EFFECT OF DIELECTRIC CONSTANT ON THE SOLVATION OF SODIUM CITRATE

Transcription

1 cadeic Sciences International Journal of Cheistry Research ISSN Vol 3, Issue 2, 212 EFFECT OF DIELECTRIC CONSTNT ON THE SOLVTION OF SODIUM CITRTE Research rticle J ISHWR BHT a * ND M N MNJUNTH b a Departent of Cheistry, Mangalore University, Mangalagangothri , b S.J.C. Institute of Technology, Chickballapur-56211, India. Eail: bhatij@yahoo.co BSTRCT Received: 18 Oct 211, Revised and ccepted: 23 Nov 211 The paper reports on the solvation of Sodiu citrate in water, water + CH 3 and water + DMSO ixtures (v/v) under varying dielectric constant at different teperature. The specific conductance data obtained was analyzed by Kraus-Bray and Shedlovsky conductivity odels. Liiting olar conductance ( ), dissociation constant/ association constant (K C/Ka) were evaluated for all the solvent copositions. The liiting olar conductance, decreases with the increase in aount of co-solvent in water, due to increased solvent-solvent interaction and decrease in dielectric constant. Due to high viscosity and olecular size of DMSO in water, lower conductance was observed in water + DMSO edia. The Ka values increases with the increase in aount co-solvent in water at all the teperatures studied. Energy of activation of the rate process and related therodynaic paraeters such as ΔH a, ΔGa and ΔSa have been evaluated. ΔGa is found to be negative indicating spontaneity of the process. Walden product ( η) and corrected Stoke s radius (ri) have also been evaluated. Fuoss- ccascina equation was used to identify the ion-pairs and ion-triplets in the syste. But the slopes of the plot were found to be less than -.5, indicating the absence of ion-pairs or ion-triplets. Born relation of solvation was verified. These data have been used to study the nature of ion-solvent interaction and solvent-solvent interactions existing in the syste. Keywords: Dielectric constant, Solvation, Liiting olar conductance, Sodiu citrate. INTRODUCTION Conductance easureent is an iportant and siplest tool to get inforation in regard to ion-solvation 1-3. The study of conductance of an electrolyte syste with different solvents of varying viscosity and dielectric constant provides satisfactory inforation regarding ion-solvation 4-7. In recent years there has been an increasing interest in the study of behavior of electrolytes in partial and non-aqueous solvents with a view of investigating the role of partial or nonaqueous solvents in the solvation of an ion 8-1. Tri-sodiu citrate (tri-sodiu 2-hydroxypropane-1, 2, 3- tricarboxylate) is a sodiu salt of citric acid. It is a coon ingredient in leon, lie and citrus soft drinks such as Ting 11, Chinotto and soe ocean spray juices. In 1914, the Belgian doctor lbert Hustin and the rgentine physician successfully used sodiu citrate as an anticoagulant in blood transfusion 11. Even today it is used in blood collection tubes and for the preservation of blood in blood banks 11. The citrate ion chelates calciu ions in the blood by foring calciu citrate coplexes, disrupting the blood clotting echanis 11. Recently, Oopvik, et.al., showed iproveent in running perforance of students over 5 k by 3 seconds 12 by the use of this copound. Sodiu citrate is used to relieve discofort in urinary tract infections such as crystitis, to reduce the acidosis seen in distal renal tubular acidosis, and can also be used as an osotic laxative. solution of sodiu citrate 13 and citric acid at ph 4. is effective in dissolving Ca 3 (PO 4) 2 (Kidney stone) stone in vitro and vivo. The sodiu citrate solution containing an acid is effectively reducing the calciu ion concentration. In the literature, no reports available on the solvation of sodiu citrate in partial and nonaqueous solvents and hence becae the subject of the present study. MTERILS ND METHODS Sodiu citrate (Loba chei, India) was used without further purification [Melting point: 298 C (Observed value), 3 C (Literature value)]. Triply distilled water (specific conductivity of the order of S c -1 ) was used throughout the study. The solvents acetonitrile and diethylsulphoxide were purified as reported 14 (specific conductivity of the order of Sc -1 ). The solutions of sodiu citrate in water and water + CH 3/ DMSO were prepared as and when required. Conductivity easureents were carried out using a digital direct reading conductivity eter (Model, CM 18, Elico ake) and a dip type calibrated15 conductivity cell of cell constant.975 c -1. ll the easureents were ade in a therostat/ cryostat aintained at the desired teperature ±.1 C. The instruent was standardized as described earlier 16. RESULTS ND DISCUSSION Liiting olar conductance The solutions of sodiu citrate in various copositions of water + CH 3 and water + DMSO (v/v) were subjected to conductivity easureent at four different teperatures (283, 293, 33 and 313 ±.1K). The specific conductance so obtained was used in the deterination of olar conductance. The conductance data was initially subjected to analysis by Kraus-Bray equation 17. Since the chosen electrolyte is of 3:1 type, the Kraus-Bray equation related to that condition was tried but the plot (C) 3 Vs 1/ was found to be non linear (Fig. not shown) and Kraus-Bray equation of 2:1 electrolyte was tried and the plot ( C) 2 Vs 1/ was found to be non linear (Fig not shown). Hence the Kraus-Bray equation related to 1:1 electrolyte was applied and the equation ay be represented as 1 1 C = + K 2 c (1) Where, is the liiting olar conductance, C is the concentration in olar and KC is the dissociation constant. On the basis of the above equation a plot was drawn by taking C along X- axis and 1/ along Y-axis. The plot was found to be linear (Fig.1, as a representative plot). This proved that the sodiu citrate behaves like 1:1 electrolyte. Fro the intercept and slope of the linear plot, the liiting olar conductance ( ) and dissociation constant (K C) were obtained and are shown in Tables 1 and 2 respectively. The easureents were done at different teperatures and various copositions of water + CH 3 and water + DMSO as it gives with a view of getting the effect of dielectric constant on or K C/ Ka.

2 Int J Che Res, Vol 3, Issue 2, 212, 6-14 But the present odel has certain inherent liitations especially the unaccountability for the effect of ionic obility and activity coefficient on conductivity. Hence a better odel which could solve the above probles was ade use. Shedlovsky odel, accounts for the above facts and expected to give absolute liiting olar conductance. The shedlovsky equation 18 ay be represented as 1 S 1 C = + 2 Sf ± 2 K a (2) Where, is the liiting olar conductance, C is the concentration, Ka is the association constant, S is the Onsager slope and f ± the ean activity co-efficient of the electrolyte. Fro the intercept and slope of the linear plot 1/S Vs C Sf ± 2 (Fig.2, as a representative plot), the liiting olar conductance ( ) and association constant (Ka) were obtained and are presented respectively in Tables 1 and K 1/ C.11.1 B /.6 33K C Fig. 1: Plot of C Vs 1/ for sodiu citrate in -2% CH 3; B-2% DMSO 7

3 Int J Che Res, Vol 3, Issue 2, 212, 6-14 Table 1: Experiental values of olar conductance at infinite dilution ( : ho c 2 ol -1 ) for sodiu citrate fro Kraus-Bray (1) and Shedlovsky (2) odels in different dielectric constant (ε at 33K) or various copositions (v/v) of water + CH 3 and water + DMSO T(K)/ ε 76.7 (%) 72.6(1%) 68.6(2%) 6.7(4%) 53.(6%) Water + CH T(K)/ ε 76.7 (%) 76.5(1%) 75.2(2%) 74.(4%) 7.5 (5%) K 1/S C Sf B K 1/S C Sf 2 Fig. 2: Plot of 1/S Vs CSf ± 2 for sodiu citrate in -2% CH3 8

4 Int J Che Res, Vol 3, Issue 2, 212, 6-14 Fro the Table 1, it is clear that the teperature enhanced the conductivity value for all copositions of water + CH3 and water + DMSO. This is expected due to the fact that the increase in teperature increases the theral energy and obility of ions leading to increase in conductivity. Liiting olar conductance decreases with increase in the aount of co-solvent (CH 3 or DMSO) due to the increased solvent-solvent interaction and decreased dielectric constant value fro 76.7 to 53. in case of water + CH 3 and 76.7 to 7.5 in case of water + DMSO at 33K. The liiting olar conductance varies with the solvent as well as with teperature as expected. The iniu conductance value was observed at 5% DMSO (ε = 7.5), whereas axiu was observed at 6% acetonitrile (ε = 53.) at all studied teperatures. Even though both the solvents are dipolar, DMSO is structure forer and acetonitrile is structure breaker 19. Relatively high viscosity and olecular size of DMSO over CH 3 have lead to lower conductivity of sodiu citrate in water + DMSO edia. Since the viscosity and conductivity are inversely related naturally the conductance should be high in the presence of acetonitrile in water. The present result is in confority with this trend. DMSO is expected to undergo self association 19 leading to varied olecular size and foration of soe interstitial cavity in water. This cavity ay trap soe of cations released fro sodiu citrate bringing down the conductance and also show increased ion-solvent interaction. Dissociation and ssociation constants The values of dissociation constant (KC) calculated fro the Kraus- Bray plot and association constant (Ka) obtained fro the Shedlovsky plot are shown in Table 2. K C increased with increase in teperature in both the case of solvent ixtures. In this case the dissociation constant did not found reciprocal to the association constant even though one expects the reciprocal relation. This ight be due to the non consideration of electrophoretic effect, relaxation effect, inter ionic activity coefficient in the case of Kraus-Bray equation. In general, the variation of these values either with teperature or with the solvent coposition is ainly due to the change in dielectric constant and the reduction in thickness of ionic atosphere surrounding the polar species. It is true that ion-solvent and solvent-solvent interactions affects liiting olar conductance and association constant which is proved by the data given in Tables 1 and 2. It is also known1,2 that they are governed by various factors like dielectric constant, viscosity, cheical effect like short range interactions (ion-dipole, ion-ion induced dipole dispersion hydrophobic and hydrophilic interactions) as well as cavity and related solvent structural effects. To prove the foration of ion pairs and ion triplets, a plot of log Vs log C was drawn (Fig. not shown) on the basis of Fuoss equilibriu equation 2. But in the present case, the slope values are found to be less than -.5 (values are not shown), as against the expected -.5 as per Fuoss for a syste with ion-pair and hence there ay not be any notable aount of ion association. Walden Product Walden product 21 relates olar conductance at infinite dilution ( ) to viscosity (η ) of the solvent as ZeF η = 6πrT (3) where r is the Stoke s olecular radius. The product was calculated at all teperatures and percentage copositions of both the solvent ixtures and the values are shown in Table 3. Walden product decreased with the increase in the aount of co-solvent in water at all teperatures. The variation of Walden product fro one coposition to other in water + CH 3 or water + DMSO ixtures clearly indicates the preferential solvation of cation by CH3 or DMSO olecules. It depends on the viscosity of the ediu, the exchange of ions with neighbouring solvent olecules and oveent of ions through the holes in hydrogen bonded solvent ixtures. Fro the eq. 3, it is clear that Walden product is directly related to Stoke s olecular radius. But in the present case corrected Stoke s radius (ri) is calculated and shown in Table 4. Corrected Stoke s radius slightly decreased with the increase in either teperature or percentage coposition, indicating that it is not fully responsible for the variation of (Table.1). So other solvent structural properties are responsible for the variation of. Table 2: Experiental values of dissociation constant (K C) and association constant (Ka) for sodiu citrate at different dielectric constant (ε at 33K) of water + CH 3 and water + DMSO ixture K C K a T(K)/ε Water + CH K C K a T(K)/ε Table 3: Coputed values of Walden product ( η: ho c 2 ol -1 poise ) for sodiu citrate at different dielectric constant (ε at 33K) of water + CH 3 and water + DMSO ixture T(K)/ε Water + CH

5 Int J Che Res, Vol 3, Issue 2, 212, 6-14 Table 4: Coputed values of corrected Stoke s radius (r i in ) for sodiu citrate at different dielectric constant (ε at 33K) of water + CH 3 and water + DMSO ixture T(K)/ε Water + CH Therodynaic Paraeters Since conductance can be related to the rate process 22, the variation of with teperature can be given by the equation =. e Ea RT Where, Ea is the energy of activation of the conducting process. Ea was calculated 22 fro the slope of the linear plot of log Vs 1/T (Fig.3) and the values are shown in Table.5. (4) The energy of activation of the rate process increases till 2% of CH3 and later decreases. In the case of water + DMSO, Ea increases with the increase in percentage coposition except at 1% (ε = 76.5) and 2% (ε = 75.2) of DMSO. ΔHa the change in enthalpy of association was calculated fro the slope of the linear plot of log Ka Vs 1/T (Fig. not shown). The coputed values are shown in Table 5. The value of ΔH a is negative except at % and 2% of CH 3 and % and 1% of DMSO. Change in free energy and change in entropy were calculated fro the equation ΔGa= -RT ln K a and ΔS a fro (ΔHa- ΔGa)/T respectively and the values are presented in the Table 5. ΔG a is found to be negative, indicating spontaneity of the process. ΔSa, the change in entropy is found to be positive in all the cases except at 4% CH 3 indicating steric hinderence in the syste. Table 5: Coputed therodynaic paraeters for sodiu citrate at different dielectric constant (ε at 33K) of water + CH 3 and water + DMSO ixture Thero dynaic paraeters Water + CH 3 ε = 76.7 ε = 72.6 ε = 68.6 ε = 6.7 ε = 53. E a ( kjol -1 ) ΔH a ( kjol -1 ) ΔG a ( kjol -1 ) ΔS a (kj K -1 ol -1 ) ε = 76.7 ε = 76.5 ε = 75.2 ε = 74. ε = 7.5 E a ( kjol -1 ) ΔH a ( kjol -1 ) ΔG a ( kjol -1 ) ΔS a (kj K -1 ol -1 ) %CH 3 1%CH 3 2%CH 3 4%CH 3 6%CH 3 log /T 1

6 Int J Che Res, Vol 3, Issue 2, 212, B log 2. %DMSO 1%DMSO 2%DMSO 4%DMSO 5%DMSO /T Fig. 3: Plot of log Vs 1/T for sodiu citrate in -water+ch 3 B-water + DMSO Therodynaics of solvation Solvation is a process where an ion gets covered by the solvent olecules and for a sheath on an ion and the nuber of solvent olecules present in this sheath is called solvation nuber. Born proposed23 a relation between therodynaics of solvation ΔG s-s, ΔH s-s and ΔS s-s and the radius of the solvated ion and the dielectric constant of the solvent as ΔG s-s ΔH s-s = ΔG s-s + T ΔSs-s (7) ( Z e ) N i 1 1 2ri ε = (5) ( Z e ) 2 N i 1 ε ΔS s-s = 2ri ε 2 T (6) Values of therodynaics of solvation coputed on the basis of the above equations and are shown in Table 6. The values are different fro those of therodynaics of association due to the involveent of different echanis in these two cases. The stability of the species in solution depends on the free energy change arising fro solute-solvent interaction. Larger the negative value higher will be the stability of the species in that solution. Observed large negative value of ΔG s-s indicate the stability of the species in solvent edia than in water which is supported by the exotheric character of the reaction indicated by the negative ΔH s-s. The ΔS s-s values are very sall, suggest the requireent of steric restriction for the foration of ion pairs or ion association. To verify the Born relation a plot of -ΔG s-s Vs 1/r i was drawn and was found to be linear (Fig. 4) proposing that the Born odel of solvation is applicable in this case. 2 Solvation nuber (S n) Solvation nuber (Sn) ay be defined as the nuber of solvent olecules around an ion that have better interaction with the central ion and is greater than to that with the other solvent olecules either in the secondary region or in the bulk of the solution. They do not have any translational degree of oveents. Solvation nuber reflects the agnitude of ion-solvent interaction in the syste. Solvation nuber can be calculated using the relation 24 d r B r sol / S sol ix n = (8) i Where db is the inter nuclear distance and r i is the corrected Stoke s radius of an ion. Difference between d B and ri gives the surplus nuber of solvent olecules present around an ion. When it is divided by the actual radius of the solvent /solvent ixture, the resulting value represents the solvation nuber. The inter nuclear distance or the closest distance of approach was calculated on the basis of the following equation24 log 2 Z Z Be = log εd kt B Here Z, ZB are the ionic charge on the cation and anion, e is the electronic charge, ε is the dielectric constant, db is the inter nuclear distance, k is the Boltzan constant and T is the absolute teperature. The plot of log versus 1/ε (was obtained by varying % coposition of water + CH3 and water + DMSO) was drawn and found to be linear at all teperatures (Fig. 5). It gave a slope of ZZBe 2 /dbkt fro which the value of inter nuclear distance db was deterined. Sn appear to be high indicating the existence of solvent separated ion pair. Calculated Sn values are shown in Table 7. (9) 11

7 Int J Che Res, Vol 3, Issue 2, 212, 6-14 Table 6: Coputed therodynaics of solvation for Sodiu citrate at different dielectric constant (ε at 33K) of water + CH 3 and water + DMSO ixture T(K)/ ε 76.7 (%) 72.6(1%) 68.6(2%) 6.7(4%) 53.(6%) Water+CH T(K)/ ε 76.7(%) 76.5(1%) 75.2(2%) 74.(4%) 7.5(5%) Water+DMSO ΔG s-s (kj ol -1 ) 2. ΔSs-s (kj K -1 ol -1 ) 3. -ΔH s-s (kjol -1 ) G s-s % 1% 2% 4% 6% /r i G s-s 33 % 1% 2% 4% 5% /r i Fig. 4: Plot of -ΔG s-s Vs 1/r i for sodiu citrate in -water + CH 3; B-water + DMSO 12

8 Int J Che Res, Vol 3, Issue 2, 212, log K /ε B log K /ε Fig. 5: Plot of log Vs 1/ε for sodiu citrate in - water + CH 3; B- water + DMSO Table 7: Coputed values of solvation nuber for the species of sodiu citrate in water + CH 3 and water + DMSO ixture at T(K) Water + CH

9 Int J Che Res, Vol 3, Issue 2, 212, 6-14 CONCLUSION The liiting olar conductance ( ) varies with the solvent as well as with teperature as expected. Lower conductance value was observed in the water + DMSO copared to water + CH3 ixture at all studied teperatures. This is due to structure foring nature of DMSO and structure breaking nature of acetonitrile. It is further proved by viscosity and olecular size of these two solvents. The viscosity and olecular size of DMSO is high copare to CH3 and hence lower conductance was observed in the case DMSO ediu. REFERENCES 1. Conway BE, Ionic hydration in Cheistry and Biophysics. sterda: Elsevier Scientific Publishing Copany; Marcus Y, Ion Solvation. New York: Wiley inter science; Burgess J, Ions in Solution. Chichester: Horwood Publishing; Radhika V, Manikyaba P. Conductance and solvation behaviour of benziidazoliu dichroate in diethylsulphoxide-water ixture. Indian J Che. 28; 47 (): Ishwara Bhat J, Sreelatha TN. Effect of ethanol on the solvation behaviour of BI 3 in acetonitrile, ethanol and DMF as a function of teperature. J. Mol. Liq. 25; 116 (3): Dip Singh Gill, nand H, Kuari, Puri JK. Study of the coparative solvation behaviour of Na+ and Cu + cations in acetonitrile + N, N, diethylforaide ixtures at K. Z. Natur.forsch 24; 59 (9): Ishwara Bhat J, Shivakuar, HR. Conductoetric Studies on Solvation behaviour of tartaric acid in various solvent ixtures. J. Mol. Liq. 24; 111: Ishwara Bhat J, Shreevaraprasad NS. Solvation behaviour of N- Chlorosucciniide species in water + Methanol. Oriental J Che. 23; 19(2): Kalidas C, Raghunath R. Preferential solvation of silver (1)- cryptand-2,2,2 perchlorate coplex in water + acetonitrile and ethanol + acetonitrile ixtures. J. Electroanalyt. Che. 1995; 389 (1-2): Sreekuar TK, Kalidas C. Ion-solvation studies of soe substituted copper (II) Benzoates in water + DMSO ixtures. J. Phys. and Che. Liq. 1994; 27(3): Wikipedia. Org /wiki/ Tri-sodiu citrate. 12. Oopik V, Saareets I, Medijainen L, Karelson K, Janson T, Tipann S. Effects of sodiu citrate ingestion before exercise on endurance perforance in well trained college runners. Br J sports Med. 23; 37: Fuller lgright, Jhonson G H. Non surgical aspects of the kidney stone proble. J.. Med. ss. 1939;113: Perin DD, rarego WLF, Perin DR. Purification of laboratory Cheicals. New York: Pergaon Press; Lind JE, Zwolenik JJ, Fuoss RM. Calibration of conductance cells at 25 C with aqueous solutions of Potassiu chloride. J.. Che. Soc.1959; 81: Levitt B P, Findlay s Practical Physical Cheistry. 9th edn. London: Longan; Kraus C, Bray WC. General relation between the concentration and the conductance of ionized substances in various solvents. J.. Che. Soc.1913; 35: Shedlovsky T, The coputation of ionization constants and liiting olar conductance values fro conductivity easureents. J. Franklin Inst. 1938; 225: Susha CB, Solvation behaviour of N-Haloaine and Perchlorate species in aqueous, partial aqueous and non-aqueous edia. PhD thesis, Mangalore University, India, Sabitha NV, Studies on the transport behaviour of chloraines under varying conditions. PhD thesis, Mangalore University, India, Walden P, bout the ionic diaeter in non-aqueous solutions. Z. norg; llg. Che. 192; 113 (1): Glasstone S, n Introduction to Electrocheistry. New York: Van Nostrand Copany; Bockris JOM, Reddy KN. Modern Electrocheistry. Vol.1, 2nd ed. New York: Plenu Press; Laidler KJ, Cheical Kinetics. 2nd ed. New Delhi: Tata Mc Graw- Hill Publishing Copany;