thallium (i) and lead (i i) with dicyclohexyl-18-crown-6 in methanol … · 2017-11-08 · lead (ii) with dicyclohexyl-18-crown-6 in methanol-water binary mixtures. Indian Journal

Indian Journal of Science • Analysis

Shah Bijal et al.Conductance and thermodynamic study on complexation of silver (i), thallium (i) and lead (ii) with dicyclohexyl-18-crown-6 in methanol-water binary mixtures,Indian Journal of Science, 2013, 4(11), 62-67, www.discovery.org.inhttp://www.discovery.org.in/ijs.htm © 2013 Discovery Publication. All Rights Reserved

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Shah Bijal A1,2, Christy Francis A2, Shrivastav Pranav S3, Sanyal Mallika2☼

1. Research Scholar, Chemistry Department, Kadi Sarva Vishwavidyalaya, Sarva Vidyalaya Campus, Sector 15/23, Gandhinagar- 382015, India2. Associate Professor, Department of Chemistry, St. Xavier’s College, Navrangpura, Ahmedabad- 380009, India3. Associate Professor, Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad- 380009, India

☼Corresponding author: Associate Professor, Department of Chemistry, St. Xavier’s College, Navrangpura, Ahmedabad 380009, India. Mail:[email protected], Mobile No: (+91)-9825972775

Received 27 June; accepted 17 August; published online 01 September; printed 16 September 2013

ABSTRACTThe complexation reactions between Ag+, Tl+ and Pb2+ cations with dicyclohexyl-18-crown-6 (DCH 18C6) were studied in methanol–water binary mixturesat different temperatures by conductometry. The conductance data shows that the stoichiometry of the complexes is 1:1 (M:L) for all three cations. Theformation constant (log Kf) of the resulting complexes observed the following sequence Tl+ > Pb2+ > Ag+. A monotonic behaviour was observed in puremethanol and methanol:water (70:30, 80:20, 90:10, mol%) binary systems and also at different temperatures (25, 35 and 45 ºC) for all three cations. Thevalues of thermodynamic parameters (∆H o, ∆So and ∆Go) for complex formation in methanol-water binary systems were obtained from temperaturedependence of formation constants of complexes using the van’t Hoff plots. In majority of cases, the complexation reactions were enthalpy stabilized andentropy destabilized. The results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixedsolvents.

Key words: Dicyclohexano-18-crown-6; Conductometry; Ag+, Tl+ and Pb+2 cations, Methanol-water solvent; Thermodynamic parameters

Abbreviations: DCH 18C6 - dicyclohexyl-18-crown-6; log Kf - formation constant

To Cite This Article:Shah Bijal A, Christy Francis A, Shrivastav Pranav S, Sanyal Mallika. Conductance and thermodynamic study on complexation of silver (i), thallium (i) andlead (ii) with dicyclohexyl-18-crown-6 in methanol-water binary mixtures. Indian Journal of Science, 2013, 4(11), 62-67

1. INTRODUCTIONMacrocyclic compounds, especially crown ethers are versatile complexing agents for a variety of cations including alkali,alkaline earths, transition and rare earth metal ions (Atwood and Lehn, 1996; Blasius and Janzen, 1985). Since the pioneeringwork of Pederson (Pederson, 1967), several new macrocyclic ligands have been synthesized and their complexing ability withcations, anions and neutral species has been investigated by different analytical techniques like NMR (Chekin et al., 2006;Karkhaneei et al., 2006), calorimetry (Gherrou et al., 2005), potentiometry (Kudo et al., 2006), polarography (Chamsaz et al.,2005; Rounaghi et al., 2002), spectrophotometry (Hasani, S. Akbari, 2007), and conductometry (Kazemi, 2010; Taheri et al.,2009; Rounaghi et al., 2006). In addition, as these compounds are readily soluble in organic diluents, they find widespreaduse as phase transfer catalysts, in transport of cations across membranes and for several other solution applications (Atwood,1990). The most striking feature of these complexes is their high selectivity and stability, which depends on several factors.This includes relative size of cation and the macrocyclic cavity, structural flexibility, nature of the substituent, nature andnumber of binding sites, acid-base character of the metal ions and nature of the solvent system (Marji and Taha, 1998).

The study of mixed solvent properties on the formation of crown ether–metal cation complexes is of interest as thesolvation capacity of the crown ether, metal cations, and even the resulting complexes change with changes in thecomposition of the mixed solvents (Józwiak, 2004). Use of mixed solvent systems can play an important role in finding newchemical possibility, especially in modern electrochemistry (Izutu, 2002). Further, studies on complexation reaction of crownethers with metal ions in different solvents demonstrate that the thermodynamic and kinetic parameters are affected by thenature and composition of the solvent system (Izatt, 1995). Metal ions such as silver (I), thallium (I) and lead (II) play animportant role in biological, environmental and industrial processes (Bremner, 1974) and comparatively less work is done inmixed solvent systems for these metal ions. The complexation reaction of Ag+ ion has been studied with dicyclohexyl 18-crown-6 (DCH 18C6) in ethanol-water (Ijeri and Srivastava, 2002), DMSO-water (Ijeri and Srivastava, 2003) and propylenecarbonate-ethylene carbonate solvent mixtures (Samant et al., 2003). Similarly, DMF-acetonitrile (Rounaghi et al., 2001;Shamsipur and Khayatian, 2001) and acetonitrile-water (Soorgi et al., 2008) binary mixtures are reported for Tl+-DCH 18C6complexes. For Pb2+, studies are done mainly in ethanol-water (Ijeri and Srivastava, 2002), acetonitrile-DMSO (Shamsipurand Pouretedal, 1999) and DMSO-water (Ijeri and Srivastava, 2003) solvent systems. So far there are no reports on the use

RESEARCH • CHEMISTRY Indian Journal of Science, Volume 4, Number 11, September 2013

Conductance and thermodynamic study on complexation of silver (i),thallium (i) and lead (ii) with dicyclohexyl-18-crown-6 in methanol-waterbinary mixtures

ScienceIndian Journal of

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Crown ethers:They are characterized ascyclic compounds havinga cavity with several ethergroups. The term "crown"refers to the resemblancebetween the structure ofcrown ether bound toa cation, anda crown sitting on aperson's head.

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of methanol-water binary solvent mixtures on the complexation of thesemetal ions with DCH 18C6 (Figure 1). Thus, the aim of the presentinvestigation is to study the role of methanol-water solvent system on thestability, selectivity and thermodynamic parameters for complexation ofAg+, Tl+ and Pb2+ with dicyclohexyl 18-crown-6 at different temperatures byconductometry. The principal advantage of conductometry is that it can bedone at very low solution concentrations with high sensitivity using asimple experimental set-up. Moreover, it is an inexpensive technique andgives reliable results (Christy and Shrivastav, 2011).

2. EXPERIMENTAL2.1. Materials and experimental proceduresDicyclohexyl 18-crown-6 (DCH18C6) of 99.9 % purity was obtained fromSigma Aldrich ( St. Louis, MO, USA). Nitrate salts of metals ion wereprocured from Central Drug House (P) Ltd. (New Delhi, India) with purity of≥ 99.6 %. HPLC grade methanol was obtained from Mallinckrodt Baker,S.A.de C.V. (Estado de Mexico, Mexico). Triple distilled water havingconductivity less than 1 × 10-7 S/cm was used in the entire study. Allstandard solutions of DCH 18C6 and metal ions were prepared by

accurately weighing known amounts of the compounds in suitable solvent systems. Theconductance measurements were carried out using Metrohm 856 conductivity module. A dip-type conductivity cell having a cell constant of 0.88/cm was used. The cell constant of theconductivity cell was determined by measuring the conductivity of aqueous potassiumchloride solutions of different concentrations (Wu and Koch, 1991). A thermostated water-bath was used to maintain a constant solution temperature at the desired value having anaccuracy of ±0.1°C. In order to determine the complex formation constants, 25 mL (5 × 10-4

M) solution of the desired metal salt solution in appropriate solvent was placed in speciallydesigned water jacketed cell equipped with a magnetic stirrer. To maintain a constanttemperature it was connected to a thermostated circulator water bath and the conductance ofthe solution was measured. Then a known amount of DCH 18C6 solution (5 × 10-3 M) wasadded in a stepwise manner and the conductivity of the mixture was measured after stirring(1.0 min) and temperature equilibration. This procedure was repeated in the same manner foreach addition until the DCH 18C6 to metal ion ratio was about 3.5:1. The same procedurewas followed at 25, 35 and 45oC, and the conductivity data was used for the calculation of theformation constant of the complexes at the desired temperatures. The conductivity data wasanalyzed using a non linear least squares curve fitting procedure for 1:1 complex formationmodel as reported previously (Marji and Taha, 1998). Analysis of log Kf versus 1000/T data

was done using a linear fitting program.

3. RESULTS AND DISCUSSIONTo study the complexation of DCH 18C6 with the nitrate salts of Ag+, Tl+ and Pb2+ ions, the molar conductance (ɅM) of thesolution was monitored as a function of mole ratio of DCH 18C6/metal ions in different methanol-water binary mixtures (mol%) and in pure methanol at different temperatures. As evident from Figures 2-4 there is a gradual decrease in ɅM withincrease in DCH 18C6/metal ions mole ratio for all three cations. This indicates that Ag+-DCH 18C6, Tl+-DCH 18C6 and Pb2+-DCH 18C6 complexes in methanol and its binary mixtures have lower mobility than free solvated Ag+, Tl+ and Pb2+ cations. Onthe other hand, the slope of the corresponding molar conductivity versus mole ratio of DCH 18C6/metal ion plots changeswhere the DCH 18C6 to cation ratio is close to unity. This suggests formation of 1:1 complex for all the three cations atdifferent temperatures. Further addition of DCH 18C6 resulted in no significant change in molar conductance. The generalreaction for 1:1 complexation with DCH 18C6 can be represented by the following equilibrium expression,

Mn+ + L ↔ ML n+

The equilibrium constant for this reaction can be expressed as,

where [MLn+], [Mn+], [L] and f represent the equilibrium molar concentrations of the complex, the free cation, the free ligandand the activity coefficients of the species indicated, respectively. Under the dilute conditions used, where the ionic strength is

less than 0.001 M, the activity coefficient of the uncharged ligand, Lf can be reasonably assumed to be unity. Based on

Debye–Huckel limiting law of electrolytes the activity coefficient of f (MLn+) f (Mn+), thus the above equation reduces to

The formation constant values for all three cations were evaluated using a nonlinear least-squares curve fitting programfrom the corresponding molar conductance vs. mole ratio plots at three different temperatures (Table 1). In all the solventmixtures and also in pure methanol the stability of complexes of Ag+, Tl+ and Pb2+ cations with DCH 18C6 follows the order Tl+> Pb2+ > Ag+. The variation of log Kf for the formation of DCH18C6-Ag+, DCH 18C6-Tl+ and DCH 18C6-Pb2+ complexesversus the ionic radii in pure methanol and methanol-water binary mixtures is shown in Figure 5. It is clearly seen that the Tl+cation forms a most stable complex with DCH18C6 than the other cations based on size-fit. This seems reasonable as theionic size of the Tl+ cation (1.50Å) is very close to the size of the DCH18C6 cavity (1.34Å - 1.55Å). Further, Tl+ is bound byion-dipole interaction with some covalent contribution (Kodama and Kimura, 1976). Pb+2 having an ionic radius of 1.19Å showgreater stability than Ag+ due to higher charge density. Since ionic size of the Ag+ ion (1.15Å) is smaller than the size of DCH

)()(]][[

)(][

LfMfLM

MLfMLK

nn

nn

f

Comparison:The study has shown the complexing ability of DCH 18C6 with Ag+, Tl+ andPb2+ cations in methanol and methanol-water binary mixtures which wasnot reported yet. Comparison of the results with published work with thesethree cations suggests that the presence of dicyclohexyl group could bethe reason for unfavorable complexation compared to its parent compound18-crown-6. Dicyclohexyl group introduces rigidity in the molecule, whichprevents efficient complexation in different solvent systems.

Content:The invention of molecular crowns- the crown ethers is one of the fewcases in the history where an unexpected finding during an experiment ledto a huge breakthrough that changed the course of scientific world. With aseemingly simple structure, the innovative concept of crown ethers had asignificant influence in the field of chemistry. The first crown ether was anunexpected byproduct made from an impurity in the reaction mixture, andit’s been said that the yield was only 0.4 %. Sir C.J. Pederson was carefulenough to keep and analyze the minute sample of crystals and succeededin the landmark discovery.

Figure 1Structure of dicyclohexyl 18-crown-6

Indian Journal of Engineering

Formation constant:A formation constant orstability constant isan equilibriumconstant for the formationof a complex in solution.It is a measure of thestrength of the interactionbetween the ligand andthe cation to form thecomplex.



Page6418C6 cavity, and it does not have a good fit condition, the Ag+ ion forms a less stable complex compared to the Tl+ and Pb+2

cations. Additionally, the complexation reactions between Ag+, Tl+ and Pb2+ cations with DCH 18C6 were sensitive to thesolvent composition such that the stability of all the complexes increased with increase in methanol concentration in themethanol-water binary mixtures (Table 1). This observation can be attributed lower solvation capacity of methanol comparedto water. In a strong solvating solvent such as water with a relatively high Gutmann donor number (DN = 33), the solvation ofthe metal cation is high compared to methanol (DN = 19.0), thus the formation of the complex is weakened as theconcentration of water increases in methanol-water binary mixtures. However, the solvent-solvent interaction betweenmethanol and water does not have any significant impact and there was a linear relationship between the log Kf of thecomplexes and the composition of methanol-water binary mixtures. Comparing the formation constant of these cations withDCH 18C6 in methanol and those reported with its parent compound 18C6 (Lee et al., 1990) shows lower values in thepresent work possibly due to the presence of cyclohexyl groups which results in unsuitable conformation of this ligand.

Table 1Formation constants and thermodynamic parameters for DCH 18C6 complexes in methanol and methanol- water binary mixtures

Crown ether –metal ion complex log Kf ΔGo

(kJ/mol)ΔHo

(kJ/mol)ΔSo

(J/mol/k)25 oC 35oC 45 oCDCH18C6-Ag+

Methanol-water (70:30, mol %)Methanol-water (80:30, mol %)Methanol-water (90:30, mol %)Pure methanol

3.703.843.954.01

3.483.623.773.96

3.203.383.663.93

-21.11-21.91-22.53-22.88

-9.09-26.46-41.88-45.58

46.26-13.06-66.63-81.83

DCH18C6-Tl+Methanol-water (70:30, mol %)Methanol-water (80:30, mol %)Methanol-water (90:30, mol %)Pure methanol

3.904.024.124.18

3.793.944.054.12

3.653.864.004.05

-22.25-22.93-23.50-23.85

-11.81-12.76-14.58-19.15

40.4436.1728.1510.53

DCH18C6-Pb2+

Methanol-water (70:30, mol %)Methanol-water (80:30, mol %)Methanol-water (90:30, mol %)Pure methanol

3.773.894.014.11

3.603.743.914.09

3.523.643.794.02

-21.51-22.19-22.82-23.45

-5.47-17.30-22.82-22.76

67.2818.69-2.01-4.00

]][[

][

LM

MLK

n

n

f

Figure 2Molar conductance versus mole ratio plots for DCH18C6-Ag+ complex in methanol and methanol-water at 25 ºC, 35 ºC and 45ºC.



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Figure 3Molar conductance versus mole ratio plots for DCH18C6-Tl+ complex in methanol and methanol-water at 25 ºC, 35 ºC and 45ºC

Figure 4Molar conductance versus mole ratio plots for DCH18C6-Pb2+ complex in methanol and methanol-water at 25 ºC, 35 ºC and 45ºC

Enthalpy:It can be defined as ameasure of thetotal energy ofa thermodynamic system,which includes thesystem's internalenergy or thermodynamicpotential (a state function),as well as its volume andpressure (an extensivequantity).

Entropy:It is a measure of disorderof a system or the numberof specific ways in which asystem may be arranged.



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In order to have a better understanding of the thermodynamic behaviorof Mn+-DCH 18C6 complexes in methanol and methanol-water binarymixtures, it is worthwhile to consider the contribution of enthalpy andentropy in these reactions. The influence of temperature on theformation constants of these metal complexes shows that the stabilitydecreases as the temperature is increased (Table 1). This correlationindicates that the process of complex formation is exothermic. The datain table shows that the values of thermodynamic parameters forcomplexation reactions vary with the nature and composition of themixed solvents. The negative values for free energy (∆Go) indicate thatthe complexation process is spontaneous. The values forthermodynamic quantities ∆Ho and ∆S o were computed from the slopesand intercepts of the corresponding ln Kf vs. 1000/temperature plots(Figure 6) by applying a linear least square analysis according to thevan’t Hoff equation,

2.303 log Kf = - (ΔH/RT) + (ΔS/R)

The values of enthalpy and entropy for complexation reactions variedmonotonically with the solvent composition. The experimental values ofΔHo and ΔSo illustrate that all complexes with Tl+ ion were enthalpy and

entropy stabilized. However, some of the Ag+ and Pb2+ complexes in pure methanol, 80 and 90 mol% methanol were entropydestabilized. Nevertheless, the unfavorable contribution of entropy was adequately compensated by the magnitude of T∆So

values, which favored the process of complexation. Since there are many factors which contribute to changes in complexationenthalpies and entropies, therefore one should not expect regularity between these parameters and the binary solventmixtures.

4. CONCLUSIONIn the present work, the results obtained for the complexation between DCH 18C6 and Ag+, Tl+ and Pb2+ cations can beexplained in terms of the size-fit concept, where the crown ether forms a most stable complex with a cation having a sizewhich fits best with its cavity size. The results also show that the formation constants depend on the composition andconcentration of the mixed solvent systems. Further, the thermodynamic parameters demonstrate that the complexationprocess for Tl+ cation is enthalpy as well as entropy stabilized. However, with increasing concentration of methanol in thesolvent mixture, the complexation process with Ag+ and Pb2+ ions was entropy destabilized.

SUMMARY OF RESEARCH1.The stoichiometry of DCH18C6-Ag+, DCH 18C6-Tl+ and DCH 18C6-Pb2+ complexes was 1:1 in methanol and methanol-water binary mixtures.2.The formation constants of all the complexes increased with increase in methanol concentration in the methanol-water binary mixtures.3.All DCH 18C6-metal ion complexation reactions were spontaneous and enthalpy stabilized.

FUTURE ISSUESBased on the results obtained it would be interesting to evaluate the stability and selectivity order for dioxane-water mixtures which is unexplored yet. Thissystem owes its status largely to the possibility of attaining a large range of dielectric constants (2.2 < ε < 78 at 25 °C) by varying the solvent composition.

DISCLOSURE STATEMENTThere is no financial support for this research work from any funding agency.

ACKNOWLEDGMENTThe authors wish to thank all our colleagues for their constructive criticism and assistance towards successful completion of the work.

Figure 5Variation of log Kf for DCH18C6-Ag+, DCH 18C6-Tl+ and DCH 18C6-Pb2+

complexes versus the ionic radii in methanol-water binary mixtures at 25 oC

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Figure 6Plot of log Kf versus 1000/T for (a) DCH18C6-Ag+, (b) DCH 18C6-Tl+ and (c) DCH 18C6-Pb2+ complexes

Free energy:The energy that canbe converted intouseful work at auniform temperatureand pressure for agiven process.



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thallium (i) and lead (i i) with dicyclohexyl-18-crown-6 in methanol … · 2017-11-08 · lead (ii) with dicyclohexyl-18-crown-6 in methanol-water binary mixtures. Indian Journal