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Some conductance and potentiometric studies in 20 mass% propylene carbonate + ethylene carbonate: application of hydrogen and quinhydrone electrodes
JOORNAL OF
m' hi'v7 7,i]IL,],, 71IL,,
r,~/r
ELSEVIER
Journal of Electroanalytical Chemistry 380 (1995) 29-33
Some conductance and potentiometric studies in 20 mass% propylene carbonate + ethylene carbonate: application of hydrogen and quinhydrone electrodes A.K. Srivastava *, R.A. Samant Department of Chemistry, Universi~" of Bombay, Vidyanagari, Santacruz (E), Bombay - 400 098, hMia Received 22 March 1994; in revised form 25 May 1994
Abstract The behaviour of methanesulfonic and 2,5-dichlorobenzenesulfonic acids in a 20 mass% propylene carbonate (PC) + ethylene carbonate (EC) mixture has been investigated by means of conductance and potentiometric measurements at 25°C. The conductance behaviour of acids in 20 mass% PC + EC is explained on the basis of simple dissociation (HA # H++ A - ) and homoconjugation ( H A + A - ~ H A ~ ) equilibria. The results of the measurements have been subsequently correlated with potentiometric studies involving the cell: Pt [H2(1 atm), HA in 20 mass% PC + EC NHgCl2(s)IHg ref. in 20 mass% PC + EC. In addition, the Hg [HgCl2(s) reference electrode has been used to determine the standard potential of the reaction: quinone + 2H++ 2e ~ hydroquinone vs. SHE in 20 mass% PC + EC. It was found to be 0.3516 _+0.0023 V.
Keywords: Conductance behaviour; Potentiometric studies 1. Introduction Propylene carbonate (4-methyl-l,3-dioxolane-2-one or PC) and ethylene carbonate (1,3-dioxolane-2-one or EC) are dipolar aprotic solvents with large dipole moments and relatively high dielectric constants [1]. Although PC has been receiving increasing attention as an electrochemical solvent [2-4] less attention has been directed towards EC as a solvent [5] probably because of its high freezing point (37°C). The dipole moment of PC ( 1 . 6 5 × 1 0 29 C m = 4 . 9 4 D ) and E C ( 1 . 6 4 × 10 -29 C m = 4.93 D) are similar and they are miscible with each other [1], however, only a few fundamental studies have been reported on binary mixtures of these two solvents [6,7]. The 20 mass% PC + EC has a dielectric constant of 87.2 at 25°C, which is higher than that of PC (64.4 at 25°C). This offers a special advantage over the low dielectric media where ionic association often creates undesirable complications [2-4,8]. The object of the present work was to study the behaviour of the methanesulfonic and 2,5-dichlorobenzenesulfonic acids through conductance and potential measurements. An arbitrary reference electrode, viz.
*
Corresponding author.
0022°0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 4 ) 0 3 5 9 8 - W
Hg/HgCl2(s) was first standardized against a P t [ H 2 electrode in 20 mass% PC + EC using solutions of the two acids. Subsequently, the study of the suitability of the quinhydrone electrode for measuring hydrogen ion activity in the solvent 20 mass% PC + EC was considered worthwhile; the standard potential of the reaction quinone + 2 H + + 2 e - ~ hydroquinone was estimated, and a general evaluation of the electrode system has been made. The results of this study are expected to broaden the scope of routine potentiometric measurements in 20 mass% PC + EC solvent.
2. Theory
2.1. Conductance data analysis The molar conductivity of methanesulfonic and 2,5dichlorobenzenesulfonic acids (HA) in 20 mass% PC + EC does not vary linearly with the square root of the acid concentration. In contrast, it indicates that these systems involve considerable ion association. Analysis of the conductance data for these acids has been attempted using the Fuoss method of 1978 [9] but satisfactory results could not be obtained. The observed conductance behaviour of these acids in 20
30
A.K Srivastat,a, R.A. Samant /Journal of Eh'ctroanalytical Chemistry 380 (1995) 29 33
mass% PC + EC is interpreted in terms of following equilibria [10]:
tance, AHA, for any given acid concentration, CHA, by making use of the approximation
HA~H++A
AHA={A,,+[H + ] + A A [A ] + A H A e [ H A R I } / c u A
(1)
KttA=aH~a A /art A
(2)
(11)
where KHA and KHA : denote the thermodynamic dissociation constant and the homoconjugation constant respectively. If it is assumed that the activity coefficients of all uncharged species are unity and those of ions H +, A and H A 2 are equal, denoted by fi, Eqs. (1) and (2) can be expressed as
where an+, a a , AHA 7 are the limiting ionic conductances. For any particular acid, a trial and error procedure was repeated with different sets of values for KHA and KUA~; then a plot of log AHA VS. log CnA was generated for each set of these constants over a range of assumed values of [H+], using the values 21.10, 21.52 and 14.64 S c m 2 mo1-1 for AH+, Amethanesul~,nate and A2,5_dichlorobenzenesulfonate respectively as determined previously [7]. The AHA2 value was taken as half of AA value as the mobility of H A 2 species is reduced to half [11] on association of A - and HA species. The simulated plots were then compared with the experimental ones, using the criterion of best fit. The specific set of values of KHA and KHA 2 which yielded the most satisfactory agreement was considered to be representative of any particular acid system.
HA + A ~- HA 2
KHA 2 = alIA2/aHAaA
KHA = [ H + ] [ A ] f i 2 / [ H A ]
(3)
KHA : = [ H A T ] / [ H A ] [ A - ]
(4)
Accordingly, the ratio follows
KHA,/KHA can be related as
KHA;/KHA = [ H A ; ] / [ H + ] [ A - ] 2f2
(5)
Now, from the charge neutrality principle [H +] = [HA2] + [ A - ]
(6)
2.2. Potentiometry
Thus, substituting for [HA 2] in Eq. (5) one obtains KHA;/KHA = {[H + ] - [ A - ] } / [ H + ] [ A - ] 2 f i 2
(7)
Eq. (7) can now be rearranged to give
KHAj[H+]A[A ] 2 + K H A [ A
]--KHA[H+]=0
(8)
If values of KHA and KHA_; are available, Eq. (8) can be conveniently solved for [A ] for a particular value of [H +] which, in the present situation also determines the ionic strength ( I ) and can, therefore, be used to obtain fi from the Debye-Huckel limiting law - l o g fi = A / i 2 1 / / ; w h e r e A = 0.4325 for 20 mass% PC + EC at 25°C. The dielectric constant and viscosity values of 20 mass% PC + EC used for calculation are 87.2 and 2.32 mPa s(cP) at 25°C respectively. The total acid concentration, cna, can be expressed using the principle of mass balance: CHA = [ A - ] + [HA] + 2[HAT]
(9)
Combining Eq. (9) with Eqs. (3) and (4), one obtains the total acid concentration in terms of [H+], [A ] and the constant K~tA and KItA2: CHA = [ a - ] +
[U+][a-]fi2/gHa
+ 2KHA;[H+][A
]2f~2/gHa
(10)
For a particular set of values of KHA and KtlA2 E q . (10) can now be used to calculate the total acid concentration (CHA) for a series of hypothetical values of [H +]. Also one is able to calculate the molar conduc-
2.2.1. Calibration of the reference electrode The reference electrode Hg IHgClz(s)was standardized vs. SHE in 20 mass% PC + EC by using the cell: CellI:
PtlH z (latm);HA[IHgC12(s)lHg
with solutions of methanesulfonic acid or 2,5-dichlorobenzenesulfonic acid as H A in 20 mass% PC + EC. At any concentration of the acid used, the emf, E, of the Cell I, at 25°C, can be expressed as E = Eref+ E~.j- 0.05916 log an+
(12)
where Eref. is the potential of the reference electrode and E~.j. represents any liquid junction potential that may be involved. Thus if the hydrogen ion activity (an+) is known for the concentration (qtA) of the acid used in Cell I, the quantity Eref.+ ELi" can be evaluated readily from the cell emf. Assuming that ELi. is negligible or that it remains constant independent of the nature of the liquid junction, the value of the sum E~ef.+ El.j. (referred to as E'ref. hereafter) determined in this manner can be taken as the potential of the reference electrode (vs. SHE) for all practical purposes. In the present method of determination of E'~ef. the emf's of Cell I were measured at different concentrations of HA, and plotted as a function of - l o g CHA. This plot was then compared with a similar one constructed on the basis of calculated values of -0.05916 log an+ and the corresponding values of - log CHA for the particular acid used. The procedure involved gen-
A.K Srit,astal a, R.A. Samant /Journal of Electroanalytical Chemis'try 380 (1995) 29-33
eration of values of CHA from the various assumed values of [H ÷] using Eq. (10) with the available equilibrium constants KHA a n d KHA F as determined by the conductance method for the two acids employed in the study.
31
Both solvents were stored in sealed containers to prevent contamination from carbon dioxide and atmospheric water. Known masses of each solvent were mixed to form a (PC + EC) mixture with 20 mass% PC.
3.2. Reagents" 2.2.2. Determination of the standard potential of the quinhydrone electrode The standard potential of the reaction: O
OH
O
OH
(O)
3.3. Procedure
(H2Q)
in 20 mass% PC + EC was determined from the measurement of emf's of the cell: Cell II:
2,5-Dichlorobenzenesulfonic acid was prepared according to a published procedure [12]; the melting point of the anhydrous product was found to be 105106°C. Methanesulfonic acid (Fluka, 9 8 - 9 9 % pure), reagent grade quinhydrone and mercuric chloride were used without any further purification. The mercury used was of triple-distilled quality.
Hg IHgCla(S ) ref. in 20 mass% PC + EC H2,5-dichlorobenzenesulfonic acid in 20 mass% PC + EC, Q + H 2 Q I P t
with the equimolar mixture of quinone and hydroquinone (quinhydrone) using several concentrations of 2,5-dichlorobenzenesulfonic acid solutions in 20 mass% PC + EC. Ignoring any dissociation of H 2 Q and assuming that the activity coefficients of both Q and H ? Q are unity, the emf of Cell II, at 25°C can be represented by Eq. (13): E - E Q/H20--Eref.+ ° ' 0.05916 Iog all+
(13)
in terms of the standard potential (vs. SHE) of the 0 t q u i n o n e / h y d r o q u i n o n e system, (Eo/H2O), Eref. and the hydrogen ion activity of the solution. In the present treatment, the value of 0.05916 log au+, necessary for 0 the calculation of Eo/n2 Q were interpolated from the negative logarithm of the concentration of 2,5-dichlorobenzenesulfonic acid mentioned earlier in connection with the standardization of the reference electrode.
3. Experimental 3.1. Solvent The purification and storage of PC have been described elsewhere [2,10]. Commercially available EC (99% pure, Fluka) was distilled three times under reduced pressure. In the distillation only 80% middle distilled product was collected. The specific conductivity of purified solvent varied between 1 and 3 × 10 7 S c m - 1 at 40°C.
3.3.1. Conductivity measurements All conductance measurements were made at 25 _+ 0.05°C using a dip-type cell (cell constant 1.010 c m - l ) with lightly platinized electrodes as described earlier [13]. All molar conductivities reported were calculated after correcting for the solvent conductivity. 3.3.2. Potential measurements Platinum wire (2.5 cm long, 1.5 mm diameter) sealed into a Pyrex glass tube lightly coated with platinum black, in conjunction with pure and dry hydrogen gas was used as the hydrogen electrode; a freshly platinized electrode was used in each measurement. The electrode used in setting up the quinhydrone half cell consisted of the usual bright platinum electrode. The reference electrode consisted of a mercury pool in contact with a saturated solution of HgC12 containing an excess of solid chloride. A period of 24 h was allowed for equilibrium before the electrode was put into use. Normally, for a particular series of measurements, a newly prepared reference electrode was used; reference electrodes were discarded 2 - 3 days after the initial period of equilibration. Duplicate electrodes prepared under identical conditions agreed to within 0.5 mV. The design of the indicator and reference half cell compartment has been described previously [14]. An Equip-Tronics digital potentiometer model E Q - D G S was used for the emf measurements in an air-bath maintained at 25 _+ 0.5°C. In course of reading the cell potential a flowing junction was maintained by allowing the solution from the reference electrode to drain slowly into the side arm of the indicator half cell. As a general procedure the emf's were noted every 2 - 3 rain for all cells over a period of ~ 20 min; the reported potentials are the average of the last two readings which normally agreed to within 0.5 mV; measured cell potentials are considered reproducible to _+2 mV. In
32
A.K. Sricastat,a, R.A. Samant /Journal of Electroanalytical Chemisto' 380 (1995) 29-33
Table 1 Results of conductivity measurements on methanesulfonic acid and 2,5-dichlorobenzenesulfonic acid in 20 mass% PC + EC at 25°C Methanesulfonic acid 104 c / M
A / S cm: mol
50.9 30.7 23.4 17.5 12.1 8.1 4.9 2.5
1.94 2.50 2.79 3.02 3.42 3.99 5.01 6.92
0 -70
2,5-dichlorobenzenesulfonic acid 1
104
Y
A / S cm2 mol [
c/M
239.9 220.4 152.6 100.7 60.1 37.4 27.9 19.3 13.0
0 .60-
7.15 8.06 9.26 10.74 12.60 14.68 16.56 18.98 21.13
t,
? g:
the case of h y d r o g e n e l e c t r o d e m e a s u r e m e n t s , an initial p e r i o d of s a t u r a t i o n with H 2 gas for 1 5 - 2 0 min was allowed; d u e to the low v a p o u r p r e s s u r e of 20 m a s s % P C + E C at the e x p e r i m e n t a l t e m p e r a t u r e no v a p o u r p r e s s u r e c o r r e c t i o n was c o n s i d e r e d necessary.
~ o
0.50
O o i
0.30
v bJ
.Mr
/
0.10
4. Results and discussion
0"5
T h e conductivity m e a s u r e m e n t s of the two acids are s u m m a r i z e d in T a b l e 1. T h e c o r r e s p o n d i n g plots of l o g A c vs. log CHA a r e shown in Fig. 1 along with the c a l c u l a t e d plots which gave t h e b e s t fit a c c o r d i n g to the p r e s e n t t r e a t m e n t . KHA for 2 , 5 - d i c h l o r o b e n z e n e sulfonic and m e t h a n e s u l f o n i c acids a r e f o u n d to be 1.1 × 10 -3 a n d 8.5 X 10 -6 respectively. T h e KHA 2 val1.4-
o•
o" eo ®
1"0"
T
3 E
O
• •
0 0
if) ~.
0.8
u
o 0.6
•&
• && 0.4
0.2
-Iog(CHA/M)
Fig. 1. Plot of log AcN A vs. -log CHA. Methanesulfonic acid: (A) Experimental; ( • ) Calculated. 2,5-dichlorobenzenesulfonic acid: (©) Experimental; (e) calculated.
/
0.20 ~
/
1.2-
o
/
~
i
I
/
/
~4r.. /
/
,
,
2"0
3.5
- log(ellA/M)
Fig. 2. Plots of E and -0.05916 log aN+ VS. log CHA:methanesulfonic acid ( a , • ) and 2,5-dichlorobenzenesulfonic acid (©, e). The solid line represents experimental data using Cell I; the dashed line refers to the plot of calculated function -0.05916 log all+.
ues for b o t h acids a r e f o u n d to b e 1.0. A c o m p a r i s o n of the dissociation c o n s t a n t s (KHA) of 2 , 5 - d i c h l o r o b e n zenesulfonic acid (1.145 × 10 - s ) a n d m e t h a n e s u l f o n i c acid (5.37 × 10 - 9 ) in PC [3] with the values 11 × 10 3 a n d 8.5 × 10 . 6 in 20 m a s s % PC + E C respectively, as o b t a i n e d in the p r e s e n t w o r k i n d i c a t e s that b o t h the acids are a p p r o x i m a t e l y 100 times s t r o n g e r in 20 m a s s % P C + E C t h a n in PC. R e s u l t s of the p r e s e n t m e a s u r e m e n t s using Cell 1 are shown in Fig. 2 in the form of E vs. - l o g CHA plots o f b o t h m e t h a n e s u l f o n i c a n d 2 , 5 - d i c h l o r o b e n z e n e s u l f o n i c acids along with the t h e o r e t i c a l plots of the function - 0 . 0 5 9 1 6 log a . + . V a l u e s of KHA and K H A 2 as d e t e r m i n e d by the conductivity m e t h o d were used for the c a l c u l a t i o n of all+ at d i f f e r e n t total conc e n t r a t i o n of acids using Eq. (10). It is w o r t h w h i l e to note that the slopes o b t a i n e d for the e x p e r i m e n t a l a n d t h e o r e t i c a l plots in case o f both the acids a r e almost the same. I n s p e c t i o n of the e x p e r i m e n t a l plots in the case of m e t h a n e s u l f o n i c acid a n d the value of 005916 log a , + at t h r e e p o i n t s in the r a n g e of c o n c e n t r a t i o n s 1 × 10 2-1 x 10 - 4 M y i e l d e d an a v e r a g e value of 0.3810 V for E'ref. a n d a value of 0.3843 V was similarly d e r i v e d from the 2 , 5 - d i c h l o r o b e n z e n e s u l f o n i c acid data. T h e overall average of the two values viz. 0.3826 _+ 0.0023 V
A.K Sr&astat'a, R.A. Samant /Journal of Electroanalytical Chemistry 380 (1995) 29-33 Table 2 Results of measurements using Cell II: HgIHgCl2(s) ref. in 20 mass% PC + EC LI2,5-dichlorobenzenesulfonic acid, Q + H 2Q IPt
E/V
CHA/M
Quinhydrone concentration 5.300×10 1.812 × 105.436× 10 1.603×10 9.060× 10 2.712× 10
4 4 3 3 2
3M
2.013×10 -3 M
0.130 0.150 0.173 0.198 0.218
0.128 0.151 0.170 0.200 0.219
was finally taken as the value of E'ref. for the Hg JHgC12(s) reference electrode vs. SHE. The potentiometric measurements using Cell II are summarized in Table 2 for the two quinhydrone concentrations studied, as the concentration of 2,5-dichlorobenzenesulfonic acid is changed. The corresponding plots of emf's of Cell II vs. logarithm of the acid concentrations are given in Fig. 3. Using the interpolated value of the function 0.05916 log an+ from Fig. 2 at three different concentrations in the range 5 ×
0,25-
0.20-
0.15 0.15-
0.10 0'0
~io
zlo
31o
4.0
- iog(CHA/M)
Fig. 3. Plots of E vs. - log CHA (Cell II). Quinhydrone concentration: (z~)2.013×10 3 M ; ( o ) 5.300×10 ~ M.
33
1 0 - 2 - 5 × 10 -4 M of acid, an average of 0.3516 _+ 0.0023 V is calculated for E Q/H?Q ° VS. S H E in 20 mass% PC + EC. It is worth mentioning here that the slope of plots of E vs. log CHA (Fig. 3) are almost identical in magnitude with that obtained from hydrogen electrode measurements of 2,5-dichlorobenzenesulfonic acid (Fig. 2) over the same concentration range. This suggests that both P t l H 2 and quinhydrone electrode are equally reliable as an H+-indicator electrode in 20 mass% PC + EC solvent.
References [1] W.H. Lee, Cyclic Carbonates, in J.J. Lagowski (Ed.), The Chemistry of Non-Aqueous Solvents, Academic Press, New York, 1976, Vol. IV. [2] L.M. Mukherjee, Crit. Rev. Anal. Chem., 4 (1975) 325. [3] K. Izutsu, I.M. Kolthoff, T. Fujinaga, M. Hattori and M.K. Chantooni Jr., Anal. Chem., 49 (1977) 503. [4] A.K. Srivastava and L.M. Mukherjee, J. Electroanal. Chem., 160 (1984) 209. [5] G. Petrella and A. Sacco, J. Chem. Soc., Faraday Trans. 1, 74 (1978) 2070. [6] R. Fong, V.S. Von and J.R. Dhan, J. Electrochem. Soc., 137 (1990) 2007. [7] A.K. Srivastava and R.A. Samant, J. Chem. Eng. Data, 39 (1994) 358. [8] A.K. Srivastava and B. Tiwari, J. Electroanal. Chem., 325 (1992) 301. [9] R.M. Fuoss, J. Phys. Chem., 82 (1978) 2427. [10] A.K. Srivastava and L.M. Mukherjee, J. Electroanal. Chem., 157 (1983) 53. [11] H.S. Harned and B.B. Owen, The Physical Chemistry of Electrolytic Solutions, Reinhold, New York, 1958, pp. 229-300. [12] J.H. Crowell and L.L. Raiford, J. Am. Chem. Soc., 42 (1920) 145. [13] A.K. Srivastava and A.R. Desai, J. Chem. Eng. Data, 37 (1992) 322. [14] (a) S. Bruckenstein and I.M. Kolthoff, J. Am. Chem. Soc., 78 (1956) 2974; (b) L.M. Mukherjee, Ph.D. Thesis, University of Minnesota (1961); (c) S. Bruckenstein and L M . Mukherjee, J. Phys. Chem., 66 (1962) 2228.
m' hi'v7 7,i]IL,],, 71IL,,
r,~/r
ELSEVIER
Journal of Electroanalytical Chemistry 380 (1995) 29-33
Some conductance and potentiometric studies in 20 mass% propylene carbonate + ethylene carbonate: application of hydrogen and quinhydrone electrodes A.K. Srivastava *, R.A. Samant Department of Chemistry, Universi~" of Bombay, Vidyanagari, Santacruz (E), Bombay - 400 098, hMia Received 22 March 1994; in revised form 25 May 1994
Abstract The behaviour of methanesulfonic and 2,5-dichlorobenzenesulfonic acids in a 20 mass% propylene carbonate (PC) + ethylene carbonate (EC) mixture has been investigated by means of conductance and potentiometric measurements at 25°C. The conductance behaviour of acids in 20 mass% PC + EC is explained on the basis of simple dissociation (HA # H++ A - ) and homoconjugation ( H A + A - ~ H A ~ ) equilibria. The results of the measurements have been subsequently correlated with potentiometric studies involving the cell: Pt [H2(1 atm), HA in 20 mass% PC + EC NHgCl2(s)IHg ref. in 20 mass% PC + EC. In addition, the Hg [HgCl2(s) reference electrode has been used to determine the standard potential of the reaction: quinone + 2H++ 2e ~ hydroquinone vs. SHE in 20 mass% PC + EC. It was found to be 0.3516 _+0.0023 V.
Keywords: Conductance behaviour; Potentiometric studies 1. Introduction Propylene carbonate (4-methyl-l,3-dioxolane-2-one or PC) and ethylene carbonate (1,3-dioxolane-2-one or EC) are dipolar aprotic solvents with large dipole moments and relatively high dielectric constants [1]. Although PC has been receiving increasing attention as an electrochemical solvent [2-4] less attention has been directed towards EC as a solvent [5] probably because of its high freezing point (37°C). The dipole moment of PC ( 1 . 6 5 × 1 0 29 C m = 4 . 9 4 D ) and E C ( 1 . 6 4 × 10 -29 C m = 4.93 D) are similar and they are miscible with each other [1], however, only a few fundamental studies have been reported on binary mixtures of these two solvents [6,7]. The 20 mass% PC + EC has a dielectric constant of 87.2 at 25°C, which is higher than that of PC (64.4 at 25°C). This offers a special advantage over the low dielectric media where ionic association often creates undesirable complications [2-4,8]. The object of the present work was to study the behaviour of the methanesulfonic and 2,5-dichlorobenzenesulfonic acids through conductance and potential measurements. An arbitrary reference electrode, viz.
*
Corresponding author.
0022°0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 4 ) 0 3 5 9 8 - W
Hg/HgCl2(s) was first standardized against a P t [ H 2 electrode in 20 mass% PC + EC using solutions of the two acids. Subsequently, the study of the suitability of the quinhydrone electrode for measuring hydrogen ion activity in the solvent 20 mass% PC + EC was considered worthwhile; the standard potential of the reaction quinone + 2 H + + 2 e - ~ hydroquinone was estimated, and a general evaluation of the electrode system has been made. The results of this study are expected to broaden the scope of routine potentiometric measurements in 20 mass% PC + EC solvent.
2. Theory
2.1. Conductance data analysis The molar conductivity of methanesulfonic and 2,5dichlorobenzenesulfonic acids (HA) in 20 mass% PC + EC does not vary linearly with the square root of the acid concentration. In contrast, it indicates that these systems involve considerable ion association. Analysis of the conductance data for these acids has been attempted using the Fuoss method of 1978 [9] but satisfactory results could not be obtained. The observed conductance behaviour of these acids in 20
30
A.K Srivastat,a, R.A. Samant /Journal of Eh'ctroanalytical Chemistry 380 (1995) 29 33
mass% PC + EC is interpreted in terms of following equilibria [10]:
tance, AHA, for any given acid concentration, CHA, by making use of the approximation
HA~H++A
AHA={A,,+[H + ] + A A [A ] + A H A e [ H A R I } / c u A
(1)
KttA=aH~a A /art A
(2)
(11)
where KHA and KHA : denote the thermodynamic dissociation constant and the homoconjugation constant respectively. If it is assumed that the activity coefficients of all uncharged species are unity and those of ions H +, A and H A 2 are equal, denoted by fi, Eqs. (1) and (2) can be expressed as
where an+, a a , AHA 7 are the limiting ionic conductances. For any particular acid, a trial and error procedure was repeated with different sets of values for KHA and KUA~; then a plot of log AHA VS. log CnA was generated for each set of these constants over a range of assumed values of [H+], using the values 21.10, 21.52 and 14.64 S c m 2 mo1-1 for AH+, Amethanesul~,nate and A2,5_dichlorobenzenesulfonate respectively as determined previously [7]. The AHA2 value was taken as half of AA value as the mobility of H A 2 species is reduced to half [11] on association of A - and HA species. The simulated plots were then compared with the experimental ones, using the criterion of best fit. The specific set of values of KHA and KHA 2 which yielded the most satisfactory agreement was considered to be representative of any particular acid system.
HA + A ~- HA 2
KHA 2 = alIA2/aHAaA
KHA = [ H + ] [ A ] f i 2 / [ H A ]
(3)
KHA : = [ H A T ] / [ H A ] [ A - ]
(4)
Accordingly, the ratio follows
KHA,/KHA can be related as
KHA;/KHA = [ H A ; ] / [ H + ] [ A - ] 2f2
(5)
Now, from the charge neutrality principle [H +] = [HA2] + [ A - ]
(6)
2.2. Potentiometry
Thus, substituting for [HA 2] in Eq. (5) one obtains KHA;/KHA = {[H + ] - [ A - ] } / [ H + ] [ A - ] 2 f i 2
(7)
Eq. (7) can now be rearranged to give
KHAj[H+]A[A ] 2 + K H A [ A
]--KHA[H+]=0
(8)
If values of KHA and KHA_; are available, Eq. (8) can be conveniently solved for [A ] for a particular value of [H +] which, in the present situation also determines the ionic strength ( I ) and can, therefore, be used to obtain fi from the Debye-Huckel limiting law - l o g fi = A / i 2 1 / / ; w h e r e A = 0.4325 for 20 mass% PC + EC at 25°C. The dielectric constant and viscosity values of 20 mass% PC + EC used for calculation are 87.2 and 2.32 mPa s(cP) at 25°C respectively. The total acid concentration, cna, can be expressed using the principle of mass balance: CHA = [ A - ] + [HA] + 2[HAT]
(9)
Combining Eq. (9) with Eqs. (3) and (4), one obtains the total acid concentration in terms of [H+], [A ] and the constant K~tA and KItA2: CHA = [ a - ] +
[U+][a-]fi2/gHa
+ 2KHA;[H+][A
]2f~2/gHa
(10)
For a particular set of values of KHA and KtlA2 E q . (10) can now be used to calculate the total acid concentration (CHA) for a series of hypothetical values of [H +]. Also one is able to calculate the molar conduc-
2.2.1. Calibration of the reference electrode The reference electrode Hg IHgClz(s)was standardized vs. SHE in 20 mass% PC + EC by using the cell: CellI:
PtlH z (latm);HA[IHgC12(s)lHg
with solutions of methanesulfonic acid or 2,5-dichlorobenzenesulfonic acid as H A in 20 mass% PC + EC. At any concentration of the acid used, the emf, E, of the Cell I, at 25°C, can be expressed as E = Eref+ E~.j- 0.05916 log an+
(12)
where Eref. is the potential of the reference electrode and E~.j. represents any liquid junction potential that may be involved. Thus if the hydrogen ion activity (an+) is known for the concentration (qtA) of the acid used in Cell I, the quantity Eref.+ ELi" can be evaluated readily from the cell emf. Assuming that ELi. is negligible or that it remains constant independent of the nature of the liquid junction, the value of the sum E~ef.+ El.j. (referred to as E'ref. hereafter) determined in this manner can be taken as the potential of the reference electrode (vs. SHE) for all practical purposes. In the present method of determination of E'~ef. the emf's of Cell I were measured at different concentrations of HA, and plotted as a function of - l o g CHA. This plot was then compared with a similar one constructed on the basis of calculated values of -0.05916 log an+ and the corresponding values of - log CHA for the particular acid used. The procedure involved gen-
A.K Srit,astal a, R.A. Samant /Journal of Electroanalytical Chemis'try 380 (1995) 29-33
eration of values of CHA from the various assumed values of [H ÷] using Eq. (10) with the available equilibrium constants KHA a n d KHA F as determined by the conductance method for the two acids employed in the study.
31
Both solvents were stored in sealed containers to prevent contamination from carbon dioxide and atmospheric water. Known masses of each solvent were mixed to form a (PC + EC) mixture with 20 mass% PC.
3.2. Reagents" 2.2.2. Determination of the standard potential of the quinhydrone electrode The standard potential of the reaction: O
OH
O
OH
(O)
3.3. Procedure
(H2Q)
in 20 mass% PC + EC was determined from the measurement of emf's of the cell: Cell II:
2,5-Dichlorobenzenesulfonic acid was prepared according to a published procedure [12]; the melting point of the anhydrous product was found to be 105106°C. Methanesulfonic acid (Fluka, 9 8 - 9 9 % pure), reagent grade quinhydrone and mercuric chloride were used without any further purification. The mercury used was of triple-distilled quality.
Hg IHgCla(S ) ref. in 20 mass% PC + EC H2,5-dichlorobenzenesulfonic acid in 20 mass% PC + EC, Q + H 2 Q I P t
with the equimolar mixture of quinone and hydroquinone (quinhydrone) using several concentrations of 2,5-dichlorobenzenesulfonic acid solutions in 20 mass% PC + EC. Ignoring any dissociation of H 2 Q and assuming that the activity coefficients of both Q and H ? Q are unity, the emf of Cell II, at 25°C can be represented by Eq. (13): E - E Q/H20--Eref.+ ° ' 0.05916 Iog all+
(13)
in terms of the standard potential (vs. SHE) of the 0 t q u i n o n e / h y d r o q u i n o n e system, (Eo/H2O), Eref. and the hydrogen ion activity of the solution. In the present treatment, the value of 0.05916 log au+, necessary for 0 the calculation of Eo/n2 Q were interpolated from the negative logarithm of the concentration of 2,5-dichlorobenzenesulfonic acid mentioned earlier in connection with the standardization of the reference electrode.
3. Experimental 3.1. Solvent The purification and storage of PC have been described elsewhere [2,10]. Commercially available EC (99% pure, Fluka) was distilled three times under reduced pressure. In the distillation only 80% middle distilled product was collected. The specific conductivity of purified solvent varied between 1 and 3 × 10 7 S c m - 1 at 40°C.
3.3.1. Conductivity measurements All conductance measurements were made at 25 _+ 0.05°C using a dip-type cell (cell constant 1.010 c m - l ) with lightly platinized electrodes as described earlier [13]. All molar conductivities reported were calculated after correcting for the solvent conductivity. 3.3.2. Potential measurements Platinum wire (2.5 cm long, 1.5 mm diameter) sealed into a Pyrex glass tube lightly coated with platinum black, in conjunction with pure and dry hydrogen gas was used as the hydrogen electrode; a freshly platinized electrode was used in each measurement. The electrode used in setting up the quinhydrone half cell consisted of the usual bright platinum electrode. The reference electrode consisted of a mercury pool in contact with a saturated solution of HgC12 containing an excess of solid chloride. A period of 24 h was allowed for equilibrium before the electrode was put into use. Normally, for a particular series of measurements, a newly prepared reference electrode was used; reference electrodes were discarded 2 - 3 days after the initial period of equilibration. Duplicate electrodes prepared under identical conditions agreed to within 0.5 mV. The design of the indicator and reference half cell compartment has been described previously [14]. An Equip-Tronics digital potentiometer model E Q - D G S was used for the emf measurements in an air-bath maintained at 25 _+ 0.5°C. In course of reading the cell potential a flowing junction was maintained by allowing the solution from the reference electrode to drain slowly into the side arm of the indicator half cell. As a general procedure the emf's were noted every 2 - 3 rain for all cells over a period of ~ 20 min; the reported potentials are the average of the last two readings which normally agreed to within 0.5 mV; measured cell potentials are considered reproducible to _+2 mV. In
32
A.K. Sricastat,a, R.A. Samant /Journal of Electroanalytical Chemisto' 380 (1995) 29-33
Table 1 Results of conductivity measurements on methanesulfonic acid and 2,5-dichlorobenzenesulfonic acid in 20 mass% PC + EC at 25°C Methanesulfonic acid 104 c / M
A / S cm: mol
50.9 30.7 23.4 17.5 12.1 8.1 4.9 2.5
1.94 2.50 2.79 3.02 3.42 3.99 5.01 6.92
0 -70
2,5-dichlorobenzenesulfonic acid 1
104
Y
A / S cm2 mol [
c/M
239.9 220.4 152.6 100.7 60.1 37.4 27.9 19.3 13.0
0 .60-
7.15 8.06 9.26 10.74 12.60 14.68 16.56 18.98 21.13
t,
? g:
the case of h y d r o g e n e l e c t r o d e m e a s u r e m e n t s , an initial p e r i o d of s a t u r a t i o n with H 2 gas for 1 5 - 2 0 min was allowed; d u e to the low v a p o u r p r e s s u r e of 20 m a s s % P C + E C at the e x p e r i m e n t a l t e m p e r a t u r e no v a p o u r p r e s s u r e c o r r e c t i o n was c o n s i d e r e d necessary.
~ o
0.50
O o i
0.30
v bJ
.Mr
/
0.10
4. Results and discussion
0"5
T h e conductivity m e a s u r e m e n t s of the two acids are s u m m a r i z e d in T a b l e 1. T h e c o r r e s p o n d i n g plots of l o g A c vs. log CHA a r e shown in Fig. 1 along with the c a l c u l a t e d plots which gave t h e b e s t fit a c c o r d i n g to the p r e s e n t t r e a t m e n t . KHA for 2 , 5 - d i c h l o r o b e n z e n e sulfonic and m e t h a n e s u l f o n i c acids a r e f o u n d to be 1.1 × 10 -3 a n d 8.5 X 10 -6 respectively. T h e KHA 2 val1.4-
o•
o" eo ®
1"0"
T
3 E
O
• •
0 0
if) ~.
0.8
u
o 0.6
•&
• && 0.4
0.2
-Iog(CHA/M)
Fig. 1. Plot of log AcN A vs. -log CHA. Methanesulfonic acid: (A) Experimental; ( • ) Calculated. 2,5-dichlorobenzenesulfonic acid: (©) Experimental; (e) calculated.
/
0.20 ~
/
1.2-
o
/
~
i
I
/
/
~4r.. /
/
,
,
2"0
3.5
- log(ellA/M)
Fig. 2. Plots of E and -0.05916 log aN+ VS. log CHA:methanesulfonic acid ( a , • ) and 2,5-dichlorobenzenesulfonic acid (©, e). The solid line represents experimental data using Cell I; the dashed line refers to the plot of calculated function -0.05916 log all+.
ues for b o t h acids a r e f o u n d to b e 1.0. A c o m p a r i s o n of the dissociation c o n s t a n t s (KHA) of 2 , 5 - d i c h l o r o b e n zenesulfonic acid (1.145 × 10 - s ) a n d m e t h a n e s u l f o n i c acid (5.37 × 10 - 9 ) in PC [3] with the values 11 × 10 3 a n d 8.5 × 10 . 6 in 20 m a s s % PC + E C respectively, as o b t a i n e d in the p r e s e n t w o r k i n d i c a t e s that b o t h the acids are a p p r o x i m a t e l y 100 times s t r o n g e r in 20 m a s s % P C + E C t h a n in PC. R e s u l t s of the p r e s e n t m e a s u r e m e n t s using Cell 1 are shown in Fig. 2 in the form of E vs. - l o g CHA plots o f b o t h m e t h a n e s u l f o n i c a n d 2 , 5 - d i c h l o r o b e n z e n e s u l f o n i c acids along with the t h e o r e t i c a l plots of the function - 0 . 0 5 9 1 6 log a . + . V a l u e s of KHA and K H A 2 as d e t e r m i n e d by the conductivity m e t h o d were used for the c a l c u l a t i o n of all+ at d i f f e r e n t total conc e n t r a t i o n of acids using Eq. (10). It is w o r t h w h i l e to note that the slopes o b t a i n e d for the e x p e r i m e n t a l a n d t h e o r e t i c a l plots in case o f both the acids a r e almost the same. I n s p e c t i o n of the e x p e r i m e n t a l plots in the case of m e t h a n e s u l f o n i c acid a n d the value of 005916 log a , + at t h r e e p o i n t s in the r a n g e of c o n c e n t r a t i o n s 1 × 10 2-1 x 10 - 4 M y i e l d e d an a v e r a g e value of 0.3810 V for E'ref. a n d a value of 0.3843 V was similarly d e r i v e d from the 2 , 5 - d i c h l o r o b e n z e n e s u l f o n i c acid data. T h e overall average of the two values viz. 0.3826 _+ 0.0023 V
A.K Sr&astat'a, R.A. Samant /Journal of Electroanalytical Chemistry 380 (1995) 29-33 Table 2 Results of measurements using Cell II: HgIHgCl2(s) ref. in 20 mass% PC + EC LI2,5-dichlorobenzenesulfonic acid, Q + H 2Q IPt
E/V
CHA/M
Quinhydrone concentration 5.300×10 1.812 × 105.436× 10 1.603×10 9.060× 10 2.712× 10
4 4 3 3 2
3M
2.013×10 -3 M
0.130 0.150 0.173 0.198 0.218
0.128 0.151 0.170 0.200 0.219
was finally taken as the value of E'ref. for the Hg JHgC12(s) reference electrode vs. SHE. The potentiometric measurements using Cell II are summarized in Table 2 for the two quinhydrone concentrations studied, as the concentration of 2,5-dichlorobenzenesulfonic acid is changed. The corresponding plots of emf's of Cell II vs. logarithm of the acid concentrations are given in Fig. 3. Using the interpolated value of the function 0.05916 log an+ from Fig. 2 at three different concentrations in the range 5 ×
0,25-
0.20-
0.15 0.15-
0.10 0'0
~io
zlo
31o
4.0
- iog(CHA/M)
Fig. 3. Plots of E vs. - log CHA (Cell II). Quinhydrone concentration: (z~)2.013×10 3 M ; ( o ) 5.300×10 ~ M.
33
1 0 - 2 - 5 × 10 -4 M of acid, an average of 0.3516 _+ 0.0023 V is calculated for E Q/H?Q ° VS. S H E in 20 mass% PC + EC. It is worth mentioning here that the slope of plots of E vs. log CHA (Fig. 3) are almost identical in magnitude with that obtained from hydrogen electrode measurements of 2,5-dichlorobenzenesulfonic acid (Fig. 2) over the same concentration range. This suggests that both P t l H 2 and quinhydrone electrode are equally reliable as an H+-indicator electrode in 20 mass% PC + EC solvent.
References [1] W.H. Lee, Cyclic Carbonates, in J.J. Lagowski (Ed.), The Chemistry of Non-Aqueous Solvents, Academic Press, New York, 1976, Vol. IV. [2] L.M. Mukherjee, Crit. Rev. Anal. Chem., 4 (1975) 325. [3] K. Izutsu, I.M. Kolthoff, T. Fujinaga, M. Hattori and M.K. Chantooni Jr., Anal. Chem., 49 (1977) 503. [4] A.K. Srivastava and L.M. Mukherjee, J. Electroanal. Chem., 160 (1984) 209. [5] G. Petrella and A. Sacco, J. Chem. Soc., Faraday Trans. 1, 74 (1978) 2070. [6] R. Fong, V.S. Von and J.R. Dhan, J. Electrochem. Soc., 137 (1990) 2007. [7] A.K. Srivastava and R.A. Samant, J. Chem. Eng. Data, 39 (1994) 358. [8] A.K. Srivastava and B. Tiwari, J. Electroanal. Chem., 325 (1992) 301. [9] R.M. Fuoss, J. Phys. Chem., 82 (1978) 2427. [10] A.K. Srivastava and L.M. Mukherjee, J. Electroanal. Chem., 157 (1983) 53. [11] H.S. Harned and B.B. Owen, The Physical Chemistry of Electrolytic Solutions, Reinhold, New York, 1958, pp. 229-300. [12] J.H. Crowell and L.L. Raiford, J. Am. Chem. Soc., 42 (1920) 145. [13] A.K. Srivastava and A.R. Desai, J. Chem. Eng. Data, 37 (1992) 322. [14] (a) S. Bruckenstein and I.M. Kolthoff, J. Am. Chem. Soc., 78 (1956) 2974; (b) L.M. Mukherjee, Ph.D. Thesis, University of Minnesota (1961); (c) S. Bruckenstein and L M . Mukherjee, J. Phys. Chem., 66 (1962) 2228.