Polarographic investigation of reduction process of some azodyes and their complexes with rare earths

221 Talanta 54 (2001) 221– 231 www.elsevier.com/locate/talanta

Polarographic investigation of reduction process of some azodyes and their complexes with rare earths Liliya Dubenska *, Halyna Levytska, Nataliya Poperechna Department of Analytical Chemistry, Faculty of Chemistry, I6an Franko National Uni6ersity of L’6i6, Kyryla and Mephodia Str. 6, 79005 L’6i6, Ukraine Received 16 March 2000; received in revised form 18 July 2000; accepted 25 July 2000

Abstract The voltammetry behavior of six azodyes (tropeolin 0, tropeolin 00, tropeolin 000, magnezon, eriochrome black T, arsenazo I) with various structures of the interface circuit and substituents in o,o%-positions, and their rare earth (RE) complexes was investigated in detail. The process of complex formation of RE(III) with some o,o%-hydroxyazodyes was studied. The reduction mechanism of o,o%-disubstituted azodyes and their complexes with RE(III) was offered. It was confirmed that all investigated compounds and complexes are adsorbed on the surface of the dropping mercury electrode. The possibility of voltammetric determination of azodyes in a water environment was shown. The results of the research were used for determination of rare earths in inorganic materials. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Rare earth; Azodye; Polarography; Reduction

1. Introduction The determination of rare earths (REs) is a very actual problem because of their wide application in industry and technology. RE(III) are important components of functional materials with valuable electric, magnetic, mechanical (resistance to vacuum, high pressure and temperature), scintillation and other properties. The characteristics of these materials essentially depend on RE(III) contents. Therefore, reliable, sensitive, selective and rapid methods of their determination are very necessary * Corresponding author. E-mail address: [email protected] (L. Dubenska).

for control quality of such objects. Several analytical techniques have been applied for the trace determination of RE, including isotope dilution mass spectroscopy, X-ray fluorescence spectrometry, atomic absorption spectrometry and inductivity-coupled plasma spectrometry [1]. Most of these techniques require costly and specialized equipment that may not be available in many laboratories. Voltammetry, the sensitive electroanalytical technique, may provide an effective and economical alternative. Although RE(III) ions in aqueous solutions give no well-defined waves before the liberation of hydrogen (with the exception of Eu and Yb), it was found that the analysis of RE(III) could be

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carried out by the reduction of their complexes with certain organic ligands. The application of organic ligands increases sensitivity and selectivity of RE(III) polarographic determination. Complexes RE(III) with azodyes are very perspective from these considerations. The polarographic behavior of some azodyes and their complexes with RE(III) has been described [2 – 6]. RE(III) can be determined successfully using o,o%-hydroxysubsti-

tuted azodyes. But there is not one thought in the literature about the structure of these complex compounds. The processes of RE(III) reduction in the presence of azo compounds that contain different substituents in the o,o%-positions or without a substituent in the o,o%-position have not been studied enough or have not been investigated at all. The reduction mechanism of the complexes RE(III) with azodyes has not been fully discov-

Table 1 Structure of azodyes investigated

Fig. 1. Oscillo voltammograms of Tr 0 (a), Tr 00 (b), Tr 000 (c) at pH 2.2 (I) and pH 10.0 (II). V =0.5 V s − 1.

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2. Experimental

Fig. 2. Oscillo voltammograms of Mag (I), EB T (II) and Ar I (III); V= 0.5 V s − 1. (a) pH 2.0, (b) pH 10.5.

ered. As for the reduction of azodyes, the authors [2,7] are most often limited by the general scheme of the reduction mechanism:

In this paper, the polarographic behavior of some azodyes with various disubstituted groups in o,o%-positions (tropeolin 0 (Tr 0), tropeolin 00 (Tr 00), tropeolin 000 (Tr 000), magnezon (Mag), eriochrome black T (EB T), arsenazo I (Ar I)) was investigated with the purpose of their application for rare earth determination.

All voltammetric curves were obtained on an oscilloscopic polarograph PO-5122 model 03 with a slowly dropping mercury working electrode (DME), saturated calomel reference electrode and platinum-wire counter electrode using a polarizing range of 0 to − 0.9 V. The characteristics of capillary of the slowly DME as determined at a potential of 0 V and a mercury column of 30.0 cm were m= 5.34× 10 − 4 g s − 1, t= 14.5 s. The dissolved oxygen was removed from the solutions by purging with pure argon. The temperature of the solutions was controlled by thermostat UTU 2/77. The pH-meter model MV 870 DIGITAL-pHMESSGERA8 T was used for pH measurement. Spectrophotometric measurements dealing with the determination of the complex stoichiometry and the number of protons involved in the complexation process were made on a spectrophotometer model SPECOL-11 with glass cuvettes of 1 cm path length. All chemicals used were of commercial analytical grade reagents. All solutions were prepared from doubly distilled water. Standard solutions of 1.0× 10 − 2 mol l − 1 RE(III) were prepared by dissolving a desired amount of metallic RE(III) in 1 ml HNO3 and 5 ml HCl, evaporating to near dryness and diluting to the scale with water. Solutions of 1.0×10 − 3 mol l − 1 dyes were prepared in the normal way with water. NH4Cl + HCl (pH 0.5 –5.0), acetic buffer (pH 3.8 –6.3) and ammonium buffer (pH 7.0 –12.0) were used as the background electrolytes. Alloys prepared at the Department of Neorganic Chemistry of Ivan Franko National University of L’viv were used for analysis.

Table 2 Influence of ionic strength on the reduction process of some azodyes (Cdye =4.0×10−5 M; V =0.5 V s−1) EB T

Ar I

v −E kp (V) I kp (mA) v −E kp (V) I kp (mA)

0.07 0.62 7.60 0.10 0.68 6.00

0.10 0.62 7.80 0.15 0.66 6.30

0.33 0.61 8.00 0.18 0.65 6.35

0.66 0.61 8.20 0.21 0.65 6.45

0.55 7.80 0.26 0.64 6.65

1.32 0.59 9.00 0.31 0.64 6.63

1.65 0.52 7.95 0.41 0.62 6.35

0.58 9.20 0.46 0.61 6.00

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Table 3 Influence of the ethanol concentration on the reduction process of some azodyes (V =0.5 V s−1) Dye

Cdye×105 (M)

…(C2H5OH) (vol.%)

−E kp (V)

I kp (mA)

Dlog i/Dlog V

Eriochrome black T

0.8

0.0 19.2 28.8 38.4 48.0 0.0 3.84 7.68 19.20 28.80 0.0 19.2 28.8 38.4 48.0

0.61 0.59 0.59 0.59 0.57 0.68 0.68 0.67 0.60 0.50 0.61 0.60 0.58 0.58 0.57

1.95 1.75 1.45 1.28 0.65 6.00 5.55 4.00 2.10 1.30 8.20 5.00 4.30 4.00 3.70

0.70 0.63 0.60 0.54 0.49 0.82 0.77 0.60 0.52 0.46 0.72 0.70 0.69 0.65 0.59

4.0

Arsenazo I

4.0

3. Results and discussion

3.1. Reduction of the azodyes The structure of investigated azodyes is shown in Table 1. All investigated dyes are reduced on the DME (Figs. 1 and 2). The potentials of their peaks become negative with an increase of pH, showing that the hydrogen ions act as a reactant in the reduction process. Potentials of peaks at pH \7 are independent of pH. In this case, the nonprotonized form is reduced or only one kind of the proton donors, water, takes part in the process. This question can be solved if the influence of ionic strength and organic solvent on the reduction process of azocompounds would be investigated. Change of the height of the reduction peaks of the azodyes (I kp) is coordinated with change of their existing forms. The dependences of I kp on concentration of all dyes look like saturation curves. The constancy of cathode currents with the increase of Cdye higher than saturation is caused by maximal adsorption of dye on DME. Meanings of rate criterion Dlog i/Dlog V \ 0.5 of the investigated processes is also the proof of adsorption nature of the reduction currents. The potential scan rate (V) was performed in the range 0.08 – 2.00 V s − 1. Since the most legible and high peaks were ob-

Fig. 3. Oscillo voltammograms of azodyes with heavy RE(III); Cdye =4.0 × 10 − 5 M, CRE =8.0 × 10 − 6 M, V =0.5 V s − 1. (a) Mag at pH 10.5, (b) EB T at pH 10.5, (c) Ar I at pH 7.8.

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Fig. 4. Results of the determination of the number of protons involved in the complexation process in the systems: (1) Tb – EB T; (2) Tb–Mag; (3) Pr–Mag; (4) Pr–EB T. Cdye = CRE(III) = 4.0 × 10 − 5 M.

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tained at V= 0.5 V s − 1, all the following investigations were carried out at such scan rate. The introduction of electron-donor groups in o,o%-positions complicates the reduction process of dyes and increases their irreversibility. The increase of ionic strength (Table 2) of a solution facilitates reduction of azodyes, which is observed only for processes with previous superficial protonization. The height of the reduction peaks of the azocompounds also increases, because the rate of their protonization increases [8]. The potential peaks of reduction become positive and the height of the cathode peaks decrease with the increasing C2H5OH contents. The increase of ethanol concentration in aqua –ethanol mixes reduces proton-donor activity of water and reduces the rate of protonization. The rate of the electrode process strongly decreases in the case of the superficial reaction of protonization that precedes an electrochemical stage, because adsorption of the basic form of the depolarizer decreases. It is confirmed by significant decreasing of the rate criterion at the increase of C2H5OH concentration in solutions (Table 3). Thus, the reduction mechanism of the investigated azodyes is represented as follows.

Fig. 5. Effect of pH on I kp complexes RE(III) with (1) EB T, (2) Mag and (3) Ar I. Cdye = 4.0× 10 − 5 M, CRE = 8.0× 10 − 6 M, V =0.5 V s − 1. Table 4 Rate criterion meanings of the reduction process of complexes RE(III) with Mag, EB T (pH 10.5) and Ar I (pH 7.8) (Cdye =4.0×10−5 M; n= 3) Dye

RE(III)

CRE(III)×105 (M)

Dlog i/Dlog V

Magnezon

Tm

Eriochrome black T

Gd

1.0 0.8 1.0

0.69 0.62 0.76

4.0 0.8 1.0 0.8 4.0 1.0

0.67 0.61 0.75 0.71 0.66 0.71

Eu

Arsenazo I

Tb Nd Tb

The dyes without a substituent in the o,o%-positions (Tr 00, Tr 000) and Tr 0 do not form with RE(III) complex compounds. Polarographic characteristics of these dyes are not changed with the addition of RE(III). When RE(III) are added to o,o%-disubstituted azodyes (Mag, EB T, Ar I) and

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Fig. 6. Electrocapillary curves: (1) ammonium buffer (pH 10.5); (2) 4.0 × 10 − 5 M EB T; (3) 4.0 × 10 − 5 M EB T+8.0 × 10 − 6 M Nd3 + ; (4) 4.0 × 10 − 5 M EB T+4.0 × 10 − 5 M Tb3 + ; (5) 4.0 ×10 − 5 M EB T+ 8.0× 10 − 6 M Tb3 + .

an ammonium buffer system, the height of the reduction peaks of the azocompounds decreases and an additional peak with a more negative potential appears (Fig. 3). Additional peaks are caused by the reduction of complexes RE(III) with these azodyes.

3.2. Composition of the complexes The stoichiometry of RE(III) – Mag and RE(III) –EB T complexes was determined by Job’s method and the method of saturation [9]. In these complexes, the molar composition ratio ligand:metal is 1:1. So, a composition formula of complexes is REInd. The number of protons involved in the complexation process was also de-

termined. In the case when a ligand is a weak acid and the reagent absorbs itself, it is possible to use the graphic Astahov’s method [10]. A solution series of equimolar concentrations of reacting components at different pH values was prepared. The absorbance of these solutions (AX ) and reagent solutions at such concentration and pH (AR) was measured. From received data, the plot of log(AX − AR)/(Amax − AX )2 against pH was constructed (Fig. 4), where Amax is the maximum value of the solution absorbance that is reached at high values of pH. The slope of this graph is equal to the number of protons involved in the complexation process. In all cases, cation RE(III) supercedes one proton at the interaction with the dyes. In general, the kind of complexation process of RE(III) with azodyes is as presented as follows. RE3 + + HInd2 − ? REInd+ H+

(5)

3.3. Reduction of the complex The effect of pH on polarographic characteristics of complexes RE(III) with Mag, EB T and Ar I was studied. The height of additional peaks is stable and shows a maximum value when the pH is between 9.8 and 10.8 for Mag and EB T or between 7.0 and 8.2 for Ar I (Fig. 5). An absence of anodic peaks on the polarograms testifies that reduction of the complex compounds is an irreversible process (Fig. 3).

Table 5 The effect of pH on DE kp (Cdye = 4.0×10−5 M, CRE =8.0×10−6 M; background electrolyte, ammonium buffer) pH

Mag −E kp, dye (V)

8.0 8.5 9.0 9.5 10.0 10.5 11.2

0.63 0.62 0.62 0.62 0.62 0.62 0.61

EB T DE kp (V) − E p, complex k (V) 0.84 0.84 0.80 0.78 0.78 0.78 0.77

0.21 0.22 0.18 0.16 0.16 0.16 0.16

−E kp, dye (V)

– 0.54 0.55 0.57 0.61 0.61 0.60

pH DE kp (V)

−E kp, dye (V)

−E p, complex k (V) – 0.84 0.83 0.82 0.80 0.80 0.78

Ar I DE kp (V) − E p, complex (V)

k

– 0.30 0.28 0.25 0.19 0.19 0.18

7.0 7.8 8.0 8.5 9.0 9.5 –

0.66 0.67 0.67 0.67 0.65 0.64 –

0.74 0.74 0.74 0.74 0.73 0.72 –

0.08 0.07 0.07 0.07 0.08 0.08 –

4.0 6.0 8.0 10.0 40.0

C(Tb3+)×106 (M)

0.62 0.62 0.62 0.62 0.62

−E kp, dye (V)

Mag

0.78 0.78 0.78 0.78 0.78

−E kp, complex (V) 0.16 0.16 0.16 0.16 0.16

DE kp (V) 0.61 0.61 0.61 0.61 0.61

−E kp, dye (V)

EB T

0.80 0.80 0.80 0.80 0.80

−E kp, complex (V)

0.19 0.19 0.19 0.19 0.19

DE kp (V)

0.67 0.67 0.67 0.67 0.67

−E kp, dye (V)

Ar I

0.75 0.75 0.74 0.74 0.74

−E kp, complex (V)

Table 6 The effect of Tb3+ concentration on DE kp at the optimal range of pH (Cdye =4.0×10−5 M; background electrolyte, ammonium buffer)

0.08 0.08 0.07 0.07 0.07

DE kp (V)

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Fig. 7. Influence of the ion nature on the height of the reduction peaks of the complexes RE(III) with Mag (I) and EB T (II) (pH 10.5); Cdye = 2.0× 10 − 5 M, CRE(III) = 8.0× 10 − 6 M, V= 0.5 V s − 1. Table 7 Metrological characteristics of the determination of RE(III) using Mag, EB T and Ar I at the optimal range of pH (Cdye = 4.0× 10−5 M, V =0.5 V s−1) Characteristic

Equation of calibration graph Dispersion of calibration graph Correlation coefficient Detection limit, M

Ligand Eriochrome black T

Magnezon

Arsenazo I

I kp = 0.03+1.03×105C 3.38×10−3 0.9900 1.72×10−6

I kp =−0.03+2.78×105C 2.42×10−3 0.9980 1.74×10−6

I kp =0.38+2.44×105C 2.91×10−3 0.9995 1.11×10−6

Meanings of the rate criteria of the reduction process of complexes are shown in Table 4, from which we can see that the complexes possess adsorptivity at DME. The electrocapillary curves EB T and EB T+ RE(III) are shown in Fig. 6. Comparing curves 2–5 with curve 1, we can see the zero charge potential shifted negatively when EB T was added to the solution, and the curves 2 – 5 were deformed. It shows that EB T and the EB T– RE(III) complex were both adsorptive on the surface of the mercury drop in the form of negative ions. The electrocapillary curves Mag, Ar I and their complexes with RE(III) are analogous. Addition of ethanol influences the reduction process of complexes similar to the free ligand. The decrease of the height of the reduction peaks and meanings of the rate criterion with increase of the ethanol content in the solutions confirms the

superficial reaction of protonization. The reaction of protonization occurs before the electrochemical stage. The reduction potentials of azodye complexes with RE(III) do not differ from each other, and with other metals (In, Al) do not differ essentially. Therefore, it can be concluded that the main contribution in the bottom vacant molecular orbital of complexes belongs to the orbitals of ligands, i.e. the ligand is reducible in the complex, but the metal ion is not reduced. There are two possible cases of the polarographic reduction of such a metal complex where the ligand is reducible [11]. 1. The complex is reduced while the metal ion remains complexed with the reduced form of the ligand. DE kp = E kp, complex − E kp, azodye is independent of the pH of the solution and the concentration of the metal ion; it is dependent

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only on the relevant stability constants of the complexes in the oxidized and the reduced forms, and the dissociation constants of the oxidized and the reduced forms of the dye. 2. The complex dissociates at the electrode surface prior to reduction and the free ligand is reduced. DE kp is not only dependent on the stability constants and dissociation constants of the oxidized form, but also depends on the concentration of metal ion and the pH of the solution. The data are shown in Tables 5 and 6, respectively, from which it may be seen that DE kp is independent of pH and the concentration of RE(III) in optimal conditions of the complexation. Therefore, it can be concluded that the reduc-

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tion mechanism of the RE –azodye complex belongs to case 1. The reaction scheme is shown in Eq. (6) and 7. REInd ? [REInd]ads

(at the surface)

(6)

Fifteen REs (except Pm) were tested by the same procedure. I kp of the reduction of complexes RE(III) with Ar I is independent of the nature of rare earth ions. The influence of the ion nature on the height of the reduction peaks of the RE –Mag and RE –EB T complexes is shown in Fig. 7. It is seen that the detection limit of heavy RE and

Table 8 Influence of the coexisting ions on the polarographic determination of RE(III) with Mag, EB T (pH 10.5) and Ar I (pH 7.8) (CRE =8.0×10−6 M) Coexisting ion

Variant of reaction RE–Mag

RE–EB T

RE–Ar I

K+ Na+ NH+ 4 Mg2+ Cu2+ Mn2+ Pb2+ Cd2+

\1:1000 \1:1000 \1:1000 1:2 1:2 1:0.1 1:0.5 1:0.2

\1:1000 \1:1000 \1:1000 1:2.5 1:6 1:0.1 1:1 1:15

Co2+ Ni2+ Fe2+ Zn2+

… 1:1 1:0.5 1:0.1

1:10 1:0.1 1:1 1:0.2

1:5000 1:5000 \1:1000 1:580 1:12 … 1:12 1:0.1 1:10 (CN−) … 1:0.3 1:1 1:0.1 1:10 (CN−) 1:10

Al3+

Fe3+ In3+ Ge(IV) PO3− 4 F− Cl− SO2− 4 EDTA

1:0.4 1:15 (F−) 1:7 (pH 8.5) 1:0.5 1:0.05 1:5 (F−) 1:25 1:5 1:100 \1:1000 1:2000 1:0.3

1:1 … 1:2 (pH 8.5) 1:0.5 1:2.5 1:100 1:4 1:250 \1:1000 1:1000 1:0.3

1:5 1:10 … 1:0.25 1:1 \1:1000 1:125000 1:0.1

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Table 9 Results of the RE (III) determination in the inorganic materials (n = 5; P =0.95) Sample

Approximate contents of REa (%)

Dye

Weight of sample (g)

Found RE (%)

Sr (%)

Tm20Cu70Ge10

39.5

EB T

Tm33Cu32Ge35

54.9

Tm10Cu60Ge30

22.0

TbAl2

74.6

TbAl4

59.6

DyAl4 KMgF3(EuF3)

60.2 :0.3

Mag Ar I Mag Ar I Mag Ar I

0.0343 0.0405 0.0154 0.0313 0.0345 0.0215 0.0062

KMgF3(Tb2O3)

:1.5

Ar I

38.25 38.27 53.42 53.50 21.05 21.10 73.18 72.72 57.76 57.33 60.16 0.32 0.32 1.46

4.47 4.20 4.60 4.55 4.00 3.91 4.11 3.52 3.38 3.52 0.23 2.08 2.07 2.12

0.0268 0.0130 1.7500 1.5650 1.9500

Analysed alloys were prepared by reacting the elemental components (all components with purity \99.99%) by arc melting in an argon atmosphere. The composition of alloys was controlled by comparison mass of the compact metals with mass of the prepared sample. The differences of mass were B1–2%. a

yttrium is the highest, and that of the light REs is the least using Mag or EB T. Similar dependencies have been received for complexes RE(III) with other o,o%-disubstituted azodyes: acid chrome blue K [4] and solochrome violet RS [5]. Martinenko [12] approved that the monotonic character in the dependence ‘property of complex – protonic number of RE’ indicates the covalent character of bonds among the central ion and ligand.

4. Application of the method The discrete adsorptive complex waves can be used for analytical purposes. Metrological characteristics of heavy RE(III) determination using Mag, EB T and Ar I are shown in Table 7. Parameters of the calibration graph were calculated by the least-squares method. A detection limit was calculated by the formula C = [(Sa + X( Sb )th ]/(b + thSb ), where Sa and Sb are the average square deviations of the calibration graph parameters a and b, respectively, th is the Student’s coefficient, a and b are parameters of the calibration graph, and X( is the value of abscissa for the average point of the calibration graph.

The effects of various coexisting ions on the determination of heavy RE(III) with Mag, EB T and all RE(III) with Ar I under the optimum experimental conditions were investigated in order to determine the selectivity of the polarographic method of RE determination using these dyes. The results of the investigation are shown in Table 8. The tolerance limit in Table 8 is the highest tolerance amount of foreign ions when relative error is not more that 5% for RE(III) determination. The method was used for the determination of RE(III) in inorganic materials: alloys of the systems RE –Cu –Ge and RE –Al from small weights of the samples and scintillation materials with low contents of RE. The results are presented in Table 9. Relative error (Sr) is not more than 5% for rare earth determination. From the results presented, it can be concluded that the proposed method is simple, rapid and stable for the determination of rare earth metals.

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