Anal. Chem. 2004, 76, 7354-7359
Lipophilic Lanthanide Tris(β-diketonate) Complexes as an Ionophore for Cl- Anion-Selective Electrodes Rakesh Kumar Mahajan,*,† Inderpreet Kaur,† Ravneet Kaur,† Aki Onimaru,‡ Satoshi Shinoda,‡ and Hiroshi Tsukube*,‡
Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India, and Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyosi-ku, Osaka 558-8585, Japan
A series of novel anion-selective electrodes were developed by incorporating lipophilic lanthanide tris(β-diketonates) into plasticized poly(vinyl chloride) membranes. The new electrodes exhibited high selectivity toward Clanion in the concentration range of Cl- anion between 1.0 × 10-5 and 1.0 × 10-1 mol/L with near-Nernstian slopes and practically low detection limits. They offered nonHofmeister anion selectivity, and interestingly discriminated Cl- anion from NO3-, ClO4-, and other anions. Since the employed lanthanide tris(β-diketonates) were confirmed to form 1:1 highly coordinated complexes with Cl- anion, the observed high selectivity for Cl- anion was attributed to the characteristics of lanthanide coordination chemistry. All the prepared sensors worked well at neutral pH with quite short response time, 6, which may be due to the simultaneous response of electrodes to Cl- and hydroxide in basic media. Although all the employed complexes were thought to be decomposed by protonation of β-diketonate ligands at low pH and hydrolyzed by hydroxide anion at high pH, the present type of Cl- anion-selective electrodes operated well in the neutral solutions. Since the pH values of most sample solutions lay in the functional pH ranges of the sensors, no pH adjustment was usually required. We used phosphate buffer solution to adjust pH as 5.8 in the cases of quantitative experiments (see Figure 3 and Tables 1 and 2), which Analytical Chemistry, Vol. 76, No. 24, December 15, 2004
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Figure 5. Variation of electrode potential of chloride sensors with pH at 1.0 × 10-1M Cl-.
Figure 6. Effect of plasticizers on Cl- ISEs based on Eu complex II.
was prepared by mixing 4.0 mL of 0.2 M Na2HPO4 and 46.0 mL of 0.2 M NaH2PO4 and diluting to 100 mL. Effect of Plasticizer on Electrode Performance. Since the nature of the plasticizer influences the dielectric property of the membrane phase, mobility of the ionophore in the PVC matrix, its selection is one of the most important tasks in designing a sensitive and selective anion-selective electrode. The responses of the chloride sensor based on Eu complex II using five plasticizers having different polarities were investigated. The plasticizers examined were DOS, DOA, DOP, TBP, and DBP, and their natures significantly affected the sensor performances of PVC membranes such as slope, linear concentration range, and detection limit (Figure 6). Among them, DOP offered the most excellent response to Cl- anion with a slope of -57.5 mV/decade over a wide concentration range of 1.0 × 10-1-1.0 × 10-5 mol/L and a lower detection limit of 1.1 × 10-5 mol/L. Selectivity of Lanthanide Tris(β-diketonate)-Based Electrode. The relative response of the lanthanide tris(β-diketonate)based electrode for primary ion over other ions present in the 7358 Analytical Chemistry, Vol. 76, No. 24, December 15, 2004
solution was evaluated in terms of potentiometric selectivity Pot ). In the present work, potentiometric selectivcoefficients (KA,B ity coefficients over interfering ions were determined using the fixed interference method. According to this method, potentioPot metric selectivity coefficients (KCl,B ) can be evaluated by carrying out potential measurements on solutions containing fixed concentration of interfering ions (i.e., 1.0 × 10-2 mol/L) and varying concentrations of Cl- anion. The logarithmic selectivity Pot coefficients, log KA,B obtained from potential data of various ClISEs are summarized in Table 2. Among the employed electrodes, E(I)-E(III) exhibited higher selectivity for Cl- anion than other common anions such as NO2-, F-, HCO3-, CH3COO-, SO42-, citrate, tartrate, perchlorate, and nitrate, and other lipophilic anions had negligible impact on the Nernstian performance of these electrodes. Since most of the previously reported Cl- anionselective electrodes exhibited comparable selectivity coefficients for Br-, I-, and SCN- anions, these anions often caused interference when present at high concentration of 1.0 × 10-2 mol/L. Pot values of our systems for these anions are Interestingly, log KA,B very much lower in the case of E(II). When the electrodes E(I)-E(VII) were compared, their potentiometric properties were largely dependent on the natures of the lanthanide complexes. Among them, E(II) based on europium tris(β-diketonate) showed the highest selectivity toward Cl- anion. The Eu complex containing electron-withdrawing fluorinated ligands was suggested to offer high Lewis acidity. Its potentiometric selectivity sequence decreases in the order of Cl> SCN- > Br- > N3- > CH3COO- > I- > NO2- > F- ∼ ClO4> HCO3- > NO3- > SO42- > CO32- ∼ tartrate > citrate. As frequently reported, the electrode selectivity is usually governed by anion lipophilicity, according to the Hofmeister series: ClO4> I- > SCN- > N3- > NO3- > Br- > NO2- > HCO3- > Cl- > CH3CO2- > F- > SO42- > CO32-. The observed selectivity pattern with lanthanide tris(β-diketonates) is significantly different from this Hofmeister series. Since the lanthanide tris(β-diketonates) were reported to form more stable complexes with Cl- anion than Br-, I-, and ClO4- anions,25 the highly coordinated complexation properties of lanthanide tris(β-diketonates) could offer unique anion selectivity. The observed anion selectivity profiles may be determined by a combination of lanthanide coordinating ability and hydrophilicity of the targeted anion, because CH3COO- and F- anions were well recognized as potential ligands toward the lanthanide center. To investigate the influence of lipophilic ionic additives on the selectivity behavior of the chloride-selective electrode, three different membranes A, B, and C were prepared, which included 0.3 wt % NaTPB, 0.3 wt % HTAB, and no additive, respectively. The selectivity coefficients and other parameters of the three chloride sensors are given in Table 3. As is clear, no improvement was observed in the slopes and selectivity coefficient values when NaTPB was removed or replaced by HTAB. Taking into account that anionic impurities are always present in plasticized PVC membranes and therefore the membrane C incorporates in fact a small amount of ionic impurities, the addition of the lipophilic ionic sites does not have a large effect.33,34 We can conclude that the presence of HTAB or NaTPB has no significant effect on electrode (30) Chaniotakis, N. A.; Chasser, A. M.; Meyerhoff, M. E. Anal. Chem. 1988, 60, 185. (31) Simon, W.; Wuthier, U. Mikrochim. Acta 1986, III, 225.
Table 3. Selectivity Coefficients and Other Parameters of Chloride Sensors Based on Eu-Complex (II) Incorporating Different Additives
membranea
F-
Br-
A B C
-2.94 -2.67 -2.78
-1.19 -0.96 -1.05
a
SO4
Log KPot Cl,B ClO4I-
NO2-
CO32-
N3-
slope (mV/decade)
linear range (mol/L)
-3.96 -3.67 -3.79
-2.94 -2.53 -2.79
-2.91 -2.49 -2.65
-3.97 -3.79 -3.85
-2.20 -2.16 -2.12
-57.50 -55.61 -56.48
1.0 × 10-1-1.0 × 10-5 1.0 × 10-1-5.0 × 10-5 1.0 × 10-1-5.0 × 10-5
2-
-2.71 -2.58 -2.64
(A) 0.3 wt % NaTPB; (B) 0.3 wt % HTAB; (C) no additive.
Table 4. Comparison of Selectivity Coefficients of Different Chloride Ion-Selective Electrodes Log KPot Cl,B
chloride carrier tridodecylmethylammonium chloride diethylhexyltin acetate chloro(5,10,15,20-tetraphenylporphyrinato)manganese(III) [9]-mercurborand-3 (MC3) Eu complex II
ref
ClO4-, +4.6; SCN-, +3.0; I-, +2.8; Br-, +0.6. NO3-, -0.9; HCO3-, -1.9; Br-, +0.1; SO42-,-1.3; I-, +0.8; SCN-, +0.6; OAc-, -0.1 SCN-, +3.4; I-, +2.4; NO3-, +1.2; Br-, +0.9; NO2-, +0.7, ClO4-, -3.6; SCN-,
-,
-,
I-,
30 31 32
Br-,
-0.16; ClO4 -2.79; NO3 -2.71; +2.34; +1.04 SCN-, -0.27; I-, -2.71; NO3-, -3.34; Br-, -1.19; NO2-, -2.91, ClO4-, -2.94.
performance, suggesting the complexes I-VII studied here behaved as neutral carriers. A comparison with PVC-based chloride sensors reported in the literature is made (Table 4). Since the best chloride-selective electrode E(II) based on Eu complex II is found to be superior in terms of selectivity than those already reported in the literature, great scope exists in developing anion sensors based on lanthanide tris(β-diketonates). To access the application of the electrode E(II), the experiments were conducted to measure Cl- anion concentration of the water sample containing a known concentration of Cl- anion (Table 5). Although the amount of Cl- anion recovered was somewhat less than the added amount, the sensor electrode E(II) was confirmed to detect Cl- anion very efficiently with 99% recovery. CONCLUSION We first demonstrated that lanthanide tris(β-diketonates) acted as Cl- anion-specific ionophores in the PVC membrane electrode systems. Their electrodes were readily prepared and exhibited high selectivity, excellent sensitivity, wide range, low detection limits, rapid response, and practical stability as Cl- anion-selective sensors. Although they had potentiometric selectivity coefficients depending on the nature of the lanthanide tris(β-diketonate) ionophores, europium tris(β-diketonate) II offered the best Cl(32) Simon, W.; Kondo, Y.; Bu ¨ hrer, T.; Seiler, K.; Fromter, E. Pflu ¨ gers. Arch. 1989, 414, 663. (33) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391. (34) Shamsipur, M.; Khayatian, G.; Tangestaninejad, S. Electroanalysis 1999, 11, 1340.
19 present work
Table 5. Determination of Chloride Anion Concentration in Synthetic Sample Using Chloride Sensor E(II) amount of Cl- (mmol/L) added
recovered
% recovery
80 50 25 5 2
79.1 48.2 24.2 4.7 1.8
98.9 96.4 96.8 94.0 90.0
ion-selective electrode performance. The present type of electrodes offered enhanced selectivity for Cl- anion over Br-, I-, ClO4-, NO3-, and SCN- anions, via non-Hofmeister selectivity. The sensors were successfully used to measure Cl- anion of the water samples containing known concentrations of chloride with high percentage recovery. Since the lanthanide tris(β-diketonates) have wide structural variations, new series of anion-selective electrodes can be developed after structural optimization. ACKNOWLEDGMENT We are thankful to the Council of Scientific and Industrial Research, New Delhi, India and also Ministry of Education, Culture, Sports, Science and Technology, Japan for financial supports. Received for review September 24, 2004.
February
8,
2004.
Accepted
AC0497858
Analytical Chemistry, Vol. 76, No. 24, December 15, 2004
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