Exploration of Solution Behaviour of Potassium Halides in Mixtures Of L-Proline And Water At 298.15, 308.15 And 318.15 K

Apparent molar volume ( V  ) and viscosity B-coefficients were estimated for potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L-proline from measured solution density ( ρ ) and viscosity ( η ) at 298.15, 308.15 and 318.15 K at various electrolyte concentrations. The experimental density data were evaluated by Masson equation and the derived data were interpreted in terms of ion–solvent and ion–ion interactions. The viscosity data has been analyzed using Jones–Dole equation and the derived parameters, B and A, have also been interpreted in terms of ion–solvent and ion–ion interactions respectively. The structure-making or breaking capacity of the electrolyte under investigation has been discussed in terms of sign 


Introduction
Studies on densities (ρ) and viscosities (η) of electrolyte solutions are of great importance in characterizing the properties and structural aspects of solutions. The addition of an electrolyte to an aqueous L-proline solution alters the pattern of ion solvation and causes phenomenal changes in the behaviour of the dissolved electrolyte. The viscosity data of solutions for the electrolytes in L-proline have been analyzed using Jones-Dole equation and A and Bcoefficients obtained from the equation are good indicative of ion-solvent and ion-ion interactions respectively. Hence studies on the limiting apparent molar volume and viscosity B-coefficients of electrolyte provide us valuable information regarding ion-ion, ion-solvent and solvent-solvent interactions [1][2][3]. L-proline is often used as asymmetric catalyst and 764 therefore it is used in many biotechnological reactions [4]. It has been found by a number of workers [5][6][7] that the addition of an electrolyte could either make or break the structure of a liquid. As the viscosity of a liquid depends on the intermolecular forces, the structural aspects of the liquid can be inferred from the viscosity of solutions at various electrolyte concentrations and temperature.
In this paper we have attempted to report the limiting apparent molar volume ( 0 V  ), experimental slopes ( V * S ) and viscosity B-coefficients for potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L-proline at 298. 15, 308.15 and 318.15 K. Since potassium ion being a common cation for all of the electrolytes under investigation, the present work enables us to have a qualitative comparison of the role of anion in aqueous L-proline in terms of various derived parameters obtained from viscosity (η) and density (ρ) measurements.

Materials
L-proline ( SD. Fine Chemicals) was purified by standard methods [8]. The purity of the solvent was checked by measuring the viscosity (η) and density (ρ) at 298.15 K which was in good agreement with the literature values. Doubly distilled, degassed and deionised water with a specific conductance of 1×10 −6 Ω −1 cm −1 was used. Potassium chloride, potassium bromide and potassium iodide (Sigma-Aldrich, Germany) were purified by re-crystallizing twice from conductivity water and then dried in a vacuum dessicator over P 2 O 5 for 24 h before use. The experimental values of viscosity (η) and density (ρ) of aqueous mixtures of 0.01 M , 0.03 M and 0.05 M L-proline at different temperatures are listed in Table 1.

Apparatus and procedure
Densities (ρ) were measured with an Anton Paar density-meter (DMA 4500M) with a precision of 0.0005 g/cm 3 . The calibration was done by double-distilled water and dry air and uncertainty in density was ±0.00005 g cm −3 .
The measurements were done in a thermostat bath controlled to ±0.01 K. Viscosity (η) was measured by means of Brookfield DV-III Ultra Programmable Rheometer with spindle size-42 having an accuracy of 1.0% and fitted to a Brookfield Digital Bath TC-500 at 298K using 765 density and viscosity values from the literature [9][10][11]. The uncertainty in viscosity measurements is within ±0.003 mPa.s. The mixtures were prepared by mixing known volume of pure liquids in airtight-stopper bottles and each solution thus prepared was distributed into three recipients to perform all the measurements in triplicate, with the aim of determining possible dispersion of the results obtained. Adequate precautions were taken to minimize evaporation loses during the actual measurements.
The electrolyte solutions studied here were prepared by mass and the conversion of molality into molarity was accomplished [3] using experimental density values. The experimental values of concentrations (c), densities (ρ), viscosities (η), and derived parameters at various temperatures are reported in Table 2.

Density calculation
The apparent molar volumes ( V  ) were determined from the solution densities using the following Eq. [3]: where M is the molar mass of the solute, c is the molarity of the solution; ρ 0 and ρ are the densities of the solvent and the solution respectively. The limiting apparent molar volumes 0 V  were calculated using a least-square treatment to the plots of V  versus √c using the following Masson equation [12]:  Table 3.

766
In these systems the ion-solvent and ion-ion interactions can be interpreted in terms of structural changes between various components of the solvent and solution systems. 0 V  can be used to interpret ion-solvent interactions. Table 3  water. This is probably due to more violent thermal agitation at higher temperatures, resulting in diminishing the force of ion-ion interactions (ionic-dissociation) [13]. This suggests that ion-solvent interactions dominate over ion-ion interactions in all the solutions and at all experimental temperatures.
The variation of 0 V  with temperature of potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L-proline follows the polynomial,  Table 4.
The apparent molar expansibilities ( 0 E  ) can be obtained by the following equation: 767 potassium chloride increases with the increase in the amount of L-proline in the mixture.
However, for potassium bromide and potassium iodide the 0 E  values were found to be rather complicated to explain. During the past few years it has been emphasized by a number of workers that V * S is not the sole criterion for determining the structure making or breaking tendency of any solute. Hepler [14] developed a technique of examining the sign of for the solute in terms of long-range structure-making and breaking capacity of the electrolytes in the mixed solvent systems. The general thermodynamic expression used is as follows is positive or small negative [15,16] the electrolyte is a structure maker and when the sign of where η 0 and η are the viscosities of the solvent and solution respectively. A and B are the coefficients estimated by least square method and are reported in Table 6. The effects of ion-solvent interactions on the solution viscosity can be inferred from the Bcoefficient [18,19]. The viscosity B-coefficient is a valuable tool to provide information concerning the solvation of the solutes and their effects on the structure of the solvent. From Table 6 [20,21] that dB/dT is a better criterion for determining the structure-making/breaking nature of any solute rather than simply the value of the B-coefficient. It is found from Table 6 that the values of the Bcoefficient increase with a rise in temperature (positive dB/dT) suggesting the structurebreaking tendency of potassium chloride, potassium bromide and potassium iodide in the solvent systems.

Conclusion
Extensive study of potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L-proline reveals that potassium iodide is more associated in L-proline than the other two halides. The ion-association is found minimum in the case of potassium chloride in L-proline. The said interaction of potassium bromide arises in the intermediary of potassium iodide and potassium chloride. The present study reveals the predominance of ionsolvent interaction over the ion-ion interaction in all the solution under investigation.     Table 3.   Table 4.
Values of the coefficients of Eq.   Table 5.
Limiting partial molar expansibilities for potassium chloride, potassium bromide and