Evaluation of apparent and partial molar volume of potassium ferro- and ferricyanides in aqueous alcohol solutions at different temperatures

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1 Indian Journal of Chemical Technology ol. 11, September 4, pp Evaluation of apparent and partial molar volume of potassium ferro- and ferricyanides in aqueous alcohol solutions at different temperatures U N Dash a, G S Roy b * & S Mohanty c a Department of Chemistry, Utkal University, Bhubaneswar 751 4, India b Department of Physics, Rajadhani College, Bhubaneswar, India c Department of Physics, S M College, Jagatsinghpur , India Received 19 September 3; revised received May 4; accepted 16 June 4 Apparent and partial molar volume of potassium ferro- and ferricyanides in aqueous alcohol solutions have been determined at four different temperatures 98.15, 33.15, and K with the objective of studying ion-solvent interaction in these systems. The transfer of volumes for the transfer of these salts from aqueous alcohol solution to water has been evaluated. Negative transfer of volume was observed and the results have been explained on the basis of electrostriction. IPC Code: G1 N 9/, C1 C 3/1 Keywords: Apparent molar volume, partial molar volume, potassium ferrocyanide, apparent molar expansibility, aqueous alcohol. Evaluation of partial molar quantities are of importance as they give a lot of informations regarding ion-solvent interaction in various complex compounds. But since these quantities are not directly experimentally determined it is difficult to throw light on molecular interaction in ternary mixtures. Further, these quantities are related to the corresponding apparent molar quantities which are directly experimentally determined and can be used for studying ion-solvent interaction in solution. The partial and apparent molar properties of potassium ferro and ferricyanides in water and water+acetone mixtures have been reported earlier 1. The same properties of these salts in water + methanol, water + ethanol and water + n-propanol mixtures (5, 1 and wt% in each case) at four different temperatures: 98.15, 33.15, and K are reported here. Further, the transfer of volumes for the transfer of these salts from aqueous alcohol solution to water have also been determined. The contributions of change in temperature, change in composition and increase of chain length have been discussed in the light of electrostriction. Theory The apparent molar volume Φ and apparent molar *For correspondence. expansibility Φ E relations, are computed by the following Φ = 1 (d c -1-1 ) (d - d ) + M d (1) and Φ Ε = α Φ + (α α ) 1 c -1 () where c is the molar concentration, d and d are the densities of solvent and solution respectively, M is the molecular weight of the solute, α and α are the co-efficients of thermal expansion of solvent and solution respectively. The Φ and Φ Ε data are fitted to Masson equation by least squares method, Φ = Φ +S c1/ (3) and Φ Ε = Φ Ε + S Ε c 1/ (4) to obtain Φ, the limiting apparent molar volume, S the experimental slope of Eq.(3), Φ Ε, the limiting apparent molar expansibility and S Ε, the experimental slope of Eq.(4). The partial molar volume and partial molar expansibility have been calculated from the relations 3, = Φ + (1 - cφ ) ( + S c3/ ) -1 S c1/ (5) and E =Φ Ε + (1 - cφ Ε ) ( + S Ε c 3/ ) -1 S Ε c 1/ (6)

2 DASH et al.: EALUATION OF MOLAR OLUME OF POTASSIUM FERRO AND FERRICYANIDES 715 The apparent molar volume at infinite dilution, also called the limiting apparent molar volume is equal to the partial molar volume at infinite dilution. The partial molar volume of transfer of the above mentioned salts from aqueous alcohol solution to water are calculated from the relation, Φ (tr) = Φ (aqueous alcohol) - Φ (water) (7) Experimental Procedure Potassium ferrocyanide and potassium ferricyanide (BDH, Anal Rs) were kept over anhydrous calcium chloride in vacuum desiccator until required. Methanol, ethanol and n-propanol (BDH, Anal Rs.) were dried over 4A molecular sieve and distilled. Conductivity water (sp.cand.~1-6 S cm -1 ) was used for preparing water + alcohol mixtures. The alcohol content in the mixed solvents was accurate to within ±.1%. The solutions were prepared on molal basis and conversion of molarity was done by using standard expression 4. The densities were measured pychnometrically (uncertainty ± kg m -3 ). Temperature was maintained by a thermostat with a precision of ±.5 K. Results and Discussion A perusal of Table 1 and Fig. 1 shows that, Φ values of ferrocyanide salt are positive in water and increases with temperature in all the solvents. With increase of alcohol concentration, the value decreases and becomes negative at certain composition. Negative value of Φ provides evidence of electrostriction 5. Again, since Φ is a measure of ionsolvent interaction, the negative value indicates weaker ion-solvent interaction. The result indicates, that, the ion-solvent interaction increases with temperature, decreases with alcohol concentration and number of -CH - groups in alcohol (i.e. chain length). The lowering of Φ values is probably due to the increased steric hindrance of the bulkier solvent molecules to the ion-solvent interaction. The presence of ion-solvent interaction between the molecules promotes the structure making effect of the salts in water + alcohol mixtures. In case of ferricyanide salt (Table, Fig. ), Φ values are positive in all the solvents and at all the four temperatures. Ion-solvent interaction of ferricyanide salt is greater than that of ferrocyanide salt which implies that ferrocyanide salt shows more structure making effect than ferricyanide salt. Fig. 1 Φ ~ c 1/ for potassium ferrocyanide in 5wt% methanol at (1) K, () K, (3) K, (4) K and in (5) 1 wt% methanol and (6) wt% methanol at K. Fig. Φ E ~ c 1/ for potassium ferricyanide in (1) 5 wt% methanol, () 1 wt% methanol, (3) wt% methanol, (4) wt% ethanol and (5) wt% n-propanol at K. As observed, S values are high and positive at every temperature and decrease with temperature for both the salts. Since, S is a measure of ion-ion interaction, the result indicates the presence of ion-ion interaction in the system at every temperature and both the salts ionize to a greater extent with increase in temperature. Ion-ion interaction increases with increase of alcohol content in the solution. This suggests that more and more solute molecules are accommodated within the void spaces left in the packing of the large associated solvent molecules and as such enhance the structure of the solvent.

3 716 INDIAN J. CHEM. TECHNOL., SEPTEMBER 4 Table 1 alues of Φ (m 3 mol -1 ), S (m 9/ mol -3/ ), Φ (tr) (m 3 mol -1 ), Φ E (m 3 mol -1 K -1 ) and S E (m 9/ mol -3/ K -1 ) for potassium ferrocyanide in water and water+alcohol system at different temperatures wt. % alcohol Temp (K) 1 6 Φ 1 9 S 1 5 Φ (tr) 1 6 Φ E -1 9 S E (methanol) (methanol) (methanol) (ethanol) (ethanol) (ethanol) (n-propanol) (n-propanol) (n-propanol) It is observed that the partial molar volume increases with concentration and temperature in all the solvents whereas decreases with increase of alcohol content in the mixed solvent. Increase of with concentration is owing to the structure breaking of the solvent molecules in concentrated solutions of high charge density ions like potassium characterized by very strong interaction forces with the solvent molecules and this interaction increases with temperature and decreases with alcohol concentration. The values of Φ (tr) are negative for both the salts. The measured partial molar volume can be considered to be a sum of the geometric volume of the solute and changes in the solvent due to its interaction with solvent. This simple approach has been widely used in many models 6 to interpret partial molar

4 DASH et al.: EALUATION OF MOLAR OLUME OF POTASSIUM FERRO AND FERRICYANIDES 717 Table alues of Φ (m 3 mol -1 ), S v (m 9/ mol -3/ ), Φ (tr) (m 3 mol -1 ), Φ E (m 3 mol -1 K -1 ) and S E (m 9/ mol -3/ K -1 ) for potassium ferricyanide in water and water+alcohol system at different temperatures. wt. % alcohol Temp (K) 1 6 Φ 1 9 S 1 5 Φ (tr) 1 6 Φ E -1 9 S E (methanol) (methanol) (methanol) (ethanol) (ethanol) (ethanol) (n-propanol) (n-propanol) (n-propanol) volume data for a broad range of solutes. When two charged centres are not separated by the distance 3-4 A, then their hydration co-spheres overlap which results in the decrease in the electrostriction. The overlap of co-spheres of two ionic species shows an increase in volume whereas overlap of hydrophobichydrophobic and ion-hydrophobic groups results in decrease in volume. In the present case there is increase of electrostrictive solvation as well as hydrophobic solvation. Hydrophobic solvation increases as the number of -CH - group increases in the alcohols. As expected, the partial molar expansibility E decreases with concentration and increases with temperature. The value of Φ Ε increases with temperature indicating the presence of caging or

5 718 INDIAN J. CHEM. TECHNOL., SEPTEMBER 4 packing effect 8. As is seen, the Φ Ε values increase with increase of alcohol content in the mixed solvent. This suggests that the structure making effect of the electrolytes studied is favoured in aqueous alcohol medium as compared to aqueous medium. References 1 Dash U N, Roy G S & Mohanty S, J T R Chem, 9(1) () 55. Harned H S & Owen B B, The Physical Chemistry of Electrolytic Solutions, 3rd Edn (Reinhold Publishing Corporation, New York), 1958, Dash U N & Nayak S K, Thermochim Acta, 3 (1979) 331; 34 (1979) Robinson R A & Stokes R H, Electrolyte Solutions (Butter Worths Scientific Publications, London), 1955, 3. 5 Davis C W, Ion Association (Butter Worths Scientific Publication, London), 196, Rohankar P G & Aswar A S, Indian J Chem, 41A () Chalikian T, Sarvzyam A P & Breslauer K J, J Phys Chem, 97 (1993) Millero F J, Structure and Transport Processes in Water and Aqueous Solutions, Ch.15 edited by Horne R A (Wiley Interscience, New York), 1971.

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