Spectrophotometric Determination of Palladium (II)
With Semixylenol Orange

Yuan Fuzhen
(Department of Chemistry, Yaitai University
Yantai, Shandong, 264005, P. R. China)

Abstract A highly selective, reproducible and rapid spectrophotometry for the determination of palladium (Ⅱ) has been worked out. This method is based on the chelation of Pd (Ⅱ) with semixylenol orange (C₂₄H₂₃NaNO₉S) in 1.00 mol.l^-1 sulphuric acid medium (H₂SO₄), to chelate a stable orange complex of Pd (Ⅱ) . Beer′s law is obeyed over the range 0.2-1.6 μg ml^-1 Palladium(Ⅱ),its maximum absorption wavelength λ_max is at 540 nm,the apparent molar absorptivityε₅₄₀ is 6.2*10⁴ l mol^-1 cm^-1.The method has been successfully applied for the determaination of Palladium(Ⅱ)in Pd-C catalyst , Al-Si-Pd and Zr-Pd alloys.

Key words Semixylenol orange,Spectrophotometry,Palladium(Ⅱ)

Introduction

Palladium is one of the common precious metals. Owing to its noble character and alloying ability, Palladium and its alloys are used in medicinal devices and in ethylene process. Literature survey reals that several spectropholmetric methods for the determination of Palladium(Ⅱ) [1-14] are not sensitive enough and rapid , because of these methods are related to the extraction of organic solvents , have the complicated reaction and coexist diverse species. However , in the present investigation , an improved method for the determination of Palladium(Ⅱ) has been worked out in a strong acidic medium which made use of semixylenol orange as a chelating agent. This method has been found to be rapid, selective, reproducible and free from the interference of a large number of precious and common metal ions, the determination results of Pd(Ⅱ)are satisfactory.

Experimental

Apparatus

Absorption spectra and absorbance measurements were obtained with a VIS-7230 digital spectrophotometer and a TG 328 automatic analytical balance ( Shanghai Analytical Instrument Plant, China),an electronic water bath pot (Shen-zhen Guohua Instrument Plant,China).

Reagents and solution

All reagents and chemical used were of analytical grade unless otherwise mentioned and distilled water was used throughout.PdCl₂ ,analytically pure (Shanghai Reagent Center Station Chemical Plant,China); Semixylenol orange, Chromatographically pure, free of xylenol orange and cresol red interferential impurities (Beijing Xu-dong Chemical Plant, China).

500 μg. ml^-1 Pd (Ⅱ) standard stock solution was prepared by dissolving 0.2083 g Palladium Chloride (PdCI₂) in 2 ml of concentrated HCI and diluting to 250 ml with distilled water .The stock solution was standardized [15] . The working standard solution (10μg/ml ) was prepared by dilution before use.

1% (m/v) semixylenol orange solution was prepared by dissolving 2.500 g semixylenol.orange in distilled water and diluting to 250 ml volumetric flask with distilled water. The 1% (m/v) solution was diluted to 0.1% (m/v) semixylenol orange solution before use.

Aqua regia ,mixed with three portions of concentrated HCI and one portion of conc HNO₃; sulphuric acid (1∶1) ; 3% (m/v) tartaric acid (H₂C₄H₄O₆) solution ; 10% (m/v) fluoboric acid (HBF₄) solution.

General procedure used for the measurement

Weighed accurately an adequate amount of Pd (Ⅱ) sample , dissolved it with aqua regia (add to HF if necessary ) in a beaker ,then added to 2.0 ml of sulphuric acid (1∶1) ,heated the sample solution until it smoked SO₃. After cooling ,added to 13.0 ml of sulphuric acid (1∶1), 5.0 ml of tartaric acid solution , 2.8 ml of fluoboric acid solution, sequentially, then mixed the sample solution in a 25 ml standard flask, diluted to the mark with distilled water and to be shaken .Take 5.00 ml of the sample solution into another 25 ml standard flask, added to 2.0 ml semixylenol orange, diluted to the mark with distilled water, and heated over a boiling-water bath for 10 minutes, cooled to room temperature. The absorbance of the sample solution was measured at 540 nm against the reagent blank in 1.0 cm path-length celles.

Results and discussion

Standard curve and regression equation

1.00, 1.50, 2.00, 2.50, 3.00, 3.50 and 4.00 ml of Pd (Ⅱ) working standard solution were taken repectively into seven 25 ml standard flasks. Added to 2.0 ml semixylenol orange to these flasks, diluted to the mark with distilled water, and heated over a boiling-water bath for 10 minutes , cooled to room temperature. The absorbance of the seven sample solution were measured at 540 nm against the reagent blank in 1.0cm path-length celles. The Palladium (Ⅱ) standard curve was drawn up (Fig.1).

Fig.1 Standard curve of Pd(Ⅱ)

Its linear regression equation is A=0.0042+0.023C

C(μg/25 ml) is the concentration of the Pd (Ⅱ) working standard solution , the related coefficient Υ=0.9996, the apparent molar absorptivity ε₅₄₀=6.2×10⁴ l.mol^-1.cm^-1.

Spectral characteristics

Absorption spectra of Pd (Ⅱ) complexes in sulphuric acid medium against reagent blank were measured in the range 450 nm to 580 nm. The absorption spectra of the orange coloured complex formed between Pd (Ⅱ) and semixylenol orange and colouring agent (semixylenol orange) are shown in Fig.2. The Pd (Ⅱ) complex and colouring agent have λ_max at 540 nm and 450 nm, respectively. In order to minimize the effect of colouring agent , the absorbance of the Pd (Ⅱ) complex was measured at 540 nm where the colouring agent shows negligible absorbance.

Choice of acid and effect of acedity

Choice of acid and its concentration are important in the determination of Pd (Ⅱ) with semixylenol orange as a chelating agent. Different acids , viz. HCI, HNO₃ and H₂SO₄ were tried, the results were shown in Table 1. Amongst the other acids (HCI & HNO₃) sulphuric acid is the most suitable because maximum absorbance of Pd (Ⅱ) complex is obtained with H₂SO₄ under the same concentration and the same working conditions.

With sulphuric acid medium , the effect of its acidity on the absorbance of Pd (Ⅱ) complex was studied in the acidity range 0.85 to 1.09 mlo.l^-1 . Results are shown and illustrated in Table 2 and Fig.3. They reveal that the absorbance is uneffected by acidity in the range 0.91 to 1.05 mol.l^-1. Thus 1.00 mol.l^-1 acidity of sulphuric acid was selected for all further studies.

Effect of the amount of semixylenol orange solution

The absorbances of a series of solution containing 15 μg. (25ml)^-1 Pd (Ⅱ) and various amount of semixylenol orange solution (0.50～3.20ml) were measured keeping all other parameters constant , an almost constant absorbance was observed in the range 1.00 to 3.00 m.l. Thus 2.00 ml of semiyxleol orange solution was selected for all further studies . Results are shown in Table 3 and illustrated in Fig.4.

Stability of Pd (Ⅱ) complex

The orange colourated complex formed between Pd (Ⅱ) and semixylenol orange , after cooling to room temperature, the absorbance of Pd (Ⅱ) complex is stable for at least 12 hr at 540 nm. Results are shown in Table 4. It indicated that the complex chelated by Pd (Ⅱ) and semixylenol orange, has a satisfactory stability in use.

Effect of diverse ion

In order to study the effect of various ions on the determination of Pd (II), a fixed amount 15 μg of

Pd (Ⅱ) was taken with different amount of foreign ions and the recommended procedure was followed. The results are summarized in Table 5. An error of ±2% in the determination of Pd (Ⅱ) was considered tolerable.

APPLICATIONS

The present method for the determination of trace amounts of Pd (Ⅱ) is highly sensitive and free from the interference of foreign diverse species ,has rather good selectivity.

The method has been applied to the determination of palladium (Ⅱ) in three different samples, seven times of measurement have been carried out for every sample, their average value Pd (Ⅱ) %, standard deviation , relative standard deviation RSD and standard value were summarized in Table 6.

Acknowledgment_The author is thankful to the Materials Laboratory of Physics-Chemistry Examination Center of Shandong Province for a generous supply of Pd-C catalyst, Al-Si-Pd and Zr-Pd alloys three samples.

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Received 9 June.1999，Revised 9 Sep.1999