稀有金属(英文版) 2015,34(08),595-599

收稿日期：25 January 2013

基金：financially supported by the Natural Science Foundation of Shandong Province (No. Y2008B26);

Determination of trace gadolinium by catalytic kinetic fluorimetry

Juan-Ping Chen Yu-Ying Liu Ying-Jie Gao

School of Chemical Engineering, Shandong University of Technology

Abstract：

There was a significant catalytic effect of trace Gd(III) ions on the oxidative reaction of potassium persulfate with Saffron T in the acetic acid–sodium acetate buffer solution. Thus, a catalytic kinetic fluorimetry method for the determination of trace Gd(III) ions was established. The factors such as acidity, concentration of reagents, reaction time, and temperature as well as influence of coexisting ions were discussed. The optimum reaction conditions were established. The apparent rate constant and apparent activation energy of the reaction were determined. The linear range is 0.02–0.10 lgáml-1,and the detection limit is 7.27 9 10-4lgáml-1. This method was used for the determination of gadolinium in the samples of lanthanum acetate with RSD of 0.9 %–3.1 %.

Keyword：

Gadolinium; Catalytic kinetic fluorimetry; Saffron T; Potassium persulfate;

Author： Yu-Ying Liu,e-mail: lyy999999@163.com;

Received： 25 January 2013

1 Introduction

Rare earth elements represent a group of elements from La to Lu, which show similar physical and chemical properties. They play an important role in various fields ranging from geology to electronics, which demands their determination from percentage level to ultra-trace level [1]. Gadolinium, at the middle of the series, is a good proxy for the lanthanides. Its electronic configuration is [Xe] 4f⁷5d¹6s². Given the large energy difference between the 5d or 6s electronic levels and the 4f level, gadolinium, like other lanthanides, is a highly reactive metal [2]. It slowly dissolves in water to produce hydrogen and Gd^3?, and is relatively stable in dry air. This element can rarely be found in nature, as it occurs in very small amounts in the mineral gadolinite. Gadolinium exhibits ferromagnetic properties below room temperature. It is chiefly used in petroleum industries to produce catalyzes, in nuclear research as neutron absorber, in steel industries, and polishing glassware. Its oxides are very important and are widely used in the preparation of optical glasses, glass fibers, radio-contrast agents, gasoline-cracking catalysts, polishing compounds, the construction of carbon arcs and in iron and steel industries to remove sulfur, carbon, and other electronegative elements.

Gadolinium detection has become necessary lately, because of the increasing utilization of gadolinium compounds in industry. A number of techniques such as inductively coupled plasma-mass spectrometry [3–5], inductively coupled plasma-mass and atomic emission spectroscopy [6–8], electron spin resonance [9], neutron activation analysis [10], X-ray fluorescence spectrometry [11], and phosphorescence [12] are used for its determination. Except the methods mentioned above, fluorescence spectrometry becomes more and more popular in the determination of gadolinium [13–15] because of its high sensitivity and selectivity, and the instrument used is uncomplicated.

In this paper, a new fluorescence system of Gd(III)– Saffron T–K₂S₂O₈for the determination of gadolinium was established. And this methodology is simple and fast and can be directly applied to the determination of trace gadolinium in the samples of lanthanum acetate with satisfying results.

2 Experimental

2.1 Apparatus

Fluorescence measurements were made on a Shimadzu RF5301 fluorescence spectrophotometer using a slit width of 1.5 nm for both excitation and emission. The excitation and emission wavelengths were set at 560 and 574 nm, respectively. All the solid reagents were weighed on an electronic balance made in Shanghai, China. All the solutions used were heated at 65 °C for 10 min with a thermostatic bath (Jiangsu, China).

2.2 Reagents

All reagents were prepared from analytical reagent grade chemicals. The gadolinium standard stock solution (1 mg?ml^-1) was prepared by dissolving 0.7176 g of Gd(NO₃)₃?6H₂O in a 250-ml volumetric flask and diluting to the mark with water. Working solution of 1 lg?ml^-1was prepared from the stock solution, Saffron T solution: 1.0 9 10^-4mol?L^-1, various oxidants with all the concentration of 0.01 mol?L^-1are as follows: potassium persulfate, potassium dichromate, ferric chloride, hydrogen peroxide, potassium bromate, and acetic acid–sodium acetate buffer solution.

2.3 Procedure

Solutions adding into a 25-ml calibrated tube is as the following order: 2.0 ml of 1.0 9 10^-4mol?L^-1Saffron T solution, 2.0 ml of acetic acid–sodium acetate buffer solution, 1.5 ml of 0.01 mol?L^-1potassium persulfate solution, and moderate gadolinium standard working solution. Dilute the mixture to the mark with distilled water, shake and heat for 10 min at 65 °C in a thermostatic bath. Then the solution was cooled by tap water for 10 min. Meanwhile, a blank experiment without the gadolinium solution was also made under the same conditions. The fluorescence intensity of catalyst system (F) and non-catalyst system (F₀) at emission wavelength 574 nm in a 1 cm quartz cell, keeping the excitation wavelength at 560 nm. Then calculate the difference DF(DF = F₀- F).

3 Results and discussion

3.1 Excitation and fluorescence spectra

The excitation and emission spectra of the fluorescence systems are shown in Fig. 1. Saffron T can emit a fluorescence spectrum with strong fluorescence intensity. Its excitation and emission wavelengths were 560 and 574 nm, respectively. And the fluorescence intensity decreases with the addition of the oxidant potassium persulfate solution. Moreover, trace amount of gadolinium ions can catalyze the redox reaction between the Saffron T and potassium persulfate.

Fig.2 Effect of p H on DF. Conditions: Gd(III), 1 lgμml-1; Saffron T, 8.0 9 10-6molμL-1; K2S2O8, 6.0 9 10-4molμL-1

Fig.1 Excitation spectra a (kex= 560.0 nm) and emission spectra b (kem= 574.0 nm). Conditions: HAc–Na Ac, p H 5.0; Saffron T, 8.0 9 10-6molμL-1; K2S2O8, 6.0 9 10-4molμL-1; Gd(III), 1 lgμml-1

3.2 Effect of p H

The p H of the solution has a great influence on the fluorescence intensity of the Gd(III)–Saffron T–K₂S₂O₈system. The p H of the solutions was adjusted using dilute HCl and/or Na OH to values between 2 and 10 (Fig. 2). The experimental results show that the highest fluorescence intensity signals are observed at a p H of approximately 5. Thus, the acetic acid–sodium acetate buffer solution was employed as optimum for all subsequent work.

Fig.3 Effect of Saffron T concentration on DF. Conditions: Na Ac– HAc, p H 5.0; K2S2O8, 6.0 9 10-4molμL-1; Gd(III), 1 lgμml-1

3.3 Effect of fluorescent reagent Saffron T

In order to determine the optimum concentration of the fluorescent reagent Saffron T, a series of solutions containing various concentrations of Saffron T (from 1.5 9 10^-6to 13.5 9 10^-6mol?L^-1) were prepared to investigate the effect on the DF. From Fig. 3, it can be seen that the catalysis of gadolinium is the most obvious when the concentration of Saffron T is 8.0 9 10^-6mol?L^-1, once it exceeds this amount, DF would decreased slowly. So 8.0 9 10^-6mol?L^-1is favorable.

3.4 Effect of oxidants

Figure 4 shows the influence of the various oxidants on the fluorescence intensity. And there is a significant effect of the oxidants on the system. By comparing the five fluorescence spectra, it can be drawn that potassium bromate is the last choice, and with the addition of it, great change of the fluorescence spectra shape would take place. While there is almost the same effect of ferric chloride and hydrogen peroxide on the fluorescence intensity and to our disappointment, the catalysis of the rare earth element gadolinium has little effect on the system with these two oxidants. In contrast with those oxidants mentioned above, both potassium persulfate and potassium dichromate have the better results relatively. The catalytic effect of gadolinium is notable with these two oxidants, but there are some changes of the spectra shape when potassium dichromate is selected, what is more, its amount is hard to control. In conclusion, potassium persulfate was chosen in the experiment. Meanwhile, the optimum concentration of this oxidant was studied from 2 9 10^-4to 1.6 9 10^-3mol?L^-1. And it shows that 6.0 9 10^-4mol?L^-1is the best choice.

Fig.4 Effect of oxidants on DF: a K2S2O8, b K2Cr2O7, c Fe Cl3, d H2O2, and e KBr O3. Conditions: all the oxidants being of 8 9 10-4molμL-1, Na Ac–HAc, p H 5.0; Gd(III), 1 lgμml-1; Saffron T, 8.0 9 10-6molμL-1

3.5 Effect of temperature and the determination of apparent activation energy

The influence of temperature on the reaction was examined over the range of 25–80 °C. The results in Fig. 5 show that the reaction rate accelerates with the temperature increasing and when it reaches 60 °C, the fluorescence intensity tends to be stable. Thus, the reaction system is heated at 65 °C in water bath to ensure it reacted completely. And the calibration curve obeys the following linear regression equation over the range of 25–60 °C: ln DF = -1.4693 9 10³9 1/t ? 5.1369, and the regression coefficient is 0.9940. According to the Arrhenius equation, the activation energy of the reaction calculated from the equation mentioned above was 12.22 k J?mol^-1.

3.6 Effect of time and the determination of the apparent reaction rate constant

Under the working conditions selected, the relationship between DF and the reaction time is represented in Fig. 6.

Fig.5 Effect of temperature on DF. Conditions: Na Ac–HAc, p H 5.0; Saffron T, 8.0 9 10-6molμL-1; K2S2O8, 6.0 9 10-4molμL-1;Gd(III), 1 lgμml-1

Fig.6 Effect of reaction time on DF. Conditions: Na Ac–HAc, p H 5.0; Saffron T, 8.0 9 10-6molμL-1, K2S2O8, 6.0 9 10-4molμL-1,Gd(III), 1 lgμml-1

And DF is directly proportional to t over the range of 0–10 min. The reaction is heated for 10 min to make it react completely. The linear regression equation is DF = 1.2085t+30.462.

The correlation coefficient c = 0.9937, the order of the reaction is zero, and the apparent rate constant k = DF/ t = 0.02 s-1.

3.7 Effect of foreign ions

In order to investigate its selectivity and potential analytical application for Gd(III), the influence of various ions on the determination of Gd(III) with the proposed method was studied. The results listed in Table 1 are the tolerated ratio of interfering ions to Gd(III) under which the foreign ions have no interference (in a deviation less than ±5 %), the interference cannot be negligible over this level.

Table 1 Effect of foreign ions on the determination of 0.08 lgμml^-1Gd(III)  下载原图

Table 1 Effect of foreign ions on the determination of 0.08 lgμml^-1Gd(III)

Fig.7 Working curve. Conditions: Na Ac–HAc, p H 5.0; Saffron T, 8.0 9 10-6molμL-1; K2S2O8, 6.0 9 10-4molμL-1

Table 2 Analytical results of samples  下载原图

Table 2 Analytical results of samples

3.8 Calibration graph

The calibration graph for the determination of Gd(III) in Fig. 7 was constructed under the optimal conditions (Dk = 1.5 nm). Excellent linearity equation DF = 144.43q_Gd(III)+12.237 (c = 0.9858, and SD = 0.035 for 11 times measurements) is obtained over the range 0.02–0.10 lgμml^-1Gd(III). The detection limit is down to 7.27 9 10^-4lgμml^-1.

4 Sample analysis

The proposed method is successfully applied to the determination of gadolinium in lanthanum acetate samples. The samples were dissolved by hydrochloric acid (volume ratio with water is 1:1) in a baker of 50 ml and were heated to make sure they were dissolved completely. The mixture is transferred into a flask of 50 ml when it is cooled down. Then it is determined and the results are summarized in Table 2.

5 Conclusion

A new catalytic fluorimetry method for the determination of trace Gd(III) ions was established on the basis of the significant catalytic effect of trace Gd(III) on the oxidative reaction of persulfate with Saffron T in the acidic medium. The accuracy and selectivity of the method were tested to be satisfied. And the system can be applied to determine the amount of gadolinium in lanthanum acetate samples without separation.The linear range is 0.02–0.10 lgμml^-1 and the detection limit is 7.27×10^-4 lgμml^-1.