Novel thiocyanate-selective membrane sensors based on di-, tetra-, and hexa-imidepyridine ionophores

Analytica Chimica Acta 482 (2003) 9–18

Novel thiocyanate-selective membrane sensors based on di-, tetra-, and hexa-imidepyridine ionophores Saad S.M. Hassan a,∗ , M.H. Abou Ghalia b , Abdel-Galil E. Amr b , Ayman H.K. Mohamed a a

Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt b National Research Center (NRC), Dokki, Cairo, Egypt

Received 15 October 2002; received in revised form 18 December 2002; accepted 27 January 2003

Abstract Potentiometric thiocyanate-selective sensors based on the use of three synthesized di-, tetra-, and hexa-imidepyridine derivatives as novel anionic neutral ionophores in plasticized poly(vinyl chloride) (PVC) membranes are described. The sensors exhibit significantly enhanced response towards thiocyanate ions over the concentration range 5×10−6 to 1.0×10−2 M with a lower detection limit of 0.3 ␮g ml−1 and slopes ranging from −55.6 to −58.3 mV per decade. Fast and stable response, good reproducibility, long-term stability, applicability over a wide pH range (2–8) and high selectivity for SCN− ion in the presence of 18 common anions are demonstrated. The sensors are used for direct potentiometric measurements of thiocyanate ions over the concentration range 0.2–580 ␮g ml−1 and for monitoring sequential titration of some metal ions (e.g. Ag+ , Tl+ , Cu2+ , Pb2+ ) in binary and ternary mixtures. Sequential binding of these metal ions with SCN− ensures share stepwise titration curves with consecutive end point breaks at the equivalent points. Recoveries of 98.5–99.1 ± 0.3% are obtained for metal ion concentrations of 0.06–4 mg ml−1 . © 2003 Elsevier Science B.V. All rights reserved. Keywords: Thiocyanate; Di-, tetra-, and hexa-imidepyridine ionophores; Poly(vinyl chloride) membranes; Potentiometric sensors; Sequential determination; Metal ions

1. Introduction Thiocyanate sensors in common use are those commercially available incorporating solid-state AgSCN membranes [1]. These membranes are ionic conductors for silver ions; and hence, suffer sever interference from species that form silver complexes or insoluble silver salts such as CN− , I− , Br− , Cl− , S2 O3 2− and S2− [1]. Liquid and solvent polymer membrane sensors based on the use of quaternary ∗ Corresponding author. Tel.: +20-2-682291; fax: +20-2-682291. E-mail address: [email protected] (S.S.M. Hassan).

ammonium or phosphonium salts have been suggested [2–4]. However, many of these sensors are not sensitive enough for measuring thiocyanate ions at levels <10−3 mol l−1 , have a short life-time, suffer from serious interference by most common ions and display a selectivity order consistent with the sequence expected from Hoffmeister series depending on the lipophilicity and hydration energy of the ions [2–4]. Thiocyanate polymeric membrane sensors consisting of a neutral carrier such as cyclopalladated amine [5], silver thiourea derivatives [6], copper diphenyl-dithiocarbazone complex [7], metal porphyrins [8,9], and metal phthalocyanines [10] have been described. Although these sensors exhibit an

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anti-Hoffmeister selectivity order, they suffer from strong interference by I− , CN− , NO2 − , IO4 − , ClO4 − , Br− , and N3 − . The recognition of anionic guest species by synthetic ligands is a rapid growing research area [11,12]. During the past three decades, few classes of ligands capable of anion binding have been developed. In spite of the fact that amide hydrogen bonding plays an important role in anion binding and recognition in natural protein binding sites [13], relatively little work has been reported on the use of synthetic amide-based anion receptors. Some neutral bis-thiourea ionophores have been shown to form complexes through hydrogen bonding with monoanions and successfully used for developing a chloride-selective solvent polymeric membrane sensor [14]. Several pyrrole-containing macrocycles have been shown to bind Cl− due to the formation of hydrogen bonds between the amide functional group and Cl− [15]. Neutral thiourea-derivatized p-t-butylcalix [4,6] arenes are known to bind halide anions exclusively through hydrogen bonding [16,17]. In this work, thiocyanate poly(vinyl chloride) (PVC) membrane sensors incorporating new synthesized imide pyridine derivatives are prepared, characterized and tested. The sensors exhibit significantly high selectivity for SCN− ions over many common anions. These sensors are satisfactorily used for accurate determination of ␮g quantities of thiocyanate and for monitoring the simultaneous titration of binary and ternary metal ions in mixtures with thiocyanate as titrant.

2.2. Synthesis of ionophores (II) and (III)

2. Experimental

Thiocyante PVC membrane sensors were prepared as described previously [19–21] using a mixture consisting of 3 mg of the ionophore, 130 mg of plasticizer (DOP), 65 mg of PVC, and 1 mg of TDMAC in about 7 ml of THF. The clear solution was poured into a glass dish (3 cm diameter) and the solvent was allowed to evaporate at room temperature for 24 h. The resulting membrane was peeled off from the glass mould and discs of 7 mm i.d. were cut out and glued onto an 8 mm i.d. PVC tube using THF. A mixed solution consisting of equal volumes of 1 × 10−3 M NaSCN and 1 × 10−3 M NaCl was used as an internal reference solution. Ag/AgCl coated wire (3 mm diameter) was

2.1. Reagent All reagents used were of analytical grade. Deionized water was used for preparing all aqueous solutions. High molecular weight PVC, dioctylphthalate (DOP), tridodecylmethylammonium chloride (TDMAC) and tetrahydrofuran (THF) were purchased from Fluka. Ionophore (I) was prepared according to the method previously published [18]. Synthetic routes for preparation of ionophores (II) and (III) are given below.

Ionophore (II) (N␣ -dipicolinoyl)-bis-[l-leucyl-dlnorvaline) was prepared by dropwise addition of sodium hydroxide (1 M, 15 ml) to a cold and stirred methanolic solution (0.619 g, 1 mmol, −5 ◦ C) of N␣ -dipicolinoyl-bis-[l-leucyl-dl-norvalylmethyl ester]. Stirring was continued for 2 h followed by stirring for 12 h at room temperature. Methanol was evaporated under reduced pressure. The reaction mixture was acidified (0.5 M HCl) to pH ∼3, and the solid obtained was filtered off, washed with cold water and recrystallized from an ethanol/ether mixture to give ionophore (II) (yield 68%, mp 135–137 ◦ C). The mass spectrum of the ionophore showed a molecular ion peak at m/z 591 (M+ , 20%) corresponding to the molecular formula C29 H45 N5 O8 and a base peak at m/z 302 (100%). Ionophore (III) (N␣ -dipicolinoyl)-bis-[l-valyl-lphenylalanine hydrazide]) was prepared by addition of hydrazine hydrate (0.5 ml, 10 mmol) to a methanolic solution of N␣ -dipicolinoyl-bis-[l-valyl-l-phenylalanyl methyl ester] (0.687 g, 1 mmol, 25 ml). The reaction mixture was refluxed for 5 h followed by evaporation under reduced pressure. The residue was triturated with ether, filtered and recrystallized from methanol/water mixture to give ionophore (III) (yield 82%, mp 107–109 ◦ C). The mass spectrum of this compound showed a molecular ion peak (M+ ) at m/z 687 (8%) corresponding to the molecular formula C35 H45 N9 O6 and a base peak (100%) at m/z 567. 2.3. Membrane preparation and sensor construction

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employed as an internal reference electrode. The sensors were soaked overnight in a solution of 1×10−2 M NaSCN for conditioning, and stored in the same solution when not in use.

and after addition of 1.0 ml of 1 × 10−2 M SCN− solution.

2.4. EMF measurements

A thiocyanate membrane sensor based on ionophore (I) as an indicator sensor and a Ag/AgCl double junction reference electrode were used for monitoring the titration of thiocyanate solutions with 1 × 10−2 M AgNO3 . The sensor and reference electrode were immersed in the unknown SCN− test solution (5.0, −15.0 ml of 1 × 10−3 M) in a 50 ml beaker. The potential reading was recorded after each addition. The equivalence point was calculated from the sharp inflection at the equivalence point or from the first derivative curves.

All measurements were made at 25 ± 1 ◦ C with a cell of the type: Ag/AgCl(s) , 4 mol l−1 KCl saturated with AgCl(s) /sample solution//membrane//internal reference solution/AgCl(s) /Ag. pot Potentiometric selectivity coefficients (KSCN,B ) were evaluated according to IUPAC guidelines using the separate solutions method [22,23] in which the potential of a cell comprising the membrane electrode and a reference electrode is measured with two separate solutions, one containing the thiocyanate ion A at the activity aA (but no B), the other containing the interfering ion B at the same activity aA = aB (but no A) and EA and EB are the measured values, respectively. Different interfering anions at a concentration of 1 × 10−3 M at pH 5.5 are utilized and the results are obtained using the equation: 
(EB − EA ) ZA pot log KA,B = log aA (1) + 1− S ZB pot

where KA,B is the potentiometric selectivity coefficient, S the slope of the calibration plot, aA the activity of thiocyanate and ZA and ZB are the charges on SCN− and the interfering anion, respectively. 2.5. Direct potentiometric determination of thiocyanate

2.6. Potentiometric titration of thiocyanate

2.7. Potentiometric titration of metal ions A series of potentiometric precipitation titrations was performed in which the sensor based on ionophore (I) was used as an indicator sensor. Test solutions containing a single metal ion (e.g. Ag+ , Tl+ , Cu2+ or Pb2+ ) or binary mixtures (e.g. Ag+ + Pb2+ or Ag+ + Tl+ ) or ternary mixtures (e.g. Cu2+ + Ag+ + Tl+ or Cu2+ + Ag+ + Pb2+ ) were titrated with 3 × 10−2 M SCN− . Equivalence points at each inflection break were located and the concentration of each metal ion was determined (1 mol SCN− = 0.5 mol Cu2+ or Pb2+ = 1 mol Ag+ or Tl+ ).

3. Results and discussion 3.1. Performance characteristics of the sensors

A thiocyanate membrane sensor based on ionophore (I) with TDMAC and Ag/AgCl double junction reference electrode was immersed to a 25 ml beaker containing 9 ml of deionized water. Portions (0.1–1.0 ml) of 10−3 to 10−2 M standard SCN− test solutions were successively added and the potential change after each addition was recorded. A calibration graph was constructed by plotting the potential change against the logarithm of the SCN− concentration. The plot was used for subsequent determination of unknown SCN− test solutions. Alternatively, the standard addition (spiking) technique [20] was used by measuring the potential of the test solution before

Three pyridine carboximide ionophores (I), (II), and (III) (Fig. 1) were synthesized and tested as novel thiocyanate ion carriers. Sensors incorporating these ionophores in membranes with the composition of 1.5 wt.% ionophore, 65 wt.% DOP plasticizer, 33 wt.% PVC, and 0.5 wt.% TDMAC were prepared, characterized and evaluated according to IUPAC recommendations [23]. TDMAC was used as a cationic discriminator. Results from replicate studies indicate near-Nernstian slopes of −56.8 ± 0.2 , −56.3 ± 0.2 , and −58.3 ± 0.1 mV per decade with lower detection limits of 5 × 10−6 , 5.5 × 10−6 , and 5 × 10−6 M for

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Fig. 1. Chemical structures of thiocyanate ionophores.

the sensors based on ionophores (I), (II), and (III), respectively. The dynamic response time of the sensors to reach ∼95% of the equilibrium response is 10–20 s for 10−6 to 10−2 M thiocyanate. Typical calibration

graphs are shown in Fig. 2 and the general response characteristics of the sensors are presented in Table 1. A study of the potential–pH curves of SCN− membrane sensors reveals good stability within the pH

Table 1 General performance characteristics of some potentiometric thiocyanate membrane sensors Ionophore

Slope (mV per decade)

Linear range (M)

Mn(III) porphyrin derivative Mn(II) porphyrin Mn(II) porphyrin Co(II), Mn(II) porphyrin Co(II) porphyrin Urea-functionalized porphyrin

−56 −64 −59 −55 to −57 −55 −58.1

7 × 10−5 to 10−1 10−5 to 10−2 10−5 to 10−2 3 × 10−5 to 10−2 10−4 to 10−1 5 × 10−5 to 10−2

Cobalt and Mn phthalocyanine Ni and Fe phthalocyanaine Solid-state AgX/AgSCN− Ag(I)-thiourea complex Tri-n-octyltin chloride Ionophore (I) Ionophore (II) Ionophore (III)

−57.5 −55 −58.5 −41.5 −60.1 −56.8 −58.3 −55.6

1 × 10−6 to 10−1 10−6 to 10−1 10−6 to 10−1 3 × 10−4 to 10−2 10−3 to 10−1 10−5 to 10−2 9 × 10−6 to 10−2 10−5 to 10−2

Limit of detection (M) 9 × 10−6 2.5 × 10−5 7 × 10−6 4.5 × 10−5 –10−2 3 × 10−4 4 × 10−5 3× 5× – 10−4 10−3 5× 5× 5×

10−6 10−7

10−6 10−6 10−6

Interferent

Reference

I− , salicylate, ClO4 − Salicylate, IO4 − Salicylate, ClO4 − , IO4 − , I− NO2 − , CN− NO2 − , ClO4 − NO3 − , NO2 − , I− , ClO4 − , Cl− , Br− Salicylate, I− and N3 − Salicylate, I− , ClO4 − I− , S2− , X− I− , S2 O3 2− , Br− NO3 − , Cl− , ClO4 − IO4 − IO4 − IO4 −

[8] [27] [9] [28] [26] [25] [10] [32] [1] [6] [19] This work This work This work

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Fig. 2. Calibration graphs for sensors based on ionophores I, II, and III.

range 2–8; the potential does not vary by more than ±2 mV. The life span of the sensors is at least 8 weeks. 3.2. Selectivities of the sensors pot

Selectivity coefficients (KSCN,B ), describing the preference of the membrane for an interfering ion B− relative to SCN− were determined by the separate solutions method [22,23] and the results are summarized in Fig. 3. Potentiometric selectivity of sensors based on ionophores (I), (II), and (III) can be related to the preferential interaction of these ionophores with thiocyanate anion over other anions. Periodate is the only interfering species. A typical selectivity pattern for a series of anions tested by ionophore (I)-based sensor − is in the order: IO− I− > HCO− 4 ∼ SCN 3 > − − 2− − CN > ClO3 > NO3 > HPO4 ∼ CH3 COO− > − − − − 2− > CrO2− 4 > Br ∼ NO2 > BrO3 > F S 2− S2 O2− ∼ SO2− 3 >SO4 3 . For ionophore (II)-based − sensor, the selectivity order is: IO− > 4 SCN − − − − − I > ClO3 > HCO3 > CN > NO3 > Br − ∼

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− − > CrO2− > CH3 COO− > NO− 2 > BrO3 > F 4 2− 2− 2− 2− > S2 O3 > HPO4 > SO4 > SO2− S 3 . For ionophore (III)-based sensor, the selectivity order is: − − − − − IO− 4 > SCN > I > HCO3 > CN > ClO3 ∼ − − − > CH COO− > HPO2− 3 4 > NO3 > NO2 > Br 2− − 2− − CrO4 > BrO3 > F S2 O3 > S2− > SO2− 3 2− SO4 . Generally, a sensor based on ionophore (I) is less affected by IO4 − than are sensors based on ionophores (II) and (III). A sensor based on ionophore (II) has good selectivity for SCN− and is less affected by SO3 2− , PO4 3− , CrO4 2− , F− , CN− , I− , HCO3 − and CH3 COO− than sensors based on ionophores (I) and (III). In the absence of IO4 − , the sensor based on ionophore (II) is recommended for general use. The potentiometric selectivities of the present pyridine imide membrane-based sensors indicate that these ionophores exhibit selectivities that deviate from those observed with the classical anion exchanger-based membranes, probably due to hydrogen bonding. It has been reported that many anions form complexes via hydrogen bonding with amides [24,25]. The selectivity with respect to some common interfering ions of the proposed membrane sensors based on ionophores (I), (II), and (III) are comparable with those previously reported. Thiocyanate sensors based on different neutral ion carriers [3,8,26–29] suffer from sever interference from NO2 − , I− , CN− , and IO4 − .The solid-state Ag/AgSCN sensor displays significant interferences from S2− , CN− , halide, and pesudohalide ions [1]. It can be seen that the proposed sensors show superior selectivity behavior and exhibit a better linear response range and lower limit of detection than many of these previously suggested membrane sensors (Table 1).

3.3. Determination of thiocyanate The method for determining thiocyanate was validated by determining the performance characteristics of the utilized procedure using quality control–quality assurance standards [30]. Six batches (six determination each) covering the concentration range 0.5–580 ␮g ml−1 thiocyanate were used for determining the accuracy, trueness, precision, range, lower detection limit, repeatability (CVw ) and between-day variability (CVb ). The results obtained are presented in Table 2. A statistical evaluation of the results indi-

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pot

Fig. 3. Selectivity coefficients (log KSCN,B ) of thiocyanate membrane sensors based on ionophores I, II, and III.

cate that at the 95% confidence level, the responses show no statistical difference (t = 0.78 ). A thiocyanate sensor based on ionophore (I) was used as an indicator electrode for the potentiometric titration of thiocyanate solutions with silver ions. Typical potentiometric titration curves of thiocyanate are shown in Fig. 4. The titration curves show sharp inflections (ca. 100 mV) at 1:1 Ag+ :SCN− ; the standard deviation is 0.3% (n = 10). 3.4. Potentiometric titration of metal ions The thiocyanate membrane sensor based on ionophore (I) was also used for monitoring the titra-

tion of some metal ions (e.g. Ag+ , Cu2+ , Tl+ and Pb2+ ), singly or in binary and ternary mixtures, with a standard thiocyanate solution. In the titration of binary and ternary metal ion mixtures (M1 , M2 , and M3 ), the reaction of M2 begins when the concentration of SCN− ions according to Eq. (2) becomes available in the solution. At this level of SCN− , the concentration of unreacted M1 is expressed by Eq. (3). Thus, a differential reaction of the two metal ions without overlapping necessitates at least two decades difference in the solubility products or stability constants of M1 SCN and M2 SCN (K1 /K2 ≤ 10−2 ). Under these conditions, at least 99% of M1 (Eq. (4)) will react before M2 starts to react, leading to consecutive

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Table 2 Potentiometric response characteristics of thiocyanate sensors based on ionophores (I), (II), and (III)a Parameter

Ionophore

Slope (mV per decade) Correlation coefficient, r (n = 6) Intercept (mV) Linear range (M) Detection limit (M) Working range (pH) Response time for 10−3 M (s) Life span (weeks) Standard deviation (σ ) Accuracy (%) Trueness (%) Repeatability, CVw (%) Between-day variability, CVb (%) a

(I)

(II)

(III)

−56.8 −0.9982 −108.2 10−5 to 10−2 5 × 10−6 3.5–7.8 <10 8 0.3 98.5 98.2 0.4 0.8

−55.6 −0.9985 −95.7 10−5 to 10−2 5.5 × 10−6 2–7 <10 8 0.2 98.6 98.1 0.4 0.6

−58.3 −0.9997 −104.7 9 × 10−6 to 10−2 5 × 10−6 2–7 <10 8 0.3 99.1 98.6 0.5 0.7

Average of five measurements.

inflection breaks. [M2 SCN] [SCN− ] = K2 [M2 ] [M1 ] = K1 [M1 ] =

[M1 SCN] [SCN− ]

K1 [M1 SCN][M2 ] K1 = [M2 ] K2 [M2 SCN] K2

(2) (3) (4)

Fortunately, the literature reveals that the differences among the solubility products of many metal thiocyanates are greater than several orders of magnitude [31]. For example, the solubility products of silver, copper, thallium and lead thiocyanates are 1.1×10−12 , 4.8 × 10−15 , 1.7 × 10−4 and 2 × 10−5 , respectively. This suggests the possibility of sequential titration of two and more metal ions in the same solution without

Fig. 4. Potentiomtric titration plots of SCN− solutions with 1 × 10−2 M AgNO3 using an ionophore (II)-based membrane sensor: (䊏) 5 ml; (䊉) 10 ml; and (䉱) 15 ml of 1 × 10−3 mol l−1 thiocyanate). Arrows indicate the theoretically expected equivalence points.

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Fig. 5. Potentiometric titration plot of the Ag+ and Pb2+ mixtures with 3 × 10−2 M SCN− using an ionophore (II)-based membrane sensor. Arrows indicate the theoretically expected equivalence points.

Fig. 6. Potentiometric titration plot of an Ag+ and Tl+ mixture with SCN− using an ionophore (II)-based membrane sensor. Arrows indicate the theoretically expected equivalence points.

prior separation, as sequential binding of the metal ions with SCN− ensures stepwise titration curves with sequential end point breaks (Figs. 5–8). For titration of binary mixtures of (Ag+ + Pb2+ ) and (Ag+ + Tl+ ) and ternary mixtures of (Cu2+ + Ag+ + Tl+ ) and (Cu2+ + Ag+ + Pb2+ ), two and three sharp successive inflection breaks at the equivalent points of these metals are obtained. Whereas Pb2+ and Cu2+ ions consumes 2 mol of SCN− mol−1 , Ag+ and Tl+ ions consume 1 mol of SCN− mol−1 . The sequence of inflections is in the order of Ag+ and Pb2+ (Fig. 5), Ag+ and Tl+ (Fig. 6), Cu2+ , Ag+ and Tl+ (Fig. 7) and Cu2+ , Ag+ and Pb2+ (Fig. 8). These are in a good agreement with the sequence of the values of the solubility products of SCN− derivatives of these metal ions [31]. Table 3 presents results obtained for determination of some mixtures of metal ions. The mean average recoveries calculated for the pooled data for silver, thallium, lead, and copper in these mixtures are 98.5 ± 0.3%, 98.6 ± 0.2%, 99.1 ± 0.3% and

Fig. 7. Potentiometric titration plot of a Cu2+ , Ag+ and Tl+ mixture with 3 × 10−2 M SCN− using an ionophore (II)-based membrane sensor. Arrows indicate the theoretically expected equivalence points.

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98.6 ± 0.2%, respectively. Other metal ions may also be simultaneously determined in their mixtures.

4. Conclusions Potentiometric thiocyanate-selective membrane sensors based on recently synthesized pyridine carboximides as neutral ionophores and TDMAC as a cationic discriminator show fast, stable, reproducible and selective characteristics. The sensors display a near-Nernstian response (−55 to −58 mV per decade), offer a wide linear response range (1 × 10−2 to 5 × 10−5 M), provide a lower limit of detection (5 × 10−6 M) and exhibit much better selectivity than previously reported SCN− sensors. The sensors have satisfactorily been used for determining of very small quantities of thiocyanate ions and for monitoring the sequential titration of some metal ions in binary and ternary mixtures. Fig. 8. Potentiometric titration plot of a 1 × 10−3 mol l−1 Cu2+ , Ag+ and Pb2+ ternary mixture with 3 × 10−2 M SCN− using an ionophore (II)-based sensor. Arrows indicate the theoretically expected equivalence points.

Table 3 Simultaneous determination of metal ion mixtures by potentiometric titration with SCN− using ionophore (II)-based membrane sensor Mixture (M1 –M2 –M3 ) M1 Ag+ –Pb2+ Ag+ –Tl+ Cu2+ –Ag+ –Tl+ Cu2+ –Ag+ –Pb2+

Added (␮g ml−1 )

Found (␮g ml−1 )

Recoverya (%)

S.D. (%)

108.0 540.0 317.5 64.0

102.6 525.5 309.5 63.5

95.0 97.3 97.5 99.2

0.2 0.2 0.2 0.3

M2 Ag+ –Pb2+ 456.5 Ag+ –Tl+ 4000.0 540.0 Cu2+ –Ag+ –Tl+ Cu2+ –Ag+ –Pb2+ 108.1

415.0 3914.0 505.5 104.5

90.9 97.9 93.6 96.8

0.4 0.4 0.2 0.3

M3 Ag+ –Pb2+ – – Ag+ –Tl+ 4000.0 Cu2+ –Ag+ –Tl+ Cu2+ –Ag+ –Pb2+ 520.0

– – 4050.0 508.5

– – 101.2 97.7

– – 0.5 0.2

S.D.: standard deviation. a Average of five measurements.

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