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Enthalpy of dissolution of potassium peroxodisulfate in water at 298.15 K
M-1084 J. Chem.
1980,
Thermodvnamics
1079-1083
Enthalpy of dissolution of potassium peroxodisulfate in water at 298.15 K V. PEKAREK,” and V. VACEK
R. RYCHLY,b
J. BALEJ,
Institute of Inorganic Chemistry of the Czechoslovak qf Sciences, 160 00 Prague 6, Czechoslovakia
(Received 16 July 1979; in revised,form
31 March
The enthalpy of dissolution of K2S,0s (potassium peroxodisulfate) measured in Calvet and LKB calorimeters in the molality range molality dependence can be expressed by AH,,,/kJ
mall’
= 68.964-16.367(m/mol
Academy
1980) in water at 298.15 K was 0.002 to 0.19 mol kg -‘. Its
kg~‘)‘2+14.353(m/mol
kg-‘).
for the molality region mentioned above. The influence of the chemical instability of aqueous solutions of potassium peroxodisulfate upon reliability of the measured values is discussed. The crystallization enthalpy of potassium peroxodisulfate estimated from the above-mentioned molality dependence was calculated.
1. introduction The molality dependence of the dissolution enthalpy of K&O8 (potassium peroxodisulfate) in water has not been measured systematically so far. A value taken from standard tables”) for the enthalpy of dissolution at infinite dilution, AH” = 54.4 kJ mol- ‘, is based on a single old experimental measurement of Berthelot.“’ Recently the enthalpy of dissolution of K&O, in water has been measured by Hu and HepIer in a narrow molality range of very dilute solutions from 0.004 to 0.01 mol kg-’ at 298.15 K. Their values lie within the interval: 67.95 to 68.45 kJ mol-’ ; a value extrapolated by them (3) to infinite dilution: AH” = (67.53 f 0.50) kJ mol- ‘, differs appreciably from that found by Berthelot.‘2’ The present work deals with the measurement of the enthalpy of dissolution of potassium peroxodisulfate in water as a function of the molality in a wide region of unsaturated solutions. The effect of potassium peroxodisulfate decomposition on measured results was evaluated.
2. Experimental Potassium peroxodisulfate of reagent grade (Lachema Brno, Czechoslovakia) after dissolution and neutralization with a solution of KOH was triply recrystallized from ’ To whom correspondence b Chemopetrol, Research 0021-9614/80/111079+05
should be addressed. Institute of Inorganic Chemistry, %01.00/O
400 00 Usti nad Labem, 1.1 1980 Academic
Czechoslovakia.
Press Inc. (London)
Ltd.
1080
V. PEKAREK.
R. RYCHLY.
J. BALEJ,
AND
V. VACEK
redistilled water, dried in an electric oven at 333 K, and then in a desiccator with phosphorus pentoxide. According to LeBlanc and Eckardt’“’ analysis, the impurity content of the resulting potassium peroxodisulfate was less than 0.05 mass per cent. Water redistilled from silicac5) was used in all experiments. The Calvet standard microcalorimeter (Setaram-France, 473 K, 15 cm3 cells) with the signal output fed to an ITC-type integration unit was used for measurements within the molality range 0.002 to 0.13 mol kg- ’ and was calibrated by the reaction enthalpy of tris(hydroxymethyl)aminomethane (NBS-724a standard reference material) with 0.1 mol kg- ’ HCl. “’ The stainless-steel reaction cells of the mentioned firm were adapted so that the substance under study placed in a mesh having blades on the bottom could be dipped into solvent and the formed solution could then be stirred by a moderate motion of the mesh. Before dipping, the substance was maintained in a water-vapor-free closed chamber to avoid hydration of the salt. The thermal effect of stirring was negligible with respect to the chosen sensitivity of measurements. The temperature changes in the calorimeter in the course of the measurements were checked several times by means of a cell with built-in thermistor, which was calibrated with a relative accuracy of +0.005 K. The temperature of measurement was (298.15 +O.Ol) K. The duration of individual experiments was 30 to 90 min. The LKB calorimeter, type 8700-l with 25 cm3 and 100 cm3 glass cells was used for measurements within the molality region 0.13 to 0.19 mol kg-i. A solution of initial molality m,(m, ranged from 0.11 to 0.17 mol kg-‘) was used as a solvent in these experiments instead of pure water. The duration of these experiments was shorter than 15 min and the results were evaluated by a standard Regnault-Pfaundler method.“’ The rate of decomposition of potassium peroxodisulfate solutions was measured in a Pyrex glass vessel thermostatted at (298.15 kO.05) K without and with the stainlesssteel calorimetric cell.
3. Results The measured values of the enthalpy of dissolution m(m) of potassium peroxodisulfate are summarized in table 1 together with relative deviations E from values calculated from a least-squares polynomial. The experimentally found dependence of m(m) on molality m is fitted as usually by an empirical equation: AH(m)
= A + Bm”’
+ Cm.
(1)
The scatter of the measured values is apparent from figure 1. The average relative deviation, i.e. 0.2 per cent for 30 measured values of AH(m), corresponds well to the expected experimental error of about 0.5 per cent. The mutual consistency of both sets of results, from Calvet and LKB calorimeters, was verified by the measurement of the dissolution enthalpy of potassium peroxodisulfate in pure water (in table 1 designated by *) on the LKB calorimeter. For the resulting molality of
ENTHALPY
OF
DISSOLUTION
OF K&O,
1081
IN WATER
TABLE 1. Enthalpies of dissolution of potassium peroxodisulfate in water at 298.15 K. AH,(m) denotes the enthalpy of dissolution of K,S,Os for given molality m and initial molality mi: E denotes the relative difference {m(m)-M(calc., m))/AH(m), the value of AH(calc.. m) following from equation (1 )
m, mol kg-’
0.1141 0.1323 0.1357 0.1607 0.1721 0.1567 0.1643
m mol kg-’ 0.00197 0.00192 0.00196 0.00209 0.00226 0.00296 0.00314 0.00553 0.00904 0.01327 0.01321 0.00968 0.02501 0.04010 0.04219 0.04360 0.02563 0.06430 0.069 11 0.06846 0.08228 0.08372 0.09650 0.11211 0.12972 0.11245 0.13430 0.14910 0.15770 0.18350 0.18870 0.16260 0.17050
AH,(m) -__ kJ molt 68.55 68.27 68.19 68.39 68.67 67.61 68.05 67.66 67.53 67.56 67.25 67.21 66.74 66.64 66.46 66.20 66.23 65.89 65.81 65.57 65.44 65.44 65.26 65.19 64.90 64.82 64.89 64.94 64.68 64.48 64.50 64.76 64.59
Instrument
’ +0.42 -0.01 -0.11 +0.21 +0.67 -0.75 - 0.06 - 0.25 0.00 to.44 -- 0.03 -0.42 +0.02 +0.58 1-0.39 + 0.05 -0.71 +0.24 +0.26 -0.14 0.00 +0.03 0.00 +0.16 -0.02 -0.40 +0.02 +0.26 - 0.05 -0.13 - 0.06 +0.1 1 - 0.07
Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calcet C&et Calvet LKB* Calvet LKB* Callet Calvet Calvet Calvet Calvet Calvet LKB’ Calbet Calvet Calvet LKB LKB LKB LKB LKB LKB LKB
0.0401 mol kg-’ the value of AH(m) = 66.64 kJ mol-’ was found. The calculated value from equation (1) is about 0.5 per cent lower [AH(calc., m) = 66.26 kJ molt ‘1 than the experimental one, which can be taken as a satisfying consistency test. A solution of molality 0.2249 mol kg-’ of K,S1O, was considered as saturated at 298.15 K,‘*’ taking the molar mass of K$,O, as 270.319 g mol-‘. The measured enthalpy change on the LKB calorimeter corresponding to the dissolution of amount of substance An, of K,S,Os in a solution of initial molality mi containing amount of substance n2 of K&Os, where the final molality is in a simple relation to the enthalpies of dissolution 4= mi(l +An,/n,) (n2 + An,)AH(n+)n,AH(mi). The value of AZ-Z(+) was calculated from this equation using measured values of the enthalpy change, with n,, An,, and AH(mi) from equation (1) by smoothing 23 points measured on the Calvet microcalorimeter in the
1082
V. PEKAREK.
R. RYCHLY, J. BALEJ. AND V. VACEK
--I
1
0
I
I
0.1 -1 m/mol kg
I
0.2
m,
FIGURE 1. Dissolution enthalpy AH(m) for K1S208 (potassium peroxodisulfate) in water at 298.15 K as a function of molality m. 0. Present work-Calvet microcalorimeter; 0, present work-LKB calorimeter; 0, 0, Hu and Hepler; t3) @ , (>, value extrapolated from equation (1). Curve 1. equation (1). Curve 2, calculated dissolution enthalpy following from the appropriate derivative of equation (1).
molality region from 0.002 to 0.13 mol kg- ‘. The resulting equation (1) was computed by fitting the whole set of 30 points, giving A = 68.964, B = - 16.367, and C = 14.253. Both dependences differed only negligibly. This fact can be taken also as a check of good mutual consistency of both sets of results, that measured on the Calvet and that on the LKB calorimeter.
4. Discussion Aqueous solutions of peroxodisulfates are thermodynamically undergo hydrolytic decomposition according to the reaction : S,Oi-(aq)+H,O
= 2SOi-(aq)+2H+
+0.50,.
unstable and can (2)
The reaction enthalpy (Al& = 168.123 kJ mol-’ at 298.15 KY” can therefore affect profoundly the measured enthalpies of dissolution. From decomposition-rate measurements of aqueous peroxodisulfate solutions, especially in the presence of the calorimetric stainless-steel cells (see table 2) we found that the rate constants k lie
ENTHALPY
OF DISSOLUTION
OF K&O8
IN WATER
1083
TABLE 2. Rate constants k for the decomposition of aqueous K,S,Os of various molalities at 298.15 K mimol kg-’ 0.136 0.0143 0.00088
0.169 0.0219
k/h-’ 1O-4 5x1o-5 1 x 1o-3 1 x 1o-6 I x 1o-4
6x
Cell material Pyrex Pyrex Pyrex Pyrex Pyrex
glass glass glass glass with stainless-steel cell glass with stainless-steel cell
within the range 10m3 to lO-‘j h-i, the highest value having been determined for the most dilute solutions. According to these results, even for such dilute solutions, the enthalpy change of reaction (2) during measurements in the Calvet microcalorimeter (not exceeding 1.5 h) should be less than 0.37 per cent of the corresponding enthalpy of dissolution, and for more concentrated solutions should be 10 to 100 times less, especially for measurements in the LKB calorimeter, where the duration of measurements was less than 15 min. We can therefore suppose that the measured enthalpies of dissolution of potassium peroxodisulfate in water are true, as the errors due to the enthalpy of the decomposition reaction (2) lie in the range of uncertainty of the calorimetric experimental method. The measured rate constants presented in table 2 are in a fairly good agreement with published values :(9) (k = 3.7 x 10m4 h-l at 298.15 K for 0.05 mol kg-’ W&W Extrapolated values of AH” presented by us and by Hu and Heplert3’ differ by about 2 per cent. We suppose that this difference is mainly due to the method of extrapolation. In reference 3 the limiting Debye-Htickel law was considered ; we did not take this into account because of insufficient accuracy of our measurements in very dilute solutions to justify the extrapolation by use of this law. REFERENCES I. Rossini, F. D.; Wagman, D. D.; Evans, W. H.; Levine, S.; Jaffe, I. Selected Vulues of’ Chemical Thermodynamic Properties, NBS 500. U.S. Government Printing Office: Washington D.C. 1952. p. 497. 2. Berthelot. M. Ann. Chim. Phys. 1892, 26, 526. 3. Hu, T.; Hepler, L. G. J. Chem. Eng. Data 1962, 7, 58. 4. LeBlanc, M.; Eckardt, M. Z. Elekrrochem. 1898-9, 5, 355. 5. Stopka, P.; Vepiek-SiSka, J. Chem. Listy (Czech) 1973, 67, 424. 6. Rychly, R.; Pekarek, V. J. Chem. Thermodynamics 1977, 9, 391. I. Wads& I. Science Tools 1966, 13, 33. X. Balej, J.; Regner, A. CON. Czech. Chem. Commun. 1960, 25, 1685. 9. Galiba, H.: Csanyi. L. J. ; Szabo. Z. G. Z. Anorg. Allgem. Chem. 1956, 287. 152.
1980,
Thermodvnamics
1079-1083
Enthalpy of dissolution of potassium peroxodisulfate in water at 298.15 K V. PEKAREK,” and V. VACEK
R. RYCHLY,b
J. BALEJ,
Institute of Inorganic Chemistry of the Czechoslovak qf Sciences, 160 00 Prague 6, Czechoslovakia
(Received 16 July 1979; in revised,form
31 March
The enthalpy of dissolution of K2S,0s (potassium peroxodisulfate) measured in Calvet and LKB calorimeters in the molality range molality dependence can be expressed by AH,,,/kJ
mall’
= 68.964-16.367(m/mol
Academy
1980) in water at 298.15 K was 0.002 to 0.19 mol kg -‘. Its
kg~‘)‘2+14.353(m/mol
kg-‘).
for the molality region mentioned above. The influence of the chemical instability of aqueous solutions of potassium peroxodisulfate upon reliability of the measured values is discussed. The crystallization enthalpy of potassium peroxodisulfate estimated from the above-mentioned molality dependence was calculated.
1. introduction The molality dependence of the dissolution enthalpy of K&O8 (potassium peroxodisulfate) in water has not been measured systematically so far. A value taken from standard tables”) for the enthalpy of dissolution at infinite dilution, AH” = 54.4 kJ mol- ‘, is based on a single old experimental measurement of Berthelot.“’ Recently the enthalpy of dissolution of K&O, in water has been measured by Hu and HepIer in a narrow molality range of very dilute solutions from 0.004 to 0.01 mol kg-’ at 298.15 K. Their values lie within the interval: 67.95 to 68.45 kJ mol-’ ; a value extrapolated by them (3) to infinite dilution: AH” = (67.53 f 0.50) kJ mol- ‘, differs appreciably from that found by Berthelot.‘2’ The present work deals with the measurement of the enthalpy of dissolution of potassium peroxodisulfate in water as a function of the molality in a wide region of unsaturated solutions. The effect of potassium peroxodisulfate decomposition on measured results was evaluated.
2. Experimental Potassium peroxodisulfate of reagent grade (Lachema Brno, Czechoslovakia) after dissolution and neutralization with a solution of KOH was triply recrystallized from ’ To whom correspondence b Chemopetrol, Research 0021-9614/80/111079+05
should be addressed. Institute of Inorganic Chemistry, %01.00/O
400 00 Usti nad Labem, 1.1 1980 Academic
Czechoslovakia.
Press Inc. (London)
Ltd.
1080
V. PEKAREK.
R. RYCHLY.
J. BALEJ,
AND
V. VACEK
redistilled water, dried in an electric oven at 333 K, and then in a desiccator with phosphorus pentoxide. According to LeBlanc and Eckardt’“’ analysis, the impurity content of the resulting potassium peroxodisulfate was less than 0.05 mass per cent. Water redistilled from silicac5) was used in all experiments. The Calvet standard microcalorimeter (Setaram-France, 473 K, 15 cm3 cells) with the signal output fed to an ITC-type integration unit was used for measurements within the molality range 0.002 to 0.13 mol kg- ’ and was calibrated by the reaction enthalpy of tris(hydroxymethyl)aminomethane (NBS-724a standard reference material) with 0.1 mol kg- ’ HCl. “’ The stainless-steel reaction cells of the mentioned firm were adapted so that the substance under study placed in a mesh having blades on the bottom could be dipped into solvent and the formed solution could then be stirred by a moderate motion of the mesh. Before dipping, the substance was maintained in a water-vapor-free closed chamber to avoid hydration of the salt. The thermal effect of stirring was negligible with respect to the chosen sensitivity of measurements. The temperature changes in the calorimeter in the course of the measurements were checked several times by means of a cell with built-in thermistor, which was calibrated with a relative accuracy of +0.005 K. The temperature of measurement was (298.15 +O.Ol) K. The duration of individual experiments was 30 to 90 min. The LKB calorimeter, type 8700-l with 25 cm3 and 100 cm3 glass cells was used for measurements within the molality region 0.13 to 0.19 mol kg-i. A solution of initial molality m,(m, ranged from 0.11 to 0.17 mol kg-‘) was used as a solvent in these experiments instead of pure water. The duration of these experiments was shorter than 15 min and the results were evaluated by a standard Regnault-Pfaundler method.“’ The rate of decomposition of potassium peroxodisulfate solutions was measured in a Pyrex glass vessel thermostatted at (298.15 kO.05) K without and with the stainlesssteel calorimetric cell.
3. Results The measured values of the enthalpy of dissolution m(m) of potassium peroxodisulfate are summarized in table 1 together with relative deviations E from values calculated from a least-squares polynomial. The experimentally found dependence of m(m) on molality m is fitted as usually by an empirical equation: AH(m)
= A + Bm”’
+ Cm.
(1)
The scatter of the measured values is apparent from figure 1. The average relative deviation, i.e. 0.2 per cent for 30 measured values of AH(m), corresponds well to the expected experimental error of about 0.5 per cent. The mutual consistency of both sets of results, from Calvet and LKB calorimeters, was verified by the measurement of the dissolution enthalpy of potassium peroxodisulfate in pure water (in table 1 designated by *) on the LKB calorimeter. For the resulting molality of
ENTHALPY
OF
DISSOLUTION
OF K&O,
1081
IN WATER
TABLE 1. Enthalpies of dissolution of potassium peroxodisulfate in water at 298.15 K. AH,(m) denotes the enthalpy of dissolution of K,S,Os for given molality m and initial molality mi: E denotes the relative difference {m(m)-M(calc., m))/AH(m), the value of AH(calc.. m) following from equation (1 )
m, mol kg-’
0.1141 0.1323 0.1357 0.1607 0.1721 0.1567 0.1643
m mol kg-’ 0.00197 0.00192 0.00196 0.00209 0.00226 0.00296 0.00314 0.00553 0.00904 0.01327 0.01321 0.00968 0.02501 0.04010 0.04219 0.04360 0.02563 0.06430 0.069 11 0.06846 0.08228 0.08372 0.09650 0.11211 0.12972 0.11245 0.13430 0.14910 0.15770 0.18350 0.18870 0.16260 0.17050
AH,(m) -__ kJ molt 68.55 68.27 68.19 68.39 68.67 67.61 68.05 67.66 67.53 67.56 67.25 67.21 66.74 66.64 66.46 66.20 66.23 65.89 65.81 65.57 65.44 65.44 65.26 65.19 64.90 64.82 64.89 64.94 64.68 64.48 64.50 64.76 64.59
Instrument
’ +0.42 -0.01 -0.11 +0.21 +0.67 -0.75 - 0.06 - 0.25 0.00 to.44 -- 0.03 -0.42 +0.02 +0.58 1-0.39 + 0.05 -0.71 +0.24 +0.26 -0.14 0.00 +0.03 0.00 +0.16 -0.02 -0.40 +0.02 +0.26 - 0.05 -0.13 - 0.06 +0.1 1 - 0.07
Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calvet Calcet C&et Calvet LKB* Calvet LKB* Callet Calvet Calvet Calvet Calvet Calvet LKB’ Calbet Calvet Calvet LKB LKB LKB LKB LKB LKB LKB
0.0401 mol kg-’ the value of AH(m) = 66.64 kJ mol-’ was found. The calculated value from equation (1) is about 0.5 per cent lower [AH(calc., m) = 66.26 kJ molt ‘1 than the experimental one, which can be taken as a satisfying consistency test. A solution of molality 0.2249 mol kg-’ of K,S1O, was considered as saturated at 298.15 K,‘*’ taking the molar mass of K$,O, as 270.319 g mol-‘. The measured enthalpy change on the LKB calorimeter corresponding to the dissolution of amount of substance An, of K,S,Os in a solution of initial molality mi containing amount of substance n2 of K&Os, where the final molality is in a simple relation to the enthalpies of dissolution 4= mi(l +An,/n,) (n2 + An,)AH(n+)n,AH(mi). The value of AZ-Z(+) was calculated from this equation using measured values of the enthalpy change, with n,, An,, and AH(mi) from equation (1) by smoothing 23 points measured on the Calvet microcalorimeter in the
1082
V. PEKAREK.
R. RYCHLY, J. BALEJ. AND V. VACEK
--I
1
0
I
I
0.1 -1 m/mol kg
I
0.2
m,
FIGURE 1. Dissolution enthalpy AH(m) for K1S208 (potassium peroxodisulfate) in water at 298.15 K as a function of molality m. 0. Present work-Calvet microcalorimeter; 0, present work-LKB calorimeter; 0, 0, Hu and Hepler; t3) @ , (>, value extrapolated from equation (1). Curve 1. equation (1). Curve 2, calculated dissolution enthalpy following from the appropriate derivative of equation (1).
molality region from 0.002 to 0.13 mol kg- ‘. The resulting equation (1) was computed by fitting the whole set of 30 points, giving A = 68.964, B = - 16.367, and C = 14.253. Both dependences differed only negligibly. This fact can be taken also as a check of good mutual consistency of both sets of results, that measured on the Calvet and that on the LKB calorimeter.
4. Discussion Aqueous solutions of peroxodisulfates are thermodynamically undergo hydrolytic decomposition according to the reaction : S,Oi-(aq)+H,O
= 2SOi-(aq)+2H+
+0.50,.
unstable and can (2)
The reaction enthalpy (Al& = 168.123 kJ mol-’ at 298.15 KY” can therefore affect profoundly the measured enthalpies of dissolution. From decomposition-rate measurements of aqueous peroxodisulfate solutions, especially in the presence of the calorimetric stainless-steel cells (see table 2) we found that the rate constants k lie
ENTHALPY
OF DISSOLUTION
OF K&O8
IN WATER
1083
TABLE 2. Rate constants k for the decomposition of aqueous K,S,Os of various molalities at 298.15 K mimol kg-’ 0.136 0.0143 0.00088
0.169 0.0219
k/h-’ 1O-4 5x1o-5 1 x 1o-3 1 x 1o-6 I x 1o-4
6x
Cell material Pyrex Pyrex Pyrex Pyrex Pyrex
glass glass glass glass with stainless-steel cell glass with stainless-steel cell
within the range 10m3 to lO-‘j h-i, the highest value having been determined for the most dilute solutions. According to these results, even for such dilute solutions, the enthalpy change of reaction (2) during measurements in the Calvet microcalorimeter (not exceeding 1.5 h) should be less than 0.37 per cent of the corresponding enthalpy of dissolution, and for more concentrated solutions should be 10 to 100 times less, especially for measurements in the LKB calorimeter, where the duration of measurements was less than 15 min. We can therefore suppose that the measured enthalpies of dissolution of potassium peroxodisulfate in water are true, as the errors due to the enthalpy of the decomposition reaction (2) lie in the range of uncertainty of the calorimetric experimental method. The measured rate constants presented in table 2 are in a fairly good agreement with published values :(9) (k = 3.7 x 10m4 h-l at 298.15 K for 0.05 mol kg-’ W&W Extrapolated values of AH” presented by us and by Hu and Heplert3’ differ by about 2 per cent. We suppose that this difference is mainly due to the method of extrapolation. In reference 3 the limiting Debye-Htickel law was considered ; we did not take this into account because of insufficient accuracy of our measurements in very dilute solutions to justify the extrapolation by use of this law. REFERENCES I. Rossini, F. D.; Wagman, D. D.; Evans, W. H.; Levine, S.; Jaffe, I. Selected Vulues of’ Chemical Thermodynamic Properties, NBS 500. U.S. Government Printing Office: Washington D.C. 1952. p. 497. 2. Berthelot. M. Ann. Chim. Phys. 1892, 26, 526. 3. Hu, T.; Hepler, L. G. J. Chem. Eng. Data 1962, 7, 58. 4. LeBlanc, M.; Eckardt, M. Z. Elekrrochem. 1898-9, 5, 355. 5. Stopka, P.; Vepiek-SiSka, J. Chem. Listy (Czech) 1973, 67, 424. 6. Rychly, R.; Pekarek, V. J. Chem. Thermodynamics 1977, 9, 391. I. Wads& I. Science Tools 1966, 13, 33. X. Balej, J.; Regner, A. CON. Czech. Chem. Commun. 1960, 25, 1685. 9. Galiba, H.: Csanyi. L. J. ; Szabo. Z. G. Z. Anorg. Allgem. Chem. 1956, 287. 152.