DETERMINATION OF ACTIVITY COEFFICIENTS USING A FLOW EMF METHOD. 2. NaCl-K 2 SO 4 -H 2 O SYSTEM AT , AND K

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2 DTRMNATON OF ACTVTY COFFCNTS USNG A FLOW MF MTHOD.. NaCl-K SO 4 -H O SYSTM AT 98.15, AND K Luzheng Zhang, Jaj Luo, o Xaohua Lu, Yanru Wang and Jun Sh Department of Chemcal ngneerng Nanjng Unversty of Chemcal Technology Nanjng 10009, P.R.Chna ABSTRACT n ths study, a flow electromotve force (MF) method proposed by the authors was extended to mxed electrolyte aqueous solutons. Ths method was orgnally used to determne actvty coeffcent of a sngle electrolyte n mxed solvent rapdly. The MF data for the ternary system NaCl(1)-K SO 4 ()-H O(3) wthout a common on were reported at 98.15, and 38.15K and at dfferent onc strength fracton, y, of K SO 4 (y =0.196, and 0.743). These new expermental results obey Harned's rule and the Gbbs- Helmholtz relaton for the effect of temperature on actvty coeffcents n mxed electrolyte solutons. NOTATON a the closest approach parameter, 4.4 Å a actvty of on A Debye-Hückel constant b coeffcent n q.(4) B Debye-Hückel constant c coeffcent n q.(4) electromotve force of a cell 0 standard electromotve force of a cell T transfer electromotve force F Faraday constant, onc strength on molalty scale k Nernstan slope pot selectvty coeffcent of an S K,j m R T Z molalty unversal gas constant temperature, K charge number of ons GRK LTTRS γ ± υ mean onc actvty coeffcent on molalty scale stochometrc number of on n an electrolyte NTRODUCTON The determnaton of mean actvty coeffcent of salts n mxed electrolyte aqueous solutons has been the object of the understandng of varous natural and ndustral processes such as waste water treatment, sea water desalnaton, manufacture of norganc chemcals and hydrometallurgcal processes durng the last few decades. The tradtonal method, for example, sopestc technques have been wdely used for the measurement of osmotc coeffcents of solvent n sngle and/or mxed electrolyte solutons [1]. However, wth the development of on-selectve electrodes (Ss) n the recent years, the electromotve force (MF) method often trends to be chosen for the determnaton of actvty coeffcents of electrolyte n a soluton. Many expermental mean onc actvty coeffcents of salt(s) n bnary [] and ternary systems, ncludng mxed solvent electrolyte solutons [3] and mxed electrolyte aqueous solutons [4,5], have been measured. n all the measurements, the expermental technques nvolved the preparaton of each ndvdual soluton to be tested by weght. Obvously, weghng sample n ths manner s very laborous and greatly decreases the speed of determnaton. n order to rapdly and accurately determne actvty coeffcents of salt n electrolyte solutons, we proposed a flow method and determned actvty coeffcents of hydrogen chlorde n the mxtures of methanol and water at 98.15, and K [6]. Recently, we also extended ths method to mxed electrolyte aqueous soluton wth a common anon, NaCl-KCl-H O system, at 98.15K [7]. n ths work, the flow method wll be further extended to a ternary system wthout a common on, NaCl-K SO 4 -H O, and the MF data for the system at dfferent onc strength fracton of K SO 4 and at 98.15, and K wll be reported. APPARATUS AND CHMCALS The apparatus used n ths study has been descrbed elsewhere [6]. The Na-S (Model 10, Chna) and Cl- S (Model 301, Chna) are used to response the ons n a soluton. The potental s measured usng an on-meter (PXJ-1C model, Chna) wth an nternal mpedance grater than Ω and resoluton around ±0.1 mv The salts, used wthout further purfcaton, are sodum chlorde (guarantee grade, 99.8%wt.) and potassum sulfate (analyss grade, 99.5%wt.). The conductvty grade water s prepared by re-dstllng water n the presence of KMnO 4, ts electrcal conductvty s less than S-cm -1. n order to get a stock mxed electrolyte aqueous soluton, NaCl-K SO 4 -H O, wth a constant onc strength fracton of K SO 4, two bnary systems NaCl-H O and K SO 4 -H O are prepared by weghng.

3 TST AND VRFCATON OF Ss n order to establsh the slope of response versus the on concentraton, the system NaCl-H O s used for the test and verfcaton of Na-S and Cl-S. The cell s descrbed as follows: Na S NaCl( m) H O Cl S (A) Transfer MF, T When the salt M ν+ X ν- concentraton change from to m molalty, based on the Nernst equaton 0 RT ν+ ν ( ν+ + ν ) = + ln( m+ m γ ±, MX ) (1) nf For NaCl-H O system, the transfer MF T of cell (A) can be obtaned: RT F m γ ±, ln( ) m γ T = 1 ±, 1 Actvty coeffcents of NaCl at K are obtaned from lterature [8]. A group of potentals of cell (A) wth the system at K fro.0 mol/kg to m molalty s measured. The mean absolute devaton between expermental and calculated transfer MF, up to.5 molalty, s 0.3mV. t proves that the Ss can accurately response the changes of MF versus salt concentratons. Nernstan slope For the cell (A) wth NaCl-H O system at K, the values of MF,, versus ln(a NaCl ) dependence has the slope k (k=rt/f), called the Nernstan slope. Several seres of versus the concentratons of NaCl n aqueous soluton at 98.15, and K are measured. The actvty coeffcents of NaCl n bnary system at these temperatures are also obtaned from lterature [8]. Table 1 gves the measured and theoretcal slope values. t s shown that the response of Na-S and Cl-S versus the on concentratons n soluton follows Nernstan law. Table 1 Measured and theoretcal Nernstan slopes at 98.15, and K T/K Measured Theoretcal Selectvty of Ss f the analyte contans response on and nterfenrent j wth actvty a j, the MF of a cell wth Ss s gven by the Nkolsky equaton [9] 0 RT po t = + ln( a + K a Z Z, j j j ) (3) nf where pot K,j s the selectvty coeffcent of the Ss for on wth respect to on j. The selectve behavor of an S follows from q.(3). f K pot Z Z j a j s suffcently low, the, j q.(3) s converted nto q.(1). The selectvty coeffcent of the Ss to NaCl wth respect to K SO 4 s determned by the separate-technques [10]. The value s about The nfluence of the nterferent K SO 4 on the MF of the cell wth Na-S and Cl-S s therefore neglected..e. the Ss stll obey the Nernstan law (q.(1)) for the ternary system, wthout a common on, NaCl-K SO 4 -H O as a analyte n the cell: () Na S NaCl( m ) K SO ( m ) H O Cl S 1 4 (B) Usng the MF of ths cell, actvty coeffcents of NaCl n the ternary system can be determned by the followng procedure. CALCULATON OF ACTVTY COFFCNT n mxed electrolyte aqueous soluton, the actvty coeffcents of salts s emprcally descrbed by the equaton: A ln γ ± = + b + c (4) 1 + Ba where s onc strength on molalty scale defned by = 1 m Z Substtutng q(4) nto q(1), we can obtan: 0 ' = / k + b + c (6) where ' = / k + A / ( 1+ Ba ) ln( ), k s the Nernst theoretcal slope at the nterested temperature. f a seres of MFs,, of cell (B) wth respect to the molaltes of NaCl,, and K SO 4, m, n that ternary system at the studed temperatures are measured, the standard MFs, 0, and the coeffcents b and c n q (6) can be obtaned by regressng the measured MF wth respect to the onc strength and the actvty coeffcents of NaCl at the temperatures are determned by substtutng the 0 nto q(1). RSULTS Frstly, the potentals of a cell contanng the system NaCl-KCl-H O wth a common anon are measured. The actvty coeffcents of NaCl n the system at K are determned usng the method of calculaton descrbed above. The comparson of the expermental to the calculated values was shown elsewhere [7]. The extenson of the flow MF method s excellent. On the bass of the work above, the MFs of the cell (B) contanng the system NaCl-K SO 4 -H O, wthout a common on, at 98.15, and K and at dfferent onc strength of K SO 4, y, (y =0.0, 0.198, and 0.743) are measured, respectvely. Table gves the standard MFs, 0, and the coeffcents b and c n q.(4). The expermental potentals wth respect to the molaltes of NaCl and K SO 4 are lsted n Table 3. Table The values of 0 /k for system NaCl- K SO 4 -H O at y =0.196, and and 98.15, , and 38.15K y 0 /k b 10 c K K K (5)

4 Table 3 The expermental potentals of a cell contanng NaCl(1)-K SO 4 ()-H O(3) at y =0.196, 0.597, and and at 98.15, , and 38.15K m m m y =0.196 y =0.597 y = K 98.15K 98.15K K K K K K

5 DSCUSSON Accordng to the Harned s rule [11], the actvty coeffcents of NaCl n the ternary system depend on lnearly the onc strength fracton, y, at a constant total onc strength. Fg. 1 shows the lnear dependence for the system at 98.15K when the total onc strength are 0.3 mol/kg and 0.7 mol/kg, respectvely. n order to demonstrate the nfluence of temperatures on the expermental values, ncludng the potentals and actvty coeffcents, Fg. provdes the measured actvty coeffcents of NaCl n NaCl-K SO 4 - H O system at 98.15, and K when the total onc strength s constant. t s shown that the relatonshp between expermental values of actvty coeffcents and temperatures s also lnear. Wth the consderaton of basc thermodynamc relatons, whch was descrbed elsewhere [6], the standard potental, 0, of cell (B) are also lnearly changed wth the temperatures. The relatve coeffcents are about Fg. 3 gves the expermental actvty coeffcents of NaCl and the values predcted by the Ptzer s model [1] at 38.15K. The agreement of experment wth the predcton s better. However, when the onc strength fracton, y, ncreases, for example y =0.743, a larger gap appears. Probably, these dstncton results from the model parameters, whch are correlated from expermental data at 98.15K. RFRNCS [1] R. M. Pykowez, Actvty Coeffcent n lectrolyte Solutons, Vol. (CRC Press, Florda, 1979) [] C.C. Brgges and T. H. Llley, J. Chem. Thermodyn., 6: 599(1974) [3] M. A. steso, O. M. Gonzalez-Daz, F. F. Mernandez- Luzand, and L. Fernandez-Merda, J. Soluton Chem. 18: 77(1989) [4] J. N. Butler and R. Houston, Anal. Chem., 4, 676(1970) [5] A. Haghtalab, and J. H. Vera, J. Soluton Chem. 0:479(1991) [6] L. Zhang, X. Lu, Y. Wang, and J. Sh, J. Soluton Chem. : 137(1993) [7] L. Z. Zhang, Ph.D. Thess, Nanjng nsttute of Chemcal Technology (1994) [8] R. A. Robnson and R. H. Stokes, lectrolyte Solutons (Butterworths Scentfc, London, 1959) [9] J. Koryta and K. Stulk, on-selectve electrode (Cambrdge Unversty Press, London, 1983) [10]. Pungor, K. Toth, and A. Hrabeczy-Pall, Pure & Chem., 51, 1913(1979) [11] H. S. Harned and B. B. Owen, Physcal Chemstry of lectrolyte Solutons, (Renhold, New York, 1958) [1] K. S. Ptzer, J. Phys. Chem. 77: 68(1973) ACKNOWLDGMNT The authors are grateful to the Natonal Natural Scence Foundaton of Chna (NNSFC) and Fok Yng- Tong ducaton Foundaton for fnancal support. ln γ NaCl = 0.3 mol/kg = 0.7 mol/kg T=98.15 K y Fg. 1. Actvty coeffcents of NaCl-K SO 4 - H O at 98.15K and at =0.3 and 0.7 mol/kg ln NaCl γ y =0.198 y =0.597 y =0.743 = 0.7 mol/kg /T (K -1 ) Fg.. The relatonshp between actvty coeffcents of NaCl n NaCl(1)-K SO 4 ()- H O(3) and temperatures at =0.7mol/kg γ NaCl T=38.15 K y =0.0 y =0.198 y =0.597 y =0.743 Predcted by Ptzer Model / 1/ Fg. 3. xpermental and predcted actvty coeffcents of NaCl n the ternary system NaCl(1)-K SO 4 ()-H O(3) at 38K