2,6-Diamino-1- N -methylpyrimidine as new fluorescent probe for palladium(II) determination using hyphenated technique

Abstract

The selective and sensitive hyphenated technique for Pd(II) fluorescence and spectrophotometric determination was proposed. The procedure was based on preliminary solid-phase extraction of [PdCl₄]²⁻ onto silica modified with quaternary ammonium salts, with the subsequent elution from the surface by 2,6-diamino-1-N-methylpyrimidine solution and measurement of the fluorescence intensity. The luminescence enhancement effect of 2,6-diamino-1-N-methylpyrimidine in the presence of Pd(II) due to the complex formation was applied to the development of sorption-spectroscopic techniques for detecting trace amounts of palladium in solution.

Keywords

Adsorption palladium desorption hyphenated technique 2

Introduction

The recovery and determination of platinum group metals (PGMs), especially palladium, in the spent catalyst and electrolytes are very actual now. Thus, it is necessary to regenerate these materials with the view to use them in the next cycle (Terrazas-Rodriguez et al., 2011). Currently, highly sensitive methods are used for the determination of PGMs, such as mass spectrometry with inductively coupled plasma, neutron activation analysis, stripping voltammetry (Bencs et al., 2003; Godlewska–Zyłkiewicz, 2004). However, their application for routine analysis is limited because of high cost of equipment and services. Therefore, the development of cost effective and interference-free hyphenated techniques for PGMs determination is a topical task in materials science (Godlewska–Zyłkiewicz, 2004).

The fluorescence and UV-Vis spectroscopy due to its simplicity and reliability would certainly contribute to solving the problem of metal ions detection (Eaton and Douglas, 2012; Godlewska–Zyłkiewicz, 2004; Lakowicz, 1999). The ability of palladium to form complexes with nitrogen containing functional groups was used in the design of new derivatives of 2,6-diaminopyrimidines (Milokhov et al., 2012). As it was found by Zaporozhets et al. (2015), 2,6-diamino-1-N-methylpyrimidine (3-(6-amino-5-(1,3-benzothiazol-2-yl)-1-methyl-2-imino-1,2-dihydro-4-pyrimidinyl)-1-propanol, HR) exhibits fluorescent properties in the weak acidic aqueous-propanol solution. We foresee that this probe can be used for palladium determination based on optical responses. The solid-phase extraction (SPE) using modified silica (SG) is known to be one of the most efficient techniques used for the separation and pre-concentration of heavy metals from water solutions (Zaporozhets et al., 1996) due to its simplicity and capability to achieve high enrichment factors. Recently, magnetic nanoparticles with anionic-exchange 3-aminopropyl surface groups (Zavoiura et al., 2015) and SG modified with quaternary ammonium salts (QAS-SG) (Volovenko et al., 2013) were successfully applied for SPE of anionic palladium complexes [PdCl₄]²⁻ from chloride solutions. The sorption-spectrophotometric method using QAS-SG for Pd(II) determination was developed.

In this study, new selective and sensitive hyphenated techniques for Pd(II) spectrophotometric and fluorescent determination using two-step adsorption-desorption procedure based on preliminary solid-phase extraction of [PdCl₄]²⁻ onto QAS-SG followed by subsequent elution from the surface by 2,6-diamino-1-N-methylpyrimidine solution were developed.

Materials and methods

Chemicals and equipment

Chloroform (CHCl₃, 98% w/w), hexane (C₆H₁₄, 98% w/w), 2-propanol (IPA, (CH₃)₂CHOH, 98% w/w) (HPLC grade) (Merck), and tetrachloropalladic acid (H₂PdCl₄; standard solution, 1 mg ml⁻¹, in HCl (2 M)) were used. Tetradecylammonium nitrate ((N(C₁₀H₂₁)₄)NO₃, QAS) (TSI, Japan) was used as 0.01 M solution in chloroform/hexane (1:16). 3-(6-Amino-5-(1,3-benzothiazol-2-yl)-1-methyl-2-imino-1,2-dihydro-4-pyrimidinyl)-1-propanol was synthesized as reported earlier (Milokhov et al., 2012), and 0.100 mM HR solution in IPA was used. Silica gel (Merck SG-60, S_BET = 490 m² g⁻¹) was used without pre-treatment.

Optical absorbance and fluorescence spectra of the solutions were obtained via SHIMADZU UV-2401PC recording spectrophotometer and Perkin Elmer LS 55 Fluorescence spectrometer. The concentration of Pd(II) in the solutions was measured by flame atomic absorption spectroscopy (FAAS) (AAS1N; Carl Zeiss, Jena, Germany). The air-acetylene flame was used to ionize the sample (excitation wavelength (λ) = 247.6 nm). Sorption was facilitated by the use of a magnetic stirrer.

Immobilization of QAS onto silica

The silica was functionalized with QAS according to the method proposed by Zaporozhets et al. (1994), but with some modifications. In all, 1.000 g of silica gel was stirred with 40.0 ml of 0.01 M QAS solutions for 15 min. The concentrations of QAS in chloroform/hexane (1:16) solution were determined spectrophotometrically by measuring the absorbance of ion-associates (QAS⁺)₂Co(SCN)₄ at 630 nm (Zaporozhets et al., 1994). The amount of the compound adsorbed (a) was calculated according to the equation:

a (mol g^{- 1}) = (C - [C]) \times {Vm}^{- 1}

(1)

where C and [C] are the initial and the equilibrium concentration of adsorbate in solution (M), V is the volume of the solution (l), and m is the mass of sorbent employed (g). Sorbents modified with QAS (QAS-SG) were separated and dried at room temperature for 24 h. The QAS-SG with the adsorption capacity a(QAS) = 25 µmol g⁻¹ was used.

Pd(II) adsorption and batch desorption studies

All adsorption experiments were carried out at room temperature according to the method suggested by Volovenko et al. (2013). The Pd(II) solutions were prepared by sequential dilution of H₂PdCl₄ standard solution. In order to adjust the ionic strength to 0.1 M and to prevent hydrolysis of Pd(II), 1 M solution of NaCl was used. The pH was adjusted with NaOH (0.1 M) and HCl (0.1 M). The batch adsorption experiments were conducted at pH 2.0 ± 0.2, with the solution volume-to-adsorbent weight ratio of 1000 ml g⁻¹. The concentration of [PdCl₄]²⁻ in solution was controlled spectrophotometrically by measuring its corresponding absorbance at 280 nm, and employing calibration graph. The amount of the Pd(II) adsorbed (a (mol g⁻¹)) was calculated as mentioned above.

It was shown (Volovenko et al., 2013) that Pd(II) adsorbed onto QAS-SG remained virtually unleached over the pH 1–3 (HCl). The batch technique was employed to study the Pd(II) elution from QAS-SG into the aqueous or organic phase. The following procedure was applied: samples of QAS-SG with Pd(II) adsorbed (100 mg) were transferred into 5.0 ml of HR in water/IPA (1:9) solution (0.01 M HCl and 0.1 M NaCl) and stirred for 1–10 min. The amount of Pd(II) eluted from the surface was detected by the spectrophotometric or fluorescence methods. The percentage (% d) of Pd (II) elution was calculated according to the equation:

d = C \times V \times 100 / a \times m

(2)

where C is the concentration of desorbate in solution (M), V is the volume of the solution (l), a is the amount of the compound adsorbed (mol g⁻¹), and m is the mass of sorbent employed (g).

Results and discussion

The solid-phase extraction of [PdCl₄]²⁻ from acidic solutions is caused by the formation of the (QAS⁺)₂[PdCl₄]²⁻ ion-associates on the QAS-SG surface (Volovenko et al., 2013). The extraction of palladium up to 25.0 µM from 100.0 ml solution with 0.100 g QAS-SG under these conditions was quantitative. It was found that Ag⁺, Au³⁺, and Pt²⁺ in two-fold excess and heavy metal ions, such as Co²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Fe³⁺ in 10-fold excess, Sn²⁺ 100-fold excess, and NO₃⁻, SO₄²⁻, EDTA in 500-fold excess did not interfere in the Pd(II) extraction from 0.1 M chloride solution at pH 2.0 ± 0.2 (Volovenko et al., 2013). The fact that the light absorbance of the sorbent varies directly with the concentration of [PdCl₄]²⁻ in solution was employed earlier for the development of sorption-spectroscopic technique for Pd(II) determination with detection limit 15 µg·l⁻¹.

Here we report about another approach. It is based on two-step procedure which includes preliminary solid-phase extraction of [PdCl₄]² onto QAS-SG followed by subsequent elution from the surface and recording of fluorescence or optical absorbance of the eluate. To avoid the dilution of the concentrate, it is advisable to desorb palladium with fluorescent reagent solution.

It has been found that the addition of Pd(II) to the HR solution in IPA (Figure 1(a)) is accompanied by decrease in the fluorescence signal intensity (Zaporozhets et al., 2015). On the other hand, optical absorption spectrum of the solution is characterized by the appearance of two absorption bands at approximately 300 and 380 nm (Figure 1(b)). Such changes in the emission and absorption spectra of the solution indicate the formation of the complex. Under these conditions, the protonated form H₂R⁺ and reactive PdCl₄²⁻ appeared to be predominant in the solution (Zaporozhets et al., 2015). The protonated form of dye is transparent in the range >375 nm (Figure 1(b)) thus further UV-Vis studies were carried out at the 380 nm.

Figure 1.

Fluorescence (a) and absorbance (b) spectra of HR in water/IPA (1:9) solution before (1) and after treatment with [PdCl₄]²⁻ (2–10). Experimental conditions: C(HR) = 50.0 µM, pH 2.0 ± 0.2, C(Pd), µmol l⁻¹: 2.0–20.0 (2–8 (a)), (2–10 (b)), C_NaCl = 0.1 M. l = 1.00 cm, λ_ех = 337 nm (a).

Hence, the fluorescence of HR at 450 nm and its absorbance at 380 nm after treatment with a solution of [PdCl₄]²⁻ was also found depend on the palladium(II) concentration in the solution. The intensity signal was in direct proportion to the palladium(II) concentration over the 2.0–20.0 µM Pd(II) range and could be described by the equations: ΔІ₄₅₀ = (0.8 ± 2.4) + (17.1 ± 0.2) × C (µM) (R = 0.999); А₃₈₀ = (0.017 ± 0.002) + (0.0134 ± 0.0002) × C (µM) (R = 0.999). The detection limit of fluorescence determination (3σ) was 45 µg l⁻¹, and detection limit of spectrophotometric determination (3σ) was 48 µg l⁻¹.

To elute palladium from the adsorbent, the QAS-SG with [PdCl₄]²⁻ was treated with 5.0 ml of 50.0 µM of HR in water/IPA (1:9) solution for 1–10 min. Figure 2 shows the efficiency of elution under such conditions. It can be seen that 90% desorption of palladium is achieved within 3 min.

Figure 2.

Desorption of Pd(II) from the QAS-SG samples with HR in water/IPA (1:9): a(Pd(II)) = 5.0 µmol.g⁻¹, m(QAS-SG) = 100 mg, C(HR) = 50.0 µM, V = 5.0 ml. T = 290 ± 1 K.

Trueness of sorption-desorption techniques was checked by the recovery functions for different detection techniques and varying masses of sorbent (Table 1).

Table 1.

Calibration data sets for the test of trueness by the recovery functions for sorption-spectrophotometric (SSP) and sorption-fluorescence (SF) methods of palladium determination.

C_Pd (µg)
Added	Found SSP^a	Found SF^a	Added	Found SSP^b	Found SF^b
2.1	1.9	2.3	4.3	4.5	4.1
3.7	3.5	3.4	7.4	7.6	7.1
5.3	5.7	5.4	10.6	11.3	10.9
8.0	8.2	8.5	16.0	16.5	16.1
10.6	11.3	10.4	21.3	21.5	20.8

m(QAS-SG) = 50 mg.

m(QAS-SG) = 100 mg.

The recovery functions could be described by the equations: C_Found, µg = (−0.5 ± 0.4) + (1.09 ± 0.03)C_Add., µg (R = 0.993) for SSP and C_Found, µg = (0.14 ± 0.26) + (1.02 ± 0.03)C_Add., µg (R = 0.996) for SF. Linearity of recovery function was proved by analysis of residuals plot and by Mandel test (Reichenbächer and Einax, 2011). At significance level α = 0.05 deviation from linearity was not observed. Closeness of the intercept and the slope of the recovery function to 0 and 1, respectively, indicate the absence of constant and proportional systematic errors. Therefore, the developed hyphenated techniques are acceptable for the analytical use. An enrichment factor of 20 was achieved (sorption from 100.0 ml of sample, and desorption into 5.0 ml of solution). Hence, the detection limits for Pd(II) were 2.4 and 2.3 µg l⁻¹ for sorption-spectrophotometric and sorption-fluorescence methods, respectively.

To check the accuracy of the sorption-desorption techniques, both spent electrolyte and catalyst solutions were examined. The results obtained were in agreement with standard FAAS method (Table 2). The techniques appeared to be fairly accurate and readily reproducible.

Table 2.

Results of palladium determination using sorption-spectrophotometric (SSP) and sorption-fluorescence (SF) methods.^a

Sample		C_Pd (µg)
Sample	Label claim of Pd	Found SSP (RSD)	Found SF (RSD)	Found FAAS (RSD)
Spent electrolyte^b	∼100 mg.l⁻¹	99 ± 10 (0.04)	94 ± 5 (0.03)	91 ± 2 (0.01)
Spent catalyst	0.1%	90 ± 5 (0.02) (0.096%)	93 ± 4 (0.02) (0.099%)	88 ± 3 (0.01) (0.094%)

n = 3, P = 0.95.

The contents (mg^.l⁻¹) of the Spent electrolyte solution were: Cu(II) (120), Fe(III) (34), Zn(II) (2).

RSD: relative standard deviation.

Conclusions

Two-step sorption-desorption method with preliminary solid-phase extraction of palladium from acidic solutions onto modified quaternary ammonium salts silica have been developed. Palladium was eluted quantitatively with 2,6-diamino-1-N-methylpyrimidine water-organic solution due to the complex formation, followed by decrease of fluorescence signal intensity of the probe and by increase of the light absorption of probe. An enrichment factor of 20 was achieved. Hence, the detection limits for Pd(II) were 2.4 and 2.3 µg l⁻¹ for hyphenated sorption-spectrophotometric and sorption-fluorescence determination, respectively, which are lower than the values of SPE methods reported previously (Losev et al., 2009; Volovenko et al., 2013).

Footnotes

Acknowledgements

This study was first presented at the 15th Ukrainian–Polish Symposium on Theoretical and Experimental Studies of Interfacial Phenomena and their Technological Applications, Lviv, Ukraine, 12–15 September 2016.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partially funded by the Ministry of Education and Sciences of Ukraine, no. 0114U003554.

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