Study on the solid phase extraction and spectrophotometric determination of gold in water and ore with 5-(p-aminobenzylidene)-thiorhodanine

Wu Xianhua, Zhong Yihui, Zhang Xin, Huang Qiling
(Department of Chemistry, Yuxi Teacher's College, Yunnan 653100,  China)

Abstract In this paper, a new chromogenic reagent, 5-(p-aminobenzylidene)-thiorhodanine (ABTR) was synthesized. A high sensitive, selective and rapid method for the determination of gold based on the rapid reaction of gold with ABTR and the solid phase extraction of the colored chelate with a reversed phase polymer-based C₁₈ cartridge has been developed. In the presence of 0.05-0.5 mol L^-1 of hydrochloric acid solution and emulsifier-OP medium, ABTR reacts with gold to form a red chelate of a molar ratio 1:3 (gold to ABTR). This chelate was enriched by the solid phase extraction with a polymer-based C₁₈ cartridge and eluted to form the chelate from cartridge with dimethyl formamide (DMF). The ABTR-Au(III)) chelate in 100 mL solution can be concentrated to 1.0 mL, and the enrichment factor of 100 was achieved. In the DMF medium, the molar absorptivity of the chelate is 1.23×10⁵ L.mol^-1.cm^-1 at 550 nm. Beer's law is obeyed in the range of 0.01- 3 mg mL^-1 in the measured solution. The relative standard deviation for eleven replicate samples of 0.5 mg L^-1 level is 2.18%. The detection limit, based on the three times of standard deviation is 0.02 mg L^-1 in the original sample. This method was applied to the determination of gold in water and ore with good results.
Keywords  Gold, solid phase extraction; spectrophotometry, 5-(p-aminobenzylidene)-thiorhodanine

1. INTRODUCTION    
Gold belongs to the elements which occur on the Earth with very low natural contents. It is one of most important noble metals due to its wide application in industry and economic activity. For this reason, a simple, sensitive and selective method for determination of trace gold was required strongly. Although several sophisticated techniques, such as inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES), electrochemical, spectrofluorimetry, neutron activation analysis and the like have widely been applied to the determination of gold ^[1-9]. But the spectrophotometric method still has the advantages for which is simple and does not need expensive or complicated test equipment. Therefore, a wide variety of spectrophotometric methods for the determination of gold have been reported, each chromogenic system has its advantages and disadvantages with respect to sensitivity, selectivity and rapidity ^[10-18]. However, the routine spectrophotometric methods are often not sensitive enough to determine low concentration of gold ion in water or ore samples, the gold concentration only m g L^-1 or ng L^-1 level. For the determination low concentration of gold ion, a preconcentration step is usually required.
    Solid phase extraction is an attractive technique because of its notable advantages. In our previous works, the determination of some trace metal ions by solid phase extraction with reserved phase silica-bond with C₁₈ cartridge was studied ^[19-23]. However, the routine reserved phase silica-bond C₁₈ cartridge only can be used in pH range of 2 - 8. The chromogenic systems in acidic or alkaline medium can not be preconcentrated by this cartridge. To meet the need of metal chelate enrichment by solid phase extraction in acid medium, in this work, the color reaction of 5-(p-aminobenzylidene)-rhodanine (ABTR) with gold and the solid phase extraction of Au-ABTR chelate with a reversed phase polymer-based C₁₈ cartridge was studied. The polymer-based C₁₈ is manufactured from a hydrophilic methacrylate polymer, which is functionalized with C₁₈ ligands. It is a reversed-phase solid phase cartridge provides a broad range of solvent choices and a pH range from 0 - 14. By using the polymer-based C₁₈ cartridge, the Au-ABTR chelate was enriched by solid phase extraction in hydrochloric acid medium and the enrichment fact of 100 was achieved. Based on this, a highly sensitive, selective and rapid method for the determination of gold in water and ore samples was developed.

2. EXPERIMENTAL
2.1 Apparatus          
A UV-160 A spectrophotometer (Shimidzu Corporation, Tokyo, Japan) equipped with 1 cm microcells (0.5 mL) was used for all absorbance measurements. The pH values were determined with a Beckman F-200 pH meter (Beckman Instruments, Fullerton, CA, USA). The extraction was performed on Waters Solid Phase Extraction (SPE) device (It can prepare twenty samples simultaneously), and a reversed phase Polymer C₁₈ polymer-based C₁₈ cartridge (The volume is 1mL, The particle size is 30 m m, The capacity of the cartridge is 30 mg, The maximal flow rate through the cartridge is 25 mL/min) from Beijing Genosys Technologies, P.R.China was used.
2.2 Reagents
The ABTR was synthesized according to our previously proposed procedure [24]: 40 mL of acetic acid was added to the sample of 1.5 g of thiorhodanine and 1.2 g of p-aminobenzaldehyde, and the mixture was heated gently to dissolve the thiorhodanine and p-aminobenzaldehyde completely. The solution was refluxed for about 1.5 h. In the course of refluxing, 1 mL of concentrated sulfuric acid was added dropwise. After the color of the solution turned red, the refluxing was stopped and the sample was poured into 200 mL of distilled water. To this solution, a small amount of aqueous ammonia was added. Thereafter, the precipitants were separated by filtration, and were recrystallized twice with absolute alcohol. The yield is 55% (m. p. 282^oC-285^oC). The structure of ABTR was verified as shown in Fig.1 by elemental analysis, IR, ¹HNMR and MS. Elemental analysis: calculated (found), 47.59 (47.23)% C, 3.19 (3.28)% H, 11.10 (11.02)% N, 38.12 (37.53)%. IR (KBr) (cm^-1): 3470, 3450, 3355 (n_N-H); 3060, 3020 (n_-c=C-H); 1628 (d_N-H); 1566, 1548, 1515, 1450 (n_C=C); 1292 (n_C-N); 1171 (n_C=S); 825 (d_Ar-H); 806 (d_C=C-H). ¹HNMR (solvent: DMSO-d₆) (d,ppm): 7.46 (1H, s, C=C-H); 7.26, 7.35 (2H, d, J≈9Hz, H-2 and H-6); 6.62, 6.72 (2H, d, J≈9Hz, H-3 and H-5); 3.36 (2H, w, -NH₂). MS (EI) (m/z): 252 (M^+·).
    All solutions were prepared with ultra-pure water obtained from a Milli-Q50 SP Reagent Water System (Millipore Corporation, USA). High purity dimethyl formamide (DMF) (Fisher Corporation, USA) was used. A 3.0×10^-4 mol L^-1 of ABTR solution was prepared by dissolving ABTR in DMF. A stock standard solution of gold (1.0 mg mL^-1) was obtained from Chinese Material Standard Center, and a work solution of 0.5 m g mL^-1 was prepared by diluting this solution. A solution of 5 mol L^-1 of hydrochloric acid was used. Emulsifier-OP solution (2.0 % (v/v)) was prepared by dissolving emulsifier-OP with water. All chemical used were of analytical grade unless otherwise stated.

Fig.1 The structure of ABTR

3.2 Effect of Acidity
Results showed that the optimal condition for the reaction of Au(III) with ABTR is in the acid medium. Therefore, the effect of hydrochloric acid, sulfuric acid, perchioric acid, phosphoric acid and the like, on the color reaction of Au(III) with ABTR was studied. Experiment shows that hydrochloric acid has the best effect, and the concentration of hydrochloric acid within a 0.05-0.5 mol L^-1 was found to give a maximum and constant absorbance, so 5 mL of hydrochloric acid was recommended.
3.3 Effect of Surfactants
The effects of surfactants on Au(III)-ABTR system were studied. The results (Table 1) showed that in the absence of surfactants, anionic surfactants or cationic surfactants, the Au(III)-ABTR chromogenic system gives a low absorption, whereas in the presence of nonionic surfactants, the absorption of the chromogenic system increases markedly. Various anionic surfactants enhance the absorbance in the following sequence: Emulsifier-OP > Tween-80 > Tween-20 > Tween-60. Accordingly, the emulsifier-OP was the best additive, and the use of 1.5 - 4 mL of emulsifier-OP solution give a constant and maximum absorbance. Consequently, the use of 3.0 mL was recommended.

Table 1 The Effect of Surfactants on Au(III)-ABTR Chromogenic System

3.4 Effect of ABTR Concentration
For up to 3.0 m g of Au(III), the use of 5 mL of 3.0×10^-4 mol L^-1 of ABTR solution was found to be sufficient for a complete reaction. Accordingly, 5.0 mL of ABTR solution were added in all further measurement.
3.5 Stability of the Chromogenic System
After mixing the components of the system, the absorbance reaches its maximum within 8 min at room temperature and remains stable for at least 5 h. After having been extracted into the DMF medium, the chelate was stable for at least 8 h.
3.6 Solid Phase Extraction
Both the enrichment and the elution were carried out on a Waters SPE device (which can prepare twenty samples simultaneously). The flow rate was set to 20 mL min^-1 when enrichment and 5 mL min^-1 when elution.
   Some experiments were carried out in order to investigate the retention of ABTR and its Au(III) chelate on the cartridge. It was found that the ABTR and its Au(III) chelate was retained on the cartridge quantitatively when they pass the cartridge as hydrochloric acid medium. The capacity of the cartridge was determined as 22 mg for Au(III)-ABTR chelate in a 100 mL of solution. In this experiment, the maximum amount gold is only 3.0 m g. Therefore, the cartridge has adequate capacity to enrich the Au(III)-ABTR chelate.

    In order to choose a proper eluant for the retained ABTR and its Au(III) chelate. Various organic solvents were studied. For eluting the Au(III)-ABTR chelates from the cartridge, the volume of the solvent needed is 0.8 mL for DMF, 1.2 mL for isopentyl alcohol, 1.4 mL for acetone, 1.4 mL for acetonitrile, 1.8 mL for ethanol, 2.0 mL for methanol. The maximal enrichment was achieved when DMF was selected as eluant. So the DMF was selected as eluant. The experiment show that it was easier to elute the retained ABTR and its Au(III) chelate in reverse direction than in forward direction(Only 0.8 mL of eluant was needed when eluted in reverse direction. However, 2.2 mL of eluant was needed when eluted in forward direction), so it is necessary to upturn cartridge when elution. 1.0 mL of DMF was sufficient to elute the ABTR and its Au(III) chelate from cartridge at a flow rate of 5 mL min^-1. The volume of 1.0 mL was selected.
3.7 Calibration Curve and Sensitivity
The calibration curve showed that Beer's law is obeyed in the concentration range of 0.01- 3 m g Au(III) per mL in the measured solution. The linear regression equation obtained was A = 0.621 C (m g mL^-1) + 0.0154 (r =0.9992). The molar absorptivity was calculated to be 1.23×10⁵ L mol^-1 cm^-1 at 550 nm. The relative standard deviation at a concentration level of 0.5 m g L^-1 of Au(III) (11 repeats determination) was 2.18%. The detection limit, based on the three times of standard deviation, is 0.02 m g L^-1 in the original sample.
3.8 Interference
The selectivity of the proposed method was investigated by the determination 0.5 m g 100 mL^-1 of Au(III) in the presence of various ions within a relative error of ± 5% are given in Table 2. The result shows that most common ions do not interfere with the determination. This method is highly selective.