Acoustical Behaviour of SodiumNitroprusside in Aquo-Organic Solvent Media at 308 . 15 K

Density and ultrasonic velocity have been measured for sodium nitroprusside in aqueous solutions of CH3OH, ethylene glycol, DMSO, and n-propanol solvents at 308.15K. A quantitative relationship has been established among the acoustical properties like ultrasonic velocity (U), adiabatic compressibility (ββ), intermolecular free length (LLff), acoustic impedance (Z), apparent molar compressibility (Kφφ), apparent molar volume (Vφφ), limiting apparent molar compressibility (KK 0 φφ) limiting apparent molar volume (VVφφ), and their constants (SSKK, SSvv). From the obtained values, molecular interaction study has been made successfully in the light of these acoustical properties through hydrogen bonding in solute and solvent mixture.


Introduction
In the recent years, ultrasonic velocity measurements are helpful to interpret solute-solvent, ion-solvent, and solventsolvent interactions in aqueous and nonaqueous medium [1][2][3][4].Recently, acoustic parameter studies of metal complexes have been carried out in our laboratory [5], and A. P. Mishra and D. K. Mishra have also reported a similar work [6].e interaction helps in better understanding the types of solute and solvent, that is, whether the added solute modi�es or distorts the structure of the solvent.Apparent molar volume gives valuable information about ion-ion and ion-solvent interactions in solution [7][8][9][10][11].e addition of organic solvent to an aqueous solvent of electrolyte brings about a change in ion solvation that results in a large change in the reactivity of dissolved electrolyte [12,13].e transition metal plays a vital role in life system because of the natural presence in vitamins, proteins, and enzymes.Sodium nitroprusside (SNP), an Fe (II) ion complex, is an effective drug which rapidly lowers blood pressure and causes vascular smooth muscle relaxation.It is also used in a urine analysis test.In the presence of buffers, it is used as a reagent for ketone strips which test the ketone level in the urine of a diabetic.It is used by forensic chemists in Simons being fast for the identi�cation of illicit substances [14].us, an attempt has been made to elucidate the ion-ion interaction between nitroprusside ions and ion-solvent interaction of SNP in aqueous solutions of methanol, ethylene glycol, npropanol, and DMSO at 308.15 K through ultrasonic velocity and density measurement.

Experimental
e entire chemicals used in this present research work are spectroscopic reagents (SRs), analytical reagent (AR) grades of minimum assay of 99.9% obtained from E-Merck, Germany, and SD �ne chemicals, India, which are used without further puri�cation.�ater used in these experiments was deionized and distilled.A synthetic method of preparation of SNP has been reported [15].e required quantity of SNP was dissolved in binary mixture of aqueous methanol, ethylene glycol, DMSO, and n-propanol, and a similar procedure has been adopted for different molalities of SNP.e density was determined using a speci�c gravity bottle by a relative measurement method.e ultrasonic velocity was measured by ultrasonic interferometer having frequency 2 MHz (Mittal Enterprises, model no.F-81).e constant temperature is mentioned by circulatory water through the double wall measuring cell made up of steel.

Theory
Using the measured data, various acoustic parameters such as adiabatic compressibility, intermolecular free length, acoustic impedance, apparent molar compressibility, apparent molar volume, limiting apparent molar compressibility, limiting apparent molar volume, and the associated constants   and   are calculated.
Intermolecular free length can be determined by where   is Jacobson constant = 2.0965 × 10 −6 .e apparent molar compressibility,   , can be computed from From the density data, the apparent molar volume   was calculated from the relation e apparent molar compressibility (  ) and the apparent molar volume (  ) thus obtained are found to vary linearly with  2 .e   and   data were �tted by least squares method where  0  and  0  are limiting apparent molar compressibility and limiting apparent molar volume, respectively.  and   are the slopes.

Result and Discussion
e desired parameters such as adiabatic compressibility (), intermolecular free length (  ), acoustic impedance (), relative association (  ), apparent molar comparability (  ), apparent molar volume (  ), limiting apparent molar compressibility ( 0  ), limiting apparent molar volume ( 0  ) and their constants (  ,   ) have been studied at 308.15 K. e density increases with the increase in the concentration of sodium nitroprusside due to the increased electrostriction in the system.e density increases in case of CH 3 OH + water, ethylene glycol + water, DMSO + water, and npropanol + water with the increase in the concentration of sodium nitroprusside, but it decreases with mole fraction of CH 3 OH and n-propanol and increases with the mole fraction of ethylene glycol and DMSO for a particular concentration of sodium nitroprusside.e decrease in density in case of CH 3 OH + water and n-propanol + water indicates the decrease in ion-solvent interaction or structure-breaking properties of sodium introprusside.But addition of ethylene glycol and DMSO to water has rigidi�ed the threedimensional structure of water forming a strong hydrogen bond between solvent moleuculs [16,17].It also indicates the structure-forming properties of sodium nitroprusside in ethylene glycol + water and DMSO + water.
Table 1 shows that ultrasonic velocity increases with the increase in the concentration of sodium nitroprusside in aqueous CH 3 OH and DMSO, but it is found to be decreased in aqueous ethylene glycol and n-propanol.Molecular association is responsible for the observed increase in ultrasonic velocity in these mixtures.e increase in ultrasonic velocity in these solutions may be attributed to the cohesion brought about by the hydration [18].
e adiabatic compressibility (Table 1) decreased with increasing the concentration of solute in all aqueous solvent solutions except n-propanol + water.e decreasing adiabatic compressibility observed for sodium introprusside in CH 3 OH + water, ethylene glycol + water, and DMSO + water con�rms the calculation drawn from the velocity data.e increasing electrostrictive compression of water around the solute molecules results in a large decrease in the compressibility of solutions.e decrease in the compressibility implies that the molecular association is enhanced in these systems with the increase in the solute content as the new entities formed due to molecular association become compact and less compressable [19].e compressibility appeared to decrease with increasing hydrogen bond strength between solute and solvent molecules.e larger decrease in compressibility of nitroprusside in DMSO + water indicates larger molecular association capacity than other solvents.But the increase of adiabatic compressibility with the increase in concentration of solution may be due to collection of solvent molecules around ion [20] supporting weak ionsolvent interaction.is indicates that there is signi�cant ionsolvent interaction.
e intermolecular free length (  ) is a predominant factor in solvation chemistry.Intermolecular free length depends on the intermolecular attractive and repulsive forces.In the present paper, the intermolecular free length is found to decrease with the increase in the concentration of nitroprusside as shown in Table 1, indicating a signi�cant molecular association between solute and solvent molecules, suggesting a structure-promoting behaviors on addition of solute.
e character that determines the restriction/backward movement of sound wave is known as acoustic impendence ().It has been estimated for nitroprusside solution of CH 3 OH + water, ethylene glycol + water, and DMSO + water which is found to increase with the increase in the concentration of sodium nitroprusside.But a reverse trend in the value of  is observed for sodium nitroprusside solution of n-propanol + water.e higher impedance in DMSO + water indicates the presence of bulkier-solvated ion due to ion-solvent/solvent-solvent interaction which restrict the free �ow of ultrasonic velocity.
T 1: e values of density (), ultrasonic velocity (), adiabatic compressibility (), intermolecular free length (  ), acoustic impedance (), and relative association (  ) of sodium nitroprusside in CH 3 OH + water, ethylene glycol + water, DMSO + water, and n-propanol + water at 308. 15  e decrease in relative association (  ) is due to the breaking up of the solvent molecules on adding the solute, whereas the increase as   may be due to the solvation of solute [21].In all the systems,   increases with the increase in the concentration due to the increase in electrostatic attraction.From this data, it can be said that ion-ion interaction between nitroprusside ions overcomes the ionsolvent interaction.e following observations have been made on   and   of sodium nitroprusside in aqueous CH 3 OH, ethylene glycol, DMSO, and n-propanol mixtures whose values are given in Table 2.
(i) e values of   are negative for all the systems over the entire range of concentration.
(ii) e values of   increase linearly with increasing the ion concentration.
(iii) e values of   are positive for CH 3 OH and npropanol whereas negative for ethylene glycol and DMSO.
Further, the negative values of   indicated electrostatic solvation of ions [22].In other words, the large negative value of   may indicate the presence of packing or caging effect [23].e decrease in the negative value indicates that with the increase in the mole fraction of aquo-organic solvents, ionsolvent interaction increases as the caging effect diminishes.
e limiting apparent molar compressibility  0  provides information regarding ion-solvent interaction.e values of limiting apparent molar compressibility  0  are negative in all the systems as the shown in Figures 1, 2   decreases with the increase the in concentration of sodium nitroprusside solution for ethylene glycol and DMSO systems but increases in case of n-propanol system.e decrease is not regular in CH 3 OH system.It implies that a little bit ionion interaction exists in CH 3 OH and more ion-ion interaction in n-propanol systems.But the large negative values for ethylene glycol and DMSO systems indicate negligible ionion interaction and stronger ion-solvent interaction.e volume behavior of solute at in�nite dilution is satisfactorily represented by  0  which is independent of ion-ion interaction and provides information concerning ion-solvent interaction.e values of  0  are positive for CH 3 OH and n-propanol shown in Figures 5 and 8, whereas these are negative for ethylene glycol and DMSO (Figures 6 and 7).us in ethylene glycol and DMSO solutions, predominance of ion-solvent interaction is again proved by negative  0  and  0  values.However, in CH 3 OH and npropanol solutions, positive values of  0  suggest that ion-ion interactions predominate.
e constant   provides information regarding ion-ion interaction.e values of   (Table 3) exhibit positive in all the systems.ese values increase in ethylene glycol and DMSO systems, but this increase is not regular in CH 3 OH con�rming insigni�cant ion-ion interaction.It decreases in n-propanol system.e value of   in n-propanol system is higher at low mole fraction, indicating more ion-ion interaction.In case of ethylene glycol and DMSO systems, ion-ion interactions are stronger because of having greater value of   .So ion-ion interaction is favoured at high mole fraction of ethylene glycol and DMSO, but it is favoured at low mole fraction of n-propanol.e   exhibits negative values in CH 3 OH and n-propanol systems suggesting the presence of more ion-ion interactions and less complex formation occurring in the systems.e values of   which exhibit positive from Table 3 in ethylene glycol and DMSO systems re�ect the existence of weak ion-ion interactions and predict the complex formation taking place in the systems.
Generally, the types of interactions occurring between sodium nitroprusside and aqueous solutions of solvents can be classi�ed as follows: (i) ionic-hydrophilic interactions between the ions of sodium nitroprusside and polar group of solvents, (ii) hydrophilic-hydrophobic interactions between the ions of sodium nitroprusside and the nonpolar side group of solvents.

Conclusion
In summary, volume and compressibility data have been determined for sodium nitroprusside in aqueous CH ethylene glycol, DMSO, and n-propanol at 308.15 K, and the results have been used to study the ion-solvent interactions present in the mixtures.From the magnitude of  0  . 0  ,   and   , it can be concluded that a larger ion-solvent interaction will exist in ethylene glycol and DMSO solvents than other solvents.us, sodium nitroprusside is an effective structure maker in aqueous ethylene glycol and DMSO solutions over other two systems.
, 3, and 4. It F 2: Variation of apparent molar with compressibility (  ) with  1/2 for sodium in different mole fraction of ethylene glycol in water at 308.15 K.  0  corresponds to its value at zero concentration.
F 4: Variation of apparent molar compressibility (  ) with  1/2 for sodium nitroprusside in different mole fraction of npropanol in water at 308.15 K.  0  corresponds to its value at zero concentration.F 6: Variation of apparent molar volume (  ) with  1/2 for sodium nitroprusside in different mole fraction of ethylene glycol in water at 308.15 K.  0  corresponds to its value at zero concentration.F 8: Variation of apparent molar volume (  ) with  1/2 for sodium nitroprusside in different mole fraction of n-propanol in water at 308.15 K.  0  corresponds to its value at zero concentration.
3 OH, F 5: Variation of apparent molar volume (  ) with  1/2 for sodium nitroprusside in different mole fraction of CH 3 OH in water at 308.15 K.  0  corresponds to its value at zero concentration.F 7: Variation of apparent molar volume (  ) with  1/2 for sodium nitroprusside in different mole fraction of DMSO in water at 308.15 K.  0  corresponds to its value at zero concentration.