Ashraf A. Mohamed*,
Eslam H. A. Mahmoud and
Mostafa M. H. Khalil
Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo-11566, Egypt. E-mail: aamohamd@sci.asu.edu.eg; Fax: +20 224831836; Tel: +20 1001578849
First published on 8th November 2019
Desktop scanners can be favorable alternatives to sophisticated spectrophotometers for the assessment of analytes in complex real samples. Distinctively, our method has been thoroughly investigated, optimized, validated and successfully applied to the assessment of silver and gold in complex real samples, applying syringal rhodanine (SR) as a novel specifically tailored chromogenic reagent and using a desktop scanner as a versatile sensor. Maximum colour absorbance was obtained in the presence of cetylpyridinium chloride (CPC) and cetyltrimethylammonium chloride (CTAC) for silver and gold chelates, respectively. For each metal ion, two ternary complexes were formed depending on the SR concentration with stoichiometries of 1:
1
:
1 and 1
:
2
:
3 (Ag–SR–CPC) and 1
:
2
:
3 and 1
:
3
:
4 (Au–SR–CTAC), respectively. The methods adhered to Beer's law for 0.15–2.5 and 0.15–2.25 μg mL−1 with detection limits of 0.0089 and 0.0163 μg mL−1 for silver and gold, respectively. The molar absorptivities were 3.63 × 104 and 6.15 × 104 L mol−1 cm−1 at 550 nm and 554 nm, with Sandell's sensitivity indexes of 0.0029 and 0.0032 μg cm−2, respectively. The method was successfully applied to the assessment of silver and gold in a wide range of complex environmental samples.
Sophisticated analytical techniques including inductively coupled plasma-mass spectrometry (ICP-MS),1–4 inductively coupled plasma optical emission spectrometry (ICP-OES),5,6 neutron activation analysis (NAA),3,7,8 X-ray fluorescence (XRF),9 and atomic absorption spectrometry (AAS)10–14 enable sensitive and selective determination of trace amounts of Ag(I) and Au(III) in a wide range of matrices. However, conventional spectrophotometry stands out as a relatively low cost, simple and easy-to-use technique. The spectrophotometric determination of silver is usually achieved using its reaction with dithizone,15 4-(2-quinolylazo)phenol (p-QAP),16 4,4′-bis(dimethylamino)thiobenzophenone,17 2-cyano-3-iminodithiobutyrate (CIDT),18 or 2′,3′-dihydroxypyridyl-4′-azobenzene-4-arsonate (DHP-4A);19 however, that of gold is achieved using dithizone,20 4-(2-pyridylazo)resorcinol (PAR),21 4-(2-thiazolylazo)resorcinol (TAR),22 methiomeprazine hydrochloride (MMH),23 4-(2-quinolylazo)phenol (p-QAP),24 or dithiodiantipyrylmethane.25 Nevertheless, the poor selectivity, sensitivity and/or precision restrains the use of most of these reagents. But, rhodanine derivatives surpassed many of these reagents and therefore, have found vast applications in the determination of silver26–29 and gold.26,29–32 However, some difficulties were noted with the use of many rhodanine derivatives, including the relatively poor solubility of reagents and their chelates and/or the poor stability of the developed colours. Thus, surfactants were used to solubilize and stabilize the pseudo-solutions containing the ligands and/or their metal-chelates; thereby enhancing both the selectivity and sensitivity of these methods.33,34
On the other hand, methods of digital image-based analysis (DIBA) can be simple, easy-to-use and low-cost alternatives.35,36 In DIBA, the analyte is allowed to react with a chromogenic reagent to give a coloured product whose digital images are captured and analysed to give the red, green and blue intensity values (IR, IG, and IB). The RGB intensities,37–44 and the RGB absorbances (AR, AG, AB)45,46 or thereof35,36 have been used as analytical signals in DIBA.
Herein, syringal-rhodanine (SR) was synthesized, characterized and applied as a novel and specifically tailored chromogenic reagent for the sensitive and highly selective assessment of silver(I) and gold(III) based on monitoring with a desktop scanner as a versatile low-cost sensor due to its relatively fixed light source intensity and the lack of outdoor light interference. Further, the reaction variables affecting the spectral characteristics of Ag–SR and Au–SR chelates were thoroughly investigated, optimized and incorporated into respective recommended procedures for the assessment of Ag(I) and Au(III) in antiseptic and burn cream, electroplating wastewaters, and rock samples.
A flat bottom 96-micro-well plate was used for colour absorbance measurements using an Hp-Deskjet F4200 all in one printer-scanner-copier. A conventional HP X7H29EA#ABV Notebook running under Windows 10 Home 64 bit was used for data treatment and analysis. Photoshop CS6 was used for digital image processing and ImageJ2x software 2.1.4.7 was used for RGB channel intensities calculations. Then, for each image, the RGB colour absorbance (AR, AG, AB) was calculated.35,36
Syringal-rhodanine (SR) was synthesized according to Julian and Sturgis47 by adding 3.0 g of freshly prepared, fused and powdered sodium acetate to 30 mL glacial acetic acid containing 0.03 mol rhodanine and 0.03 mol 4-hydroxy-3,5-dimethoxy benzaldehyde (syringaldehyde). The mixture was refluxed for 4 hours with occasional shaking. The solid crude SR was poured into a 1 L beaker containing iced water, triturated vigorously and then filtered. To the filtrate, Na2CO3 was added stepwise until no effervescence was observed. The resulting second crop of crude SR was collected and added to the first crop. The melting point of crude SR was 244–248°. Double recrystallization from glacial acetic acid yielded orange-yellow fine crystals of pure SR with mp 255–256° which is in excellent agreement with the previously reported data.48 The purity of the reagent was confirmed by thin-layer chromatography with benzene–petroleum ether (1:
4 v/v) as eluent. A single yellow band was obtained for the pure product.
An ethanolic stock solution of 5.0 mmol L−1 SR was prepared by dissolving the appropriate amount of SR in absolute ethanol. This solution was stable for more than 2 months. A 1.0 mmol L−1 SR working solution was prepared daily by appropriate dilution of the stock solution with absolute ethanol.
A stock standard 0.01 mol L−1 Ag(I) solution was prepared by dissolving the suitable amount of dried silver nitrate in 0.02 mol L−1 nitric acid. More dilute working silver(I) solutions were prepared daily by appropriate dilution of its stock solution. A stock standard 1000 μg mL−1 Au(III) solution in hydrochloric acid, was purchased from Sigma-Aldrich (St. Louis, MO, USA). More dilute working gold(III) solutions were prepared by appropriate dilution of its stock solution.
A composite surfactant-buffer solution that is 4.0 mmol L−1 cetylpyridinium chloride (CPC) and 50.0 mmol L−1 borate buffer of pH 9.80 was prepared by dissolving the appropriate amounts of CPC and borax in water and adjusting the pH to 9.80 ± 0.05.
Cetyltrimethylammonium chloride (CTAC) was prepared as a 20.0 mmol L−1 solution by dissolving the appropriate amount in deionized water.
A borate buffer series of pH 7.8–10.8 was prepared by adjusting the pH of 50.0 mmol L−1 sodium tetraborate solution with hydrochloric acid or sodium hydroxide solution using a pre-calibrated pH meter.
From each flask, a 300 μL aliquot was pipetted into the micro-well plate and an image was scanned using the desktop scanner. As previously reported,35,36 various scan resolutions of 300, 600 and 1200 dpi gave essentially the same response; therefore, a scan resolution of 300 dpi was adopted for simplicity. From each image, a representative homogenous circular area (60 pixels in diameter) was cropped using Photoshop CS6, and the RGB colour intensities (IR, IG, IB) were registered by ImageJ2x and then the colour absorbance for each channel (AR, AG, AB) was calculated from [ARGB = log(Io/I)]; where Io and I are the channel intensity of the blank and sample, respectively.35,36 The silver or gold concentration was determined from a similarly prepared calibration graph obtained with a series of standard solutions.
Therefore, four species are involved in the SR acid dissociation process, according to the above equation. The respective dissociation constants were determined spectrophotometrically in 1:
1 ethanol–water. Where, K1, K2, K3 were assigned to the deprotonation of SH+, NH, and OH groups, respectively. The deprotonation constant K1, couldn't be determined due to the instability of the reagent in strongly acidic HCl media (pH < 3.0). However, K2 and K3 were determined by spectral measurements of 0.03 mmol L−1 SR solutions adjusted to various pH values between 3.0–13.0, Fig. 1A. The obtained spectral data were treated with Datan software,50 Fig. 1B. The molar ratios distribution of various dissociable species showed intersections corresponding to the dissociation constants K2 and K3 with values of 6.61 and 10.87, respectively. However, the dissociation constants can be also derived graphically or mathematically from the conventional A–pH relation at the respective wavelength maxima.
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Fig. 3 Effect of some cationic surfactants on the spectral characteristics of Ag–SR and Au–SR chelates. CTAB, cetyltrimethyl ammonium bromide; CPB, cetyl pyridinium bromide; except for the abscissa variable, other conditions and symbols are those of Fig. 2. (A) Spectrophotometric data. (B and C) Corresponding DIBA results for the Ag–SR–CPC and Au–SR–CTAC chelates, respectively. (D) Spectrophotometric data for the effects of surfactant's concentration. (E and F) Corresponding DIBA results for Ag–SR and Au–SR chelates, respectively. |
The binary Ag–SR and Au–SR chelates were solubilized in presence of ≥0.5 mmol L−1 of CPC or CTAC and using other conditions of the recommended procedure. Moreover, the maximum absorbances of the sensitized complexes were obtained at concentrations of 2.0 and 1.0 mmol L−1 CPC and CTAC, respectively, and these were therefore adopted in the recommended procedure.
Further, the sensitization of the colour reactions was exhibited at surfactant's concentrations laying well below its own critical micellar concentration. (The c.m.c. of CPC and CTAC have been reported as 0.90 and 0.98 mmol L−1, respectively51). Consequently, the surfactant interacts with the binary chelate to form true ternary complexes.
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Fig. 4 Molar ratio and continuous variation plots of the Ag–SR (A–D) and Au–SR (E–H) ternary complexes. |
Similarly, the surfactant ratios in the ternary complexes were determined in a similar way by molar ratio method, by increasing the CPC or CTAC concentration while maintaining a constant M:
L ratios of 1
:
1 Ag–SR, 1
:
2 Ag–SR, 1
:
2 Au–SR, or 1
:
3 Au–SR complexes. The study confirmed the formation of 1
:
1
:
1 and 1
:
2
:
3 (Ag–SR–CPC) and 1
:
2
:
3, 1
:
3
:
4 (Au–SR–CTAC) complexes, respectively.
Further, the conditional stability constants of the M–L complexes were determined from the molar ratio and the continuous variation plots using the equation52
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Fig. 5 Calibration plots for the determination of Ag(I) and Au(III); (A) spectrophotometrically; (B) and (C) DIBA-based data; (D) and (E) are scanned and cropped images of Ag–SR–CPC and Au–SR–CTAC chelates, respectively. Except for the abscissa variable, other conditions are those of Fig. 2. Captured images were arbitrarily compressed to fit into the page margins; however, for image processing, the original uncompressed images were used. |
Despite the extraordinary selectivity of SR towards silver and gold, few precious metal ions interfered with the determination.
Therefore, some masking agents were adopted to impart extra selectivity to the developed methods. The results obtained are illustrated in Table 1. However, in the implemented method, an EDTA–citrate masking solution was adopted to enhance the tolerance limits of the proposed highly selective methods towards some metal ions, e.g., Cu(II) and Fe(II, III). Dimethylglyoxime (DMG) was effective in masking Pd(II) and Pt(II). Ascorbic acid was used to mask Hg(II) and Au(III) in the determination of silver.
Ternary silver complex method | Ternary gold complex method | ||
---|---|---|---|
Foreign ionb | Tolerance limit [ion]/[Ag] | Foreign ionb | Tolerance limit [ion]/[Au] |
a Conditions were those of the recommended procedure using 2 μg mL−1 Ag(I) or Au(III).b Masking agents: (a) 1 mL 0.01 mol L−1 EDTA/citrate solution; (b) 0.1 mL of 0.25 mol L−1 ethanolic solution of DMG; (c) 0.1 mL of 0.1 mol L−1 ascorbic acid; (d) 0.1 mL of 0.1 mol L−1 iodide/Cl–NH3OH. | |||
Glycine, tartrate, citrate, succinate, acetate, benzoate | >5000 | Glycine, tartrate, citrate, succinate, acetate, benzoate, Cl− | >5000 |
Ascorbic acid, Mo(VI), W(VI) | 3000 | Mo(VI), W(VI), Br−, I− | 4000 |
Ca(II)a, Cu(II)a, NO2− | 2500 | Ca(II)a, Cu(II)a, NH2OH·HCl | 2500 |
Mg(II)a, Mn(II)a, Co(II)a, Ni(II)a, Zn(II)a, Cd(II)a, PO42−, | 1500 | PO42−, SO42− | 2000 |
Fe(II)a Al(III)a, NO3−, Cl− | 500 | Mg(II)a, Mn(II)a, Co(II)a, Ni(II)a, Zn(II)a, Cd(II)a, NO2− | 1500 |
Pb(II)a, SO42−, CO32− | 250 | Al(III)a, Fe(II)a, NO3, CO32− | 500 |
ClO3−, SCN−, NH4+ | 150 | Pb(II)a, Ag(I)d, Hg(II)d | 200 |
Pt(II)b, Pd(II)b, Hg(II)c, Au(III)c, BrO3− | 100 | Pt(II)b, Pd(II)b, ClO3−, NH4+ | 150 |
BrO3−, SCN− | 100 |
A 0.1 mol L−1 solution of sodium iodide was used to mask Hg(I, II) and Ag(I) in the determination of gold. The presence of hydroxylamine hydrochloride didn't affect the spectral characteristics of the Au–SR ternary complex indicating that Au(III) has been already reduced to Au(I). However; the presence of ascorbic acid severely suppressed the absorbance of the complex denoting the reduction of Au(III) to the metallic gold. This finding replenishes the findings of Cotton and Woolf,30 and Borissova.53 Traces of an oxidizing agent such as hydrogen peroxide altered the spectral characteristics of the complex consolidating the previous findings.
Thus, in the implemented methods most interfering species were tolerated at three orders of magnitude compared to silver and gold. Moreover, a 100-fold excess of Hg(II), Pt(II), and Au(III) did not interfere in the determination of Ag(I). In addition, a 200-fold excess of Ag(I), and Hg(II) and 150-fold excess of Pt(II), and Pd(II) did not interfere in the determination of Au(III); showing the extraordinary selectivity of the implemented methods.
The result obtained for silver in the pharmaceutical sample, as determined by the proposed procedure for seven replicates, was 0.987 ± 0.019 whereas the corresponding values obtained using ICP-AE spectrometry was 1.010 ± 0.004. For the determination of gold in two samples of electroplating wastewater, the results obtained were 1.285 ± 0.011 and 0.546 ± 0.003 and the corresponding values obtained with the ICP-AES were 1.271 ± 0.007 and 0.529 ± 0.001 for seven replicates each. The results of silver and gold determination in the goldmines samples obtained using the proposed DIBA and spectrophotometric methods along with those of the ICP-AES method are summarized in Table 2.
Source | ICP | Spectrophotometer | DIBA (G) | ||||||
---|---|---|---|---|---|---|---|---|---|
Mean ± SD (μg g−1) | Mean ± SD (μg g−1) | F0.05 Ftable = 6.388 | t0.05 ttable = 2.306 | P0.05 | Mean ± SD (μg g−1) | F0.05 Ftable = 6.388 | t0.05 ttable = 2.306 | P0.05 | |
Gold in presence of other precious metals | |||||||||
Umm urayyat | 31.66 ± 0.36 | 31.56 ± 0.36 | 1.00 | 0.43 | 0.67 | 31.57 ± 0.36 | 1.00 | 0.41 | 0.69 |
Haimour | 12.58 ± 0.01 | 12.49 ± 0.02 | 4.00 | 1.31 | 0.06 | 12.61 ± 0.01 | 1.00 | 1.18 | 0.08 |
Mongul | 16.18 ± 0.09 | 16.24 ± 0.12 | 1.78 | 0.96 | 0.09 | 16.42 ± 0.14 | 2.42 | 0.98 | 0.10 |
Fatiri | 3.57 ± 0.01 | 3.62 ± 0.01 | 1.00 | 1.59 | 0.09 | 3.63 ± 0.02 | 4.00 | 1.63 | 0.08 |
Abu Marawat | 12.74 ± 0.02 | 13.03 ± 0.03 | 2.25 | 0.63 | 0.61 | 13.13 ± 0.04 | 4.00 | 0.87 | 0.57 |
Um Atoud | 20.12 ± 0.14 | 20.36 ± 0.14 | 1.00 | 1.63 | 0.11 | 20.52 ± 0.15 | 1.15 | 1.80 | 0.11 |
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Silver in presence of other precious metals | |||||||||
Um qurayyat | 0.52 ± 0.01 | 0.51 ± 0.01 | 1.619 | 0.63 | 0.53 | 0.50 ± 0.01 | 1.691 | 0.66 | 0.51 |
Haimour | 3.37 ± 0.08 | 3.32 ± 0.07 | 1.433 | 0.97 | 0.21 | 3.29 ± 0.08 | 1.024 | 0.99 | 0.19 |
Mongul | 5.69 ± 0.11 | 5.66 ± 0.09 | 1.471 | 1.82 | 0.06 | 5.57 ± 0.09 | 1.535 | 1.98 | 0.07 |
Fatiri | 12.48 ± 0.30 | 12.10 ± 0.24 | 1.623 | 1.21 | 0.09 | 12.23 ± 0.15 | 3.896 | 1.52 | 0.10 |
Abu Marawat | 59.75 ± 0.66 | 60.34 ± 0.86 | 1.699 | 2.00 | 0.11 | 60.97 ± 0.77 | 1.367 | 2.13 | 0.13 |
Um Atoud | 0.74 ± 0.02 | 0.73 ± 0.01 | 2.367 | 1.96 | 0.06 | 0.71 ± 0.02 | 1.562 | 1.99 | 0.08 |
The statistical t-test and F-test at 95% confidence level clearly show that there is no statistical difference between the means and variances of the proposed methods and the standard ICP-AES methods.
A comparison between the analytical characteristics of the proposed method, some of the well-established reagents for silver and gold, and some of the well-known rhodanine derivatives is clarified in Table 3. This work introduces competitive selectivity and precision and even greater simplicity, affordability and sensitivity compared to most of the previously reported spectrophotometric methods.
Reagent | pH | λmax, nm | ε × 10−4 L mol−1 cm−1 | Sensitivity μg cm−2 | Comments | Ref. |
---|---|---|---|---|---|---|
Selected methods of silver determination | ||||||
Dithizone | ≤4 mol L−1 H2SO4 | 462 | 3.1 | 0.003 | Extraction with CCl4 | 15 |
4-(2-Quinolylazo)phenol | 9.2 | 530 | 8.3 | 0.001 | Cu, co, Fe, Ni, and Pd interfere | 16 |
4,4′-Bis(dimethylamino)thiobenzophenone | 3 | 520 | 9.3 | 0.001 | Noble metals interfere | 17 |
2-Cyano-3-iminodithiobutyrate | 4–6 | 565 | 1.3 | 0.008 | Hg interferes | 18 |
2′,3′-Dihydroxypyridyl-4 ′-azobenzene-4-arsenate | 535 | 3.0 | 0.004 | Complex formation in strongly alkaline medium | 19 | |
p-Dimethylaminobenzylidenerhodanine | 0.05 mol L−1 HNO3 | 580 | 2.0 | 0.005 | With long pathlength cuvettes | 26 |
5-(4-Hydroxybenzylidene)rhodanine | Citrate buffer | 490 | 1.5 | 0.007 | Pt metal interferes | 27 |
5-[4-(2-Methyl-3-hydroxy-5-hydroxymethyl)pyridylene]rhodanine | 8.2 | 530 | 1.5 | 0.007 | Pt metal interferes | 28 |
5-(2,4-Dihydroxybenzylidene)rhodanine | 10 | 547 | 7.1 | 0.002 | 29 | |
Syringal rhodanine–CPC | 9.8 | 550 | 3.63 | 0.003 | Spectrophotometer is not needed | This work |
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Selected methods of gold determination | ||||||
Dithizone | 420 | 2.8 | 0.007 | Extraction with chloroform | 20 | |
4-(2-Pyridylazo)resorcinol | 2.5 | 540 | 8.3 | 0.002 | Extraction; strong interferences | 21 |
4-(2-Thiazolylazo)resorcinol | 1.5 | 520 | 1.5 | 0.013 | Extraction; strong interferences | 22 |
5-(4-Dimethylaminobenzylidene)rhodanine | 0.12 mol L−1 HCl | 500 | — | — | 26 | |
5-(2.4-Dihydroxybenzylidene)rhodanine | 10 | 558 | 8.5 | 0.002 | 29 | |
5-(4-Dimethylaminobenzylidene)rhodanine | 0.12 mol L−1 HCl | 515 | 3.9 | 0.005 | Extraction with isoamyl acetate | 30 |
5-(4-Dimethylaminobenzylidene)rhodanine | 3 | 515 | 3.8 | 0.005 | Pt metal interferes | 31 |
5-(o-Hydroxyphenyl)methylenerhodanine | 446 | 0.9 | 0.022 | Pt metal interferes | 32 | |
5-(6-Methylpyridyl)methylenerhodanine | 420 | 1.1 | 0.018 | Pt metal interferes | 32 | |
Syringal rhodanine–CTAC | 10 | 554 | 6.15 | 0.003 | Spectrophotometer is not needed | This work |
Footnote |
† Electronic supplementary information (ESI) available: Additional figures and tables. See DOI: 10.1039/c9ra06840f |
This journal is © The Royal Society of Chemistry 2019 |