Spectrophotometric Determination of Palladium(II) Ions Using a New Reagent: 4-(N’-(4-Imino-2-oxo-thiazolidine-5-ylidene)- hydrazino)-benzoic Acid (p-ITYBA)

-e simple, rapid spectrophotometric method for palladium(II) ions determination using a new analytical reagent is described. -e interaction of Pd(II) ions with a reagent, of the class of azolidones, 4-(N′-(4-imino-2-oxo-thiazolidine-5-ylidene)-hydrazino)benzoic acid, in water medium results in the formation of a complex. -e Pd(II)-p-ITYBA complex shows maximum absorbance at a wavelength of 450 nm. -e molar absorptivity is 4.30×10 L·mol·cm. -e optimal pH for complex formation is 7.0. -e developed method has a wide linearity range of 0.64–10.64 μg·mL for Pd(II). -e detection limit is 0.23 μg·mL. It was found that Co(II), Ni(II), Zn(II), Fe(III), Cu(II), Al(III), and many anions do not interfere with the Pd(II) determination. -e proposed method was tested in the analysis of model solutions and successfully applied for the determination of palladium in catalyst. -e obtained results show that this method can be used for serial determinations of palladium in various objects.


Introduction
Among the all platinum metals, the palladium has the widest practical application in various branches of industries. Palladium is used in the production of autocatalysts and jewelry, as well as in the orthopedic stomatology for the manufacture of dental prostheses. Due to its high stability and good conductivity, Pd is used in many electrical devices, which makes it an alternative to gold in galvanic elements. Palladium is also used to create drugs for cancer treatment [1][2][3][4].
In the chemical industry, palladium is a component of the scheme for the manufacture of nitric acid and petroleum. It is widely used in the production of synthetic polymers, in particular rubber and nylon. Alloys that contain palladium are used during fuel cells creation [4][5][6].
Significant use of palladium in the chemical industry and automotive catalysts has led to an increase in the concentration of this metal in the objects of the environment. Due to the significant growth of palladium objects, there is a need for the development and improvement of methods for its determination. e control of the content of palladium in mineral raw materials, industrial products, and waste production is an important task of analytical chemistry [4,6]. e spectrophotometric methods are widely used to determine the palladium. Palladium provides intense-colored complexes with hydrazones, oximes, thiosemicarbazones, dithiocarbonates, thiazolilazo, nitroso dyes, and others [2,7,8].
However, existing methods have a number of limitations, such as temperature control, a narrow range of pH, and low selectivity. erefore, it is necessary to find new selective, sensitive, available reagents that would allow the determination of the content of palladium in complex objects. In analytical chemistry, organic reagents containing the functional-analytical group NH...X (X � S, N, O) are successfully used for element determination. ese reagents are azolidones [9]. is class of compounds can be used not only as reagents for spectrophotometric determination of elements, but they can also be considered as analytes, since there is a problem of their determination in biological substrates because of their biological activity.
Rhodanine is used for noble and heavy metals identification since it belongs to the class of azolidones and its derivatives. We can achieve a very high sensitivity determination (ε = 10 6 L·mol −1 ·cm −1 ) of platinoids [10][11][12] using azorhodanine, azothiopropiorhodanine, and their derivatives as a reagent for the platinum metals determination in a form other than chloride.
e obtained results were characterized by good metrological characteristics; therefore, we continue to synthesize new reagents of the class of azolidones and investigate them as analytical reagents. is work is devoted to the research of palladium ion interaction with reagent 4-(N′-(4-imino-2oxo-thiazolidine-5-ylidene)-hydrazino)-benzoic acid (p-ITYBA), which is a representative of the azolidones and was synthesized for the first time by our research group [30].

Materials and Methods
2.1. Apparatus. All spectrophotometric measurements were performed using a computerized spectrophotometer ULAB 108-UV with 1.0 cm quartz cuvettes. Acidity of the solution was measured by using a pH-meter model pH-150N equipped with a combination glass electrode. A CLA-03 oscillopolarograph and computerized polarographic setup equipped with linear potential scan in three-electrode cell (dropping mercury electrode: indicator electrode; saturated calomel electrode: reference electrode; and platinum: auxiliary electrode) were used for voltammetric measurements. 1 H NMR spectra were obtained using a spectrometer Varian Mercury VX-400 (400 MHz) in DMSO-d 6 with tetramethylsilane as reference.

Reagents.
All reagents used were of analytical grade.
A stock solution of palladium(II) was prepared by dissolving this metal (99.999% purity) in a mixture of concentrated nitric and hydrochloric acids (1 : 3, v/v) by heating on a sand bath. A standard working solution of Pd(II) was prepared by diluting the stock solution with 1.0 mol·L −1 HCl.
Solutions of NaOH (4.0 mol·L −1 ) and NaCl (2.0 mol·L −1 ) were prepared by dissolving 39.997 g and 29.22 g, respectively, in distilled water in a 250.0 mL volumetric flask. e solution of HCl was prepared by dilution of concentrated acid. e universal buffer solution (UBS) was made by mixing solutions of acetic, boric, and phosphoric acids. 2− , and Sal − ) were prepared by dissolving the appropriate sodium salts in distilled water.

General Procedure for Determination of Pd(II) Ions.
To sample solutions containing 0.64-10.64 µg·mL −1 of Pd(II) in a 25.0 mL volumetric flask, 0.75 mL of 2.5 × 10 −3 mol·L −1 p-ITYBA, 1.25 mL of 2.0 mol·L −1 NaCl, 1.0 mL of 0.5 mol·L −1 UBS, and distilled water (∼20 mL) were added. en pH was adjusted by NaOH to pH = 7.0, and distilled water was added to complete volume. e absorbance of the Pd(II)-p-ITYBA complex at 450 nm against a reagent blank was measured.

Effect of Reagent Concentration.
Various volume (0.125-12.5 mL) of 1.0 × 10 −3 mol·L −1 p-ITYBA was added to the solution containing 0.5 mL of 2.5 × 10 −3 mol·L −1 Pd(II). en, 1.25 mL of 2.0 mol·L −1 NaCl, 1.0 mL of 0.5 mol·L −1 UBS, and distilled water∼20 mL were added. e pH was adjusted to pH = 7.0 by NaOH and diluted to a volume of 25.0 mL with water. e absorbance was measured in a 1.0 cm cell at 450 nm.

Determination of the Composition of the Pd(II)-p-ITYBA Complex.
e mole-ratio method was performed in the following way: into a series of 25.0 mL volumetric flasks, an aliquot (0.125-6.25 mL) of 1.0 × 10 −3 mol·L −1 Pd(II) solution, p-ITYBA solution with fixed concentration (0.5 mL of 2.5 × 10 −3 mol·L −1 ), 1.25 mL of 2.0 mol·L −1 NaCl, 1.0 mL of 0.5 mol·L −1 UBS, and distilled water ∼ 20 mL were added. e pH was adjusted to pH = 7.0 by adding NaOH and diluted up to the mark with water. en, the absorbance at 450 nm was measured.
In Job's method of continuous variation, the equimolar solutions of Pd(II) and p-ITYBA (1.0 × 10 −3 mol·L −1 ) were used. en, 1.25 mL of 2.0 mol·L −1 NaCl, 1.0 mL of 0.5 mol·L −1 UBS, and distilled water (∼20 mL) were added, and the pH was adjusted to 7.0 by NaOH. e absorbance values were recorded at 450 nm.

Procedure of Catalyst Preparation for Pd(II)
Determination.
e sample of catalyst was dissolved in a mixture of concentrated nitric and hydrochloric acid (1 : 3, v/ v) by heating on a sand bath with further decantation. Pd(II) ions were converted to the chloride form by evaporation of the resulting solution to wet salts and the addition of 10.0 mL of concentrated hydrochloric acid until the nitrogen oxides were lost [31]. After that, the resulting solution was transferred quantitatively into a volumetric flask of 50.0 mL and brought to the mark with distilled water. e working solution was prepared by dilution of the stock 100 times. Suitable aliquots of this solution were used for the determination of palladium according to the recommended procedure.
is reagent is a crystalline powder of yellow-sand color. e melting point is 543 K. It is poorly soluble in water and ethanol, but is well soluble in dimethylformamide and dimethyl sulfoxide.
e maximum absorption of a reagent does not depend on the acidity of the medium and corresponds to a wavelength of 390 nm. e imino group is hydrolyzed in a highly alkaline medium (pH > 11.0). Our previous work [30] provides more details about the spectral characteristics of this reagent.

Investigation of the Formation of the Pd(II)-p-ITYBA
Complex. It was found that Pd(II) ions form a yellow-orange complex compound with p-ITYBA.
e electronic absorption spectra of dye solutions and the complex compound Pd(II)-p-ITYBA are shown in Figure 2. At a pH 1.0-10.0, there is a decrease in the maximum of absorption of the reagent when palladium(II) was added to p-ITYBA.
is indicates that there is an interaction between palladium(II) ions and p-ITYBA. At higher values of pH, this effect is not observed. We can also see the formation of the shoulder of absorption in the range of wavelengths 430-480 nm. All further studies of complex formation were carried out at a wavelength of 450 nm, where the maximum difference between the absorption of the complex and the reagent is observed. Figure 3 shows the results of studies of the pH effect on the maximum yield of colored complex compound Pd(II)-p-ITYBA against sodium chloride background. e formation of complex Pd(II) with p-ITYBA is observed in a wide range of pH. e maximum absorbance of the compound Pd(II)-p-ITYBA is achieved at pH = 7.0.

Effect of Heating Time.
e results of studies have shown that the complexation of palladium(II) ions with p-ITYBA occurs at room temperature (∼291-296 K) immediately after establishing the acidity of the medium. e heating of solutions in a boiling water bath (∼371 K) causes a decrease in the absorption of solutions, but the complex compound Pd(II)-p-ITYBA is not completely destroyed even when heated for 60 min (Figure 4). Absorption of the obtained product remains constant for more than 48 h.

Stoichiometric Ratio of Pd(II)-p-ITYBA.
e ratio of components in the system Pd(II)-p-ITYBA was determined by two methods: continuous variations (Job's method) and the mole-ratio method. ese methods confirm the formation of a complex with the ratio of components Pd(II) : p-ITYBA = 1 : 1. It was calculated that the formal stability constant of the complex relative to the palladium aqua hydroxychloride complex is 1.96 × 10 5 . e 3-fold excess of the reagent is optimal for the maximum yield of the colored compound of Pd(II) with p-ITYBA.

Calibration Curve.
e analytical signal of palladium(II) determination using p-ITYBA linearly depends on the concentration of metal in the solution and is described by the equation ∆A = (0.009 ± 0.001) + (0.0416 ± 0.0004) × C, where C is the concentration of Pd(II) in µg·mL −1 (the correlation coefficient is 0.9997). From the calibration curve, it was found that Beer's law is obeyed over the Pd(II) concentration range of 0.64 to 10.64 µg·mL −1 . e detection limit is 0.23 µg·mL −1 . e molar absorptivity is 4.30 × 10 3 L·mol −1 ·cm −1 .

Effect of Foreign Ions.
As shown in the literature [7,8,20,[32][33][34][35][36][37], most spectrophotometric methods of palladium determination are characterized by not very high selectivity towards such cations as Ni(II), Co(II), Cu(II), Fe(III), Fe(II), Ag(I), REE (rare earth elements), platinum metals, and others. We investigated the effect of platinum metals and heavy and alkaline earth metal ions that can be present in the studied samples or can be introduced during dissolution. Also typical masking anions (fluoride, citrate, tartrate, and salicylate ions) were investigated for the possibility of palladium(II) determination using p-ITYBA (see Table 1).
It  determination.
e most influence of the metals of the platinum group can be explained by the similar electronic structure of the outer electronic levels of platinum metals, which compete with the palladium ions in the process of formation of a colored complex compound. e anions that were studied do not affect the determination of Pd(II), even when present in 100-fold excess, so they can be used to mask certain metals to improve selectivity.
is method has higher selectivity that most of the earlier reported spectrophotometric methods ( Table 2) Journal of Chemistry 5 towards many cations and anions, such as: Rh(III), Ru(IV), Ga(III), Au(III), Cu(II), Co(II), Ni(II), and others.

Determination of Pd(II) in Model
Solutions. e correctness of spectrophotometric methods for the Pd(II) determination using p-ITYBA was verified by analyzing model solutions in the "added-found" method. e results are shown in Table 3.
e main problem of photometric methods for the palladium determination is their insufficient selectivity towards platinum metals. In order to show a possibility of palladium determination using p-ITYBA in the presence of platinum metals ions, we added them into the model solutions.
e concentration of palladium in the studied solutions was determined by a salibration curve.
As can be seen from the obtained results, there are good reproducibility and accuracy (no systematic error); therefore, the developed method for the palladium determination using p-ITYBA can be effectively used in the analysis of various industrial and natural objects.

Determination of Palladium in a Catalyst.
Palladium and its alloys are often used as catalysts in chemical reactions, in particular in the reactions of hydration and the production of pharmaceutical preparations [38]. Palladium exhibits better catalytic properties in the processes of hydrogenation of organic substances than nickel [39]. e developed method for spectrophotometric determination of Pd(II) by means of p-ITYBA was used to determine the contents of the palladium in the palladiumrhodium catalyst on a ceramic basis. As the amount of rhodium in this catalyst is about 0.1% [31], it did not interfere with the determination. ere was no need to use masking agents, since rhodium and silicate ions do not interfere with the determination of palladium in large quantities.
e results of the analysis are presented in Table 4.
As can be seen from Table 4, the results of palladium determination using the spectrophotometric method are well consistent with its content determined by the voltammetric method per reduction peak of its ammonium complexes [7]. e value of the relative standard deviation does not exceed the error in spectrophotometry; therefore, the presence of ions in the solution does not affect the results of the analysis. e developed method, which is described in this paper, has better selectivity when compared to some spectrophotometric methods shown in Table 2. is method is rapid, sufficiently sensitive, and does not need extraction or use of masking agents.

Conclusions
e proposed method of spectrophotometric determination of Pd(II) using p-ITYBA is simple, rapid, reproducible, sufficiently sensitive, selective, and inexpensive. is method does not require the use of heating and extraction, which greatly simplifies the determination. ese advantages allow using the developed method in routine analysis for Pd(II) determination in various objects.