Fan Zhefeng
(Center of Analysis and Testing, Shanxi Normal University, Linfen, Shanxi, 041004, China)

Abstract A solid phase extraction system for separation and preconcentration of platinum and palladium in environmental materials is proposed, It is based on the adsorption of platinum(IV) and palladium (II) as ion associates of their chlorocomplexes onto an Amberlite XAD-4 resin loaded with diphenylthiourea (DPTU) reagent. The parameters such as the effect of acidity on the platinum and palladium extraction, the effect of flow rate and sample volume on the extraction, the sorption capacity of the loaded resin, the platinum and palladium desorption from the resin and the analytical characteristics of the procedure were studied. The results demonstrate that palladium and platinum in the concentration range 10-500 ng ml^-1, and 5 mol L^-1 HCl contained in a sample volume of 25-100 ml, can be extracted by using 0.5 g Amberlite XAD-4 resin loaded with diphenylthiourea (DPTU) reagent. The adsorbed platinum and palladium were eluted from the resin by using 5 ml 0.4 mol L^-1 thiourea in 1 mol L^-1 HCl. The extractor system has a sorption capacity of 1.59 mmol for platinum and 1.81 mmol for palladium per g of Amberlite XAD-4 resin loaded with diphenylthiourea (DPTU). The precision of the method, evaluated as the RSD obtained after analyzing a series of seven replicates, was 2.5% for platinum, 3.6 % for palladium in a concentration of 25 ng ml^-1. Detection limits were 12 ng g^-1 for palladium and 18 ng g^-1 for platinum. The proposed procedure was used for platinum and palladium in environmental materials using an inductively coupled plasma atomic emission spectroscopy technique (ICP-AES).
Keyword　

1. INTRODUCTION
The determination of trace concentration of platinum and palladium in environmental and biological samples has gained a considerable importance because of their toxicity and increasing occurrence, which is mainly due to industrial and automobile catalysts ^[1-5]. The wide variety of environmental matrices and the ultra trace concentration of these metals in them require reliable and efficient analytical methods. Inductively coupled plasma atomic emission spectrometry ^[6,7] (ICP-AES), inductively coupled plasma mass spectrometry ^[8,9](ICP-MS), electrothermal atomic absorption spectrometry ^[10] (ETAAS) and neutron activation analysis ^[11] (NAA) are the most sensitive techniques and find the wider application. However, the direct analysis of platinum and palladium by these techniques are considerably restricted owing to interference caused by matrix elements, especially in the case of environmental materials. The separation and preconcentration steps prior to the final detection are necessary.
    Many separation methods such as ion-exchang^[12-15,23], sorbent extraction ^[7,16,17], extraction with dithizone ^[18], activated charcoal ^[19] and modified silica gel ^[20] have been proposed. The interest of analysts has been focused on the wider application of various sorbents and complexes for the effective separation and preconcentration of these metals. Enhancement of selectivity may be achieved owing to differences in the tendency of individual metals to form complexes in various media. An on-line sorbet extraction preconcentration system has been developed for the determination of platinum and rhodium by ETAAS and ICP-AES after accumulation of their bis(carboxymethyl)dithiocarbamate (CMDTC) chelate on a micro-column packed with Amberlite XAD-4 resin ^[7]. Methods of separation of the platinum metal based on liquid chromatography after their extraction in the form of chelate with 1-(2-pyridylazo)-2-naphthol (PAN) have been presented ^[16,17]. The separation and determination of Pt, Os, Ir, Ru, Co, Ni with 2-(6-methyl-2-benzothiazolylazo) -5-diethylaminophenol (MBTAE) has been described ^[21]. Separation of Pt, Pd and Ru quinolin-8-olates on silasorb-600 with choroformpropan-2-o1 (98+2) has been examined ^[22].
    This paper proposes an analytical procedure for the preconcentration and determination of platinum and palladium in environmental materials using inductively coupled plasma atomic emission spectrometry, after chelation onto a column containing Amberlite XAD-4 resin loaded with diphenylthiourea (DPTU).

2.EXPERIMENTAL
2.1 Apparatus
Measurements were performed using Atomic Scan 25 inductively coupled plasma atomic emission spectrometry (Thermo Jarrell-Ash, USA). Parameters of instrument and operating conditions are presented below. The ICP source was a generator with frequency 27.12 MHz operated at a power input of 1.2 kW. Gas flows (Ar) were adjusted to the following values: plasma 15.0 l min^-1, auxiliary 0.5 l min^-1, carrier 0.8 l min^-1. The V-groove nebulizer was fed by a peristaltic pump (1.2 ml min^-1), integration time was 15 s and each result was the average of three measurements, the spectral lines (nm) were used: pt II 214.432; pd II 340.458. A peristaltic pump IFIS-C (Xian Remax Electronic Science-Tech Co. Ltd, China) was used for preconcentration. A microwave device MDS 2000 (CEM, USA) was used for sample preparation.
2.2 Reagents
All reagents were of super-pure grade chemical unless otherwise stated. The standard stock solution containing 500 mg ml^-1 of Pt and Pd in 20% HCl. Double distilled water was used for the preparation of solutions. Diluted Pt and Pd solutions, ranging from 10-500 ng ml^-1, containing interfering elements were freshly prepared in a mixture of 5 M HCl + 0.2 M HNO₃ from stock standard solutions. The thiourea elute (0.4 mol L^-1) solution was freshly made up by dissolving 1.52 g of thiourea in 1 mol L^-1 HCl in a volume 50 ml. The DPTU solution (0.10%) was prepared by dissolving 0.25 g DPTU in 250 ml ethanol.
2.3 Preparation of the Amberlite XAD-4 Column loaded with DPTU 　
Amberlite XAD-4 was treated with an ethanol-hydrochloric acid-water (2:2:1 ) solution over night. ter, the resin was rinsed with distilled water until it was neutral, being dried in an oven at a temperature of 110ºC for 3h.
    The packing of the column must be done using ethanol as elute because with water the grains of resin float. The resin is satuted with the reagent by sorption of 15 ml of a 0.10 % DPTU solution in ethanol at a flow rate of 0.2 ml min^-1. Later it is rinsed with distilled water until the complete elimination of excess reagent occurs. All experiments were done in glass column with a 0.5 cm i.d. and length of 5.0 cm containing 0.5 g Amberlite XAD-4. Before the sample sorption, the column must be preconditioned by passing a 10 ml of 5 mol L^-1 HCl + 0.2 mol L^-1 HNO₃ at flow rate 2.0 ml min^-1.
2.4 Procedure for the sorption of Pt and Pd on the AmberliteXAD-4 　 　　　　　
The mixed standard solutions of Pt and Pd are pipeted into beaker, the final volume was made up with 5 mol^-1 HCl +0.2 mol L^-1 HNO₃ solution. This solution must be passed through the column at a flow rate of 2.0 ml min^-1. After passing this solution the column was rinsed with 10ml of 5 M HCl +0.2 M HNO₃. The adsorbed Pt and Pd on the column was eluted with 5 ml of 0.4 mol L^-1 thiourea in 1 mol L^-1 HCl at a flow rate of 0.50 ml min^-1. The elute solution was collected in a 5 ml volumetric flask and the Pt and Pd determined by the ICP-AES.
2.5 Samples preparation
3 g of the samples were leached in a microwave device with 25 ml of HNO₃+HCl+ HF (3:1:1). The blank solutions characterizing the whole procedure were prepared in the same way. After cooling, sample were filtered and transferred into 100 ml volumetric flasks. The final volumes were made up with 5 mol L^-1 HCl +0.2 mol L^-1 HNO₃.

Fig.1 Effect of acidity on the Pt and Pd sorption on the Amberlite XAD-4 column

3.RESULTS AND DISCUSSION
3.1 effect of acidity on adsorption 　
The mixed stock standard solutions containing 25 ng ml^-1of Pt and Pd are concentrated by means of the column procedure described. The results demonstrated that it is maximum and quantitative in the acidity rang 4-6 mol L^-1 HCl (recoveries >95%) and that the recoveries of both elements are >97% at 5 mol L^-1 HCl, as can be seen in Fig. 1. In order to determine these elements simultaneously, 5 mol L^-1 HCl is selected.
3.2 Effect of flow rate and sample volume
The effect of flow rate on Pt and Pd retention was examined by varying the flow rate from 0.5 to 5.0 ml min^-1under optimum conditions. The results demonstrated that the retention of Pt and Pd on the resin is quantitative (>95%) and for a flow rate lower than 2.0 ml min^-1. The effect of the sample volume on the Pt and Pd extraction was investigated by passing 25, 50, 100, 200 and 250 ml through the column at a constant flow-rate of 2.0 ml min^-1. The recovery obtained was higher than 95% in the sample volume than 100 ml.
3.3 Influence of desorption acidity
In order to determine the Pt and Pd desorption from resin, 5.0 ml of 1-3 mol L^-1 HCl +0.4 mol l^-1thiourea were tested. The results demonstrated that the desertion is acceptable (>95%) for solutions with 1 mol l^-1 HCl +0.4 mol l^-1 thiourea solutions. In this procedure a concentration of 1 mol l^-1 HCl +0.4 mol l^-1 thiourea is recommended.
3.4 Effect of desorption flow rate 　　　　　　　　　
When the column procedure is used, the influence of the flow rate on desorption of the analytes from the column with 5 ml of 1 mol l^-1 HCl +0.4 mol l^-1 thiourea is investigated. The results show that desorption recoveries of Pt and Pd are >97% for a flow rate of 0.5 ml min^-1. A 0.5 ml min^-1 flow rate is selected for the elute flow rate.
3.5 Sorption capacity
The sorption capacity of the Amberlite XAD-4 resin loaded with DPTU for the extraction of Pt and Pd was also determined. Increasing amounts of Pt and Pd were added to a column containing 0.5g of loaded resin. The adsorption capacity of the resin is calculated and is found to be 1.56 mmol g^-1 for Pt and 1.81 mmol g^-1 for Pd.
3.6 Interference of other ions　　　　　　　
Different potential interfering ions are added to dilute analytes standards (25 ng ml^-1 each of Pt and Pd). The analytes are preconcentrated and determined as described above. The tolerance limits of various concomitant ions are determined, when a relative error of ±5% is used as the criterion the following ions have no effect: 1000-fold of by weight Ca(II), Mg(II), Cu(II), Zn(II), Fe(II) and Al(III); 500-fold Mn(II), Ni(II), Cd(II) and Bi(III); 10-fold Pb(II). The results show that these various concomitant precious and heavy metals do not have the interference.
3.8 Application
The proposed procedure can be applied to the preconcentration and separation of Pt and Pd, in the concentration range, contained in a solution volume of 100 ml by using of 0.5 g Amberlite XAD-4 resin loaded with DPTU reagent. The precision of the method, evaluated by the RSD obtained after analyzing a series of seven replicates was 2.5% for Pt and 3.6% for Pd in a concentration of 25 ng ml^-1. The limits of detection (LOD) calculated as 3s is 12 ng g^-1 for Pd and 18 ng g^-1 for Pt. The method proposed was applied for Pt and Pd determination in environmental materials. The standard addition technique was applied and the recoveries obtained revealed that the proposed procedure has good accuracy. The results are described in Table 1.

Table.1 Determination and recovery of Pt and Pd in environmental samples (n=3)

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