Diamond paste based electrodes for the determination of Ag(I)

Abstract

Diamond paste electrodes based on three types of monocrystalline diamond were used for the assay of Ag(I). Ag(I) was determined using differential pulse voltammetry (DPV) at +80 mV (vs. Ag/AgCl). The detection limits were very low. Silver was accurately determined from residual water samples.

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1. Introduction

Polycrystalline diamond thin film electrodes have been used up to now in the electrochemical sensors design due to their superior physical and electronic properties such as hardness, chemical inertness, optical transparency, high thermal and electrical conductivity, low background current, wide potential range, lack of adsorption, and high overpotential for oxygen evolution by water oxidation in aqueous electrolyte solution.^1–5 Monocrystalline diamond proved to be a good alternative for the polycrystalline material due to its special properties, such as improved holes and electron mobilities.^6,7 The reliability obtained for the electrical properties of these single-crystal diamonds is encouraging for research in electrochemical sensors based on high quality monocrystalline diamond. Such electrodes proved to have lower limits of detection and noise than the carbon paste based electrodes.⁸ They were utilized for speciation of Cr,^9,10 Fe,^8,11 determination of I⁻¹² and creatine and creatinine¹³ and after immobilization of an enzyme or antibody, for the enantioanalysis of pipecolic acid¹⁴ and determination of azidothymidine.¹⁵

Silver is widely distributed in sulfide ores of which silver glance (argentite) is the most important.¹⁶ Native silver is sometimes associated with these ores as a result of their chemical reduction, while the action of salt water is probably responsible for their conversion into “horn silver”, AgCl, which is found in Chile and New South Wales. The chemistry of silver is very important for its quantitative determination because of its use in photography, silverware, jewellery, electricity, silvering mirrors and as dental amalgam.¹⁶

The fire assay^17–19 method is the most commonly used for the determination of silver in ores and ore dressing products. Among the other well known alternatives are atomic absorption spectrometry (AAS)^20,21 and wet chemical analysis^22,23 of the lead assay button in ores and concentrates of these two precious metals. Cathodic and anodic stripping voltammetry²⁴ based on the introduction of power ultrasound at highly boron doped diamond (BDD) electrode was developed as a sensitive technique for the analysis of trace silver ions. Instrumental neutron activation analysis (INAA), thick target proton induced X-ray emission (TT-PIXE) and inductively coupled plasma-mass spectrometry (ICP-MS)^25–27 and also flow injection analysis²⁸ were also proposed for the analysis of silver.

In this paper diamond paste based electrodes are proposed for the assay of Ag(I) in residual waters.

2. Experimental

2.1. Diamond paste electrode design

The diamond paste electrode was prepared by mixing 0.1 g of diamond powder with 20 μL of paraffin oil. A portion of the paste was then filled into a plastic pipette tip (3 mm). The diameter of the sensing part was 2.3 mm. Electric contact was made by inserting a silver wire (0.5 mm in diameter) into the diamond paste electrode. Before use, the electrode surface was smoothed by polishing with an alumina paper (polishing strips 30144-001, Orion). When not in use, the diamond paste electrode was stored at room temperature.

2.2. Reagents and materials

All chemicals were of analytical grade. All solutions were prepared by using de-ionised water. Phosphate buffer (pH = 9.00) was prepared from KH₂PO₄ (SAARCHEM-HOLPRO ANALYTIC) and Na₂HPO₄ (Chemical Suppliers). 0.1 mol L⁻¹ pyrophosphate solution was used as supporting electrolyte.

All the solutions of Ag(I) were freshly prepared everyday from silver nitrate. Monocrystalline natural 1 μ and synthetic diamond: 50 μ (synthetic-1) and 1 μ (synthetic-2) powders were purchased from Aldrich (Milwaukee, WI, USA) while the paraffin oil was purchased from Fluka (Buchs, Switzerland).

2.3. Apparatus

Differential pulse voltammograms were performed with a 663 VA Stand (Metrohm, Herisau, Switzerland) connected to a PGSTAT 20 and a Ecochemie Software Version 4.8. A platinum electrode and a Ag/AgCl (0.1 mol L⁻¹ KCl) electrode served as counter and reference electrode in the cell, respectively.

2.4. Recommended procedures: Direct DPV

The technique used for the direct voltammetric assay was differential pulse voltammetry with an applied potential pulse amplitude of 25 mV vs. Ag/AgCl. The diamond paste as a working electrode together with the reference electrode (Ag/AgCl), and an auxiliary platinum electrode were dipped into a cell containing phosphate buffer (pH = 9.0) and sodium pyrophosphate solution 0.1 mol L⁻¹ as supporting electrolyte in a ratio 3.5 : 1. All solutions were deoxygenated for 5 min before each measurement with high purity N₂. The peak height measured at 80 mV vs. Ag/AgCl was plotted versus the concentration of Ag(I). The unknown concentrations of Ag(I) were determined from the corresponding calibration graph.

3. Result and discussion

3.1. Equation of calibration

The relationship between the peak height and the concentration of Ag(I) was linear over a wide concentration range using all the diamond electrodes and can be described by the following equations of calibration: a. natural diamond; b. synthetic diamond-1 and c. synthetic diamond-2

a. H = 4.55 (±0.50) + 5.53 (±0.25) C; r = 0.9887

b. H = 1.97(±0.90) + 3.67(±0.35) C; r = 0.9999

c. H = 2.52(±0.85) + 27.30(±0.30) C; r = 0.9964

where H is the peak height (μA), C is the concentration of Ag(I) (μmol L⁻¹) and r is the regression coefficient. The DPV peak currents were proportional for a wide concentration range: natural diamond, 10⁻⁹ to 10⁻⁶ mol L⁻¹ with detection limit of 10⁻¹¹; synthetic-1, 5 × 10⁻⁹ to 10⁻⁶ mol l⁻¹ with detection limit of 10⁻¹⁰mol L⁻¹ and synthetic-2, 10⁻⁹ to 10⁻⁷ mol L⁻¹ with a detection limit of 10⁻¹⁰ mol L⁻¹.

The detection limits (DL) were calculated according to Otto:²⁹

where I_B is the background current recorded, σ_s is the standard deviation for the measurement of the background current, a is the intercept of the calibration equation, and S is the slope of the calibration equation. The lowest value of the background currents obtained for natural diamond based electrode, when used for the Ag(I) assay, explained that the value of the limit of detection is lower for this electrode than for the one based on diamond-1.

Some of the peak profiles obtained for the assay of Ag(I) with the three electrodes are shown in Fig. 1. The reproducibility of peak currents was good (RSD less than 1%, n = 10) for a period longer than 6 months, when measurements were performed every day.

Fig. 1 Peak profiles when DPV method was used with the electrodes based on: A. Natural diamond (I C_Ag(I) = 10⁻⁶ mol L⁻¹; II C_Ag(I) = 10⁻⁷ mol L⁻¹); B. Synthetic-1 (I C_Ag(I) = 10⁻⁸ mol L⁻¹; II C_Ag(I)= 10⁻⁹ mol L⁻¹); and C. Synthetic-2 (I C_Ag(I) = 10⁻⁷ mol L⁻¹; II C_Ag(I) = 10⁻⁸ mol L⁻¹).

3.2. Selectivity studies

The effect of various ions on a differential pulse voltammogram peak current of 1 × 10⁻⁶mol L⁻¹ Ag(I) was examined for all diamond paste electrodes using mixed solutions method. The amperometric selectivity coefficients were calculated accordingly with the method proposed by Wang.³⁰ The concentration of the possible interfering ions was 1 × 10⁻⁵ mol L⁻¹. The results obtained for the amperometric selectivity coefficient (Table 1) indicate that all the ions investigated, such as Fe²⁺, Mg²⁺, Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cr⁶⁺, Pb²⁺and Fe³⁺did not interfere with the determination of Ag(I). Furthermore, none of these ions present a peak on the given working conditions and on the scanning area where the peak of Ag(I) was shown when measured in the presence or absence of the Ag(I).

Table 1 Amperometric selectivity coefficients. All values are the average of ten determinations

Interfering Species/J	Kamp
	Natural Diamond	Synthetic Diamond-1	Synthetic Diamond-2
Fe^2+	1.79 × 10^−3	6.02 × 10^−3	4.58 × 10^−3
Mg^2+	3.30 × 10^−3	4.60 × 10^−3	4.54 × 10^−3
Cr^3+	5.67 × 10^−3	7.75 × 10^−4	4.16 × 10^−3
Mn^2+	2.83 × 10^−3	8.91 × 10^−3	3.43 × 10^−3
Cu^2+	3.00 × 10^−4	9.28 × 10^−4	3.68 × 10^−3
Zn^2+	6.93 × 10^−3	4.95 × 10^−3	5.17 × 10^−3
Cr^6+	1.09 × 10^−3	6.48 × 10^−3	2.03 × 10^−3
Pb^2+	6.34 × 10^−3	2.61 × 10^−3	2.35 × 10^−3
Fe^3+	1.81 × 10^−3	5.04 × 10^−3	2.62 × 10^−3

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4. Analytical applications

The response characteristics as well as the selectivity of the diamond paste electrodes for silver assay at a predetermined potential range made it suitable for the determination of Ag(I) in residual water samples. The results obtained for the quantitative determination of silver in five water samples are shown in Table 2. Ag(I) proved to be reliably assayed from these samples with high average recovery and low RSD% values (< 1%). Although the average values determined for Ag(I) assay are all lower than the standard values, the ten values determined, for each sample, using the proposed electrodes were some slightly higher than the standard values showing that it is not a systematic error in Ag(I) determination using these electrodes.

Table 2 Determination of Ag(I) in residual samples using the three paste electrodes of diamond and a standard method. All values are the average of ten determinations

Standard method^31 Average Recovery, μg Ag(I)/L	Average Recovery, μg Ag(I)/L
	Natural Diamond	Synthetic Diamond-1	Synthetic Diamond-2
0.539	0.538 ± 0.06	0.528 ± 0.06	0.530 ± 0.07
1.29	1.27 ± 0.07	1.21 ± 0.08	1.26 ± 0.06
5.39	5.33 ± 0.06	5.25 ± 0.08	5.30 ± 0.06
1.29	1.19 ± 0.07	1.24 ± 0.09	1.28 ± 0.07
1.83	1.78 ± 0.07	1.76 ± 0.08	1.76 ± 0.08

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5. Conclusion

The diamond paste electrodes presented in this paper provides excellent features for the detection of Ag(I) in geological and environmental samples. The design of the diamond paste based electrodes is simple, fast and reproducible. The reproducibility of the analytical information is assured by the low RSD % values obtained in the recovery tests.

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