Ultrasonic investigation of ion-solvent interactions in aqueous and non-aqueous solutions of transition and inner transition metal ions

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 45, February 2007, pp Ultrasonic investigation of ion-solvent interactions in aqueous and non-aqueous solutions of transition and inner transition metal ions V Kannappan * & S Chidambara Vinayagam Postgraduate Department of Chemistry, Presidency College, Chennai Received 20 February 2006; revised 5 May 2006; accepted 8 December 2006 Ultrasonic velocities (u) and densities (ρ) have been measured for aqueous and dimethyl sulphoxide (DMSO) solutions of manganous(ii) sulphate, cobalt(ii) chloride, nickel(ii) sulphate, ferrous sulphate, copper(ii) sulphate, zinc(ii) sulphate, lanthanum(iii) nitrate, cerium(iv) nitrate, uranyl nitrate and thorium(iv) nitrate in a wide range of concentrations at 303 K. Acoustical parameters, such as, adiabatic compressibility (β), free length (Lf) and solvation number (S n ) have been computed to assess the ion-solvent interaction in these solutions. It is found that the ion-solvent interactions depend on concentration, ionic size and nature of metal ion. The transition and inner transition metal ions behave as structure breaker for the associated clusters of water molecules, especially in dilute solution, as evidenced from the trend in the solvation number with molarity. The strength of ion-dipole interaction in aqueous solution of the metal ion is also concentration dependent, which is also investigated in the present work. Keywords: Ultrasonics, Inner transition metal ions, Ultrasonic velocity, Solvation number, Molarity IPC Code: B01J19/10 1 Introduction Ultrasonic velocity measurements are helpful to study the ion-solvent interactions in aqueous and nonaqueous solutions in recent years 1-7. Ultrasound has been extensively used to determine the ion solvent interactions in aqueous solutions containing electrolytes 8. A number of researchers have studied acoustical properties of solutions containing transition metal ions 9,10. In solution of ionic solute the attraction between the solute and solvent is essentially of iondipole type. The ion-dipole interaction depends mainly on ion size and polarity of the solvent. The strength of ion-dipole attraction is directly proportional to the size of the ion, charge and the magnitude of the dipole; but inversely proportional to the distance between the ion and the dipolar molecule 11,12. The dissolution of electrolyte in a solvent causes a volume contraction due to interaction between ions and solvent molecules and this may influence other acoustical properties in solution 13,14. The acoustical parameters such as adiabatic compressibility (β), free length (L f ), solvation number (S n ), acoustical impedance (Z) have been calculated from measured ultrasonic velocity (U) and density (ρ) values of solutions containing certain transition and inner transition metal ions. In this paper, the factors, which affect the ion-solvent interactions in aqueous and dimethyl sulphoxide (DMSO) media at 303K have been analyzed. 2 Experimental Details The hydrated salts used in the present studies, namely, MnSO 4.7H 2 O, FeSO 4.7H 2 O, CoCl 2.6H 2 O, NiSO 4.7H 2 O, CuSO 4.5H 2 O, ZnSO 4.7H 2 O, La(NO 3 ) 2. 6H 2 O, Ce(NO 3 ) 3.6H 2 O, UO 2 (NO 3 ) 2.6H 2 O, and Th(NO 3 ) 4.6H 2 O were of AnalaR (SDS) grade. Water used in the present investigation was prepared by distilling distilled water over alkaline potassium permanganate in an all glass distillation flask. Dimethylsulphoxide (DMSO) was distilled under reduced pressure before use (341K/10 mm of Hg). All aqueous solutions were prepared in double distilled water. The ultrasonic velocity was measured by a single crystal interferometer operating at a frequency of 2 MHz. Measurements in aqueous solution were conducted in the presence of 0.8M HClO 4 to prevent hydrolysis. The temperature was maintained constant at 303.0±0.1 K. Densities of the solutions were determined accurately using 10 ml specific gravity bottle and electronic balance (accuracy ± 0.1 mg).

2 144 INDIAN J PURE & APPL PHYS, VOL 45, FEBRUARY Results and Discussion Ultrasonic investigations have been carried out on aqueous solutions containing six different transition metal ions at 303K in a wide range of concentration ( M) to investigate the ion-solvent interactions in very dilute and moderate concentrations. As these metal ions are susceptible for hydrolysis in aqueous solutions, measurements were made in the presence of 0.8M perchloric acid. Table 1 presents the acoustical parameters for aqueous solutions containing Mn and Fe ions. The data for Co and Ni are presented in Table 2. Table 3 presents U, ρ, β, S n, Z values for aqueous solutions containing Cu and Zn ions. In these systems, the slight decrease in U in very dilute solutions suggests that the transition metal ions behave as structure breaker for cluster of water molecules. However, in the moderate concentrations, the ultrasonic velocity Table 1 Acoustical parameters for aqueous solutions of Mn and Fe at various concentrations Temp. = 303K U/ms -1 ρ/kgm -3 ß/10-10 Kg -1 ms -2 Lf/10-10 m Sn Z/10 6 Kgm -2 S -1 Mn Fe Mn Fe Mn Fe Mn Fe Mn Fe Mn Fe Table 2 Acoustical parameters for aqueous solutions of Co and Ni at various concentrations Temp=303K U/ms -1 ρ/kgm -3 ß/10-10 Kg -1 ms -2 Lf/10-10 m Sn Z/10 6 Kgm -2 S -1 Co Ni Co Ni Co Ni Co Ni Co Ni Co Ni

3 KANNAPPAN & VINAYAGAM: ULTRASONIC STUDY OF ION-SOLVENT INTERACTIONS 145 Table 3 Acoustical parameters for aqueous solutions of Cu and Zn at various concentrations Temp. = 303K U/ms -1 ρ/kgm -3 ß/10-10 Kg -1 ms -2 Lf/10-10 m Sn Z/10 6 Kgm -2 S -1 Cu Zn Cu Zn Cu Zn Cu Zn Cu Zn Cu Zn increases with concentration, which suggest that ionsolvent interaction also increases with concentration. The plots of U versus concentration of these six systems are given in Fig. 1. It is found that the plot is steaper for Ni and Zn indicating stronger iondipole interactions in the aqueous solutions containing these metal ions. It may be due to size effect in the case of Ni and uniform charge distribution in the case of Zn, which has 3d 10 as its outer electronic configuration. The ion-solvent interaction is relatively weak in the case of aqueous solution containing Cu ions. The trend in the adiabatic compressibility values also confirms this. There is a regular decrease in L f values with concentration in the six systems. When the concentration of electrolyte is increased, the thickness of oppositely charged ionic atmosphere may decrease due to increase in ionic strength. This is suggested by the increase in acoustic impedance with concentration in the six systems investigated. Ultrasonic investigations were also carried out in aqueous solution containing two iso-electronic lanthanide (La +3 and Ce +4 ) ions and two iso-electronic actinide (Th +4 and U +6 ) ions at 303K. In these four systems the concentrations are restricted to M due to solubility reason. The ultrasonic velocities and other acoustical parameters for these systems are presented in Table 4. The non-linear variations in the ultrasonic velocity values in all the four systems indicate the presence of strong iondipole interaction in aqueous solutions containing inner transition metal ions even in dilute solutions. The change in ultrasonic velocity is greater in the case of Ce +4 system than in La +3, suggesting that stronger ion-dipole interactions are there in Ce +4 system than in La +3 solution. This may be due to the smaller size and larger ion charge of Ce +4. In the case of actinide ions the solute-solvent attractive forces are stronger in Th +4 solution than in solution containing UO 2 ions. This is due to larger size and smaller charge on uranyl ion. Ultrasonic velocity measurements can be used to investigate the primary and secondary sheath of solvation. The solvent molecules are attached to the ion by strong coordination bond in primary sheath of solvation and there are weak forces of attraction between solute and solvent species in secondary sheath. The solvation number in the primary sheath corresponds to coordination number and it is concentration independent, while the solvation number in the secondary sheath is concentration dependent. The values of solvation number in aqueous solutions of the six transition metal ions are given in Tables 1-3. Plots of S n values against concentration are given in Fig. 2 for aqueous solutions, while Fig. 4 contains similar plots in

4 146 INDIAN J PURE & APPL PHYS, VOL 45, FEBRUARY 2007 Fig. 1 Plots of ultrasonic velocity versus concentration in M for transition metal ions and Inner transition metal ions in aqueous solutions at 303K Fig. 2 Plots of solvation number versus concentration in M for transition metal ions and Inner transition metal ions in aqueous solutions at 303K Table 4 Acoustical parameters for aqueous solutions of La +3, Ce +4, Th +4 and UO 2 at various concentrations Temp. = 303K U/ms -1 ρ/kgm -3 ß/10-10 Kg -1 ms -2 La +3 Ce +4 Th +4 UO 2 La +3 Ce +4 Th +4 UO 2 La +3 Ce +4 Th +4 UO (Contd.)

5 KANNAPPAN & VINAYAGAM: ULTRASONIC STUDY OF ION-SOLVENT INTERACTIONS 147 Table 4 Acoustical parameters for aqueous solutions of La +3, Ce +4, Th +4 and UO 2 at various concentrations Temp. = 303K (Contd ) Lf/10-10 m Sn Z/10 6 Kgm -2 S Fig. 3 Plots of solvation number versus concentration in M for transition metal ions and Inner transition metal ions in DMSO at 303K Fig. 4 Plots of solvation number versus concentration in M for transition metal ions and Inner transition metal ions in DMSO at 303K

6 148 INDIAN J PURE & APPL PHYS, VOL 45, FEBRUARY 2007 DMSO medium. The solvation number in a given system decreases with increase in concentration and it may attain the primary solvation in pure crystalline state. Thus, the S n values obtained from ultrasonic velocity measurements give extent of solvation in both primary and secondary sheath of solvation. Similar observations are made in aqueous solution of inner transition metals also. It may be pointed out the S n values of inner transition metal ions in DMSO solution are much greater than those of transition metal ions. This indicates that the inner transition metal ions contain more number of solvent molecules in secondary sheath of solvation which may be due to the presence of strong dispersive forces in these systems. The solvation number values are both negative and positive for Ce +4 and Th +4 system in aqueous solutions indicating structure breaking behaviour of these two Table 5 Acoustical parameters for solutions of Mn,Fe and Co in DMSO at various concentrations Temp=303K U/ms -1 ρ/kgm -3 ß/10-10 Kg -1 ms -2 Mn Fe Co Mn Fe Co Mn Fe Co Lf/10-10 m Sn Z/10 6 Kgm -2 S -1 Mn Fe Co Mn Fe Co Mn Fe Co Table 6 Acoustical parameters for solutions of Ni, Cu and Zn in DMSO at various concentrations Temp=303K U/ms -1 ρ/kgm -3 ß/10-10 Kg -1 ms -2 Ni Cu Zn Ni Cu Zn Ni Cu Zn Lf/10-10 m Sn Z/10 6 Kgm -2 S -1 Ni Cu Zn Ni Cu Zn Ni Cu Zn

7 KANNAPPAN & VINAYAGAM: ULTRASONIC STUDY OF ION-SOLVENT INTERACTIONS 149 Table 7 Acoustical parameters for solutions of La +3, Ce +4, Th +4 and UO 2 in DMSO at various concentrations Temp=303K U/ms -1 ρ/kgm Lf/10-10 m Sn La +3 Ce +4 Th +4 UO 2 La +3 Ce +4 Th +4 UO ß/10-10 Kg -1 ms -2 Z/10 6 Kgm -2 S -1 La +3 Ce +4 Th +4 UO 2 La +3 Ce +4 Th +4 UO systems. The solvation number is maximum at specific concentration in each system and the ionsolvent interaction is stronger at this concentration. However, the solvation number in DMSO medium is positive in the concentration range investigated. There is a regular decrease in solvation number with increase in concentration indicating the decrease in the size of the secondary sheath of solvation with increase in concentration in all the four systems in DMSO medium. 4 Conclusions Ultrasonic investigations in aqueous and DMSO solutions containing transition and inner transition metal ions indicate the presence of strong ion-dipole interactions. The strength of the solute-solvent interaction depends on concentration, size, charge, symmetry of charge distribution of the ion and polarity of the medium. These investigations can be used to analyse the primary and secondary sheath of solvation. The solvent molecules in secondary sheath decrease with increase in concentration as indicated by the decrease in solvation number. Acknowledgement One of the authors (S Chidambara Vinayagam) is grateful to University Grants Commission, New Delhi, for award of a Teacher Fellowship. References 1 Jayakumar S, Karunanidhi N & Kannappan V, Indian J Pure & Appl Phys, 34 (1996) Ali A & Nain A K, Acoustics Lett, 19 (1996) Tewari K, Patra C & Chakraborrty V, Acoustics Lett, 19 (1996) Tabhane V A, Sharda Ghosh & Pranjala S W, Indian J Phys, 23 (1985) 502.

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