoring of pentavalent arsenic are optimized:
5.5 M HCl + 0.1 M KI + 0.02 M N ₂ H ₄ • ₂ HCl + 100 mg / L Cu ²⁺ .

In FIG. Figure 6 shows the inverse DIP curves recorded in the indicated background during the analysis of solutions containing As (III), As (V) and their mixture. It is seen that in this background, As (V) quantitatively passes into As (III) and both peaks are fixed at the same potential (E _p = -0.76B). Using this background allows you to control the total content of arsenic, regardless of its valence state in the analyzed solution. Calibration curves of As (V) in the selected background linear in concentration ranges from 0 0,01 mg / l ((τ _nak. 200 c) 0 to 1.0 mg / l ((τ _nak. 15). The graphs constructed from the results of the analysis of standard solutions of As (V) and As (III) against the background:
5.5M HCl + 0.1M KI + 0.02M N ₂ H ₄ • 2HCl + 100 mg / L Cu ²⁺ , practically coincide (Fig. 7).

The effect of accompanying impurities on the determination of arsenic in aqueous solutions of complex composition was studied. The possibility of analysis was established at the following ion concentration ratios: [Zn] [As] 10000: 1; [Ni] [As] 500: 1; [Cd] [As] 400: 1; [Fe] [As] 200: 1 with a lower detection limit of 5 • 10 ^-4 mg / L.

 The method can be implemented on various domestic and foreign polarographic analyzers, providing measurements in inversion volt-amperemetry modes on stationary mercury electrodes, in particular, on devices ППТ-1, ПУ-1, ПЛС, АЖЭ-11, АЖЭ-12, АЖЭ -13, PAR-174, PAR-374, etc.

The method is as follows. 10 ml of the analyzed water sample is introduced into a 25 ml volumetric flask, 2.5 ml of 6M HCl solution are added; 0.5 ml of a solution of 2M N ₂ H ₄ • 2HCl and 1.25 ml of a solution of 1 g / l Cu ²⁺ (prepared from CuCl ₂ ) and adjust the volume of the solution in the flask to the mark with double distillate. The resulting mixture is poured into an electrochemical cell, the mixer is turned on, and arsenic is electroconcentrated at a potential of -0.55B. The accumulation duration is set depending on the selected range of measured concentrations. Stop mixing the solution and, after the sedation step (within 10 s), take the cathode curve with a potential linearly varying in time in the range (-0.55) - (- 1.0) B. The potential scan is performed at a speed of 10 mB / s. A series of 3,5 curves is recorded and, measuring the height of the cathode peaks at a potential of -0.72B, the content of As (III) in the solution is determined according to the calibration curve.

Transfer the solution analyzed for the content of As (III) from the electrochemical cell into a 100 ml beaker and 22 ml of a solution of 12M HCl are successively added to it; 0.75 ml of a solution of 1 g / l Cu ²⁺ (prepared from CuCl ₂ ) and 2 ml of a solution of 2.5 M KI. After mixing, part of the resulting solution is poured into an electrochemical cell and the total arsenic content is determined. The analyzer operating mode remains the same, as in the As (III) control, with the exception of the cathode sweep speed (20 mB / s). The accumulation duration is set in accordance with the selected range of measured concentrations. The total arsenic content is determined by the calibration graph, measuring the height of the cathode peaks at a potential of -0.76B. The concentration of As (V) in the sample is calculated by the difference between the results of the analysis of the solution on the total content of arsenic and the content of As (III). The total duration of the control of one water sample for the content of As (V) and As (III) during manual sample preparation does not exceed 5 10 minutes.

 When conducting an analysis in automatic mode, the sample is mixed with the background electrolyte and the analyzed solution is fed into the cell of the voltammetric sensor using three-chamber metering devices of the membrane type DZZh-4 made using corrosion-resistant materials (see Borovkov G.A. Zaretsky L.S. Automatic voltammetric determination of indium in technological solutions of the Electrozinc plant. Factory Laboratory, 1986. T. 52, No. 11, p. 7 10).

Example. Quantitative determination of heterovalent forms of arsenic in the wastewater of the Electrozinc plant, containing 0.02 0.1 mg / l of arsenic, 0.5 to 50 mg / l of Zn ²⁺ ; 0.01 0.1 Cd ²⁺ .

Pipette 10 ml of the analyzed wastewater and place into a 25 ml volumetric flask, add 2.5 ml of a solution of 6M HCl; 0.5 ml of a solution of 2M N ₂ H ₄ • 2HCl and 1.25 ml of a solution of 1 g / l Cu ²⁺ and add the flask to the mark twice distilled with distilled water. The resulting mixture is transferred to the electrochemical cell of the sensor with a stationary mercury electrode and arsenic is electronically accumulated at a potential (-0.55 V) for 90 s with the stirrer switched on. At the end of the accumulation, the mixer is turned off and, after a 10-second stage of calming, the cathode dip-curve of arsenic is recorded in the voltage range (-0.55) (-1.0) B with a sweep speed of 10 mB / s. The peak height is measured at a potential of -0.72B and the content of As (III) in the analyzed solution is determined according to the calibration graph. In total, at least three DIP peaks are removed, while the concentration of trivalent arsenic in the wastewater sample is calculated from the arithmetic mean of all analysis results.

The solution analyzed for the content of As (III) is transferred from the electrochemical cell into a 100 ml beaker and 22 ml of a solution of 12M HCl are successively added to it; 0.75 ml of a solution of 1 g / l Cu ²⁺ and 2 ml of a solution of 2.5 M KI. The contents of the beaker are mixed and the solution is again placed in the electrochemical cell of the sensor. The stirrer is turned on and electroconcentration is carried out at a potential of -0.55V for 30 s. Stirring is stopped and, after 10 seconds of calming, the dipole curve of arsenic is recorded at a cathode sweep speed of 20 mB / s in the voltage range (-0.55) - (-1.0) B. The height of the inversion peak is measured at a potential of -0.76 B and the total arsenic content in the analyzed solution is determined according to the calibration graph. In total, at least 3 peaks are recorded and the arsenic content in the wastewater sample is calculated from the arithmetic average of the results of all measurements taken. The concentration of As (V) in wastewater is determined by the difference in the results of the analysis of the sample for the total content of arsenic and the content of As (III).

It is also possible to conduct an analysis with a complete separation of sample preparation in determining the total and trivalent arsenic. In this case, part of the water sample is treated with reagents to form a background electrolyte of the composition 0.6M HCl + 0.04M N ₂ H ₄ • 2HCl + 50 mg / L Cu ²⁺ and subsequent inversion voltammetric determination of As (III). Another part of the sample, after appropriate separate processing with reagents, is analyzed for the total arsenic content against the background of 5.5 M HCl + 0.1 M KI + 0.02 M N ₂ H ₄ • ₂ HCl + 100 mg / L Cu ²⁺ . Available technical means, for example, the two-channel volt-amperemeter analyzer AZHE-13, manufacturer of JSC "Yugtsvetmetavtomatika" (see article: G.A. Borovkov, O.G. Vinogradov, V.V. Zelensky and others. Experience in the development and implementation of automatic electrochemical analyzers of the ionic composition of flotation pulps: Non-ferrous metals, 1990, N 9, p. 108 112) allow the simultaneous voltammetric determination of total and trivalent arsenic with separate sample preparation of the solution arriving for analysis. The computing device included in this analyzer, as a result of processing the output signals of two measurement channels, makes it possible to additionally obtain information on the content of As (V) in the analyzed solution.

 The block diagram of the automatic volt-amperemeter analyzer AZHE-13 of general, tri- and pentavalent arsenic in wastewater is shown in FIG. 8. The analyzer has an industrial cabinet design and consists of an IZU-3 measuring device (1) and a DTSCh-20 volt-ampere sensor (2). The measuring device includes two electronic converters PP-92 (3 and 4), a computing device VU (5), three secondary devices KSP-3 (6, 7 and 8), two control units NV-96 (9 and 10), two NP-22 memory blocks (11 and 12) and a light panel (13). The DTSCh-20 sensor includes a control unit BU (14), two DTSCh-17 electrochemical transducers (15 and 16), two DZZH-4 liquid dispensers (17 and 18) and two canisters with a background electrolyte (19 and 20). Sampling is carried out from the receiving tank (21), taken outside the analyzer.

 The most important components of the analyzer are the DTSCh-17 electrochemical converters with a working stationary mercury-droplet electrode, which is a capillary mercury dispenser with a needle-type valve. The dimensions of the formed mercury droplet are determined by the geometrical parameters of the capillary and the duration of the inclusion of the electromagnetic valve actuator, at the moment of opening of which a free source of mercury from the tank connected with the atmosphere is provided. Drop dropping is carried out by force with the help of a mechanical blade with an electromagnetic drive. The presence of an electric motor in the DTSCh-17 electrochemical converter ensures the use of a blade as well as a mixer of the analyzed solution.

 The scheme of the electrochemical cell is three-electrode (the reference electrode is silver chloride, the auxiliary glassy carbon electrode). When the analyzer operates in differential or normal pulse polarography mode, the entire measurement process is performed on one mercury drop. This not only ensures a minimum mercury consumption, but also makes it possible to conduct analysis in the inverse voltammetry mode with preliminary electrochemical or adsorption accumulation of a controlled substance on the surface of a mercury drop.

 The PP-92 converter provides voltammetric analysis in the modes of differential, normal and inverse polarography. The NV-96 control unit programs the operation of all measuring devices, as well as the drives of the DTSCh-17 electrochemical converter.

 The DZZh-4 dosing device made of chemically resistant materials ensures the selection of a water sample and background electrolyte from 10-liter canisters, mixing the sample with the background in a predetermined ratio, feeding the solution prepared for analysis into the flow cell of the electrochemical converter, followed by dumping the analyzed solution into the drainage through the drive spent mercury.

 The signals coming from the DTSCh-17 electrochemical converter, proportional to the content of the controlled component in the solution, are sent after processing in the PP-92 electronic converter to the NP-22 memory. The memory unit has adjustable gain and zero offset and is connected to the corresponding recording device KSP-3, calibrated in units of concentration of one of the monitored ionic components. The NP-22 memory blocks of each of the two measurement channels are electrically connected to the WU computing device, which provides the necessary mathematical processing of the output signals of the PP-92 transducers and is connected with the corresponding KSP-3 device. Programming the operation of all nodes of the analyzer is provided using a time relay K1, K2, K3 (not shown in the diagram) installed on the front panel of the control unit BU. Indication of block programs (pause, sample preparation, measurement) is carried out on a light panel ST located on the front panel of the cabinet of the measuring device IZU-3.

 The analyzer can operate in continuous or discrete measurement mode. The resolution of the analysis is determined by the duration of the time delay (pause), set using relay K1. The duration of the inclusion of dosing devices DZZH-4, providing sample preparation, is set using the relay K2, and the duration of the measurement (operating time of the converter PP-92 and DTSCh-17) using relay K3.

The control of water samples for the content of arsenic by the claimed method is as follows. At the command of control unit 14, the DZZh-4 liquid dispensers (17 and 18) are simultaneously turned on, which ensure sampling, sample preparation and the supply of the analyzed solution to the flow cells of the DTSCh-17 sensors (15 and 16). The solution prepared for analysis for the total arsenic content against the background of 5.5 M HCl + 0.1 M KI + 0.02 M N ₂ H ₄ • ₂ HCl + 100 mg / L Cu ²⁺ enters the electrochemical cell of the 1st measuring channel. A solution prepared for analysis for the content of As (III) against a background of 0.6 M HCl + 0.04 M N ₂ H ₄ • ₂ HCl + 50 mg / L Cu ²⁺ enters the electrochemical cell of the 2nd measuring channel. After updating the solution in the cells of the ДТЩ-17 sensors, the DZZh-4 dispensers are turned off and the inversion volt-amperemetric determination of arsenic is performed. Moreover, in accordance with the programs of each of the NV-96 control units (9 and 10), the spent and fresh mercury droplets are discharged, the stirrer is turned on and arsenic is electrochemically accumulated on the mercury electrodes for specified periods of time, the mixer is turned off, the calming, registration and processing inverse cathode peaks. In this case, an electric signal is proportional to the total content of As (III) and As (V) in the analyzed solution from the memory block NP-22 (11) of the 1st measuring channel to the recording device KSP-3 (6). An electric signal proportional to the concentration in the sample As (III) is supplied from the memory unit NP-22 (12) of the second measuring channel to the corresponding secondary device KSP-3 (8). At the same time, the output signals of the NP-22 memory blocks (11 and 12) are fed to the VU computing device (5), the output of which is connected to the KSP-3 recording device (7), calibrated in units of concentration As (V). At the end of the measurement, the control unit BU (14) turns off the measuring instruments of the analyzer and after a corresponding time pause completely repeats the analysis cycle.

 Thus, the inventive method allows to simplify the sample preparation of the controlled solution, eliminating operations difficult for automation from it, and can be implemented on analyzers mastered by the industry.

 Tests of the proposed method were carried out on industrial wastewater samples of the Electrozinc plant in the chemical laboratory of the specialized inspection of analytical control of the Ministry of Environmental Protection and Natural Resources (Ministry of Natural Resources) of North Ossetia-Alania. The raw materials processed at the Electrozinc plant usually contain arsenic compounds, which inevitably leads to the release of arsenic into the wastewater of this enterprise. Industrial wastewater entering a treatment plant is contaminated with compounds of both tri- and pentavalent arsenic, which differ significantly in their toxicity (see, for example, Handbook. Harmful substances in industry. T. 3. Inorganic substances. L. Chemistry, 1977, 608 pp.) . This necessitates the analysis of wastewater for the content of multivalent forms of water-soluble arsenic.

During testing, the concentration of arsenic in controlled samples ranged from 0.01 to 0.1 mg / L. As a control, the method of standard additives was used. During the tests, 31 samples of aqueous solutions were analyzed. In this case, the standard error of arsenic analysis did not exceed 5.5%
The inventive method can be used for automatic or rapid analysis of arsenic in wastewater and circulating waters, as well as in technological solutions of non-ferrous metallurgy.

 The implementation of the proposed method in the analytical practice of zinc production will provide full or partial automation of the analysis of arsenic in wastewater and technological solutions and will improve the environmental situation in the zone of operation of non-ferrous metallurgy enterprises. Moreover, the use of the proposed method in the analytical practice of zinc production will also contribute to improving safety during such processes as neutral leaching, in violation of which it is possible to release toxic arsenic hydrogen into the atmosphere.

Claims (1)

A method of inversion-voltammetric determination of heterovalent forms of arsenic in aqueous solutions, based on the electron accumulation of As (III) on a stationary mercury electrode in the presence of Cu ^{2 +} ions and the subsequent recording of the cathodic reduction curve of concentrated copper arsenide, including determining the content of As (III) against background 0, 6 mol HCl + 0.04 mol N ₂ H ₄ • 2HCl + 50 mg / L Cu ^{2 +} in height of the inversion cathode peak at a potential of -0.72 V, chemical reduction of As (V) to As (III), measurement of the total content water-soluble arsenic and ODA the determination of As (V) content by the difference in the concentrations of total and trivalent arsenic, characterized in that HCl, KI and Cu ^{2 +} are additionally introduced into the solution analyzed for the content of As (III), chemical reduction of As (V) to As (III) carried out in the background electrolyte with a composition of 5.5 mol HCl + 0.1 mol KI + 0.02 mol N ₂ H ₄ • 2HCl + 100 mg / l Cu ^{2 +} , arsenic is electronically accumulated at a potential of -0.55 ± 0.01 V , the cathode current-voltage curve is recorded in the voltage range from -0.55 to -1.0 V, and the total content of arsenic in the solution is determined by the height of the inversion peak at a potential of -0.76 ± 0.01 V.

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