Synthesis, spectral characterization, thermal investigation and electrochemical evaluation of benzilbis(carbohydrazone) as Cd(II) ion selective electrode

2.1 Materials

Analytical reagent grade chemicals and doubly distilled water were used for preparing all aqueous solutions. Benzil, carbohydrazide, high molecular weight poly (vinylchloride) powder (PVC), dibutyl phthalate (DBP), dioctyl phthalate (DOP) and tetrahydrofuran (THF) were obtained from Sigma. Tetradodecylammoniumtetrakis(4-chlorophenyl) borate (ETH 500), tris(2-ethylhexyl)phosphate and 2-nitrophenyl octyl ether (NPOE), were obtained from Fluka. Salts of metal nitrates or chlorides and solvents like methanol, ethanol, acetone, dioxane, nitric acid and hydrogen peroxide (all from Merck) were of the highest purity available and used without any further purification. All the metal nitrate solutions were freshly prepared by accurate dilution from their stock solution of 0.1 M, with distilled, de-ionized water.

2.2 Synthesis of benzil bis(carbohydrazone) (BBC)

An aqueous ethanolic solution of carbohydrazide (0.02 mol, 1.8 g) was treated with 1 ml glacial acetic acid followed by slow addition of a solution of benzil (0.01 mol, 2.1 g) in 20 ml ethanol. The resulting mixture was heated under reflux for about 12 h. On cooling overnight at 0 °C, white crystalline precipitate of BBC was separated out, which was filtered, washed with cold ethanol and dried under vacuum. Yield 78%, m.p. 210 °C. Elemental analysis: C₁₆H₁₈N₈O₂, Found (Calc.) C: 53.78 (54.23), H: 4.78 (5.08), N: 31.29 (31.64).

2.3 Fabrication of electrodes with ligand BBC

For analytical application, the membranes have been fabricated as suggested by Craggs et al. (1974) and others (Chandra et al., 2010, 2012). The PVC-based membranes have been prepared by dissolving appropriate amounts of BBC, anionic additive and plasticizers TEHP, DBP, o-NPOE, DOP and PVC in THF (∼5 mL). The components were added in terms of weight percentages. The homogeneous mixture was obtained after complete dissolution of all the components, concentrated by evaporating THF and it has been poured into polyacrylate rings placed on a smooth glass plate. The viscosity of the solution and solvent evaporation was carefully controlled to obtain membranes with reproducible characteristics and uniform thickness otherwise the response of the membrane sensors has shown a significant variation. Membranes of 0.4-mm thickness were removed carefully from the glass plate and glued to one end of a “Pyrex” glass tube. The membrane was studied under a polarized optical microscope (Fig. 1).

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Figure 1 Membrane picture under polarized optical microscope (a) before use as long tubular structure (b) after use with blocked capturing sites.

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2.4 Equilibration of membrane and potential measurement

The time of contact and concentration of equilibrating solution were optimized so that the sensors generated stable and reproducible potential at relatively short response time.

The polymeric membrane electrode was equilibrated for ∼2 days in 1.0 × 10⁻¹ M Cd(NO₃)₂ solution. The potentials were measured by varying the concentration of Cd(NO₃)₂ in test solution in the range of 1 × 10⁻⁹ M–1.0 × 10⁻¹ M. Standard Cd(NO₃)₂ solutions were obtained by gradual dilution of 10⁻¹ M Cd(NO₃)₂ solution.

The emf measurements with the polymeric membrane electrode were carried out on a digital potentiometer at 25 ± 1 °C using saturated calomel electrode (SCE) as reference electrode with the following cell assemblies:

Hg–Hg₂Cl₂, KCl (sat.) ‖ internal solution 1.0 × 10⁻¹ M Cd²⁺ | PVC membrane | test solution ‖ Hg₂Cl₂–Hg, KCl (sat.).

4.1 IR spectra

The assignments of the main IR absorption bands of the ligand are shown in Fig. 2. The ligand displays IR bands at 3391, 3320, 3171 and 3081 cm⁻¹ corresponding to the υ_as(NH₂), υ_s(NH₂), υ_as(NH) and υ_s(NH) stretching vibrations, respectively. The spectrum exhibits the IR bands at 1684 and 1497 cm⁻¹ which may be assigned to the amide I [υ(C⚌O)] and amide II [υ(C⚌N) + δ(NH)] stretching vibrations (Chandra et al., 2010; Kataria et al., 2010).

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Figure 2 IR spectra of ligand BBC.

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4.2 ¹H NMR spectra

The ¹H NMR spectrum of ligand was recorded in deuterated DMSO (DMSO-d₆) at 300 MHz. The spectrum (Fig. 3) shows that different non-equivalent protons resonate at different values of applied field which are discussed below:

1. A distorted singlet at δ 4.0 ppm appears in the spectrum which is assigned to the four protons of two amino groups. The appearance of signal as a broad peak may be due to the nuclear quadrupole broadening by nitrogen (Mohan, 2001).

2. Another distorted singlet at δ 5.8 ppm corresponding to the 2H appeared due to the amide protons.

3. The spectrum exhibits a multiplet between δ 6.8 and δ 7.9 ppm equivalent to the 10H of two phenyl groups.

4. The spectrum displays a sharp singlet at δ 8.1 ppm equivalent to 2H representing the enolic protons of ligand in solution (Tamboura et al., 2004).

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Figure 3 ¹H NMR spectra of ligand BBC.

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4.3 Keto-enol tautomerism

The ligand may show keto-enol tautomerism (Fig. 4) because it contains the amide bonds. The IR spectrum does not show a υ(OH) band at ca. 3550 cm⁻¹ but shows the bands at 3391 and 3081 cm⁻¹ corresponding to υ_as(NH) and υ_s(NH), indicating that in solid state, the ligand exists in the keto form. However, the ¹H NMR spectrum of ligand exhibits a sharp singlet at δ 8.1 ppm due to enolic –OH which indicates that the amide groups are transformed into iminol groups in solution.

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Figure 4 Five possible isomeric forms of ligand BBC.

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4.4 Mass spectra

The mass spectrum of ligand is given in Fig. 5. The spectrum shows the molecular ion peak (M⁺) at m/z = 354 (78%) and a weak peak at m/z = 355 due to ¹³C isotope. The amide characteristic peak appears at m/z = 59 and the moderate high intense peak (81%) at m/z = 177 appears in the spectrum due to the six membered cyclic positive ion which results from γ δ C–C bond cleavage and followed by cyclization. The other different ions give the peaks of different mass numbers like 295, 134, 118, 44 and 16. The intensities of peaks are in accordance with the abundance of the ions (Chandra and Sharma, 2009a,b; Chandra et al., 2011). The fragmentation path of ligand is given in Fig. 6.

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Figure 5 Mass spectra of ligand BBC.

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Figure 6 Mass fragmentation pattern of ligand BBC.

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4.5 Thermogravimetric analyses

Benzil bis(carbohydrazone) was studied by thermogravimetric analyses from ambient temperature to 700 °C in nitrogen atmosphere. The TG curve was redrawn as % mass loss versus the temperature (TG) curve. Typical TG curve is presented in Fig. 7. Thermal techniques, such as thermogravimetric analysis (TGA and DTA), have been successfully employed for the study of the energetic interactions of metal cations with biological species, such as amino acids (Burger, 1973). The weight loss profiles are analysed by the amount or percent of weight loss at any given temperature, and the temperature ranges of the degradation process were determined. Thermal stability domains, melting points, decomposition phenomena and their assignments for the benzil bis(carbohydrazone) are summarized below. The overall loss of mass from the TG curves is 100% for benzil bis(carbohydrazone). In the first step the compound undergoes decomposition with a weight loss exp. 2.52–2.79% (ca. 2.59–2.75%), between 170 and 190 °C due to the loss of N₂H₄ moiety. In the second step decomposition of the compound shows weight loss exp. 60.584–61.743% (ca. 60.877–61.58%) in the temperature range of 210–240 °C due to the organic moiety C₉H₁₀N₆O. DTG maximum temperature is 176.69 °C.

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Figure 7 TG-DTA Curve for benzil bis(carbohydrazone).

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4.6 Electrochemical sensors

4.6.1

4.6.1 Response of electrode based on Cd²⁺ions

The existence of nitrogen and oxygen as donor atoms in the ligand with high lipophilic character seems to form strong complexes with transition metal ions (Gupta and Chandra, 2007; Chandra and Gautam, 2009; Chandra et al., 2010). In order to study the potential response and selectivity of the ionophore for different metal ions, BBC was used as neutral carriers to prepare PVC based membrane electrodes for a variety of metal ions including alkali, alkaline earth, transition and heavy metal ions. The potential responses obtained are shown in Fig. 8. The sensitivities and selectivities obtained for a given membrane depend significantly on the membrane ingredients and the nature of plasticizer and additives used. Therefore, 10 membranes of different compositions (Table 1) have been prepared and their response characteristics were evaluated according to the IUPAC recommendations (Guilbault et al., 1976).

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Figure 8 Potential response of ligand BBC to Mⁿ⁺ ion.

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Table 1 Optimization of membrane composition.

S. No.	PVC (wt.%)	Plasticizer (wt.%)	Ionophore (wt.%)	ETH 500 (wt.%)	Slope (mV/decade)	Detection limit (M)	Linear range (M)
1.	31	66 (NPOE)	3.0	–	18.8	1.0 × 10−5	1.0 × 10−3–1.0 × 10−7
2.	31	65 (TEHP)	4.0	–	26.0	1.6 × 10−5	1.0 × 10−2–1.0 × 10−5
3.	33	65 (DOP)	4.0	–	22.0	6.0 × 10−5	1.0 × 10−3–1.0 × 10−6
4.	33	64 (DBP)	3.0	–	41.2	5.0 × 10−5	1.0 × 10−2–1.0 × 10−6
5.	30	65 (NPOE)	3.5	1.5	24.5	5.0 × 10−7	1.0 × 10−1–1.0 × 10−7
6.	32	63 (DBP)	3.0	2.0	39.8	1.0 × 106	1.0 × 10−4–1.0 × 10−7
7.	30	65 (DOP)	3.5	1.5	27.2	5.0 × 10−6	1.0 × 10−3–1.0 × 10−7
8.	30	65 (TEHP)	4.0	1.0	24.5	1.0 × 10−5	1.0 × 10−1–1.0 × 10−6
9.	30	65 (TEHP)	3.5	1.5	29.7	3.2 × 10−8	1.0 × 10−1–1.0 × 10−8
10.	31	64 (TEHP)	3.0	2.0	35.6	3.0 × 10−5	1.0 × 10−2–1.0 × 10−7

The best performance was obtained with a membrane composition of 30% poly (vinyl chloride), 65% TEHP, 3.5% BBC and 1.5% Tetradodecylammoniumtetrakis(4-chlorophenyl) borate (ETH 500). The electrode exhibits a Nernstian behaviour (with slope of 29.7 mV per decade) over a very wide concentration range of 1.0 × 10⁻¹–1.0 × 10⁻⁸ M with a detection limit of 3.2 × 10⁻⁸ M. This sensor shows very good selectivity and sensitivity towards cadmium ion over a wide variety of cations, including alkali, alkaline earth, transition and heavy metal ions. In this work, the selectivity coefficient of the sensor towards different cationic species (Mⁿ⁺) was evaluated by using the fixed interference method (FIM) (Gupta et al., 2008; Chandra and Singh, 2009). In FIM, the selectivity coefficient was evaluated from potential measurement on solutions containing a fixed concentration of interfering ion (1.0 × 10⁻² M) and varying amount of Cd²⁺ ions. It is given by the expression. $$\log K^{\text{pot}}{\text{Cd}}^{2+}\text{,B}=\log ({\text{a}}_{\text{Cd}}^{2+})-\log {({\text{a}}_{\text{B}})}^{\text{Z}}{\text{Cd}}^{2+\text{Z}}/\text{B}$$ where, Z_Cd²⁺and Z_B are the charge on Cd²⁺ ion and interfering ion.

The resulting K^pot_Cd²⁺values are summarized in Table 2.

Table 2 Potentiometric selectivity coefficient log K^pot_Cd²⁺, _B) for interfering ions.

Interfering ion	log KpotCd2+,B	Interfering ion	log KpotCd2+,B
K+	−2.98	Cr3+	−2.82
NH4+	−2.01	Ag+	−2.98
Na+	−3.41	Zn2+	−3.45
Ni2+	−3.09	Pb2+	−3.29
Cu2+	−2.93	Sr2+	−3.33
Co2+	−3.72	Hg2+	−3.62
Mn2+	−3.98	Ce3+	3.99
Fe3+	−3.96	Ca2+	−3.84
Cr2+	−2.13	Mg2+	−3.59

The effect of pH of the test solution on the response of the membrane electrode was examined at two Cd²⁺ ion concentrations. As illustrated in Fig. 9, the potentials remained constant from pH 2.0 to 9.0. However, outside this range the electrode responses at pH < 2.0 seem ascribable to the competitive binding of proton to the ligand at the surface of the membrane electrode, while the diminished potential at higher pH (>9), the potential change may be due to the hydroxylation of Cd²⁺ ions.

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Figure 9 pH Effect on the response of Cd²⁺-ISE.

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Dynamic response time is an important factor for any ion-selective electrode. In this study, the practical response time was recorded by immediately changing the Cd²⁺ ion concentration from 1.0 × 10⁻⁸ to 1.0 × 10⁻² M. The actual potential-time trace is shown in Fig. 10(a). As is seen, in the whole concentration range, the sensor reaches the equilibrium response in a very short time (∼8 s).

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Figure 10 (A) Response time curve for Cd²⁺-ISE at different concentrations. (B) Reversibility study for Cd²⁺-ISE at different concentrations.

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To evaluate the reversibility of the electrode, a similar procedure in the opposite direction was adopted. The measurements have been performed in the sequence of high-to-low from (1.0 × 10⁻² to 1.0 × 10⁻⁸ M) sample concentrations. The results (Fig. 10(b)) showed that, the potentiometric response of the electrodes was reversible; although the time needed to reach equilibrium values (40 s) was longer than that of low-to high sample concentrations.

The high lipophilicity of ionophore and plasticizer ensures stable potentials and longer lifetime (Chandra et al., 2010) for the membrane. Hence, the lifetime is dependent upon the components of the solution and the measured specimens. The lifetime of the electrode was determined by performing calibrations periodically with standard solutions and calculating the slopes over the concentration ranges of 1.0 × 10^–2–1.0 × 10^–8 M Cd²⁺ solutions. The experimental results showed that the lifetime of the present sensor was over 75 days. During this time, the detection limit and the slope of the electrode remained almost constant. Afterwards, the electrochemical behaviour of the electrode gradually deteriorated, which may be due to ageing of the polymer (PVC), the plasticizers, and leaching of the ionophore. Therefore, the electrode can be used for at least 2 and half months without a considerable change in their response characteristic towards Cd²⁺.

4.6.1.1

4.6.1.1 Interaction mechanism on the basis of UV spectroscopy

The interaction mechanism of Cd²⁺ ion with the ionophore was discussed on the basis of UV–Visible spectrophotometry. Thus, in order to investigate such mechanism UV–visible spectra of carrier without and with cadmium ion were recorded. A UV–visible spectrum of BBC of 0.01 M concentration dissolved in DMSO was recorded first. It was then used for the quantitative complexation with Cd²⁺ ion by adding an increasing amount of the Cd²⁺ ion up to 0.01 M concentration. The spectra of the ionophore BBC show the absorption bands at 266 nm (Fig. 11(a)). In Fig. 11(b) shift in the peaks was observed and the new peaks were observed at 329–397 nm with increased absorptivity when Cd²⁺ ion interacted with the carrier, which shows that the size of the complex increases on interaction of Cd²⁺ with the carrier. The absorption intensity changes as a function of molar ratio [Cd²⁺]/[BBC] (Rawat et al., 2008).

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Figure 11 UV–Visible interaction mechanism of (a) BBC (1 × 10⁻² M) in DMSO and (b) BBC with Cd²⁺ (1 × 10⁻² M) in DMSO.

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4.6.1.2

4.6.1.2 Electrochemical impedance spectroscopy

Impedance measurements were carried out on the membrane with and without ionophore, and 1.0 × 10⁻²–1.0 × 10⁻³ M solutions of cadmium nitrate as the internal and external solutions of the electrode were used, respectively. Finally their complex plan plots were generated and compared. As shown in Fig. 12, the charge transfer resistance of the membrane without ionophore (36.431 Mohm) is more than that of the membrane with ionophore (14.215 Mohm). These results can be attributed to increasing the electrical conductivity with the available ionophore that can reduce a charge transfer resistance (R_ct). This explanation confirms the results obtained from the UV–vis and potentiometric studies.

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Figure 12 Complex plan plots of the membrane with and without ionophore.

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