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Interaction of gold(I) with thiosulfate–sulfite mixed ligand systems
Inorganica Chimica Acta 358 (2005) 2183–2190 www.elsevier.com/locate/ica
Interaction of gold(I) with thiosulfate–sulfite mixed ligand systems W. Nimal Perera, Gamini Senanayake *, Michael J. Nicol A.J. Parker Cooperative Research Centre for Hydrometallurgy, Department of Mineral Science and Extractive Metallurgy, Murdoch University, Perth, WA 6150, Australia Received 19 February 2004; accepted 22 September 2004
Abstract The AuðIÞ–SO3 2 –S2 O3 2 system was studied at 25 C and at I = 0.1 M NaClO4 using hydrodynamic voltammetry, gold potentiometry, UV–Vis spectrophotometry and Raman spectroscopy. The presence of two mixed-ligand species, Au(S2O3)(SO3)3 and AuðS2 O3 ÞðSO3 Þ2 5 , was detected from the Raman experiments and supported by the gold potentiometric experiments. The stepwise formation constant, log K11r, for the reaction AuðS2 O3 Þ2 3 þ rSO3 2 () AuðS2 O3 ÞðSO3 Þr ð2rþ1Þ þ S2 O3 2 was found to be 1.1 (r = 1) and 4.8 (r = 2) from the hydrodynamic voltammetric experiments. 2004 Elsevier B.V. All rights reserved. Keywords: Gold(I); Sulfite; Thiosulfate speciation
1. Introduction Gold(I) complexation with S2 O3 2 and SO3 2 has applications in non-cyanide gold extraction [1] and industrial plating processes [2]. In particular, such solutions in alkaline media have potential applications in environmentally safe hydrometallurgical processes for gold extraction. In the case of gold electroplating, the addition of sulfite is critical in obtaining good results and maintaining the stability of the solutions [2]. Despite its application in this area, the reasons for the extra stability of the AuðIÞ–S2 O3 2 –SO3 2 system on the basis of chemical speciation have not been previously investigated. In a recent study, the mixed-speciation behaviour of Au(I) with NH3 and S2 O3 2 was found along with the observation of increased sensitivity of the Au(I) electrode process in the presence of NH3. This led to the possibility of using hydrodynamic voltammetry for studying such systems [3]. Using this same experimental technique, the interaction of gold(I) with S2 O3 2 and SO3 2 was investigated. *
Corresponding author. Fax: +61 8 9360 6343.
0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.09.058
To provide further supporting information confirmation, gold potentiometric data, UV–Vis Raman spectra of solutions of AuðS2 O3 Þ2 3 SO3 2 were acquired. The possibility of forming of these ionic species was also examined.
and and with salts
2. Experimental 2.1. Materials Solutions were made with calibrated grade A volumetric glassware using high-purity water (Millipore, Milli-Q system). High-purity NaClO4 (Aldrich Chemical Company, ‘‘99.99%’’ grade) was used as the swamping electrolyte source. The NaClO4 stock solutions were prepared and standardised as described elsewhere [4]. Ammonia stock solutions (2.0 M) were prepared from concentrated ammonia solution (25% BDH, AR, UK) and calibrated (±0.1%) against standard 1.0 M HCl (BDH, UK, concentrated volumetric standard) using methyl orange as the indicator [5]. The stock solutions of Na3Au(S2O3)2 and Na2S2O3 (0.1 M) were prepared
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by dissolving accurately weighed Na3Au(S2O3)2(s) (Alfa-Aesar, AR, US) and Na2S2O3(s) (BDH, AR, UK) using a Sartorius balance (2024 MP max 100 g ± 0.01 mg). The purity of Na2S2O3 stock solutions was determined using the standard iodometric (±0.2%) titration [5].
potential E [3]. From the general current–potential equations for a rotating disc electrode, the Levich and the Nernst equations, the difference between (E1/2)C for the two complexes C1 and C2 of Au(I) [Eqs. (1) and (2)] was used to estimate the stepwise formation constant (K1mj) for the conversion of the gold complex C1 to C2 [3,6]
2.2. Potentiometry
MXpðþnpbÞ þ jYc $ MXðpmÞ Yj
Typically, 50 mL of 1 M NaCl and 0.01 M NaOH with 0.1 mM Na3Au(S2O3)2 was placed in a doublejacketed potentiometric cell setup with a gold wire electrode and a Ag(s)/AgCl(s)/Ag(I)/5 M NaCl (potential corrected against the SCE) reference electrode in conjunction with a 1 M NaCl salt bridge. The cell temperature was maintained at 25 ± 1 C with a circulation water system (Ratek isothermal bath) and a N2 blanket was kept over the solution in the cell. To this solution, small accurately weighed quantities of Na2S2O3(s) or Na2SO3(s) were added. After each addition of Na2S2O3(s) or Na2SO3(s), the gold potential values were recorded against time and the final stable Au potentials were used for the graphical analysis using the Nernst equation to estimate the stoichiometry of the complexes formed [6]. 2.3. Hydrodynamic voltammetry The hydrodynamic voltammetry (HV) experiments were performed using a voltammetry stand of in-house construction consisting of a rotating Au disc electrode (3 mm diameter) set-up dipping into a double-walled 50 ml electrochemical cell. The cell temperature was maintained at 25 ± 1 C with a circulation water system (Ratek isothermal bath). With a Ag(s)/AgCl(s)/Ag(I)/ NaCl reference electrode in conjunction with a 0.1 M NaClO4 salt bridge and platinum counter electrode in place, the gold electrode rotation speed for the experiments was maintained at 1000 ± 5 rpm [3]. The current measurements in the 60.1 mA range were carried out using a RDE3 Scanner (Pine Instruments) with a potential sweep of 5 mV/s and results were automatically recorded in a personal computer using a LABView program [3]. Starting solutions of 25–50 mL of 0.1 M NaClO4 and 0.01 M NaOH were pipetted into the cell and N2 bubbling began prior to the addition of NH3 (to make 0.5 M). During the experiments, a N2 blanket was maintained over the solution. To minimise air oxidation of the solutions, analyte Au(I) solutions were made in situ with the stepwise addition of accurately weighed quantities of gold thiosulfate (4 mg to make 1 mM), sodium thiosulfate (Univar, AR, to make 2 mM) crystals and small portions of sodium sulfite (Univar AR, 6 mg to make 0.2 mM onwards). The E1/2 values were estimated from the minimum peaks from 20-point moving average graphs of di/dE against the
fþnðpmÞbjcg
þ mXb ; ð1Þ
ðE1=2 ÞC1 ðE1=2 ÞC2 ¼ ð0:0592=nÞflog K 1mj m log½Xb þ j log½Y c g; fþnðpmÞbjcg
where K 1mj ¼ ½MXðpmÞ Yj b m
½X
ð2Þ
j =½MXðþnpbÞ ½Yc ; p
ð3Þ
n = 1 for Au+ and b = c = 2 for S2 O3 2 and SO3 2 . Here, the minimum peak heights are proportional to the limiting current and the relation between the two [Eq. (4)] provides a method to test the reversibility of the system [3,7]. fdi=dEgE1=2 ¼ ðnF =4RT ÞI C :
ð4Þ
2.4. Raman spectroscopy Small (0.03 g) weighed quantities of Na3Au(S2O3)2(s) (0.1 g) were dissolved in milliQ water (1.00 mL) and portioned into several known volumes with an autopipette (0.195 mL) in a short 5 mm NMR tube. Similar volumes of Na2(SO3)2 solutions prepared separately (0.2–1.5 M) were added to this NMR tube and placed in the sample holder of a Nicolet Magna-IR 850 (Series II) spectrometer with the Fourier transform Raman module. Coherent radiation at 1064 nm was generated by a 1.5 W YVO4 laser, with a calcium fluoride beamsplitter and an indium gallium arsenide detector (resolution = 14 cm1). Subsequent signal processing and recording of the spectra (typically 128 scans) was carried out using the Windows based Ohmic software. The spectra were subsequently transferred to the EXCEL spreadsheet format for display. 2.5. UV–Vis spectrophotometry Experiments were conducted using 50 mL of 0.1 M NaClO4 and 0.01 M NaOH placed in the 0.74 cm spectrophotometric cell setup [3] with the addition of accurately weighed quantities of Na3Au(S2O3)2 and Na2SO3. The spectrum of the solution was recorded after each addition of Na2SO3(s). The UV–Vis data were analysed using the SPECFIT software package [8,9] after background corrections of the spectra were made. To obtain convergence for the calculations, the value of
W.N. Perera et al. / Inorganica Chimica Acta 358 (2005) 2183–2190
log b120 ðAuðS2 O3 Þ2 3 Þ was taken as 26.0 [10] and the spectral characteristics of Na2S2O3(aq) and Na2SO3(aq) were factored into the model used.
0.35 0.3
against [S2O32-] in the
0.25
EAu(I)/Au/V
3. Results and discussion
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presence of [SO32- ] = 1.4 mM
0.2
y = -0.0611x + 0.0756
0.15 0.8 mM Au(S2O3)23against [SO32- ]
0.1
y = -0.0914x - 0.0332
0.05
From previous work, it was found that the sensitivity of the Au electrode can be increased in the presence of excess Cl or NH3 [3]. Since no interaction between SO3 2 and Cl or NH3 was expected, to obtain reasonable qualitative Au potentiometric and quantitative hydrodynamic voltammetry results, experiments were done in the presence of 1 M NaCl (I = 1 M) and 0.5 NH3 (I = 0.1 M (NaClO4)), respectively. Separate Au potentiometric and UV–Vis spectrophotometric experiments showed insignificant interaction between Cl and AuðS2 O3 Þ2 3 in the concentration range examined here. However, a decrease in the gold electrode potential with respect to added quantities of Na2SO3 was seen (Fig. 1) in the potentiometric study at I = 1 (NaCl). This decrease in potential with respect to log½SO3 2 was linear with a slope of 0:091 V= log½SO3 2 , implying the formation of a new species with a coordination number of 1–2 with respect to SO3 2 . The possible presence of the mixed complex AuðS2 O3 Þq ðSO3 Þr ð2qþ2r1Þ (r = 1 and 2) according to [Eq. (5)] was thus assumed AuðS2 O3 Þ2 3 þ rSO3 2 () AuðS2 O3 Þq ðSO3 Þrð2qþ2r1Þ þ ð2 qÞS2 O3 2
ð5Þ
In the presence of excess SO3 2 , the gold potential decreased with respect to added S2 O3 2 with a slope of 0:059 V= log½S2 O3 2 , indicating that q = 1 (Fig. 1) in the mixed complex AuðS2 O3 Þq ðSO3 Þr ð2qþ2r1Þ .
0 -4
-3.5
-3
-2.5
-2
-1.5
-1
log [ligand]
Fig. 1. Variation of gold(I)/gold(0) electrode potential values (corrected for the hydrogen reference electrode) plotted against log ½SO3 2 ([Na3Au(S2O3)2] = 0.79 mM at pH = 12) and against log ½S2 O3 2 ([Na3Au(SO3)2] = 0.15 mM, [Na2SO3] = 1.4 mM at pH = 10). All experiments were conducted at 25 C with I = 1 (NaCl).
From previous work [3], a reversible hydrodynamic voltammetry process was shown to exist (Fig. 2) for the reductive scan of gold thiosulfate solutions in the presence of NH3. From the speciation calculations using the information from the same study [3] at 3.2 mM Na2S2O3, I = 0.1 (NaClO4) with 1 mM Na3Au(S2O3)2, in the presence of [NH3] 0.5 M, the predominant species present in solution was AuðS2 O3 Þ2 3 . Hence, it is possible to use such ammonia (0.5 M) containing solutions to examine the interaction between AuðS2 O3 Þ2 3 and SO3 2 . With the addition of 1.6 mM Na2SO3 to this solution, two processes were seen to exist with the presence of two minima for the differential curve plots (E vs di/dE, Fig. 3(a)). Although these curves were not obvious in the usual i–E plots, by estimating the diffusion current for each of these processes the estimated value of n (number of electrons involved in the reaction, Eq. (4)) was found to be close to 1 based on both
E/V 0
0 -0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4 -0.0002
-0.05 -0.0004
-0.0006
-0.0008 -0.15 -0.001 -0.2 -0.0012
-0.0014
-0.25
-0.3
-0.35
Since ([di/dE]1/2 x 102.7)/id = n (di/dE)E1/2 = 0.0015x50microA/V id =0.15x50microA n = 1.0
di/dE x50microA/V
i x50microA
-0.1
-0.0016
-0.0018
-0.002
Fig. 2. Representative hydrodynamic voltammogram of 1 mM Na3Au(S2O3)2, 3.2 mM Na2S2O3, 0.01 M NaOH, and 0.5 M NH3, I = 0.1 (NaClO4) at 25 C with the calculated differential curve. Reversibility is indicated when ([di/dE]max · 102.7)/id 1.
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(a)
E/V 0 -0.3
-0.2
-0.1
0
0.1
0.2
0.3
Process 1
0 0.4 -0.0002
-0.05 -0.0004 -0.0006 -0.0008
-0.15 For process 1 (di/dE)E1/2 = 0.0005 x50 microA/V id = 0.05 x50 microA n = 1.0
Process 2 -0.2
-0.25
-0.001 -0.0012 -0.0014
For process 2 (di/dE)E1/2 = 0.0009 x50 microA/V id =0.09x50 microA n = 1.0
-0.3
-0.35
(b)
di/dE x50microA/V
i x50microA
-0.1
-0.0016 -0.0018 -0.002
0 -0.25
-0.2
-0.15
-0.1
i x50microA/V
-0.3
(di/dE)E1/2 = 0.0012 x50microA/V id = 0.115 x50microA n = 1.1
-0.02
0 0.1 -0.0002
-0.04
-0.0004
-0.06
-0.0006
-0.08
-0.0008
-0.1
-0.001
-0.12
-0.0012
-0.14
-0.0014
-0.16
-0.0016
-0.18
-0.0018
-0.2
-0.002
-0.05
0
0.05
di/dE x50microA/V
E/V
Fig. 3. Representative hydrodynamic voltammogram of 1 mM Na3Au(S2O3)2, 3.2 mM Na2S2O3, (a)1.6 mM Na2SO3, (b) 13 mM Na2SO3, 0.01 M NaOH, and 0.5 M NH3, I = 0.1 (NaClO4) at 25 C with the calculated differential curve. Two processes are seen with reversibility indicated when ([di/dE]max · 102.7)/id 1 for both processes.
Figs. 3(a) and (b) indicating reversibility. With the addition of more sulfite (13 mM Na2SO3) to the solution, the E1/2 value shifted to more negative potentials and the disappearance of the two step process was seen (Fig. 3, Figs. S1 and S2). The mixed complex AuNH3 S2 O3 is found only at very high concentration ratios of the two ligands ½NH3 =½S2 O3 2 > 105 [3]. Since appreciable amounts of the mixed AuðIÞ–NH3 –S2 O3 2 species were not expected for the concentration ratio of ½NH3 =½S2 O3 2 < 105 used in the present study, the interaction of AuðS2 O3 Þ2 3 with SO3 2 could be deduced from graphs of E1/2 against log½S2 O3 2 (Fig. 4). From the slope of the two curves obtained in such graphs
(Fig. 4), the formation of AuðS2 O3 ÞðSO3 Þr ð2rþ1Þ ; r ¼ 1 or 2 at higher sulfite concentrations was implied. Raman studies were conducted semi-quantitatively to examine the two species formed with respect to S2 O3 2 in solutions containing SO3 2 . In separate experiments, the lack of changes in the Raman bands of SO3 2 and S2 O3 2 when mixed showed no evidence for the interaction of SO3 2 with S2 O3 2 . By examining the solution containing Na3Au(S2O3)2 and Na2SO3, the Raman spectroscopic results showed the formation of possibly two new species within the concentration range used (Fig. 5). It could be inferred that, during the formation of the first species, equimolar concentrations of free thio-
W.N. Perera et al. / Inorganica Chimica Acta 358 (2005) 2183–2190
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0.2 E1/2 values from the less reductive minimum k
0.15
y = -0.0511x - 0.0196 R2 = 0.9758
0.1
E1/2/V
0.05 0 E1/2 values from the more reductive minimum -0.05 y = -0.13x - 0.3465 R2 = 0.9985
-0.1 -0.15 -0.2 -4
-3.5
-3
-2.5
-2
-1.5
log [S2O32-] Fig. 4. Representative graph of E1/2 against log½S2 O3 2 from 1 mM Na3Au(S2O3)2, 3.2 mM Na2S2O3, 0.01 M NaOH, and 0.5 M NH3, I = 0.1 (NaClO4) solutions with various [Na2SO3] at 25 C.
sulfate (as seen with peaks at 999 and 447 cm1, Fig. 5) to the initial gold thiosulfate were released, i.e., AuðS2 O3 Þ2 3 þ SO3 2 () AuðS2 O3 ÞðSO3 Þ3 þ S2 O3 2 ð6Þ However, the concentration of the free thiosulfate was not appreciably changed when the second species was formed with the addition of more sulfite. Moreover, upon the addition of sulfite to maintain molar concentration ratios [Na2SO3]/[Na3Au(S2O3)2] of 1:1 and 2:1, no free sulfite was seen as revealed by the absence of the SO3 2 peaks at 964.2 and 474.3 cm1, Fig. 5, i.e., AuðS2 O3 Þ2 3 þ 2SO3 2 () AuðS2 O3 ÞðSO3 Þ2 5 þ S2 O3 2 ð7Þ This result did not change with the total gold(I) concentration [Au(I)]T from 0.1 to 0.4 M. Hence, with the information from previous experiments, it was inferred that the two species formed are Au(S2O3)(SO3)3 [Eq. (6)] and AuðS2 O3 ÞðSO3 Þ2 5 [Eq. (7)], with the value of q = 1. These two new species can be identified by the presence of Raman bands at 388 cm1 for Au(S2O3)(SO3)3 and at 386 and 372 cm1 for AuðS2 O3 ÞðSO3 Þ2 5 . In addition to these different bands, Raman peaks formed as a result of AuðIÞ–S2 O3 2 interactions were still seen in the presence of these two new species, implying the presence of S2 O3 2 moieties in both of these complexes [Eqs. (6) and (7)]. Attempts to perform experiments using AuðSO3 Þ2 3 as the starting complex were not made as it was difficult to obtain stable and pure Na3Au(SO3)2 [11,12].
Using these results to set up the speciation model, the stepwise formation constants log K11r of Au(S2O3)(SO3)3 and AuðS2 O3 ÞðSO3 Þ2 5 were calculated from plots of E1/2 (hydrodynamic) vs log½S2 O3 2 (Figs. 4 and 6) to be 1.09 ± 0.06 (r = 1) and 4.8 ± 0.3 (r = 2), at I = 0.1 M (NaClO4, 25 C). For a change in [Au(I)]T (44%), no significant change in the log K11r values was observed (<10%), discounting the presence of polynuclear species. To further support the above results, the data from UV–Vis spectrophotometric experiments were analysed using SPECFIT [8,9] with various speciation models. Separate experiments to evaluate the stability of AuðS2 O3 Þ2 3 solutions during aeration over a period of 2 days showed no changes in the UV–Vis spectrum. Although these experiments did not provide an independent model and no characteristic absorption peaks were seen, the presence of at least 3 species (including AuðS2 O3 Þ2 3 ) could be inferred from the changes in the spectra with the addition of Na2SO3 in the range 6 mM P [Au(I)] P 0.23 mM. These changes in absorbances were independent of [Au(I)]T, confirming the absence of polynuclear species. Using the values of equilibrium constants obtained from the hydrodynamic experiments, the UV–Vis spectra of the two new species were calculated (Fig. 7). Although the formation of Au(S2O3)(SO3)3 is anticipated, the increased stability of this species with respect to that of AuðS2 O3 Þ2 3 and the formation of the tricoordinated AuðS2 O3 ÞðSO3 Þ2 5 complex is unexpected. Since AuðSO3 Þ2 3 is much less stable (log bðAu ðSO3 Þ2 3 Þ 10 compared with log bðAuðS2 O3 Þ2 3 Þ
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(a)
2.5
0.19 M Na3Au(S2O3)2 + [Na2SO3] /M
Raman intesity / arbitrary units
2
No sulphite 0.10 0.19
1.5
0.45 0.85 1.13 1.13M sulphite only 1
0.19M thiosulphate only
0.5
0 1040
1020
1000
980
960
940
920
900
(b)
4
Raman intensity / arbitrary units
wavenumbers /cm-1
3.5
0.19 M Na3Au(S2O3)2 +[Na2SO3] /M No sulphite 0.10 0.19 0.45 0.85 1.13 1.13M sulphite only 0.19M thiosulphate only
3
2.5
2
1.5
1
0.5
0 500
480
460
440
420
400
380
wavenumbers /cm
360
340
320
300
-1
Fig. 5. Raman spectra of various solutions containing Na3Au(S2O3)2(aq) and Na2SO3(aq) (a) from 1050 to 900 cm1 and (b) from 500 to 300 cm1. For comparison purposes, the spectra of Na3Au(S2O3)2, Na2S2O3 and Na2SO3 are also included.
26) [13] based on the mixed speciation behaviour of Au(I) [3], the stability of both Au(S2O3)(SO3)3 and AuðS2 O3 ÞðSO3 Þ2 5 is predicted to be less than AuðS2 O3 Þ2 3 . Although tri-coordinated Au(I) complexes are known, they are not common with the tendency of Au(I) to form linear complexes [14]. The change from the usual coordination behaviour of this system may also explain the greater stability seen for these mixed complexes which in turn may be advantageous in gold hydrometallurgical processes used for gold extraction
or electroplating. However, complications arising from the presence of two complexes in such solutions must be noted. With this increased stability, the possibility of forming crystalline salts of these two species was examined. Colourless solutions which contained Na3Au(S2O3)2: Na2SO3 at molar ratios of 1:1 and 1:2, following the ratios of the two species expected in solution (Fig. 6), were evaporated under vacuum. Salts prepared with a molar ratio of 1:1 were white, whereas salts with ratios
W.N. Perera et al. / Inorganica Chimica Acta 358 (2005) 2183–2190
2189
100 Au(S2O3)23-
90
AuS2O3(SO3)23-
80
AuS2O3SO33-
% species
70 60 50 40 30 20 10 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
[SO32-] / m M
Fig. 6. Speciation diagram based on the results obtained from the hydrodynamic voltammetric results at [Au(I)] = 0.23 mM.
3000
absorbtivities/ cm-1M-1
2500 AuS2O3(SO3)23-
2000
1500
AuS2O3SO33-
1000
Au(S2O3)23-
500 S2O3
2-
SO32-
0 244
249
254
259
264 wavelength /nm
269
274
279
Fig. 7. UV–Vis spectra of various individual species as calculated from SPECFIT.
of 1:2 were yellow suggesting the presence of at least two different species.
article can be found, in the online version at doi:10.1016/j.ica.2004.09.058.
Acknowledgement
References
The authors thank A.J. Parker Cooperative Research Centre for Hydrometallurgy for financial assistance.
Appendix A. Supplementary data Figures S1 and S2 are available as supplementary information. Supplementary data associated with this
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[7] J. Heyrovsky, J. Kuta, Principles of Polarography, Academic Press, New York, 1966. [8] M. Maeder, A.D. Zuberbuhler, Anal. Chem. 62 (1990) 2220. [9] H. Gampp, M. Maeder, C.J. Meyer, A.D. Zuberbu¨hler, Talanta 32 (1985) 257. [10] J. Pouradier, M.C. Gadet, J. Chimie Physique 66 (1969) 109. [11] T. Morris, H. Copeland, G. Szulczewski, Langmuir 18 (2002) 535.
[12] M.O. Faltens, D.A. Shirley, Chem. Phys. 53 (1970) 4249. [13] E. Hogfeldt, Stability Constants of Metalion Complexes, 2nd supplement, Part A, Inorganic Ligands, IUPAC Chemical Data Series No. 21, Pergamon, Oxford, 1982. [14] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, sixth ed., Wiley Interscience, New York, 1999.
Interaction of gold(I) with thiosulfate–sulfite mixed ligand systems W. Nimal Perera, Gamini Senanayake *, Michael J. Nicol A.J. Parker Cooperative Research Centre for Hydrometallurgy, Department of Mineral Science and Extractive Metallurgy, Murdoch University, Perth, WA 6150, Australia Received 19 February 2004; accepted 22 September 2004
Abstract The AuðIÞ–SO3 2 –S2 O3 2 system was studied at 25 C and at I = 0.1 M NaClO4 using hydrodynamic voltammetry, gold potentiometry, UV–Vis spectrophotometry and Raman spectroscopy. The presence of two mixed-ligand species, Au(S2O3)(SO3)3 and AuðS2 O3 ÞðSO3 Þ2 5 , was detected from the Raman experiments and supported by the gold potentiometric experiments. The stepwise formation constant, log K11r, for the reaction AuðS2 O3 Þ2 3 þ rSO3 2 () AuðS2 O3 ÞðSO3 Þr ð2rþ1Þ þ S2 O3 2 was found to be 1.1 (r = 1) and 4.8 (r = 2) from the hydrodynamic voltammetric experiments. 2004 Elsevier B.V. All rights reserved. Keywords: Gold(I); Sulfite; Thiosulfate speciation
1. Introduction Gold(I) complexation with S2 O3 2 and SO3 2 has applications in non-cyanide gold extraction [1] and industrial plating processes [2]. In particular, such solutions in alkaline media have potential applications in environmentally safe hydrometallurgical processes for gold extraction. In the case of gold electroplating, the addition of sulfite is critical in obtaining good results and maintaining the stability of the solutions [2]. Despite its application in this area, the reasons for the extra stability of the AuðIÞ–S2 O3 2 –SO3 2 system on the basis of chemical speciation have not been previously investigated. In a recent study, the mixed-speciation behaviour of Au(I) with NH3 and S2 O3 2 was found along with the observation of increased sensitivity of the Au(I) electrode process in the presence of NH3. This led to the possibility of using hydrodynamic voltammetry for studying such systems [3]. Using this same experimental technique, the interaction of gold(I) with S2 O3 2 and SO3 2 was investigated. *
Corresponding author. Fax: +61 8 9360 6343.
0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.09.058
To provide further supporting information confirmation, gold potentiometric data, UV–Vis Raman spectra of solutions of AuðS2 O3 Þ2 3 SO3 2 were acquired. The possibility of forming of these ionic species was also examined.
and and with salts
2. Experimental 2.1. Materials Solutions were made with calibrated grade A volumetric glassware using high-purity water (Millipore, Milli-Q system). High-purity NaClO4 (Aldrich Chemical Company, ‘‘99.99%’’ grade) was used as the swamping electrolyte source. The NaClO4 stock solutions were prepared and standardised as described elsewhere [4]. Ammonia stock solutions (2.0 M) were prepared from concentrated ammonia solution (25% BDH, AR, UK) and calibrated (±0.1%) against standard 1.0 M HCl (BDH, UK, concentrated volumetric standard) using methyl orange as the indicator [5]. The stock solutions of Na3Au(S2O3)2 and Na2S2O3 (0.1 M) were prepared
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by dissolving accurately weighed Na3Au(S2O3)2(s) (Alfa-Aesar, AR, US) and Na2S2O3(s) (BDH, AR, UK) using a Sartorius balance (2024 MP max 100 g ± 0.01 mg). The purity of Na2S2O3 stock solutions was determined using the standard iodometric (±0.2%) titration [5].
potential E [3]. From the general current–potential equations for a rotating disc electrode, the Levich and the Nernst equations, the difference between (E1/2)C for the two complexes C1 and C2 of Au(I) [Eqs. (1) and (2)] was used to estimate the stepwise formation constant (K1mj) for the conversion of the gold complex C1 to C2 [3,6]
2.2. Potentiometry
MXpðþnpbÞ þ jYc $ MXðpmÞ Yj
Typically, 50 mL of 1 M NaCl and 0.01 M NaOH with 0.1 mM Na3Au(S2O3)2 was placed in a doublejacketed potentiometric cell setup with a gold wire electrode and a Ag(s)/AgCl(s)/Ag(I)/5 M NaCl (potential corrected against the SCE) reference electrode in conjunction with a 1 M NaCl salt bridge. The cell temperature was maintained at 25 ± 1 C with a circulation water system (Ratek isothermal bath) and a N2 blanket was kept over the solution in the cell. To this solution, small accurately weighed quantities of Na2S2O3(s) or Na2SO3(s) were added. After each addition of Na2S2O3(s) or Na2SO3(s), the gold potential values were recorded against time and the final stable Au potentials were used for the graphical analysis using the Nernst equation to estimate the stoichiometry of the complexes formed [6]. 2.3. Hydrodynamic voltammetry The hydrodynamic voltammetry (HV) experiments were performed using a voltammetry stand of in-house construction consisting of a rotating Au disc electrode (3 mm diameter) set-up dipping into a double-walled 50 ml electrochemical cell. The cell temperature was maintained at 25 ± 1 C with a circulation water system (Ratek isothermal bath). With a Ag(s)/AgCl(s)/Ag(I)/ NaCl reference electrode in conjunction with a 0.1 M NaClO4 salt bridge and platinum counter electrode in place, the gold electrode rotation speed for the experiments was maintained at 1000 ± 5 rpm [3]. The current measurements in the 60.1 mA range were carried out using a RDE3 Scanner (Pine Instruments) with a potential sweep of 5 mV/s and results were automatically recorded in a personal computer using a LABView program [3]. Starting solutions of 25–50 mL of 0.1 M NaClO4 and 0.01 M NaOH were pipetted into the cell and N2 bubbling began prior to the addition of NH3 (to make 0.5 M). During the experiments, a N2 blanket was maintained over the solution. To minimise air oxidation of the solutions, analyte Au(I) solutions were made in situ with the stepwise addition of accurately weighed quantities of gold thiosulfate (4 mg to make 1 mM), sodium thiosulfate (Univar, AR, to make 2 mM) crystals and small portions of sodium sulfite (Univar AR, 6 mg to make 0.2 mM onwards). The E1/2 values were estimated from the minimum peaks from 20-point moving average graphs of di/dE against the
fþnðpmÞbjcg
þ mXb ; ð1Þ
ðE1=2 ÞC1 ðE1=2 ÞC2 ¼ ð0:0592=nÞflog K 1mj m log½Xb þ j log½Y c g; fþnðpmÞbjcg
where K 1mj ¼ ½MXðpmÞ Yj b m
½X
ð2Þ
j =½MXðþnpbÞ ½Yc ; p
ð3Þ
n = 1 for Au+ and b = c = 2 for S2 O3 2 and SO3 2 . Here, the minimum peak heights are proportional to the limiting current and the relation between the two [Eq. (4)] provides a method to test the reversibility of the system [3,7]. fdi=dEgE1=2 ¼ ðnF =4RT ÞI C :
ð4Þ
2.4. Raman spectroscopy Small (0.03 g) weighed quantities of Na3Au(S2O3)2(s) (0.1 g) were dissolved in milliQ water (1.00 mL) and portioned into several known volumes with an autopipette (0.195 mL) in a short 5 mm NMR tube. Similar volumes of Na2(SO3)2 solutions prepared separately (0.2–1.5 M) were added to this NMR tube and placed in the sample holder of a Nicolet Magna-IR 850 (Series II) spectrometer with the Fourier transform Raman module. Coherent radiation at 1064 nm was generated by a 1.5 W YVO4 laser, with a calcium fluoride beamsplitter and an indium gallium arsenide detector (resolution = 14 cm1). Subsequent signal processing and recording of the spectra (typically 128 scans) was carried out using the Windows based Ohmic software. The spectra were subsequently transferred to the EXCEL spreadsheet format for display. 2.5. UV–Vis spectrophotometry Experiments were conducted using 50 mL of 0.1 M NaClO4 and 0.01 M NaOH placed in the 0.74 cm spectrophotometric cell setup [3] with the addition of accurately weighed quantities of Na3Au(S2O3)2 and Na2SO3. The spectrum of the solution was recorded after each addition of Na2SO3(s). The UV–Vis data were analysed using the SPECFIT software package [8,9] after background corrections of the spectra were made. To obtain convergence for the calculations, the value of
W.N. Perera et al. / Inorganica Chimica Acta 358 (2005) 2183–2190
log b120 ðAuðS2 O3 Þ2 3 Þ was taken as 26.0 [10] and the spectral characteristics of Na2S2O3(aq) and Na2SO3(aq) were factored into the model used.
0.35 0.3
against [S2O32-] in the
0.25
EAu(I)/Au/V
3. Results and discussion
2185
presence of [SO32- ] = 1.4 mM
0.2
y = -0.0611x + 0.0756
0.15 0.8 mM Au(S2O3)23against [SO32- ]
0.1
y = -0.0914x - 0.0332
0.05
From previous work, it was found that the sensitivity of the Au electrode can be increased in the presence of excess Cl or NH3 [3]. Since no interaction between SO3 2 and Cl or NH3 was expected, to obtain reasonable qualitative Au potentiometric and quantitative hydrodynamic voltammetry results, experiments were done in the presence of 1 M NaCl (I = 1 M) and 0.5 NH3 (I = 0.1 M (NaClO4)), respectively. Separate Au potentiometric and UV–Vis spectrophotometric experiments showed insignificant interaction between Cl and AuðS2 O3 Þ2 3 in the concentration range examined here. However, a decrease in the gold electrode potential with respect to added quantities of Na2SO3 was seen (Fig. 1) in the potentiometric study at I = 1 (NaCl). This decrease in potential with respect to log½SO3 2 was linear with a slope of 0:091 V= log½SO3 2 , implying the formation of a new species with a coordination number of 1–2 with respect to SO3 2 . The possible presence of the mixed complex AuðS2 O3 Þq ðSO3 Þr ð2qþ2r1Þ (r = 1 and 2) according to [Eq. (5)] was thus assumed AuðS2 O3 Þ2 3 þ rSO3 2 () AuðS2 O3 Þq ðSO3 Þrð2qþ2r1Þ þ ð2 qÞS2 O3 2
ð5Þ
In the presence of excess SO3 2 , the gold potential decreased with respect to added S2 O3 2 with a slope of 0:059 V= log½S2 O3 2 , indicating that q = 1 (Fig. 1) in the mixed complex AuðS2 O3 Þq ðSO3 Þr ð2qþ2r1Þ .
0 -4
-3.5
-3
-2.5
-2
-1.5
-1
log [ligand]
Fig. 1. Variation of gold(I)/gold(0) electrode potential values (corrected for the hydrogen reference electrode) plotted against log ½SO3 2 ([Na3Au(S2O3)2] = 0.79 mM at pH = 12) and against log ½S2 O3 2 ([Na3Au(SO3)2] = 0.15 mM, [Na2SO3] = 1.4 mM at pH = 10). All experiments were conducted at 25 C with I = 1 (NaCl).
From previous work [3], a reversible hydrodynamic voltammetry process was shown to exist (Fig. 2) for the reductive scan of gold thiosulfate solutions in the presence of NH3. From the speciation calculations using the information from the same study [3] at 3.2 mM Na2S2O3, I = 0.1 (NaClO4) with 1 mM Na3Au(S2O3)2, in the presence of [NH3] 0.5 M, the predominant species present in solution was AuðS2 O3 Þ2 3 . Hence, it is possible to use such ammonia (0.5 M) containing solutions to examine the interaction between AuðS2 O3 Þ2 3 and SO3 2 . With the addition of 1.6 mM Na2SO3 to this solution, two processes were seen to exist with the presence of two minima for the differential curve plots (E vs di/dE, Fig. 3(a)). Although these curves were not obvious in the usual i–E plots, by estimating the diffusion current for each of these processes the estimated value of n (number of electrons involved in the reaction, Eq. (4)) was found to be close to 1 based on both
E/V 0
0 -0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4 -0.0002
-0.05 -0.0004
-0.0006
-0.0008 -0.15 -0.001 -0.2 -0.0012
-0.0014
-0.25
-0.3
-0.35
Since ([di/dE]1/2 x 102.7)/id = n (di/dE)E1/2 = 0.0015x50microA/V id =0.15x50microA n = 1.0
di/dE x50microA/V
i x50microA
-0.1
-0.0016
-0.0018
-0.002
Fig. 2. Representative hydrodynamic voltammogram of 1 mM Na3Au(S2O3)2, 3.2 mM Na2S2O3, 0.01 M NaOH, and 0.5 M NH3, I = 0.1 (NaClO4) at 25 C with the calculated differential curve. Reversibility is indicated when ([di/dE]max · 102.7)/id 1.
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(a)
E/V 0 -0.3
-0.2
-0.1
0
0.1
0.2
0.3
Process 1
0 0.4 -0.0002
-0.05 -0.0004 -0.0006 -0.0008
-0.15 For process 1 (di/dE)E1/2 = 0.0005 x50 microA/V id = 0.05 x50 microA n = 1.0
Process 2 -0.2
-0.25
-0.001 -0.0012 -0.0014
For process 2 (di/dE)E1/2 = 0.0009 x50 microA/V id =0.09x50 microA n = 1.0
-0.3
-0.35
(b)
di/dE x50microA/V
i x50microA
-0.1
-0.0016 -0.0018 -0.002
0 -0.25
-0.2
-0.15
-0.1
i x50microA/V
-0.3
(di/dE)E1/2 = 0.0012 x50microA/V id = 0.115 x50microA n = 1.1
-0.02
0 0.1 -0.0002
-0.04
-0.0004
-0.06
-0.0006
-0.08
-0.0008
-0.1
-0.001
-0.12
-0.0012
-0.14
-0.0014
-0.16
-0.0016
-0.18
-0.0018
-0.2
-0.002
-0.05
0
0.05
di/dE x50microA/V
E/V
Fig. 3. Representative hydrodynamic voltammogram of 1 mM Na3Au(S2O3)2, 3.2 mM Na2S2O3, (a)1.6 mM Na2SO3, (b) 13 mM Na2SO3, 0.01 M NaOH, and 0.5 M NH3, I = 0.1 (NaClO4) at 25 C with the calculated differential curve. Two processes are seen with reversibility indicated when ([di/dE]max · 102.7)/id 1 for both processes.
Figs. 3(a) and (b) indicating reversibility. With the addition of more sulfite (13 mM Na2SO3) to the solution, the E1/2 value shifted to more negative potentials and the disappearance of the two step process was seen (Fig. 3, Figs. S1 and S2). The mixed complex AuNH3 S2 O3 is found only at very high concentration ratios of the two ligands ½NH3 =½S2 O3 2 > 105 [3]. Since appreciable amounts of the mixed AuðIÞ–NH3 –S2 O3 2 species were not expected for the concentration ratio of ½NH3 =½S2 O3 2 < 105 used in the present study, the interaction of AuðS2 O3 Þ2 3 with SO3 2 could be deduced from graphs of E1/2 against log½S2 O3 2 (Fig. 4). From the slope of the two curves obtained in such graphs
(Fig. 4), the formation of AuðS2 O3 ÞðSO3 Þr ð2rþ1Þ ; r ¼ 1 or 2 at higher sulfite concentrations was implied. Raman studies were conducted semi-quantitatively to examine the two species formed with respect to S2 O3 2 in solutions containing SO3 2 . In separate experiments, the lack of changes in the Raman bands of SO3 2 and S2 O3 2 when mixed showed no evidence for the interaction of SO3 2 with S2 O3 2 . By examining the solution containing Na3Au(S2O3)2 and Na2SO3, the Raman spectroscopic results showed the formation of possibly two new species within the concentration range used (Fig. 5). It could be inferred that, during the formation of the first species, equimolar concentrations of free thio-
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0.2 E1/2 values from the less reductive minimum k
0.15
y = -0.0511x - 0.0196 R2 = 0.9758
0.1
E1/2/V
0.05 0 E1/2 values from the more reductive minimum -0.05 y = -0.13x - 0.3465 R2 = 0.9985
-0.1 -0.15 -0.2 -4
-3.5
-3
-2.5
-2
-1.5
log [S2O32-] Fig. 4. Representative graph of E1/2 against log½S2 O3 2 from 1 mM Na3Au(S2O3)2, 3.2 mM Na2S2O3, 0.01 M NaOH, and 0.5 M NH3, I = 0.1 (NaClO4) solutions with various [Na2SO3] at 25 C.
sulfate (as seen with peaks at 999 and 447 cm1, Fig. 5) to the initial gold thiosulfate were released, i.e., AuðS2 O3 Þ2 3 þ SO3 2 () AuðS2 O3 ÞðSO3 Þ3 þ S2 O3 2 ð6Þ However, the concentration of the free thiosulfate was not appreciably changed when the second species was formed with the addition of more sulfite. Moreover, upon the addition of sulfite to maintain molar concentration ratios [Na2SO3]/[Na3Au(S2O3)2] of 1:1 and 2:1, no free sulfite was seen as revealed by the absence of the SO3 2 peaks at 964.2 and 474.3 cm1, Fig. 5, i.e., AuðS2 O3 Þ2 3 þ 2SO3 2 () AuðS2 O3 ÞðSO3 Þ2 5 þ S2 O3 2 ð7Þ This result did not change with the total gold(I) concentration [Au(I)]T from 0.1 to 0.4 M. Hence, with the information from previous experiments, it was inferred that the two species formed are Au(S2O3)(SO3)3 [Eq. (6)] and AuðS2 O3 ÞðSO3 Þ2 5 [Eq. (7)], with the value of q = 1. These two new species can be identified by the presence of Raman bands at 388 cm1 for Au(S2O3)(SO3)3 and at 386 and 372 cm1 for AuðS2 O3 ÞðSO3 Þ2 5 . In addition to these different bands, Raman peaks formed as a result of AuðIÞ–S2 O3 2 interactions were still seen in the presence of these two new species, implying the presence of S2 O3 2 moieties in both of these complexes [Eqs. (6) and (7)]. Attempts to perform experiments using AuðSO3 Þ2 3 as the starting complex were not made as it was difficult to obtain stable and pure Na3Au(SO3)2 [11,12].
Using these results to set up the speciation model, the stepwise formation constants log K11r of Au(S2O3)(SO3)3 and AuðS2 O3 ÞðSO3 Þ2 5 were calculated from plots of E1/2 (hydrodynamic) vs log½S2 O3 2 (Figs. 4 and 6) to be 1.09 ± 0.06 (r = 1) and 4.8 ± 0.3 (r = 2), at I = 0.1 M (NaClO4, 25 C). For a change in [Au(I)]T (44%), no significant change in the log K11r values was observed (<10%), discounting the presence of polynuclear species. To further support the above results, the data from UV–Vis spectrophotometric experiments were analysed using SPECFIT [8,9] with various speciation models. Separate experiments to evaluate the stability of AuðS2 O3 Þ2 3 solutions during aeration over a period of 2 days showed no changes in the UV–Vis spectrum. Although these experiments did not provide an independent model and no characteristic absorption peaks were seen, the presence of at least 3 species (including AuðS2 O3 Þ2 3 ) could be inferred from the changes in the spectra with the addition of Na2SO3 in the range 6 mM P [Au(I)] P 0.23 mM. These changes in absorbances were independent of [Au(I)]T, confirming the absence of polynuclear species. Using the values of equilibrium constants obtained from the hydrodynamic experiments, the UV–Vis spectra of the two new species were calculated (Fig. 7). Although the formation of Au(S2O3)(SO3)3 is anticipated, the increased stability of this species with respect to that of AuðS2 O3 Þ2 3 and the formation of the tricoordinated AuðS2 O3 ÞðSO3 Þ2 5 complex is unexpected. Since AuðSO3 Þ2 3 is much less stable (log bðAu ðSO3 Þ2 3 Þ 10 compared with log bðAuðS2 O3 Þ2 3 Þ
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(a)
2.5
0.19 M Na3Au(S2O3)2 + [Na2SO3] /M
Raman intesity / arbitrary units
2
No sulphite 0.10 0.19
1.5
0.45 0.85 1.13 1.13M sulphite only 1
0.19M thiosulphate only
0.5
0 1040
1020
1000
980
960
940
920
900
(b)
4
Raman intensity / arbitrary units
wavenumbers /cm-1
3.5
0.19 M Na3Au(S2O3)2 +[Na2SO3] /M No sulphite 0.10 0.19 0.45 0.85 1.13 1.13M sulphite only 0.19M thiosulphate only
3
2.5
2
1.5
1
0.5
0 500
480
460
440
420
400
380
wavenumbers /cm
360
340
320
300
-1
Fig. 5. Raman spectra of various solutions containing Na3Au(S2O3)2(aq) and Na2SO3(aq) (a) from 1050 to 900 cm1 and (b) from 500 to 300 cm1. For comparison purposes, the spectra of Na3Au(S2O3)2, Na2S2O3 and Na2SO3 are also included.
26) [13] based on the mixed speciation behaviour of Au(I) [3], the stability of both Au(S2O3)(SO3)3 and AuðS2 O3 ÞðSO3 Þ2 5 is predicted to be less than AuðS2 O3 Þ2 3 . Although tri-coordinated Au(I) complexes are known, they are not common with the tendency of Au(I) to form linear complexes [14]. The change from the usual coordination behaviour of this system may also explain the greater stability seen for these mixed complexes which in turn may be advantageous in gold hydrometallurgical processes used for gold extraction
or electroplating. However, complications arising from the presence of two complexes in such solutions must be noted. With this increased stability, the possibility of forming crystalline salts of these two species was examined. Colourless solutions which contained Na3Au(S2O3)2: Na2SO3 at molar ratios of 1:1 and 1:2, following the ratios of the two species expected in solution (Fig. 6), were evaporated under vacuum. Salts prepared with a molar ratio of 1:1 were white, whereas salts with ratios
W.N. Perera et al. / Inorganica Chimica Acta 358 (2005) 2183–2190
2189
100 Au(S2O3)23-
90
AuS2O3(SO3)23-
80
AuS2O3SO33-
% species
70 60 50 40 30 20 10 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
[SO32-] / m M
Fig. 6. Speciation diagram based on the results obtained from the hydrodynamic voltammetric results at [Au(I)] = 0.23 mM.
3000
absorbtivities/ cm-1M-1
2500 AuS2O3(SO3)23-
2000
1500
AuS2O3SO33-
1000
Au(S2O3)23-
500 S2O3
2-
SO32-
0 244
249
254
259
264 wavelength /nm
269
274
279
Fig. 7. UV–Vis spectra of various individual species as calculated from SPECFIT.
of 1:2 were yellow suggesting the presence of at least two different species.
article can be found, in the online version at doi:10.1016/j.ica.2004.09.058.
Acknowledgement
References
The authors thank A.J. Parker Cooperative Research Centre for Hydrometallurgy for financial assistance.
Appendix A. Supplementary data Figures S1 and S2 are available as supplementary information. Supplementary data associated with this
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[7] J. Heyrovsky, J. Kuta, Principles of Polarography, Academic Press, New York, 1966. [8] M. Maeder, A.D. Zuberbuhler, Anal. Chem. 62 (1990) 2220. [9] H. Gampp, M. Maeder, C.J. Meyer, A.D. Zuberbu¨hler, Talanta 32 (1985) 257. [10] J. Pouradier, M.C. Gadet, J. Chimie Physique 66 (1969) 109. [11] T. Morris, H. Copeland, G. Szulczewski, Langmuir 18 (2002) 535.
[12] M.O. Faltens, D.A. Shirley, Chem. Phys. 53 (1970) 4249. [13] E. Hogfeldt, Stability Constants of Metalion Complexes, 2nd supplement, Part A, Inorganic Ligands, IUPAC Chemical Data Series No. 21, Pergamon, Oxford, 1982. [14] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, sixth ed., Wiley Interscience, New York, 1999.