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Mercury-mercurous propionate electrode: standard potentials at different temperatures and related thermodynamic quantities in dioxan-water media
Ekrrorhimicn 6
wn
lczo, Vol. 24. pp. 275-278. Prar Ltd. 1979. Prinitd m Great 3ritain.
0019486/79/03014275
sOZJXJ,U
MERCURY-MERCUROUS PROPLONATE ELECTRODE: STANDARD POTENTIALS AT DIFFERENT TEMPERATURES AND RELATED THERMODYNAMIC QUANTITIES IN DIOXAN-WATER MEDIA ARUN EC. BASU and S. ALNTYA of Applied Chemistry, University of Calcutta, Calcutta-700009,
Department
(Received 3 October
India
1978)
Abtract - The standard molal potentials (EZV)
of the Hg/Hg,(OPr),, OPr- elEctrode at 15,20,25,30 and 35°C have been determined in dioxan-water media. The values of Ek as a function of the Celsius temperature, t. in different solvent composition may be represented by the following equations : (i)
20”% Dioxan, EP = 0.4804 - 4.64 X lo-& (f/“C - 25) + 3.5294 X 10-G (C/“C - 25)2.
(ii)
30% Dioxan, Ep = 0.4679 - 5.98 x 1O-4 (t/“C - 25) - 5.5294 x 10-e (tpc - 25)2.
(iii) 45% Dioxan, Ep = 0.4317 - 7.04 x LO-.’ (t/T - 25) - 3.5294 X Lo-6 (t/“C - 25)2. (iv) 704/, Dioxan, Ep = 0.3262 - il.18 x lo-’ (t/T - 25) - 7.1760 X 10-e (ti”c - 25)f.
(v) 82% Dioxan,
En
= 0,2278 - 15.52 x lo-’
(t/T - 25) - 8.7059 x 1O-6 (t/“C - 25)2.
The related thermodynamic quantities have been calculated. The dependence of the standard potentials at 25°C on the dielectric constant of the media (in the light of the Born equation), the volume fraction (@,) and the mole fraction (N,) of water [based on Feakins-French relationship) have been examined. The variation ofthe thermodynamicquantities AC”, AS” and AH” with various parameters associated with solvent media have also been examined. The observations are similar to those reported in our previous work on mercury-mercurous acetate electrode in the same media. AC; for the cell reaction has been calculated.
mentioned cell is
INTRODUClTON
In a previous communication[l] the potentials of the mercury-mercurous propionate electrode and related thermodynamic quantities for the cell reaction in aqueous medium at different temperatures have been reported. The present communication is an extension of the same in dioxan-water media. For the purpose, the emfof the ceil PtIH&)
HOPr(m,),
NaOPr(mz)
Dioxan(X),
Water(lOGX)
-
H* + H&l) + OPr-. of the cell is given by the
E-E;-kkogK-klogm,y, = E; - k log K - k log(m, where k = (RT/F)ln
- m’)yur
(1)
10, (R : gas constant, T: thermo-
dynamic temperature, F : Faraday constant), m,, = molality of unionized propionic acid and m’ is given sufficiently accurately by K(mi/mz) where K is the ionization constant of propionic acid in dioxan-water media and the values were taken from the work of Harned and co-workers[2]. For the buffer solutions (electrolyte solution in the cell) the effect of the salt upon the activity coefficient of propionic acid can be represented by the following equation[3] ;
Hg,(O~),
@)/I%
were measured at 15, 20, 25, 30 and 35°C. EXPERIMENTAL
*Hz + fHg,(OPr),(s)
In the present case, the e@(E) equation,
RESULTS
The materials, preparation of electrodes, technique and accuracy of the measurements are same as described in our previous conununication[ 11. The measured electromotive forces (in V) of the cell corrected to 1 atm pressure of hydrogen are listed in Table 1 with respective molalities of propionic acid (ml) and sodium propionate (m2)_ The cell reaction for the above
logy, = s.1, (2) where S is known as salting coefficient and I is the ionic strength of the electrolyte solution. 275
ARUNK. B~su ANDS. ADITYA
276 Table 1. Emjvalues
of the cell (E/V) corrected to 1 atm pressure of H, at different temperatures (t/C) dioxan-water mixtures g/v
m,/mol kg-’
m,/mol kg-’
15°C
20°C
25°C
30°C
-.._
-35°C
(a) 20% Dioxan 1.1929 1.0483 0.9033 0.7584 0.7272 0.6137 0.4690 0.4322 0.3486 0.2649 0.1811
0.0995 0.0874 0.0753 0.0632 0.0480 0.0512 0.0391 0.0285 0.0230 0.0175 0.0120
0.80222 0.80497 0.80817 0.81122 0.81127 0.81542 0.82147 0.82297 0.82752 0.83417 0.84302
0.80373 0.80653 0.80993 0.81298 0.81313 0.81748 0.82368 0.82458 0.83003 0.83633 0.84553
0.80578 0.80883 0.81258 0.81563 0.81538 0.82003 0.82638 0.82658 0.83243 0.83908 0.84833
0.80814 0.81144 0.81519 0.81824 0.81794 0.82259 0.82899 0.82479 0.83514 0.84184 0.85084
0.81178 0.81408 0.81783 0.82108 0.821 t8 0.82558 0.83203 0.83328 0.83853 OS4538 0.85488
(b) 30% Dioxon 1.0854 0.9952 0.9051 0.8150 0.7251 0.5445 0.3469 0.2632
0.1481 0.1358 0.1235 0.1112 0.0989 0.0743 0.0229 0.0174
0.80582 0.80752 0.80937 0.81102 0.81367 0.81947 0.82837 0.83482
0.80751 0.80881 0.81146 0.81331 0.81596 0.82231 0.83136 0.83801
0.81013 0.81168 0.81393 0.81588 0.81873 0.82548 0.83453 0.84173
0.81314 0.81439 0.8 1669 0.81844 0.82169 0.82794 0.83699 0.84464
0.81538 0.81668 0.81923 0.82148 0.82413 0.83083 0.84103 0.84848
0.0684
0.0612 0.0539 0.0466 0.0394 0.0321 0.0312 0.0167 0.0140 0.0156
0.82306 0.82521 0.82826 0.83101 0.83481 0.83936 0.85231 0.85411 0.85846 0.86911
0.82401 0.82686 0.83051 0.83306 0.83711 0.84126 0.85531 0.85726 O.S6171 0.87216
0.82654 0.82949 0.83214 0.83569 0.83954 0.84494 0.85834 0.86079 0.86569 0.87669
0.82886 0.83156 0.8343 1 0.8389 1 0.84196 0.84776 0.86151 0.8643 1 0.86991 0.88016
0.83191 0.83496 0.83791 0.84211 0.84586 0.85131 0.86591 0.86806 0.87371 0.88406
(d) 70% Dioxan 0.3948 0.3596 0.2964 0.2542 0.2116 0.1695 0.0726
0.0539 0.049 1 0.0404 0.0347 0.0289 0.0231 0.0102
0.85973 0.86218 0.86478 0.86848 0.87193 0.87678 0.895 38
0.86204 0.86384 0.867 19 0.87099 0.87504 0.88034 0.89989
0.86438 0.86683 0.86993 0.87373 0.87823 0.88328 0.90408
0.86673 0.86908 0.87 308 0.87743 0.88078 0.88683 0.90838
0.86932 0.87182 0.87577 0,88002 0.88407 0.89022 0,91187
(e) 82% Dioxan 0.2790 0.2667 0.2463 0.2295 0.2133 0.1968 0.1802 0.1637
0.0380 0.0364 0.0336 0.03 13 0.029 1 0.0268 0.0246 0.0223
0.87303 0.87378 0.87533 0.87748 0.87878 0.87978 0.88248 0.88403
0.87485 0.87665 0.87815 0.87910 0.88160 0.88315 0.8SS60 0.88720
0.87741 0.8784 1 0.88076 0.88176 0.88391 0.88556 0.88861 0.89041
0.88001 0.88186 0.88391 0.88496 0.88796 0.88926 0.89226 0.89401
0.88351 0.88541 0.88721 0.88826 0.89146 0.89271 0.89566 0.8976t
(c) 45~oDioxan 0.8199 0.7333 0.6463 0.5593 0.4723 0.3852 0.2222 0.2006 0.1679 0.1110
Thus, defining E”’ = E f
squares, to an equation of the form k log K + k log(m,
- m’),
(3)
we have E”‘=E;-k.S.I.
in
(41
E; was obtained by the method of least squares with the values of E”’ for different values of I.
The standard molal potentials (Eg) at various temperatures were fitted, by the method of least
ED
= a0 + b&/T
-
25) + c,(t,‘“C - 25)‘.
(5)
Table 2 represents the values of a,-,, b,, and ce in different dioxan-water media. EL computed at each temperature using (5) are given in column 3 of Table 3 and those from the experimental measurements in column 2 of the same table. The standard Gibbs energy change, AG”, for the cell reaction at different temperatures have been calculated
Mercury-mercurous Table 2. Values of the constants of equation ED @PC - 25) + c0 (!/“C - 25)2
wt%
dioxan
a0
20 30 45 70 82
0.4804 0.4679 0.43 17 0.3262 0.227X
-6.64 x lo-“ - 5.98 X 10-b -7.04 x 10-4 - 11.18 x 1o-4 - 15.52 X 1o-4
propionate = a0 + b.
+3.5294x - 5.5294 x - 3.5294 x -7.1760%
10mb 1O-6 1O-6 lo-’
- 8.7059 X 10-e
AG” = -FE;.
(6)
and the standard enthalpy AH”, with the equation,
for the reaction,
AH” = AG” + T AS”.
cme3)
(9)
E; = El - 2k log(kgmol-l/M,,),
(10)
and
(7)
change
277
E,” = Ei + 2k log&/g
entropy change for the with the equation,
AS” = -&AG”)/sT,
potentials
The values of AG”, AS” and A.W at different temperatures in dioxan-water media are given in Table 3. The uncertainty in AS” is from f0.6 to -f l.OJ K-l mol-l as the organic component increases. The values of AH” over the temperature range 15-35”~ give average values of AC;-for the reaction as - 188, 13 14, - 209, -408 and -502 J K-’ mol-’ in 20. 30. 45. 70 and 82 wt % of d&an, respectively. ’ ’ ’ For each temperature, values of EONand Ez against l/D (Born plot), E,” against the logarithm ofthe volume fraction of water (Q,) and Ei against the lagarithm of mole fraction of water (N,) (Feakins and French plot[4]) were plotted. Standard potentials on the molar (E,“) and mole fraction (EL) scale were calculated from that on molar scale (Eg) with the following equations :
with the equation,
The values of the standard reaction, AS”, were calculated
electrode : Standard
where d, is the density of the pure solvent mixture at the particular temperature and M,, is the mean molecular weight of the solvent, defined by the re-
(8)
Table 3. Standard molal potentials (ED) for the Hg/Hg,(OPr),, OPrthermodynamic quantities for the cell reaction+ at different temperatures
electrode and derived
(r/“C) in dioxan-water
mixtures EW
Temp. Exp.
Cak.
AG”/J mol - ’
ASO,/JK-‘moI-’
0.4874 0.4837 0.4804 0.4769 0.4742
0.4874 0.4838 0.4804 0.4772 0.4741
- 47029 - 46682 - 46356 -46019 -45758
-
30% Diaxun 15 20 25 30 35
0.4733 0.4706 0.4679 0.4641 0.4616
0.4733 0.4707 0.4679 0.4647 0.4614
- 45670 - 45408 -45149 -44783 - 44540
-47.1 - 52.3 - 57.7 - 63.0 - 68.4
-
(c) 45% Dioxun 15 20 25 30 35
0.4385 0.4348 0.4317 0.4280 0.4243
0.4384 0.4351 0.4317 0.428 1 0.4243
-42312 - 41956 -41657 -41299 -40942
-61.1 - 64.5 - 68.0 -71.4 - 74.8
-59915 -Ml860 - 61905 - 62950 - 63975
0.3366
0.3366 0.3316 0.3262 0.3204 0.3 143
- 32479 - 32025 - 31477 - 30956 -30318
_
-94.1 101.0 107.9 114.8 121.7
-
0.2424 0.2353 0.2278 0.2198 0.2114
- 23389 - 22743 -21981 - 21276 - 20389
-
133.0 141.4 149.8 158.2 166.6
VC
AH”/J mol-
(a) 20% Dioxan :z 25 30 35 (b)
(d) 70% Dioxan 15 20 25 30 35 (e) 82% Dioxan 15
l
0.3319 0.3262 0.3208 0.3142 0.2424
20 25
0.2357 0.2278
30 35
0.2205 0.2113
fH2 + fHg>(OPr),(s)
--t H + + Hg(l) + (OPr)-
57.3 60.7 64.0 67.4 70.9
- 63535 -64475 - 65460 - 66465 - 67595
59225 60775 62345 63890
59580 61630 63640 65755 67825
- 61695 -64185 - 66630 - 69225 -71715
1
ARUN K. BXXJ AND S. ADITYA
278 lation, 100/M, = X/M, + = wt % of organic component
(100-X)/M,
where X
I
of molar mass M, and MY = molar mass of water. It is seen that the Born plot shows appreciable deviation from the linearity as expected from Born equation at about 40% dioxan media. A similar behaviour was noted for mercury/mercurous acetate electrode[S, 61. Oiwa[7] and Das and co_workers[8] also noted similar trends for the Ag/AgCI electrode in methanol-water mixtures and Ag/AgBr electrode in ethylene glycol-water mixtures, respectively. The Feakins and French plots[4] are almost linear as predicted. The linearity is better for the .Ek against -log N, plot. This type of linearity has been reported for the Ag/AgCl and Ag/AgBr electrodes in ethylene glycol-water media[4], [S] and electrode in dioxan-water b/Hg,(OAe), media[5,6], The plots of standard Gibbs energy, entropy and
enthalpy changes for the cell reaction at 25°C against l/D, log Qrn log N, and wt % of dioxan were made. In all the plots the correspondingvalues in water medium for this electrode were taken from our previous work[ 11. The vaiues are 0.503 1 V, - 48546 J mol - I, - 73175 J mol- ’ and - 177.5 J -82.6J K-l mol-‘, K- ’ mol-’ for EL, AG”, AS”, AH” and AC;, respectively. In all the cases the changes in entropy (ASO), and enthalpy (AHO) pass through a maximum (nearly at 30% for AS” and 3%40% by weight of dioxan for AH”). Typical plots of AG” and AS” against l/D, log QI,, etc. are shown in Figs 1 and 2, respectively. The same type of behaviour has been reported by McIntyre and Amis[iO]. We also reported[6] this type of
Fig. 2. Plot of A.Y/J Km ’ mol - 1 against solvent parameters.
variation
in AH” and
AS” for the reaction in our em_f
study of the cell, Pt/H,@,
1 atm) 1 HOAc(m,),
NaOAc(m,)
1
Hg,OW,W/H~ in dioxan-water media. The maximum appears, in case of the propionate cell reaction, at higher percentage of dioxan ( z 30- 40%) than that ( %200/,) for acetate cell reaction.
REFERENCES I.
2. 3. 4. 5. 6. 7. 8. 9. Fig. 1. Plot of AG” against
solvent
parameters
at 25°C.
10.
Arun K. Basu and S. Aditya, Electrochim. Acta 23, 1341 (1978). H. S. Harued and T. R. Dedell, J. Am. c/rem. Sot. 63,33d8 (1941). F. A. Long and W. F. McDevit, Chem. Rev. 51,119 (1952). D. Feakins and C. M. French, 1. &em. Sot. 2581 (1957). A. K. Basu and S. Aditya, J. Indian &em. SW. 48, 129 (1971). A. K. Basu and S. Aditya, ibid. 48, 155 (1971). I. T. Oiwa, J. p&s. Chem. Irhaca 60, 754 (1956). S. K. Banerji, K. K. Kundu and M. N. Das, J.&em. Sot.. Sect. A. Inora. ohvs. Them-v 161 119671. H. S. Harnedahd-J. 0. Mdrrisioi-j. ..&I. c&m. Sot. 58, 1908 (1936). J. M. McIntyre and E. S. Amis, J. them. engng Data 13, 371 (1978).
wn
lczo, Vol. 24. pp. 275-278. Prar Ltd. 1979. Prinitd m Great 3ritain.
0019486/79/03014275
sOZJXJ,U
MERCURY-MERCUROUS PROPLONATE ELECTRODE: STANDARD POTENTIALS AT DIFFERENT TEMPERATURES AND RELATED THERMODYNAMIC QUANTITIES IN DIOXAN-WATER MEDIA ARUN EC. BASU and S. ALNTYA of Applied Chemistry, University of Calcutta, Calcutta-700009,
Department
(Received 3 October
India
1978)
Abtract - The standard molal potentials (EZV)
of the Hg/Hg,(OPr),, OPr- elEctrode at 15,20,25,30 and 35°C have been determined in dioxan-water media. The values of Ek as a function of the Celsius temperature, t. in different solvent composition may be represented by the following equations : (i)
20”% Dioxan, EP = 0.4804 - 4.64 X lo-& (f/“C - 25) + 3.5294 X 10-G (C/“C - 25)2.
(ii)
30% Dioxan, Ep = 0.4679 - 5.98 x 1O-4 (t/“C - 25) - 5.5294 x 10-e (tpc - 25)2.
(iii) 45% Dioxan, Ep = 0.4317 - 7.04 x LO-.’ (t/T - 25) - 3.5294 X Lo-6 (t/“C - 25)2. (iv) 704/, Dioxan, Ep = 0.3262 - il.18 x lo-’ (t/T - 25) - 7.1760 X 10-e (ti”c - 25)f.
(v) 82% Dioxan,
En
= 0,2278 - 15.52 x lo-’
(t/T - 25) - 8.7059 x 1O-6 (t/“C - 25)2.
The related thermodynamic quantities have been calculated. The dependence of the standard potentials at 25°C on the dielectric constant of the media (in the light of the Born equation), the volume fraction (@,) and the mole fraction (N,) of water [based on Feakins-French relationship) have been examined. The variation ofthe thermodynamicquantities AC”, AS” and AH” with various parameters associated with solvent media have also been examined. The observations are similar to those reported in our previous work on mercury-mercurous acetate electrode in the same media. AC; for the cell reaction has been calculated.
mentioned cell is
INTRODUClTON
In a previous communication[l] the potentials of the mercury-mercurous propionate electrode and related thermodynamic quantities for the cell reaction in aqueous medium at different temperatures have been reported. The present communication is an extension of the same in dioxan-water media. For the purpose, the emfof the ceil PtIH&)
HOPr(m,),
NaOPr(mz)
Dioxan(X),
Water(lOGX)
-
H* + H&l) + OPr-. of the cell is given by the
E-E;-kkogK-klogm,y, = E; - k log K - k log(m, where k = (RT/F)ln
- m’)yur
(1)
10, (R : gas constant, T: thermo-
dynamic temperature, F : Faraday constant), m,, = molality of unionized propionic acid and m’ is given sufficiently accurately by K(mi/mz) where K is the ionization constant of propionic acid in dioxan-water media and the values were taken from the work of Harned and co-workers[2]. For the buffer solutions (electrolyte solution in the cell) the effect of the salt upon the activity coefficient of propionic acid can be represented by the following equation[3] ;
Hg,(O~),
@)/I%
were measured at 15, 20, 25, 30 and 35°C. EXPERIMENTAL
*Hz + fHg,(OPr),(s)
In the present case, the e@(E) equation,
RESULTS
The materials, preparation of electrodes, technique and accuracy of the measurements are same as described in our previous conununication[ 11. The measured electromotive forces (in V) of the cell corrected to 1 atm pressure of hydrogen are listed in Table 1 with respective molalities of propionic acid (ml) and sodium propionate (m2)_ The cell reaction for the above
logy, = s.1, (2) where S is known as salting coefficient and I is the ionic strength of the electrolyte solution. 275
ARUNK. B~su ANDS. ADITYA
276 Table 1. Emjvalues
of the cell (E/V) corrected to 1 atm pressure of H, at different temperatures (t/C) dioxan-water mixtures g/v
m,/mol kg-’
m,/mol kg-’
15°C
20°C
25°C
30°C
-.._
-35°C
(a) 20% Dioxan 1.1929 1.0483 0.9033 0.7584 0.7272 0.6137 0.4690 0.4322 0.3486 0.2649 0.1811
0.0995 0.0874 0.0753 0.0632 0.0480 0.0512 0.0391 0.0285 0.0230 0.0175 0.0120
0.80222 0.80497 0.80817 0.81122 0.81127 0.81542 0.82147 0.82297 0.82752 0.83417 0.84302
0.80373 0.80653 0.80993 0.81298 0.81313 0.81748 0.82368 0.82458 0.83003 0.83633 0.84553
0.80578 0.80883 0.81258 0.81563 0.81538 0.82003 0.82638 0.82658 0.83243 0.83908 0.84833
0.80814 0.81144 0.81519 0.81824 0.81794 0.82259 0.82899 0.82479 0.83514 0.84184 0.85084
0.81178 0.81408 0.81783 0.82108 0.821 t8 0.82558 0.83203 0.83328 0.83853 OS4538 0.85488
(b) 30% Dioxon 1.0854 0.9952 0.9051 0.8150 0.7251 0.5445 0.3469 0.2632
0.1481 0.1358 0.1235 0.1112 0.0989 0.0743 0.0229 0.0174
0.80582 0.80752 0.80937 0.81102 0.81367 0.81947 0.82837 0.83482
0.80751 0.80881 0.81146 0.81331 0.81596 0.82231 0.83136 0.83801
0.81013 0.81168 0.81393 0.81588 0.81873 0.82548 0.83453 0.84173
0.81314 0.81439 0.8 1669 0.81844 0.82169 0.82794 0.83699 0.84464
0.81538 0.81668 0.81923 0.82148 0.82413 0.83083 0.84103 0.84848
0.0684
0.0612 0.0539 0.0466 0.0394 0.0321 0.0312 0.0167 0.0140 0.0156
0.82306 0.82521 0.82826 0.83101 0.83481 0.83936 0.85231 0.85411 0.85846 0.86911
0.82401 0.82686 0.83051 0.83306 0.83711 0.84126 0.85531 0.85726 O.S6171 0.87216
0.82654 0.82949 0.83214 0.83569 0.83954 0.84494 0.85834 0.86079 0.86569 0.87669
0.82886 0.83156 0.8343 1 0.8389 1 0.84196 0.84776 0.86151 0.8643 1 0.86991 0.88016
0.83191 0.83496 0.83791 0.84211 0.84586 0.85131 0.86591 0.86806 0.87371 0.88406
(d) 70% Dioxan 0.3948 0.3596 0.2964 0.2542 0.2116 0.1695 0.0726
0.0539 0.049 1 0.0404 0.0347 0.0289 0.0231 0.0102
0.85973 0.86218 0.86478 0.86848 0.87193 0.87678 0.895 38
0.86204 0.86384 0.867 19 0.87099 0.87504 0.88034 0.89989
0.86438 0.86683 0.86993 0.87373 0.87823 0.88328 0.90408
0.86673 0.86908 0.87 308 0.87743 0.88078 0.88683 0.90838
0.86932 0.87182 0.87577 0,88002 0.88407 0.89022 0,91187
(e) 82% Dioxan 0.2790 0.2667 0.2463 0.2295 0.2133 0.1968 0.1802 0.1637
0.0380 0.0364 0.0336 0.03 13 0.029 1 0.0268 0.0246 0.0223
0.87303 0.87378 0.87533 0.87748 0.87878 0.87978 0.88248 0.88403
0.87485 0.87665 0.87815 0.87910 0.88160 0.88315 0.8SS60 0.88720
0.87741 0.8784 1 0.88076 0.88176 0.88391 0.88556 0.88861 0.89041
0.88001 0.88186 0.88391 0.88496 0.88796 0.88926 0.89226 0.89401
0.88351 0.88541 0.88721 0.88826 0.89146 0.89271 0.89566 0.8976t
(c) 45~oDioxan 0.8199 0.7333 0.6463 0.5593 0.4723 0.3852 0.2222 0.2006 0.1679 0.1110
Thus, defining E”’ = E f
squares, to an equation of the form k log K + k log(m,
- m’),
(3)
we have E”‘=E;-k.S.I.
in
(41
E; was obtained by the method of least squares with the values of E”’ for different values of I.
The standard molal potentials (Eg) at various temperatures were fitted, by the method of least
ED
= a0 + b&/T
-
25) + c,(t,‘“C - 25)‘.
(5)
Table 2 represents the values of a,-,, b,, and ce in different dioxan-water media. EL computed at each temperature using (5) are given in column 3 of Table 3 and those from the experimental measurements in column 2 of the same table. The standard Gibbs energy change, AG”, for the cell reaction at different temperatures have been calculated
Mercury-mercurous Table 2. Values of the constants of equation ED @PC - 25) + c0 (!/“C - 25)2
wt%
dioxan
a0
20 30 45 70 82
0.4804 0.4679 0.43 17 0.3262 0.227X
-6.64 x lo-“ - 5.98 X 10-b -7.04 x 10-4 - 11.18 x 1o-4 - 15.52 X 1o-4
propionate = a0 + b.
+3.5294x - 5.5294 x - 3.5294 x -7.1760%
10mb 1O-6 1O-6 lo-’
- 8.7059 X 10-e
AG” = -FE;.
(6)
and the standard enthalpy AH”, with the equation,
for the reaction,
AH” = AG” + T AS”.
cme3)
(9)
E; = El - 2k log(kgmol-l/M,,),
(10)
and
(7)
change
277
E,” = Ei + 2k log&/g
entropy change for the with the equation,
AS” = -&AG”)/sT,
potentials
The values of AG”, AS” and A.W at different temperatures in dioxan-water media are given in Table 3. The uncertainty in AS” is from f0.6 to -f l.OJ K-l mol-l as the organic component increases. The values of AH” over the temperature range 15-35”~ give average values of AC;-for the reaction as - 188, 13 14, - 209, -408 and -502 J K-’ mol-’ in 20. 30. 45. 70 and 82 wt % of d&an, respectively. ’ ’ ’ For each temperature, values of EONand Ez against l/D (Born plot), E,” against the logarithm ofthe volume fraction of water (Q,) and Ei against the lagarithm of mole fraction of water (N,) (Feakins and French plot[4]) were plotted. Standard potentials on the molar (E,“) and mole fraction (EL) scale were calculated from that on molar scale (Eg) with the following equations :
with the equation,
The values of the standard reaction, AS”, were calculated
electrode : Standard
where d, is the density of the pure solvent mixture at the particular temperature and M,, is the mean molecular weight of the solvent, defined by the re-
(8)
Table 3. Standard molal potentials (ED) for the Hg/Hg,(OPr),, OPrthermodynamic quantities for the cell reaction+ at different temperatures
electrode and derived
(r/“C) in dioxan-water
mixtures EW
Temp. Exp.
Cak.
AG”/J mol - ’
ASO,/JK-‘moI-’
0.4874 0.4837 0.4804 0.4769 0.4742
0.4874 0.4838 0.4804 0.4772 0.4741
- 47029 - 46682 - 46356 -46019 -45758
-
30% Diaxun 15 20 25 30 35
0.4733 0.4706 0.4679 0.4641 0.4616
0.4733 0.4707 0.4679 0.4647 0.4614
- 45670 - 45408 -45149 -44783 - 44540
-47.1 - 52.3 - 57.7 - 63.0 - 68.4
-
(c) 45% Dioxun 15 20 25 30 35
0.4385 0.4348 0.4317 0.4280 0.4243
0.4384 0.4351 0.4317 0.428 1 0.4243
-42312 - 41956 -41657 -41299 -40942
-61.1 - 64.5 - 68.0 -71.4 - 74.8
-59915 -Ml860 - 61905 - 62950 - 63975
0.3366
0.3366 0.3316 0.3262 0.3204 0.3 143
- 32479 - 32025 - 31477 - 30956 -30318
_
-94.1 101.0 107.9 114.8 121.7
-
0.2424 0.2353 0.2278 0.2198 0.2114
- 23389 - 22743 -21981 - 21276 - 20389
-
133.0 141.4 149.8 158.2 166.6
VC
AH”/J mol-
(a) 20% Dioxan :z 25 30 35 (b)
(d) 70% Dioxan 15 20 25 30 35 (e) 82% Dioxan 15
l
0.3319 0.3262 0.3208 0.3142 0.2424
20 25
0.2357 0.2278
30 35
0.2205 0.2113
fH2 + fHg>(OPr),(s)
--t H + + Hg(l) + (OPr)-
57.3 60.7 64.0 67.4 70.9
- 63535 -64475 - 65460 - 66465 - 67595
59225 60775 62345 63890
59580 61630 63640 65755 67825
- 61695 -64185 - 66630 - 69225 -71715
1
ARUN K. BXXJ AND S. ADITYA
278 lation, 100/M, = X/M, + = wt % of organic component
(100-X)/M,
where X
I
of molar mass M, and MY = molar mass of water. It is seen that the Born plot shows appreciable deviation from the linearity as expected from Born equation at about 40% dioxan media. A similar behaviour was noted for mercury/mercurous acetate electrode[S, 61. Oiwa[7] and Das and co_workers[8] also noted similar trends for the Ag/AgCI electrode in methanol-water mixtures and Ag/AgBr electrode in ethylene glycol-water mixtures, respectively. The Feakins and French plots[4] are almost linear as predicted. The linearity is better for the .Ek against -log N, plot. This type of linearity has been reported for the Ag/AgCl and Ag/AgBr electrodes in ethylene glycol-water media[4], [S] and electrode in dioxan-water b/Hg,(OAe), media[5,6], The plots of standard Gibbs energy, entropy and
enthalpy changes for the cell reaction at 25°C against l/D, log Qrn log N, and wt % of dioxan were made. In all the plots the correspondingvalues in water medium for this electrode were taken from our previous work[ 11. The vaiues are 0.503 1 V, - 48546 J mol - I, - 73175 J mol- ’ and - 177.5 J -82.6J K-l mol-‘, K- ’ mol-’ for EL, AG”, AS”, AH” and AC;, respectively. In all the cases the changes in entropy (ASO), and enthalpy (AHO) pass through a maximum (nearly at 30% for AS” and 3%40% by weight of dioxan for AH”). Typical plots of AG” and AS” against l/D, log QI,, etc. are shown in Figs 1 and 2, respectively. The same type of behaviour has been reported by McIntyre and Amis[iO]. We also reported[6] this type of
Fig. 2. Plot of A.Y/J Km ’ mol - 1 against solvent parameters.
variation
in AH” and
AS” for the reaction in our em_f
study of the cell, Pt/H,@,
1 atm) 1 HOAc(m,),
NaOAc(m,)
1
Hg,OW,W/H~ in dioxan-water media. The maximum appears, in case of the propionate cell reaction, at higher percentage of dioxan ( z 30- 40%) than that ( %200/,) for acetate cell reaction.
REFERENCES I.
2. 3. 4. 5. 6. 7. 8. 9. Fig. 1. Plot of AG” against
solvent
parameters
at 25°C.
10.
Arun K. Basu and S. Aditya, Electrochim. Acta 23, 1341 (1978). H. S. Harued and T. R. Dedell, J. Am. c/rem. Sot. 63,33d8 (1941). F. A. Long and W. F. McDevit, Chem. Rev. 51,119 (1952). D. Feakins and C. M. French, 1. &em. Sot. 2581 (1957). A. K. Basu and S. Aditya, J. Indian &em. SW. 48, 129 (1971). A. K. Basu and S. Aditya, ibid. 48, 155 (1971). I. T. Oiwa, J. p&s. Chem. Irhaca 60, 754 (1956). S. K. Banerji, K. K. Kundu and M. N. Das, J.&em. Sot.. Sect. A. Inora. ohvs. Them-v 161 119671. H. S. Harnedahd-J. 0. Mdrrisioi-j. ..&I. c&m. Sot. 58, 1908 (1936). J. M. McIntyre and E. S. Amis, J. them. engng Data 13, 371 (1978).