Determination of trace amounts of palladium by solution scanometric method

Peer review under responsibility of Taibah University.

Abstract

In this study, solution scanometry is presented as a highly sensitive, simple and rapid method for the colorimetric determination of palladium ion in water samples and walnut leaf. The method is based on the formation of a rose-brown complex between Pd²⁺ ion and an excess of iodide in acidic medium in Plexiglas cells, which are scanned by conventional flatbed-scanner, and the colour of each cell is analysed by software written in Visual Basic (VB 6) media to determine red, green and blue values. The relative standard deviation and linear range of the method were determined to be 1.0% and 0.1–72.0 μg mL⁻¹, respectively. The proposed method was successfully applied for the determination of Pd²⁺ ion in water samples and walnut leaf.

Keywords:

• Palladium
• Solution scanometric method
• Plexiglas^® cell

1 Introduction

Palladium is an economically important metal of economic importance due to its increasing applications in the production of dental and medicinal devices and jewellery [1,2]. Palladium is also used for the purification of hydrogen gas [2] and in the electrical industry as contacts in telephone relays [3]. In recent years, the extensive use of palladium in automotive catalytic converters and in the chemical industry has led to increasing concentrations of this metal in the environment [4]. It is a metal with low toxicity but may still have deleterious effects on the health of the general population [5], such as causing asthma, allergy and other serious health problems [6,7]. Pd²⁺ ion shows a high ability to form complexes and react with reagents of practically all types [8].

Many methods are known for the determination of palladium, such as spectrophotometry [9], atomic absorption spectrometry [3], inductively coupled plasma-mass spectrometry [10] and inductively coupled plasma-optical emission spectrometry [11].

The flatbed scanner was used for the first time by Suslik et al. in the development of a colorimetric sensor array for the detection and identification of volatile organic compounds [12,13]. It was then used to scan the TLC (Thin Layer Chromatography) strip in the paptode method. TLC acts as a substrate for the reaction between an analyte and a reagent [14]. The solution scanometric method was introduced recently by Abbaspour et al. [15]. In this method, the reaction is performed in a solution in Plexiglas^® cells. The solution is scanned, and finally, the RGB (red, green and blue) colour model is used in colour monitoring [16–18]. This method has been used for the detection and determination of metal ions [19,20].

The solution scanometric method has the advantages of being easy to handle and having the capability to test various compounds simultaneously, with a high scanning speed, a quick response and limited interferences. Moreover, it is inexpensive, has portable systems and, unlike spectrometry, eliminates the need to find λ_max. Finally, this method in comparison to other scanning methods such as paptode, can produce more clear, symmetrical and homogenous images.

In this work, solution scanometry was developed as a simple, fast and inexpensive method to determine Pd²⁺ ion based on the formation of a coloured complex between Pd²⁺ ion and iodide in acidic medium and in the presence of sodium sulphite as a reducing agent for iodine.

2 Experimental

2.1 Apparatus

The cells used were built from sheets of Plexiglas, according to the previous report [15]. A flatbed CanoScan LIDE 200 with a 3-colour (RGB) LED (cold cathode fluorescent lamp (CCFL)) and charge-coupled device (CCD) as the light source and detection system, respectively, was used for scanning. The horizontal and vertical resolution of the scanner were both set at 300 dpi. The colour of each cell was analysed with software written in Visual Basic 6 (VB 6) media to determine the red, green and blue values. Three Biohit Proline pipettes with different volumes (0.5–10, 10–100 and 100–1000 μL) were used for injecting samples into the cells.

2.2 Chemicals and reagents

All chemicals used in this work were of analytical grade. In addition, double distilled water was used throughout. Palladium chloride and potassium iodide were purchased from Merck Company (E. Merck, Darmstadt, Germany). A laboratory stock solution (100 μg mL⁻¹) was made by dissolving 0.0416 g palladium chloride in hydrochloric acid and diluting to 250 mL in a volumetric flask. The analytical-grade sodium sulphite used in this study was obtained from Fluka Company (Fluka, Buchs, Switzerland).

2.3 Principles of the red, green and blue (RGB) colour systems

The RGB colour model is an additive colour model. In this model, a colour is described by specifying the intensity levels of the colours red, green, and blue. The typical range of intensity values for each colour is 0–255, which gives 16,777,216 different colours. In the RGB system, any colour is represented in the form of (R, G, B), such that black is represented by the RGB value ‘0 0 0’ (R = 0, G = 0 and B = 0), while white is represented by the RGB value ‘255 255 255’ (R = 255 G = 255 and B = 255). The values of all three colours are converted to a single integer value using this formula:$\text{V}=\text{R}+256\text{G}\text{+}{\text{256}}^2\text{B}$ where R, G and B are the red, green and blue values of the main colour, and V is the basis of the RGB system and is defined this way to identify a specific number for each colour. For black and white, V is equal to 0 and 16 777 216, respectively. The flowchart, R, G and B values of V for any colour can be extracted as described in the literature [15].

It should also be mentioned that the R, G or B colour value that has the same colour as the solution is usually useless, as this value is often higher than 180 and does not change significantly during the experiment. In this experiment, two values of the RGB system (green and blue) changed in different concentrations of the complex, and therefore the one with the highest change, that is, ″B″, was chosen.

2.4 Procedure

First, 125 μl of 100 μg ml⁻¹ palladium solution, 100 μl of 6 mol L⁻¹ hydrochloric acid, 125 μl of 4 mol L⁻¹ iodide and 50 μl of 0.02 mol L⁻¹ sodium sulphite as an reducing agent for iodine were transferred into each cell, and an appropriate volume of double distilled water was added for a total volume of 500 μL in each cell. A cell containing the desired volume of acid, the colour-forming reagent solution and sodium sulphite without any palladium (total volume of 500 μL) was set as the blank. The cells were shaken to obtain a homogenous colour, and the Plexiglas sheet was scanned with the scanner. The image was transferred into the computer, and any colour changes in each cell were analysed using a program written in VB 6 media.

In this program, the colour of each cell is analysed based on the RGB system to obtain R, G and B values. In the colour analysis program, a specific area for analysis containing approximately 10000–300000 pixels was selected. The noise could thereby be vividly decreased. The effective intensity for any colour value was calculated as follows:${\text{A}}_{\text{x}}=-\log \left({\text{C}}_{\text{s}}/{\text{C}}_{\text{b}}\right)$ where A_x is the effective intensity for red, green or blue. C_s and C_b are the red, green or blue colour values of a sample and the blank, respectively.

2.5 Preparation of real samples

2.5.1 Water samples

First, 2 mL of 1 mol L⁻¹ HNO₃ was added to water samples, which were used after 1 hour. The standard addition method was applied to determine Pd²⁺ ion in the water samples according to the procedure given in the general procedure section. The Pd²⁺ ion in these samples was also determined by atomic absorption spectroscopy (AAS). The conditions for determination of Pd²⁺ by AAS were as follows: wavelength, 244.8 nm; slit, 0.2 nm; flame; C₂H₂–Air, using a hollow cathode lamp was used.

2.5.2 Walnut leaf

The walnut leaf sample was dried for 48 h at 70 °C in an electric oven, and 4.0000 g of dry sample was subsequently kept in a furnace for 1 h at 550 °C until it ashed. The ash was digested with 4 mol L⁻¹ HNO₃ and heated on a hotplate. After dissolution, the contents were filtered by filter paper and diluted to a final volume of 50 mL with double distilled water. Finally, 200 μl of the resulting solution was analysed according to the described procedure. The Pd²⁺ ion was also determined in this sample by atomic absorption spectroscopy (AAS).

3 Results and Discussion

In the reaction of Pd²⁺ ion with iodide, the precipitate of palladium iodide has been produced. However, if a large iodide excess is added, the palladium iodide precipitate dissolves because of the formation of a soluble PdI₄²⁻ complex with rose-brown colour, according to the following reaction:${\text{PdI}}_{\text{2}}+2{\text{I}}^{-}\leftrightarrow {\text{PdI}}_4^{2-}\left(\text{rose-browncolour}\right)$

The B parameter of the picture of this colour has a good effective intensity. Accordingly, the determination of Pd²⁺ ion in this research is based on the change in this parameter.

3.1 Optimization of the system parameters

To obtain the maximum effective intensity of B as a signal, it was necessary to investigate the effects of reaction time, type and the concentration of reagents.

3.1.1 Effect of time

To attain the maximum signal, it was necessary for the interaction between reagents to be completed. The effect of the reaction time was studied in the range of 5–55 min. To examine the effect of reaction time on the determination of palladium, A cell containing 500 μl of a solution consisting of 25 μg ml⁻¹ palladium, 1 mol L⁻¹ iodide, 1.32 mol L⁻¹ HCl and 2 × 10⁻³ mol L⁻¹ sodium sulphite was scanned at 5 minute increments at room temperature, and after scanning of the sheet, the image of the cell was analysed using the software. The results, shown in Fig. 1, illustrate that the effective intensity is maximum at an analysis time of 20 minutes.

Fig. 1 Effect of time on effective intensity (Conditions: 500 μL solution containing 25 μg mL⁻¹ Pd²⁺ ion, 1 mol L⁻¹ iodide, 1.32 mol L⁻¹ HCl and 2 × 10⁻³ mol L⁻¹ sodium sulphite).

3.1.2 Effect of iodide, sulphite and acid concentrations

Fig. 2 illustrates the effect of iodide and sodium sulphite concentration as well as the effects of HCl and H₂SO₄ and their concentrations. The effective intensity B increased with the addition of iodide and decreased from the concentration of 1 mol L⁻¹ onwards, at which concentration the interaction between iodide and Pd²⁺ ion was complete. Therefore, 1 mol L⁻¹ of iodide was selected as the optimum concentration.

Fig. 2 Effect of iodide and sodium sulphite concentration as well as the effect of HCl and H₂SO₄ and their concentration on effective intensity (Conditions: 500 μL solution containing 25 μg mL⁻¹ Pd²⁺ ion, 1 mol L⁻¹ iodide, 1.32 mol L⁻¹ HCl, 0.42 mol L⁻¹ H₂SO₄, 2 × 10⁻³ mol L⁻¹ sodium sulphite; reaction time 20 min). For the other two parameters, the investigated parameter was changed, and the others were fixed similarly.

Because acid iodide solution slowly liberates iodine on standing, a quantity of sodium sulphite was added to the solution to reduce the iodine [21]. Optimization of the concentration of sodium sulphite is one of the key optimization steps. According to observations without sodium sulphite in the reaction medium, the false blue signal attributed to the released iodine was increased. The highest signal was obtained at 2 × 10⁻³ mol L⁻¹ of sodium sulphite. Thus, to provide sufficient sodium sulphite, this amount was selected as the optimum concentration.

The effect of HCl and H₂SO₄ and their concentrations on the formation of complex of Pd²⁺ ion with iodide was also examined. As shown in Fig. 2, at 0.42 mol L⁻¹ H₂SO₄ and 1.32 mol L⁻¹ HCl, the maximum colour intensity was observed. Because the effective intensity of the B parameter in the presence of HCl acid at the optimum concentration is greater than for H₂SO₄, HCl was accordingly used in subsequent studies. In previous reports, because of obtaining higher sensitivity and lower interference in the matrix, different acids have been used for the measurement of palladium with iodide reagent [21–23].

3.2 Analytical performance

Fig. 3 illustrates the calibration curve under optimum conditions. In the range of 0.1–72.0 μg mL⁻¹, a proper linear relationship was obtained between the effective intensity of the product and the concentration of Pd²⁺ ion. The equation for the regression line was Y = 0.0084C + 0.0234 (for B), where C is the concentration of Pd²⁺ ion in μg mL⁻¹. To determine the repeatability of the method, 5 sample solutions of 25 μg mL⁻¹ Pd²⁺ ion were analysed, and 1.0 was the RSD% (Relative Standard Deviation) for the B colour value. The detection limit was found to be 0.089 μg mL⁻¹ based on the standard deviation of six blanks.

Fig. 3 Calibration curve for determination of Pd²⁺ ion under optimum conditions.

3.3 Interference study

To study the selectivity of the proposed method for the determination of Pd²⁺ ion, the effect of various ions on the determination of 25 μg ml⁻¹ Pd²⁺ ion was tested under optimum conditions. The tolerance limit was defined as the concentration of added ion that causes less than ±5% relative error of the B value. The results are shown in Table 1. In this method, most of the species do not interfere, because the scanometry is based on the reflective properties of the samples.

Table 1 Effect of interfering species on the determination of 25 μg ml⁻¹ Pd²⁺ under optimum conditions.

Interfering species	Added as	Tolerance Conc. (μg mL^−1)
Ca^2+, Ba^2+, Cu^2+	Nitrate salt	5000
ClO4^−, SO4^2−	Sodium or Potassium salt	5000
Ni^2+, Na^+	Nitrate salt	2500
Hg^2+	Chloride salt	2500
Cl^−	Potassium salt	2500
Al^3+, K^+	Nitrate salt	1000
Br^−	Potassium salt	1000
Cr^3+, Cd^2+, Co^2+, Mg^2+, Pb^2+	Nitrate salt	500
Fe^3+, Ag^+	Nitrate salt	25

3.4 Application

The applicability of the proposed method in determining Pd²⁺ ion was examined with walnut leaf and three different water samples, including tap (Yasouj city), river (Beshar in Yasouj city) and spring (Ghallat village in Yasouj city) water, under optimum conditions. The results are presented in Table 2. The results obtained by the proposed method were found to be in excellent agreement with the results obtained by atomic absorption spectrometry (AAS).

Table 2 Determination of Pd²⁺ ion in real samples.

sample	Solution scanometric method					AAS
	Regression equation	R^2	Added (μg mL^−1)	Found (μg mL^−1)	Recovery %	Found (μg mL^−1)
Tap water (Yasouj city)	A = 0.0069X − 0.0007	0.9915	00.00	Nd	–	Nd
			20.00	19.48	97.38	–
			32.00	31.97	99.90	–
Spring water (Ghalat village in Yasouj city)	A = 0.0062X + 0.0003	0.9988	00.00	Nd	–	Nd
			20.00	19.73	98.63	–
			32.00	31.48	98.36	–
River water (Beshar, Yasouj city)	A = 0.0051X − 0.0043	0.9934	00.00	Nd	–	Nd
			20.00	19.91	99.53	–
			32.00	32.10	100.30	–
Walnut leaf	A = 0.0062X + 0.0039	0.9927	00.00	Nd	–	Nd
			20.00	19.79	98.95	–
			32.00	31.71	99.10	–

4 Conclusions

Trace amounts of Pd²⁺ ion were determined using solution scanometry as a new, simple, fast and inexpensive method. Table 3 compares the characteristics of the proposed method with some other previous reported methods for the determination of Pd²⁺ ion. As shown in Table 3, the method has a wide linear range, good precision and an acceptable limit of detection in comparison to the other methods. In addition, the method has other advantages, such as simplicity of operation, rapidity and low cost. The proposed method can be successfully applied to determine trace amounts of Pd²⁺ ion in real samples.

Table 3 Comparison of the results of solution scanometry and other methods for determination of Pd²⁺ ion.

Method	Reagent	Linear range (μg mL^−1)	LOD (μg mL^−1)	RSD%	Ref
Spectrophotometry	Bromide	40.0–200.0	----	----	[24]
Spectrophotometry	Azide	1.3–4.6	0.03	----	[25]
Spectrophotometry	Promazine hydrochloride	2.0–17.0	----	----	[8]
Spectrophotometry	Cefixime	0.75–16.50	0.07	----	[26]
Solution scanometry	Iodide	0.1–72.0	0.089	1.0	This work

Acknowledgement

We gratefully acknowledge the support of Yasouj University Research for this study.

Notes

Peer review under responsibility of Taibah University.