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Electrochemical characterization and analytical application of arsenopyrite mineral in non-aqueous solutions by voltammetry and potentiometry
Polyhedron 30 (2011) 702–707
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Electrochemical characterization and analytical application of arsenopyrite mineral in non-aqueous solutions by voltammetry and potentiometry Zorka Stanic´ a,⇑, Tijana Dimic´ a, Zoran Simic´ a, Ljiljana Jakšic´ b, Stella Girousi c a
Department of Chemistry, Faculty of Science, University of Kragujevac, R. Domanovic´ 12, P.O. Box 60, 34000 Kragujevac, Serbia Faculty of Mining and Geology, University of Belgrade, Ðušina 7, Belgrade, Serbia c Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University, Thessaloniki 54124, Greece b
a r t i c l e
i n f o
Article history: Received 22 November 2010 Accepted 2 December 2010 Available online 16 December 2010 Keywords: Arsenopyrite mineral Sensor Voltammetry Potentiometry Non-aqueous solutions
a b s t r a c t Based on the cyclic voltammetric method, in the present study we have employed carbon paste for arsenopyrite mineral characterization in non-aqueous solution. Arsenopyrite yields well-defined cyclic voltammetric responses with well-defined oxidation (in the potential region from 0.7 to 0.7 V, versus Ag/AgCl) and reduction (from 1.0 to 0.8 V) peaks using this electrode. In addition, arsenopyrite mineral was studied as a new indicator electrode for the potentiometric titrations of acids (benzoic, anthranilic and salicylic acids) and bases (N,N0 -diphenylguanidine, tributylamine and collidine) in acetonitrile and propionitrile. Potassium hydroxide, tetrabutylammonium hydroxide (TBAH) and perchloric acid proved to be very suitable titrating agents for these titrations. The investigated electrode showed a linear dynamic response for p-toluenesulfonic acid concentrations in the range 0.1–0.001 M, with a Nernstian slope of 38.5 mV in acetonitrile and 44.6 mV in propionitrile. The electrode showed a relatively fast response time and can be used without any time limit or without considerable divergence in potentials. The response time of the electrode was less than 10 s in both solvents. The standard deviation of the determination of the investigated acids and bases was less than 0.6% from those obtained with a glass electrode. The advantages of the electrodes are long-term stability, fast response and reproducibility, while the sensors are easy to prepare and are of low cost. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Arsenopyrite is the most abundant naturally existing arsenic mineral. In some sulfide concentrates, arsenopyrite contains ‘invisible’ gold that can be released only by chemical or biological oxidation in acidic medium [1]. Furthermore, chemical and biological oxidation of arsenopyrite present in mining tailings results in the generation of acid rock drainage, the main pollutant produced by the mining industry [2]. Previous reports have focused on the reaction mechanisms involved in the oxidation of arsenopyrite in alkaline and acidic media [3–5], and the kinetic parameters involved in this oxidation [6]. Several studies have paid special attention to the surface characterization of arsenopyrite using spectroscopic and electrochemical techniques [7–9]. X-ray photoelectron spectroscopy (XPS) has been extensively used to characterize the surface composition of arsenopyrite when either exposed to oxidizing environments [7,8] or chemically conditioned [9]. This spectroscopic technique has permitted identification of different iron and arsenic species present ⇑ Corresponding author. E-mail address: [email protected] (Z. Stanic´). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.12.009
on the mineral surface and the detection of elemental sulfur only in the acidic media. Electrochemical techniques provide a possible alternative to allow a rough characterization of the surface state of arsenopyrite. Several publications report the electrochemical characterization of the arsenopyrite surface in alkaline media [3,4], while fewer document characterization in acidic media [5]. The majority of these studies utilized a crystal sample as the arsenopyrite electrode. However, due to the polishing conditions and the intrinsic characteristics of the crystal material, this type of electrode presents the problem of poor reproducibility of the electrochemical response. For a carbon paste electrode (CPE) with a non-conducting binder, the sensitivity is high and its residual currents are low, due to the small double layer capacitance at the electrode–electrolyte interface. The hydrophobic binder used in the CPE lowers the double layer capacitance, and consequently the residual current decreases. Therefore, penetration of the electrolyte into the CPE is unlikely, due to its hydrophobic nature; thus, electrochemical reactions take place entirely at the interface [10,11]. The concept of percolation is the probability that any given region of the CPE is sufficiently well connected to the rest to be available for conduction [12,13], playing an important role in the electroactive
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response of the semiconductor minerals. For that reason, the electrochemical characterization using cyclic voltammetry with carbon paste electrodes containing mineral particles (CPE-mineral) has been, for the past decade, an effective tool to study the overall reactivity of the minerals [11,14–20]. Accordingly, the purpose of this study was to establish the use of cyclic voltammetry on a carbon paste electrode as a technique for the characterization of the surface conditions of arsenopyrite in non-aqueous media. Potentiometric sensors can offer an inexpensive and convenient method for the analyses of some weak acids and bases in nonaqueous solutions. A glass electrode is most frequently used as the indicator electrode, both in aqueous and non-aqueous media. In previous studies [21,22], many deficiencies of the glass electrode (high resistance, fragility, contamination of the membrane, alkaline error and dehydration of the glass membrane in non-aqueous media) were eliminated using natural monocrystalline pyrite, chalcopyrite and galena as electrochemical sensors for the potentiometric titrations of weak acids in pyrrolidone, formamide, c-butyrolactone and propylene carbonate. In order to make potentiometric titrations in non-aqueous solutions easier and more reliable, it seemed reasonable to investigate new sensors for nonaqueous systems. Arsenopyrite is sulfide mineral with the formula FeAsS. The electrochemical behavior of arsenopyrite under different current conditions has been intensively investigated [3–9,11, 14–20]. However, the mentioned investigations did not encompass a study of the properties of an arsenopyrite sensor, although arsenopyrite, being a semiconductor, may be expected to behave as an electrochemical sensor. Furthermore, in recent years, non-aqueous solvents have been widely used in analytical chemistry with the rapid development of non-aqueous analytical techniques and methods. Among the non-aqueous solvents, nitriles have often been used as the medium in numerous investigations [23]. Of all the nitriles, acetonitrile has the widest application, and it has been the most fully examined. Propionitrile’s properties are similar to those of acetonitrile, so that it is often used in an admixture with it, or instead of it [23]. Considering all the above-mentioned findings, we considered that it would be of interest to investigate the behavior of arsenopyrite as a sensor in the potentiometric determination of some weak acids (bases) in acetonitrile and propionitrile. The electrochemical investigations have been carried out by performing voltammetry and potentiometry experiments.
2. Experimental 2.1. Reagents All of the chemicals used in the present study were of analytical grade from Merck, Fluka or AppliChem. Acetonitrile and propionitrile were puriss p.a. purity (99.5). Solutions of the acids (benzoic, anthranilic and salicylic acid) were prepared by weighing a definite amount of acid and dissolving it in the titration solvent. The concentration of solutions of the acid was controlled by titration with standard 0.1 M Bu4NOH (tetrabutylammonium hydroxide) by visual or potentiometric end-point detection by means of a glass electrode-modified SCE couple. p-Toluenesulfonic acid monohydrate was dried under vacuum over P2O5 at 70–80 °C for several days. Before use, the liquid bases (tributylamine and collidine) were dried over fused potassium hydroxide and then distilled under reduced pressure. The concentration of the solutions of bases was checked by titration with H+ ions generated by the oxidation of hydrogen dissolved in palladium [24]. Tetrabutylammonium hydroxide (laboratory-reagent grade), 0.1 M in 2-propanol–methanol (10:1 v/v) (Fluka), was standardized against benzoic acid with Thymol Blue as the indicator. A Thymol Blue solution was prepared
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by dissolving 0.1 g of Thymol Blue in 100 mL of methanol. Potassium hydroxide (laboratory-reagent grade), 0.1 M in ethanol, was standardized against benzoic acid with Thymol Blue as the indicator. All measurements were performed at room temperatures between 20 and 25 °C. The experiments were carried out with a sample of natural arsenopyrite crystal from the Veliki Krivelj copper mine (Bor, Serbia). Graphite powder was purchased from Fluka (50870), p.a. purity (99.9%) and particle size <0.1 mm. The supporting electrolyte was 0.05 M tetrabutylammonium perchlorate (Bu4NClO4) in acetonitrile (propionitrile) for measurements with an arsenopyrite-powder modified carbon paste electrode (FeAsS-CPE). The voltammetric experiments were performed in a solution of the same composition and conditions as those for the potentiometric titration. 2.2. Apparatus Cyclic voltammetric measurements were performed using a PalmSens potentiostat purchased from IVIUM Technologies (The Netherlands, www.ivium.nl) and PalmSensPC software. The working electrode was a FeAsS-CPE with 3 mm inner and 9 mm outer diameter of the PTFE sleeve. The reference electrode was saturated Ag/AgCl and the counter electrode was a platinum wire. The apparatus used to follow the potential changes of the arsenopyrite electrode with time and for end-point detection with an arsenopyrite electrode-SCE couple was described in an earlier paper [25]. The potential changes during titration were followed with a Digital 870 pH-meter, Dresden. The same apparatus with an additional temperature-controlled cell was used to follow the potential changes of the employed electrode as a function of the concentration of p-toluenesulfonic acid. 2.3. Electrodes 2.3.1. Arsenopyrite-powder modified carbon paste electrode The bare carbon paste (CP) was prepared in the usual way by handmixing graphite powder and silicone oil to form homogeneous mixture using a pestle and mortar. The ratio of graphite powder to silicone oil was 4:1. The arsenopyrite used to prepare the FeAsS-CPE was obtained from pure crystals ground in an agate mortar and dry sieved using a standard Tyler sieve, until a particle size of less than 0.1 mm was obtained. The arsenopyrite-powder modified carbon paste electrode was prepared by mixing 0.5 g freshly made CP with 0.1 g arsenopyrite powder. The resulting paste was packed tightly into a PTFE sleeve and the electrode surface was polished to a smooth finish on a piece of weighing paper. After polishing, the electrode was ready to use, without any further mechanical, chemical or electrochemical pretreatment. Electrical contact was established with a stainless steel screw. The constructed electrode was washed with distilled water and then was transferred to the supporting electrolyte solution. Ultra pure argon was bubbled through the solutions to remove dissolved oxygen. Initially the solution was bubbled with argon for 5 min and then again for 1 min before each measurement. 2.3.2. Indicator arsenopyrite electrode The indicator arsenopyrite electrode was prepared in the following manner: A quadratic piece of arsenopyrite (a = 0.5 cm) was used as the electrode material. The arsenopyrite electrode was made by polishing the arsenopyrite crystal with diamond paste, and the best polished side was used as the working surface of the electrode. A narrow glass tube was fixed with glue to the other side of the electrode and then filled with mercury. One end of a cupper wire was immersed in the mercury and this device was mounted into a wider glass tube (ø = 1 cm), which was then cemented with a cold
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sealing mass based on methyl methacrylate. After solidification of this mass, the working surface of the electrode was polished to a high glow. The electrode was then rinsed with distilled water and alcohol, and dried in air, after which it was ready for use. The response of the indicator arsenopyrite electrode in acetonitrile and propionitrile was compared with that of a conventional glass electrode, Type G-202C (Radiometer, Copenhagen). The glass electrode was conditioned in an appropriate solvent before use. The reference electrode was a modified SCE. The SCE electrode of the Type 401 (Radiometer) was modified by complete replacement of its inner solution with a methanolic potassium chloride solution. The solution was vigorously stirred with a magnetic stirrer during titration. 2.4. Procedures 2.4.1. Voltammetric measurements by cyclic voltammetry (CV) The modified electrode was placed into a voltammetric cell. The measurement was carried out in the background electrolyte, in acetonitrile (propionitrile), with different scan rates (0.01–0.1 V s1). The conditions were as follows: Ebegin = 1.5 V, Eend = 1.5 V and Estep = 0.005 V. 2.4.2. Potentiometric measurements Stationary potential measurements of the arsenopyrite electrode were carried out in a series of p-toluenesulfonic acid concentrations, in the concentration range 0.1–0.001 M. The potential of the arsenopyrite electrode with time was followed in a temperature-controlled cell (25 ± 0.1 °C). The ionic strength of the solution was maintained with 0.05 M tetrabutylammonium perchlorate. The potential values determined in this way were used to calculate the slopes. The change in the potential of the arsenopyrite electrode with time was followed in the required solvent. This indicator electrode was coupled with a modified SCE as the reference electrode. The titration procedure was described in a previous paper [25]. 3. Results and discussion 3.1. Study of the electrochemical characteristics of the FeAsS-CP electrode using CV The electrochemical behavior arsenopyrite mineral has been widely studied by several authors [3,19,20,26,27]. Some of them [3,27] found that the anodic oxidation of arsenopyrite in alkaline and acid media is carried out in two stages. Beattie and Poling [3] determined that the oxidation of arsenopyrite at a pH greater than 7 results in the formation of ferric hydroxide deposits on the surface of the mineral. Arsenic is oxidized to arsenate and sulfur is oxidized to sulfate. Below pH 7, soluble iron species are formed and the surface becomes increasingly covered with elemental sulfur when decreasing the pH. In this paper, for characterizing the electrochemical behaviour of the system containing arsenopyrite, it was necessary to proceed with the characterization of the electrochemical processes pertaining to arsenopyrite in acetonitrile media, by cyclic voltammetry. Optimum results, based on the sensitivity and reproducibility of the peaks, using an arsenopyrite-powder modified carbon paste electrode were obtained by using 0.05 M Bu4NClO4 as a supporting electrolyte and a scan rate of 50 mV s1. The oxidation of arsenopyrite involves two steps. The surface arsenopyrite, exposed to the atmosphere in the course of making arsenopyrite-powder modified carbon paste electrode, gives a thin oxidized zone, reaction 1 (the first step). Fig. 1 presents the cyclic voltammogram of the arsenopyrite, where the potential scan took place in the anodic direction:
Fig. 1. Cyclic voltammogram for an arsenopyrite-powder modified carbon paste electrode in acetonitrile medium (0.05 M Bu4NClO4 as the supporting electrolyte). v = 50 mV s1. The scan potential is initiated in the positive direction.
three oxidation processes became apparent (the second step). The first peak, appearing at Ea(I) = 0.661 V versus Ag/AgCl, could be assigned to the oxidation of As(III) species (H3AsO3) to arsenate (H2AsO4), the second weakly-defined anodic oxidation process had a peak associated at Ea(II) = 0.251 V versus Ag/AgCl that could be attributed to the oxidation of S0 to SO42, while the third peak appeared at the potential Ea(III) = 0.644 V which is characteristic of the complete oxidation of Fe(II) species to Fe(III), according with the most probable reactions 1–3 and the results shown in the paper [27]:
FeAsS þ 6H2 O ! FeðOHÞ3 þ H3 AsO3 þ S0 þ 6Hþ þ 6e
ð1Þ
H3 AsO3 þ H2 O ! H2 AsO4 þ 3Hþ þ S0 þ 4H2 O ! SO2 4 þ 8H þ 6e
ð2Þ
þ 2e
ð3Þ
When the potential scan was reversed, a reduction process became evident with three well-defined reduction peaks at Ec(I0 ) = 0.824 V, Ec(II0 ) = 0.236 V and Ec(III0 ) = 1.056 V versus Ag/AgCl. During the reverse scan, a reduction peak I0 has been related to the reduction of iron hydroxide and peak II0 with the reduction of SO42. A reduction peak III0 could be associated to the regeneration of the FeAsS. 3.2. Mechanism of the indicator arsenopyrite electrode The anodic oxidation of arsenopyrite has been found to be a complex process with the initial formation of arsenic(III), iron(III) and sulfur as the products of oxidation [28]. Partial passivation of the process is caused by the precipitation of a layer of amorphous ferric hydroxide on the surface of the mineral [8]. The products that are formed during complete oxidation (Fe(II), Fe(III), S0, SO42, H3AsO3 and H2AsO4) depend on numerous factors, such as the nature of the oxidant, pH value, temperature, nature and concentration of present cations, anions and other chemical species, etc. Iron ions hydrolyze, and the hydrolysis of the iron ions can be represented by the following equation: ðnkÞþ
Fenþ þ 2kH2 O ! FeðOHÞk
þ kH3 Oþ
ð4Þ
The formation of hydroxide will take place on the surface of the arsenopyrite electrode. Since arsenopyrite is known to be a semiconductor, the system Fe(OH)k(nk)+/FeAsS behaves as a sensor. The equation for the potential of such a sensor can be defined as:
E ¼ E0Ox þ RT=nF ln aOx akH3 Oþ
ð5Þ
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where Ox = Fe(OH)k(nk)+/FeAsS. If hydrolysis leads to the formation of solid particles in the vicinity of the arsenopyrite surface, the equation for the potential of the arsenopyrite electrode can be simplified as:
E ¼ E0Ox þ RT=nF ln akH3 Oþ
ð6Þ
From Eq. (6), it follows that the potential of the arsenopyrite electrode depends on the activity of the H3O+ particles and it is the basis for the employment of sulphide minerals in both aqueous and non-aqueous environments [22]. 3.3. Characteristics of the indicator arsenopyrite electrode If an electrode is to be applied as a sensor for quantitative measurements in a non-aqueous environment, the following conditions should be fulfilled: stable potential, sufficient slope, short response time and long lifetime. 3.3.1. Potential of the arsenopyrite electrode The stationary potential of the arsenopyrite electrode in acetonitrile and propionitrile was measured by direct potentiometry at 25 ± 0.1 °C in a freshly prepared 0.05 M solution of p-toluenesulfonic acid in the appropriate solvent. All measurements were performed in the presence of a background electrolyte of constant ionic strength (0.05 M tetrabutylammonium perchlorate) in order to minimize the effect of streaming and diffusion potentials in the streaming sample solution. In both of the investigated solutions, a stable potential was attained in less than 5 min (Fig. 2). 3.3.2. Slope of the potential response of the electrode The potential of the arsenopyrite electrode was determined using a series of p-toluenesulfonic acid concentrations, in the concentration range 0.1–0.001 M in acetonitrile and propionitrile in a temperature-controlled cell (25 ± 0.1 °C). The ionic strength of the solutions was maintained with 0.05 M tetrabutylammonium perchlorate. It was found that the arsenopyrite electrode shows a linear dependence with a slope of 38.5 mV per decade in acetonitrile and 44.6 mV per decade in propionitrile (Fig. 3). Since the arsenopyrite electrode exhibits sub-Nernst dependences, it cannot be used for measuring the pH of a solution. However, the potential of an arsenopyrite electrode as an indicator electrode is very stable with respect to time (Fig. 2); hence, it can be successfully applied to the titration of acids and bases with acetonitrile and propionitrile as the solvents. 3.3.3. Response time of the electrode The response time of the arsenopyrite electrode was determined by recording the time elapsed before a stable potential va-
Fig. 3. Plots of the arsenopyrite electrode potential versus log c (concentrations) p-toluenesulfonic acid in propionitrile (1) and acetonitrile (2).
lue was attained after the arsenopyrite electrode and the reference electrode (modified SCE) were immersed in calibration solutions, from highly acidic (0.05 M of p-toluenesulfonic acid) to highly basic (0.05 M tetrabutylammonium hydroxide) solutions. The change of the electrode potential from the acidic to basic region ranged from 9 to 410 mV in acetonitrile and 6 to 610 mV in propionitrile. The arsenopyrite electrode showed a relatively fast response time in the investigated solvents (less than 10 s). 3.3.4. Long-term stability (lifetime) and repeatability The lifetime of the electrode was determined by raising the potential values of the calibration solution (p-toluenesulfonic acid) and plotting the calibration curves for a period of 1 year. The slope of the electrode remained constant. When the electrode was not used for titrations, it was kept in a dry place protected it from dust. Before the next use, the electrode was kept in the investigated solvent for half an hour. However, if the electrode had been used frequently and for a long time, it was necessary to rub the crystal arsenopyrite with aluminum oxide, wash the electrode and continue with use. In order to establish the efficiency of use of the arsenopyrite electrode in potentiometric titrations and the repeatability of the results obtained, the titration of anthranylic acid with TBAH was selected as a model and it was repetitively carried out six times, and the end point was monitored by using this electrode. The results obtained for the titration of anthranylic acid with TBAH are shown in Fig. 4. The relative standard deviation (RSD) for the end point determination of the titration was found to be 0.38% (Table 1). 3.4. Application to an end-point indication of the electrode
Fig. 2. Plots of the arsenopyrite electrode potential versus time in acetonitrile (1) and propionitrile (2).
Tetrabutylammonium hydroxide is most often used as a titration medium for the titration of acidic substances in nitriles as solvents with a glass electrode as an indicator electrode. Due to the alkaline error of glass electrodes, they are less frequently used as base titrants, since base titration means that they contain potassium ions in their molecules (KOH). Since we proposed a new sensor of arsenopyrite for the determination of the equivalence point (TEP), this in addition to TBAH and KOH in anhydrous methanol were used as titration means in all of our investigations. The practical utility of the proposed sensor was tested by its use as an indicator electrode for the titration of acids and bases of different strengths, such as benzoic, anthranilic, salicylic acid, N,N0 -
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Fig. 4. Five titration curves for 2 mL anthranylic acid solution (0.1 mol L1), by a standard TBAH solution of 0.1 mol L1, using an arsenopyrite indicator electrode in acetonitrile.
Fig. 5. The effect of the arsenopyrite indicator electrode on the shape of the endpoint inflexion in the potentiometric titration of some acids in acetonitrile (a) and propionitrile (b) using KOH as the titrant: (1) benzoic acid, (2) anthranilic acid and (3) salicylic acid.
Table 1 Potentiometric titration of different types of compounds in acetonitrile and propionitrile using an arsenopyrite electrode. Solvent titrant
Titrated compound
na
Taken (mg)
Found (%)
Standard deviation (%)
Acetonitrile Potassium hydroxide
Benzoic acid
6
28.79
100.06
±0.32
Anthranylic acid Salicylic acid Benzoic acid Anthranylic acid Salicylic acid N,N0 diphenylguanidine Tributylamine Collidine
6 6 6 6 6 5
27.41 25.06 28.79 27.41 25.06 42.30
99.98 99.90 99.75 100.06 99.89 99.05
±0.50 ±0.45 ±0.26 ±0.38 ±0.32 ±0.42
7 7
37.11 24.26
100.26 99.59
±0.45 ±0.56
Benzoic acid
6
25.61
99.84
±0.16
Anthranylic acid Salicylic acid Benzoic acid Salicylic acid N,N0 diphenylguanidine Tributylamine Collidine
6 6 6 6 6
26.97 27.24 25.61 27.24 43.23
100.07 99.87 99.79 100.10 99.66
±0.49 ±0.38 ±0.19 ±0.48 ±0.48
7 6
36.75 24.10
99.46 99.82
±0.41 ±0.59
TBAH
HClO4
Propionitrile Potassium hydroxide
TBAH HClO4
a
Number of determinations.
diphenylguanidine, tributylamine, collidine with methanolic potassium hydroxide, TBAH and HClO4 solution. The titration curves of benzoic acid, anthranilic and salicylic acid in acetonitrile and propionitrile as the solvents with potassium hydroxide as the titrant, and with the application of the electrode couples FeAsS-SCE, are shown in Fig. 5. The titration curves of N,N0 -diphenylguanidine, tributylamine and collidine in acetonitrile and propionitrile as the solvents with HClO4 as titrant, and with the application of the electrode couples FeAsS-SCE, are shown in Fig. 6. While titrating salicylic acid with KOH, for example, the potential jump at the TEP amounted to 138 mV/0.3 mL in acetonitrile and 228 mV/0.3 mL in propionitrile (Table 2). When salicylic acid was titrated with TBAH, the potential jumps at the TEP amounted to 170 mV/0.3 mL in acetonitrile and 177 mV/0.3 mL in propionitrile. While titrating tributylamine, the potential jumps at the TEP in acetonitrile amounted to 143 mV/0.3 mL (252 mV/0.3 mL in propionitrile).
Fig. 6. The effect of the arsenopyrite indicator electrode on the shape of the endpoint inflexion in the potentiometric titration of some bases in acetonitrile (a) and propionitrile (b) using HClO4 as the titrant: (1) N,N0 -diphenylguanidine, (2) tributylamine, (3) collidine.
Table 2 Potential jumps (mV/0.3 mL) at the end-point in the potentiometric titration of different types of compounds in acetonitrile and propionitrile using an arsenopyrite electrode. Solvent
Titrated compound
Titrating agent
FeAsS-SCE
Acetonitrile
Benzoic acid Benzoic acid Anthranylic acid Anthranylic acid Salicylic acid Salicylic acid N,N0 -diphenylguanidine Tributylamine Collidine
KOH TBAH KOH TBAH KOH TBAH HClO4 HClO4 HClO4
96 177 117 217 138 170 145 143 65
Propionitrile
Benzoic acid Benzoic acid Anthranylic acid Salicylic acid Salicylic acid N,N0 -diphenylguanidine Tributylamine Collidine
KOH TBAH KOH KOH TBAH HClO4 HClO4 HClO4
167 112 124 228 177 230 252 46
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nitrile is reported for the first time. The main observations and conclusions have been summarized as follows: (1) The results obtained in this work confirm that arsenopyrite can be successfully applied as indicator electrode for the titration of weak acids and bases in acetonitrile and propionitrile. (2) The equilibrium potential in the course of the titration and at the equivalence point were found to be rapidly established. (3) This electrode exhibited useful features for the determination of weak acids and bases with respect to its mechanical resistance, very low cost, simple preparation, great hardiness and chemical inertness to the working mediums. Acknowledgement Fig. 7. Potentiometric titration curves of benzoic acid in acetonitrile obtained by using arsenopyrite as an indicator electrode and TBAH as the titrant: (1) nonaqueous media, (2) 0.5%, (3) 1.0%, and (4) 5.0% water in acetonitrile.
This work is supported by the Ministry of Science and Technological Development of the Republic of Serbia (Project No. 172036). References
When the arsenopyrite electrode was applied as an indicator electrode in acetonitrile, and propionitrile, the potentials during the titration and at the equivalence point were rapidly established (within a few minutes) and the change in the potential at the TEP coincided with that of the applied indicator color. The presence of water lowered the potential jumps at the TEP in the applied solvents. Benzoic acid titration curves in acetonitrile with arsenopyrite as the sensor and TBAH as the titrant in the presence of different concentrations of water are shown in Fig. 7. If the solvent contained 0.5% water, the potential jump at the TEP was 175 mV/0.3 mL. If 1% and 5% water were present, the jumps were 157 mV/0.3 mL and 134 mV/0.3 mL, respectively. A more significant decrease of the potential jump was obtained when the content of water was increased by 5%. The impact of water on the decrease of the potential was much stronger in the titrations of very weak acids. The results obtained with the acids, bases, solvents and the arsenopyrite indicator electrode investigated in this research (Table 1) deviated on average by 0.16–0.59% from those obtained with a glass electrode.
4. Conclusion In this study we have employed a novel arsenopyrite-powder modified carbon paste electrode for arsenopyrite characterization by cyclic voltammetry in acetonitrile media. One of the products that is formed during oxidation of arsenopyrite mineral is Fe(III). The formation of hydroxide takes place on the surface of the arsenopyrite electrode and the system Fe(OH)k(nk)+/FeAsS behaves as a sensor. In this paper, using arsenopyrite as an electrochemical sensor for potentiometric acid–base titrations in acetonitrile and propio-
[1] R.W. Lawrence, in: H.L. Erlich, C.L. Brierley (Eds.), Microbial Mineral Recovery, McGraw-Hill Inc., New York, 1990, p. 127. [2] R.W. Lawrence, in: M.G. Monroy, A. Ldpez (Eds.), Short Course Notes on Hydrometallurgy and Biohydrometallurgy, UASLP, San Luis Potosi, S.L.P., 1994. [3] M.J.V. Beatie, G.W. Poling, Int. J. Miner. Process. 20 (1987) 87. [4] V.M. Sanchez, J.B. Hiskey, Metall. Trans. B 19 (1988) 943. [5] G.K. Sisenov, V.A. Bogdanovskaya, M.R. Tarasevich, Sov. Electrochem. 24 (1988) 729. [6] V.G. Papangelakis, G.P. Demopoulos, Can. Metall. Q. 29 (1990) 13. [7] A.N. Buckley, W. Walker, Appl. Surf. Sci. 35 (1988) 227. [8] H.W. Nesbitt, I.J. Muir, A.R. Pratt, Geochim. Cosmochim. Acta 59 (1995) 1773. [9] S. Richardson, D.J. Vaughan, Miner. Mag. 53 (1989) 223. [10] J. Wang, B.A. Freiha, Talanta 30 (1983) 317. [11] E. Ahlberg, J. Asbjornsson, Hydrometallurgy 34 (1993) 171. [12] S.R. Broadbent, J.M. Hammersley, Proc. Cambridge Philos. Soc. 53 (1957) 629. [13] J.M. Hammersley, Proc. Cambridge Philos. Soc. 53 (1957) 642. [14] R. Cruz, R.M. Luna-Sanchez, G.T. Lapidus, I. Gonzalez, M. Monroy, Hydrometallurgy 78 (2005) 198. [15] G. Urbano, A.M. Melendez, V.E. Reyes, M.A. Veloz, I. Gonzalez, Int. J. Miner. Process. 82 (2007) 148. [16] J.L. Nava, J.L. Oropeza, I. Gonzalez, Electrochim. Acta 47 (2002) 1513. [17] I. Cisneros, M.T. Oropeza, I. Gonzalez, Electrochim. Acta 45 (2000) 2729. [18] P. Velazquez, D. Leinen, J. Pascual, J.R. Ramos-Barrado, P. Grez, H. Gomez, R. Schrebler, R. Del Rio, R. Cordova, J. Phys. Chem. B 109 (2005) 4977. [19] M.C. Costa, A.M. Botelho do Rego, L.M. Abrantes, Int. J. Miner. Process. 65 (2002) 83. [20] Yu. Mikhlin, D. Shipin, A. Kuklinskiy, I. Asanov, in: F.M. Doyle, G.H. Kelsall, R. Woods (Eds.), Electrochemistry in Mineral and Metals Processing VI, vol. 18, The Electrochemical Society Inc., NJ, 2003, p. 155. [21] Lj. Mihajlovic´, S. Nikolic´-Mandic´, B. Vukanovic´, R. Mihajlovic´, J. Solid State Electrochem. 13 (2009) 895. [22] R. Mihajlovic´, Z. Stanic´, J. Solid State Electrochem. 9 (2005) 558. [23] Lj. Mihajlovic´, S. Nikolic´-Mandic´, B. Vukanovic´, R. Mihajlovic´, Anal. Sci. 25 (2009) 437 (and references therein). [24] R. Mihajlovic´, Z. Stanic´, M. Antonijevic´, Anal. Chim. Acta 497 (2003) 143. [25] Lj.V. Mihajlovic´, R.P. Mihajlovic´, M.M. Antonijevic´, B.V. Vukanovic´, Talanta 64 (2004) 879. [26] R. Cruz, I. Lazaro, M.J. Rodrıguez, M. Monroy, I. Gonzalez, Hydrometallurgy 46 (1997) 303. [27] I. Lazaro, R. Cruz, I. Gonzalez, M. Monroy, Int. J. Miner. Process. 50 (1997) 63. [28] C.M.V.B. Almeida, B.F. Giannetti, Phys. Chem. Chem. Phys. 5 (2003) 604.
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Electrochemical characterization and analytical application of arsenopyrite mineral in non-aqueous solutions by voltammetry and potentiometry Zorka Stanic´ a,⇑, Tijana Dimic´ a, Zoran Simic´ a, Ljiljana Jakšic´ b, Stella Girousi c a
Department of Chemistry, Faculty of Science, University of Kragujevac, R. Domanovic´ 12, P.O. Box 60, 34000 Kragujevac, Serbia Faculty of Mining and Geology, University of Belgrade, Ðušina 7, Belgrade, Serbia c Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University, Thessaloniki 54124, Greece b
a r t i c l e
i n f o
Article history: Received 22 November 2010 Accepted 2 December 2010 Available online 16 December 2010 Keywords: Arsenopyrite mineral Sensor Voltammetry Potentiometry Non-aqueous solutions
a b s t r a c t Based on the cyclic voltammetric method, in the present study we have employed carbon paste for arsenopyrite mineral characterization in non-aqueous solution. Arsenopyrite yields well-defined cyclic voltammetric responses with well-defined oxidation (in the potential region from 0.7 to 0.7 V, versus Ag/AgCl) and reduction (from 1.0 to 0.8 V) peaks using this electrode. In addition, arsenopyrite mineral was studied as a new indicator electrode for the potentiometric titrations of acids (benzoic, anthranilic and salicylic acids) and bases (N,N0 -diphenylguanidine, tributylamine and collidine) in acetonitrile and propionitrile. Potassium hydroxide, tetrabutylammonium hydroxide (TBAH) and perchloric acid proved to be very suitable titrating agents for these titrations. The investigated electrode showed a linear dynamic response for p-toluenesulfonic acid concentrations in the range 0.1–0.001 M, with a Nernstian slope of 38.5 mV in acetonitrile and 44.6 mV in propionitrile. The electrode showed a relatively fast response time and can be used without any time limit or without considerable divergence in potentials. The response time of the electrode was less than 10 s in both solvents. The standard deviation of the determination of the investigated acids and bases was less than 0.6% from those obtained with a glass electrode. The advantages of the electrodes are long-term stability, fast response and reproducibility, while the sensors are easy to prepare and are of low cost. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Arsenopyrite is the most abundant naturally existing arsenic mineral. In some sulfide concentrates, arsenopyrite contains ‘invisible’ gold that can be released only by chemical or biological oxidation in acidic medium [1]. Furthermore, chemical and biological oxidation of arsenopyrite present in mining tailings results in the generation of acid rock drainage, the main pollutant produced by the mining industry [2]. Previous reports have focused on the reaction mechanisms involved in the oxidation of arsenopyrite in alkaline and acidic media [3–5], and the kinetic parameters involved in this oxidation [6]. Several studies have paid special attention to the surface characterization of arsenopyrite using spectroscopic and electrochemical techniques [7–9]. X-ray photoelectron spectroscopy (XPS) has been extensively used to characterize the surface composition of arsenopyrite when either exposed to oxidizing environments [7,8] or chemically conditioned [9]. This spectroscopic technique has permitted identification of different iron and arsenic species present ⇑ Corresponding author. E-mail address: [email protected] (Z. Stanic´). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.12.009
on the mineral surface and the detection of elemental sulfur only in the acidic media. Electrochemical techniques provide a possible alternative to allow a rough characterization of the surface state of arsenopyrite. Several publications report the electrochemical characterization of the arsenopyrite surface in alkaline media [3,4], while fewer document characterization in acidic media [5]. The majority of these studies utilized a crystal sample as the arsenopyrite electrode. However, due to the polishing conditions and the intrinsic characteristics of the crystal material, this type of electrode presents the problem of poor reproducibility of the electrochemical response. For a carbon paste electrode (CPE) with a non-conducting binder, the sensitivity is high and its residual currents are low, due to the small double layer capacitance at the electrode–electrolyte interface. The hydrophobic binder used in the CPE lowers the double layer capacitance, and consequently the residual current decreases. Therefore, penetration of the electrolyte into the CPE is unlikely, due to its hydrophobic nature; thus, electrochemical reactions take place entirely at the interface [10,11]. The concept of percolation is the probability that any given region of the CPE is sufficiently well connected to the rest to be available for conduction [12,13], playing an important role in the electroactive
Z. Stanic´ et al. / Polyhedron 30 (2011) 702–707
response of the semiconductor minerals. For that reason, the electrochemical characterization using cyclic voltammetry with carbon paste electrodes containing mineral particles (CPE-mineral) has been, for the past decade, an effective tool to study the overall reactivity of the minerals [11,14–20]. Accordingly, the purpose of this study was to establish the use of cyclic voltammetry on a carbon paste electrode as a technique for the characterization of the surface conditions of arsenopyrite in non-aqueous media. Potentiometric sensors can offer an inexpensive and convenient method for the analyses of some weak acids and bases in nonaqueous solutions. A glass electrode is most frequently used as the indicator electrode, both in aqueous and non-aqueous media. In previous studies [21,22], many deficiencies of the glass electrode (high resistance, fragility, contamination of the membrane, alkaline error and dehydration of the glass membrane in non-aqueous media) were eliminated using natural monocrystalline pyrite, chalcopyrite and galena as electrochemical sensors for the potentiometric titrations of weak acids in pyrrolidone, formamide, c-butyrolactone and propylene carbonate. In order to make potentiometric titrations in non-aqueous solutions easier and more reliable, it seemed reasonable to investigate new sensors for nonaqueous systems. Arsenopyrite is sulfide mineral with the formula FeAsS. The electrochemical behavior of arsenopyrite under different current conditions has been intensively investigated [3–9,11, 14–20]. However, the mentioned investigations did not encompass a study of the properties of an arsenopyrite sensor, although arsenopyrite, being a semiconductor, may be expected to behave as an electrochemical sensor. Furthermore, in recent years, non-aqueous solvents have been widely used in analytical chemistry with the rapid development of non-aqueous analytical techniques and methods. Among the non-aqueous solvents, nitriles have often been used as the medium in numerous investigations [23]. Of all the nitriles, acetonitrile has the widest application, and it has been the most fully examined. Propionitrile’s properties are similar to those of acetonitrile, so that it is often used in an admixture with it, or instead of it [23]. Considering all the above-mentioned findings, we considered that it would be of interest to investigate the behavior of arsenopyrite as a sensor in the potentiometric determination of some weak acids (bases) in acetonitrile and propionitrile. The electrochemical investigations have been carried out by performing voltammetry and potentiometry experiments.
2. Experimental 2.1. Reagents All of the chemicals used in the present study were of analytical grade from Merck, Fluka or AppliChem. Acetonitrile and propionitrile were puriss p.a. purity (99.5). Solutions of the acids (benzoic, anthranilic and salicylic acid) were prepared by weighing a definite amount of acid and dissolving it in the titration solvent. The concentration of solutions of the acid was controlled by titration with standard 0.1 M Bu4NOH (tetrabutylammonium hydroxide) by visual or potentiometric end-point detection by means of a glass electrode-modified SCE couple. p-Toluenesulfonic acid monohydrate was dried under vacuum over P2O5 at 70–80 °C for several days. Before use, the liquid bases (tributylamine and collidine) were dried over fused potassium hydroxide and then distilled under reduced pressure. The concentration of the solutions of bases was checked by titration with H+ ions generated by the oxidation of hydrogen dissolved in palladium [24]. Tetrabutylammonium hydroxide (laboratory-reagent grade), 0.1 M in 2-propanol–methanol (10:1 v/v) (Fluka), was standardized against benzoic acid with Thymol Blue as the indicator. A Thymol Blue solution was prepared
703
by dissolving 0.1 g of Thymol Blue in 100 mL of methanol. Potassium hydroxide (laboratory-reagent grade), 0.1 M in ethanol, was standardized against benzoic acid with Thymol Blue as the indicator. All measurements were performed at room temperatures between 20 and 25 °C. The experiments were carried out with a sample of natural arsenopyrite crystal from the Veliki Krivelj copper mine (Bor, Serbia). Graphite powder was purchased from Fluka (50870), p.a. purity (99.9%) and particle size <0.1 mm. The supporting electrolyte was 0.05 M tetrabutylammonium perchlorate (Bu4NClO4) in acetonitrile (propionitrile) for measurements with an arsenopyrite-powder modified carbon paste electrode (FeAsS-CPE). The voltammetric experiments were performed in a solution of the same composition and conditions as those for the potentiometric titration. 2.2. Apparatus Cyclic voltammetric measurements were performed using a PalmSens potentiostat purchased from IVIUM Technologies (The Netherlands, www.ivium.nl) and PalmSensPC software. The working electrode was a FeAsS-CPE with 3 mm inner and 9 mm outer diameter of the PTFE sleeve. The reference electrode was saturated Ag/AgCl and the counter electrode was a platinum wire. The apparatus used to follow the potential changes of the arsenopyrite electrode with time and for end-point detection with an arsenopyrite electrode-SCE couple was described in an earlier paper [25]. The potential changes during titration were followed with a Digital 870 pH-meter, Dresden. The same apparatus with an additional temperature-controlled cell was used to follow the potential changes of the employed electrode as a function of the concentration of p-toluenesulfonic acid. 2.3. Electrodes 2.3.1. Arsenopyrite-powder modified carbon paste electrode The bare carbon paste (CP) was prepared in the usual way by handmixing graphite powder and silicone oil to form homogeneous mixture using a pestle and mortar. The ratio of graphite powder to silicone oil was 4:1. The arsenopyrite used to prepare the FeAsS-CPE was obtained from pure crystals ground in an agate mortar and dry sieved using a standard Tyler sieve, until a particle size of less than 0.1 mm was obtained. The arsenopyrite-powder modified carbon paste electrode was prepared by mixing 0.5 g freshly made CP with 0.1 g arsenopyrite powder. The resulting paste was packed tightly into a PTFE sleeve and the electrode surface was polished to a smooth finish on a piece of weighing paper. After polishing, the electrode was ready to use, without any further mechanical, chemical or electrochemical pretreatment. Electrical contact was established with a stainless steel screw. The constructed electrode was washed with distilled water and then was transferred to the supporting electrolyte solution. Ultra pure argon was bubbled through the solutions to remove dissolved oxygen. Initially the solution was bubbled with argon for 5 min and then again for 1 min before each measurement. 2.3.2. Indicator arsenopyrite electrode The indicator arsenopyrite electrode was prepared in the following manner: A quadratic piece of arsenopyrite (a = 0.5 cm) was used as the electrode material. The arsenopyrite electrode was made by polishing the arsenopyrite crystal with diamond paste, and the best polished side was used as the working surface of the electrode. A narrow glass tube was fixed with glue to the other side of the electrode and then filled with mercury. One end of a cupper wire was immersed in the mercury and this device was mounted into a wider glass tube (ø = 1 cm), which was then cemented with a cold
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sealing mass based on methyl methacrylate. After solidification of this mass, the working surface of the electrode was polished to a high glow. The electrode was then rinsed with distilled water and alcohol, and dried in air, after which it was ready for use. The response of the indicator arsenopyrite electrode in acetonitrile and propionitrile was compared with that of a conventional glass electrode, Type G-202C (Radiometer, Copenhagen). The glass electrode was conditioned in an appropriate solvent before use. The reference electrode was a modified SCE. The SCE electrode of the Type 401 (Radiometer) was modified by complete replacement of its inner solution with a methanolic potassium chloride solution. The solution was vigorously stirred with a magnetic stirrer during titration. 2.4. Procedures 2.4.1. Voltammetric measurements by cyclic voltammetry (CV) The modified electrode was placed into a voltammetric cell. The measurement was carried out in the background electrolyte, in acetonitrile (propionitrile), with different scan rates (0.01–0.1 V s1). The conditions were as follows: Ebegin = 1.5 V, Eend = 1.5 V and Estep = 0.005 V. 2.4.2. Potentiometric measurements Stationary potential measurements of the arsenopyrite electrode were carried out in a series of p-toluenesulfonic acid concentrations, in the concentration range 0.1–0.001 M. The potential of the arsenopyrite electrode with time was followed in a temperature-controlled cell (25 ± 0.1 °C). The ionic strength of the solution was maintained with 0.05 M tetrabutylammonium perchlorate. The potential values determined in this way were used to calculate the slopes. The change in the potential of the arsenopyrite electrode with time was followed in the required solvent. This indicator electrode was coupled with a modified SCE as the reference electrode. The titration procedure was described in a previous paper [25]. 3. Results and discussion 3.1. Study of the electrochemical characteristics of the FeAsS-CP electrode using CV The electrochemical behavior arsenopyrite mineral has been widely studied by several authors [3,19,20,26,27]. Some of them [3,27] found that the anodic oxidation of arsenopyrite in alkaline and acid media is carried out in two stages. Beattie and Poling [3] determined that the oxidation of arsenopyrite at a pH greater than 7 results in the formation of ferric hydroxide deposits on the surface of the mineral. Arsenic is oxidized to arsenate and sulfur is oxidized to sulfate. Below pH 7, soluble iron species are formed and the surface becomes increasingly covered with elemental sulfur when decreasing the pH. In this paper, for characterizing the electrochemical behaviour of the system containing arsenopyrite, it was necessary to proceed with the characterization of the electrochemical processes pertaining to arsenopyrite in acetonitrile media, by cyclic voltammetry. Optimum results, based on the sensitivity and reproducibility of the peaks, using an arsenopyrite-powder modified carbon paste electrode were obtained by using 0.05 M Bu4NClO4 as a supporting electrolyte and a scan rate of 50 mV s1. The oxidation of arsenopyrite involves two steps. The surface arsenopyrite, exposed to the atmosphere in the course of making arsenopyrite-powder modified carbon paste electrode, gives a thin oxidized zone, reaction 1 (the first step). Fig. 1 presents the cyclic voltammogram of the arsenopyrite, where the potential scan took place in the anodic direction:
Fig. 1. Cyclic voltammogram for an arsenopyrite-powder modified carbon paste electrode in acetonitrile medium (0.05 M Bu4NClO4 as the supporting electrolyte). v = 50 mV s1. The scan potential is initiated in the positive direction.
three oxidation processes became apparent (the second step). The first peak, appearing at Ea(I) = 0.661 V versus Ag/AgCl, could be assigned to the oxidation of As(III) species (H3AsO3) to arsenate (H2AsO4), the second weakly-defined anodic oxidation process had a peak associated at Ea(II) = 0.251 V versus Ag/AgCl that could be attributed to the oxidation of S0 to SO42, while the third peak appeared at the potential Ea(III) = 0.644 V which is characteristic of the complete oxidation of Fe(II) species to Fe(III), according with the most probable reactions 1–3 and the results shown in the paper [27]:
FeAsS þ 6H2 O ! FeðOHÞ3 þ H3 AsO3 þ S0 þ 6Hþ þ 6e
ð1Þ
H3 AsO3 þ H2 O ! H2 AsO4 þ 3Hþ þ S0 þ 4H2 O ! SO2 4 þ 8H þ 6e
ð2Þ
þ 2e
ð3Þ
When the potential scan was reversed, a reduction process became evident with three well-defined reduction peaks at Ec(I0 ) = 0.824 V, Ec(II0 ) = 0.236 V and Ec(III0 ) = 1.056 V versus Ag/AgCl. During the reverse scan, a reduction peak I0 has been related to the reduction of iron hydroxide and peak II0 with the reduction of SO42. A reduction peak III0 could be associated to the regeneration of the FeAsS. 3.2. Mechanism of the indicator arsenopyrite electrode The anodic oxidation of arsenopyrite has been found to be a complex process with the initial formation of arsenic(III), iron(III) and sulfur as the products of oxidation [28]. Partial passivation of the process is caused by the precipitation of a layer of amorphous ferric hydroxide on the surface of the mineral [8]. The products that are formed during complete oxidation (Fe(II), Fe(III), S0, SO42, H3AsO3 and H2AsO4) depend on numerous factors, such as the nature of the oxidant, pH value, temperature, nature and concentration of present cations, anions and other chemical species, etc. Iron ions hydrolyze, and the hydrolysis of the iron ions can be represented by the following equation: ðnkÞþ
Fenþ þ 2kH2 O ! FeðOHÞk
þ kH3 Oþ
ð4Þ
The formation of hydroxide will take place on the surface of the arsenopyrite electrode. Since arsenopyrite is known to be a semiconductor, the system Fe(OH)k(nk)+/FeAsS behaves as a sensor. The equation for the potential of such a sensor can be defined as:
E ¼ E0Ox þ RT=nF ln aOx akH3 Oþ
ð5Þ
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705
where Ox = Fe(OH)k(nk)+/FeAsS. If hydrolysis leads to the formation of solid particles in the vicinity of the arsenopyrite surface, the equation for the potential of the arsenopyrite electrode can be simplified as:
E ¼ E0Ox þ RT=nF ln akH3 Oþ
ð6Þ
From Eq. (6), it follows that the potential of the arsenopyrite electrode depends on the activity of the H3O+ particles and it is the basis for the employment of sulphide minerals in both aqueous and non-aqueous environments [22]. 3.3. Characteristics of the indicator arsenopyrite electrode If an electrode is to be applied as a sensor for quantitative measurements in a non-aqueous environment, the following conditions should be fulfilled: stable potential, sufficient slope, short response time and long lifetime. 3.3.1. Potential of the arsenopyrite electrode The stationary potential of the arsenopyrite electrode in acetonitrile and propionitrile was measured by direct potentiometry at 25 ± 0.1 °C in a freshly prepared 0.05 M solution of p-toluenesulfonic acid in the appropriate solvent. All measurements were performed in the presence of a background electrolyte of constant ionic strength (0.05 M tetrabutylammonium perchlorate) in order to minimize the effect of streaming and diffusion potentials in the streaming sample solution. In both of the investigated solutions, a stable potential was attained in less than 5 min (Fig. 2). 3.3.2. Slope of the potential response of the electrode The potential of the arsenopyrite electrode was determined using a series of p-toluenesulfonic acid concentrations, in the concentration range 0.1–0.001 M in acetonitrile and propionitrile in a temperature-controlled cell (25 ± 0.1 °C). The ionic strength of the solutions was maintained with 0.05 M tetrabutylammonium perchlorate. It was found that the arsenopyrite electrode shows a linear dependence with a slope of 38.5 mV per decade in acetonitrile and 44.6 mV per decade in propionitrile (Fig. 3). Since the arsenopyrite electrode exhibits sub-Nernst dependences, it cannot be used for measuring the pH of a solution. However, the potential of an arsenopyrite electrode as an indicator electrode is very stable with respect to time (Fig. 2); hence, it can be successfully applied to the titration of acids and bases with acetonitrile and propionitrile as the solvents. 3.3.3. Response time of the electrode The response time of the arsenopyrite electrode was determined by recording the time elapsed before a stable potential va-
Fig. 3. Plots of the arsenopyrite electrode potential versus log c (concentrations) p-toluenesulfonic acid in propionitrile (1) and acetonitrile (2).
lue was attained after the arsenopyrite electrode and the reference electrode (modified SCE) were immersed in calibration solutions, from highly acidic (0.05 M of p-toluenesulfonic acid) to highly basic (0.05 M tetrabutylammonium hydroxide) solutions. The change of the electrode potential from the acidic to basic region ranged from 9 to 410 mV in acetonitrile and 6 to 610 mV in propionitrile. The arsenopyrite electrode showed a relatively fast response time in the investigated solvents (less than 10 s). 3.3.4. Long-term stability (lifetime) and repeatability The lifetime of the electrode was determined by raising the potential values of the calibration solution (p-toluenesulfonic acid) and plotting the calibration curves for a period of 1 year. The slope of the electrode remained constant. When the electrode was not used for titrations, it was kept in a dry place protected it from dust. Before the next use, the electrode was kept in the investigated solvent for half an hour. However, if the electrode had been used frequently and for a long time, it was necessary to rub the crystal arsenopyrite with aluminum oxide, wash the electrode and continue with use. In order to establish the efficiency of use of the arsenopyrite electrode in potentiometric titrations and the repeatability of the results obtained, the titration of anthranylic acid with TBAH was selected as a model and it was repetitively carried out six times, and the end point was monitored by using this electrode. The results obtained for the titration of anthranylic acid with TBAH are shown in Fig. 4. The relative standard deviation (RSD) for the end point determination of the titration was found to be 0.38% (Table 1). 3.4. Application to an end-point indication of the electrode
Fig. 2. Plots of the arsenopyrite electrode potential versus time in acetonitrile (1) and propionitrile (2).
Tetrabutylammonium hydroxide is most often used as a titration medium for the titration of acidic substances in nitriles as solvents with a glass electrode as an indicator electrode. Due to the alkaline error of glass electrodes, they are less frequently used as base titrants, since base titration means that they contain potassium ions in their molecules (KOH). Since we proposed a new sensor of arsenopyrite for the determination of the equivalence point (TEP), this in addition to TBAH and KOH in anhydrous methanol were used as titration means in all of our investigations. The practical utility of the proposed sensor was tested by its use as an indicator electrode for the titration of acids and bases of different strengths, such as benzoic, anthranilic, salicylic acid, N,N0 -
Z. Stanic´ et al. / Polyhedron 30 (2011) 702–707
706
Fig. 4. Five titration curves for 2 mL anthranylic acid solution (0.1 mol L1), by a standard TBAH solution of 0.1 mol L1, using an arsenopyrite indicator electrode in acetonitrile.
Fig. 5. The effect of the arsenopyrite indicator electrode on the shape of the endpoint inflexion in the potentiometric titration of some acids in acetonitrile (a) and propionitrile (b) using KOH as the titrant: (1) benzoic acid, (2) anthranilic acid and (3) salicylic acid.
Table 1 Potentiometric titration of different types of compounds in acetonitrile and propionitrile using an arsenopyrite electrode. Solvent titrant
Titrated compound
na
Taken (mg)
Found (%)
Standard deviation (%)
Acetonitrile Potassium hydroxide
Benzoic acid
6
28.79
100.06
±0.32
Anthranylic acid Salicylic acid Benzoic acid Anthranylic acid Salicylic acid N,N0 diphenylguanidine Tributylamine Collidine
6 6 6 6 6 5
27.41 25.06 28.79 27.41 25.06 42.30
99.98 99.90 99.75 100.06 99.89 99.05
±0.50 ±0.45 ±0.26 ±0.38 ±0.32 ±0.42
7 7
37.11 24.26
100.26 99.59
±0.45 ±0.56
Benzoic acid
6
25.61
99.84
±0.16
Anthranylic acid Salicylic acid Benzoic acid Salicylic acid N,N0 diphenylguanidine Tributylamine Collidine
6 6 6 6 6
26.97 27.24 25.61 27.24 43.23
100.07 99.87 99.79 100.10 99.66
±0.49 ±0.38 ±0.19 ±0.48 ±0.48
7 6
36.75 24.10
99.46 99.82
±0.41 ±0.59
TBAH
HClO4
Propionitrile Potassium hydroxide
TBAH HClO4
a
Number of determinations.
diphenylguanidine, tributylamine, collidine with methanolic potassium hydroxide, TBAH and HClO4 solution. The titration curves of benzoic acid, anthranilic and salicylic acid in acetonitrile and propionitrile as the solvents with potassium hydroxide as the titrant, and with the application of the electrode couples FeAsS-SCE, are shown in Fig. 5. The titration curves of N,N0 -diphenylguanidine, tributylamine and collidine in acetonitrile and propionitrile as the solvents with HClO4 as titrant, and with the application of the electrode couples FeAsS-SCE, are shown in Fig. 6. While titrating salicylic acid with KOH, for example, the potential jump at the TEP amounted to 138 mV/0.3 mL in acetonitrile and 228 mV/0.3 mL in propionitrile (Table 2). When salicylic acid was titrated with TBAH, the potential jumps at the TEP amounted to 170 mV/0.3 mL in acetonitrile and 177 mV/0.3 mL in propionitrile. While titrating tributylamine, the potential jumps at the TEP in acetonitrile amounted to 143 mV/0.3 mL (252 mV/0.3 mL in propionitrile).
Fig. 6. The effect of the arsenopyrite indicator electrode on the shape of the endpoint inflexion in the potentiometric titration of some bases in acetonitrile (a) and propionitrile (b) using HClO4 as the titrant: (1) N,N0 -diphenylguanidine, (2) tributylamine, (3) collidine.
Table 2 Potential jumps (mV/0.3 mL) at the end-point in the potentiometric titration of different types of compounds in acetonitrile and propionitrile using an arsenopyrite electrode. Solvent
Titrated compound
Titrating agent
FeAsS-SCE
Acetonitrile
Benzoic acid Benzoic acid Anthranylic acid Anthranylic acid Salicylic acid Salicylic acid N,N0 -diphenylguanidine Tributylamine Collidine
KOH TBAH KOH TBAH KOH TBAH HClO4 HClO4 HClO4
96 177 117 217 138 170 145 143 65
Propionitrile
Benzoic acid Benzoic acid Anthranylic acid Salicylic acid Salicylic acid N,N0 -diphenylguanidine Tributylamine Collidine
KOH TBAH KOH KOH TBAH HClO4 HClO4 HClO4
167 112 124 228 177 230 252 46
Z. Stanic´ et al. / Polyhedron 30 (2011) 702–707
707
nitrile is reported for the first time. The main observations and conclusions have been summarized as follows: (1) The results obtained in this work confirm that arsenopyrite can be successfully applied as indicator electrode for the titration of weak acids and bases in acetonitrile and propionitrile. (2) The equilibrium potential in the course of the titration and at the equivalence point were found to be rapidly established. (3) This electrode exhibited useful features for the determination of weak acids and bases with respect to its mechanical resistance, very low cost, simple preparation, great hardiness and chemical inertness to the working mediums. Acknowledgement Fig. 7. Potentiometric titration curves of benzoic acid in acetonitrile obtained by using arsenopyrite as an indicator electrode and TBAH as the titrant: (1) nonaqueous media, (2) 0.5%, (3) 1.0%, and (4) 5.0% water in acetonitrile.
This work is supported by the Ministry of Science and Technological Development of the Republic of Serbia (Project No. 172036). References
When the arsenopyrite electrode was applied as an indicator electrode in acetonitrile, and propionitrile, the potentials during the titration and at the equivalence point were rapidly established (within a few minutes) and the change in the potential at the TEP coincided with that of the applied indicator color. The presence of water lowered the potential jumps at the TEP in the applied solvents. Benzoic acid titration curves in acetonitrile with arsenopyrite as the sensor and TBAH as the titrant in the presence of different concentrations of water are shown in Fig. 7. If the solvent contained 0.5% water, the potential jump at the TEP was 175 mV/0.3 mL. If 1% and 5% water were present, the jumps were 157 mV/0.3 mL and 134 mV/0.3 mL, respectively. A more significant decrease of the potential jump was obtained when the content of water was increased by 5%. The impact of water on the decrease of the potential was much stronger in the titrations of very weak acids. The results obtained with the acids, bases, solvents and the arsenopyrite indicator electrode investigated in this research (Table 1) deviated on average by 0.16–0.59% from those obtained with a glass electrode.
4. Conclusion In this study we have employed a novel arsenopyrite-powder modified carbon paste electrode for arsenopyrite characterization by cyclic voltammetry in acetonitrile media. One of the products that is formed during oxidation of arsenopyrite mineral is Fe(III). The formation of hydroxide takes place on the surface of the arsenopyrite electrode and the system Fe(OH)k(nk)+/FeAsS behaves as a sensor. In this paper, using arsenopyrite as an electrochemical sensor for potentiometric acid–base titrations in acetonitrile and propio-
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