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Thermal instability of cerium(IV) sulphuric acid solutions
J. lnorg. Nucl. Chem., 1964. Vol. 26, pp. 337 to 346. Pergamon Press Ltd. Printed in Northern Ireland
THERMAL
INSTABILITY OF CERIUM(IV) ACID SOLUTIONS
SULPHURIC
D. GRANT* Glasgow University, Scotland (Received 22 March 1963; in revtsed form 25 June 1963) ~et--Autoreduction of cerium(IV) sulphate in aqueous sulphuric acid occurs above 40 ° in the presence of a glass surface which acts as a catalyst. The reaction, which is approximately zero order, stops when the products of the reaction deactivate the catalyst surface. Cerium(IV) containing solids and added Ag(I) and Hg(II) ions also catalyse the reaction. No demonstrable dependence on the bulk cerium(IV) or cerium(III) concentrations could he found. Although no clear cut dependence on temperature was observed from 40 ° to 100 ° the reaction proceeds notably faster at reflux temperature.
WILLARDand YOUNG introduced the use of ceric sulphate as a quantitative reagent in analytical chemistry, tl~ Since these workers found little or no autoreduction of cerium(IV) in sulphuric acid solutions on heating, subsequent workers have largely ignored the possibility of such a reaction. It has now been found that a small amount of reduction of cerium(IV) occurs on heating cerium(IV) sulphuric acid solutions c.f. reference (2), and it was of interest to elucidate the factors influencing this reaction particularly in view of the extensive use of cerium(IV) sulphuric acid solutions in analytical chemistry.
Reaction occurring The most probable process leading to loss of titrable cerium(IV) on heating cerium(IV) sulphuric acid solution is the oxidation of water by the cerium(IV) species. 4Ce(IV) q- 2H20---* 4Ce(III) q- 4H + + O~t. It was verified that in the present case cerium(IV) was reduced to cerium(Ill). This was accomplished by comparing heated and non-heated samples when the final total cerium present in solution was oxidised to the cerium(IV) state by means of sodium bismuthate, is) The titrable cerium(IV) in both cases was now the same. CATALYSIS
OF THE REACTION
A number of experiments were conducted to find the effects of possible factors in the cerium(IV) ---*cerium(III) reaction as a function of the amount of reduction of cerium(IV) under the same conditions, at reflux temperature. * Present address: Monsanto Chemicals Ltd., Ruabon, N. Wales. t~ H. H. WZLLARO,J. P. YOUNG,3". Amer. Chem. Soc. 50, 1322 (1928); series through J. Amer. Chem. Soc. 55, 3260 (1933). ~ D. GRANT, D. S. PAYNE, Analyt. chim. Acta 25, 4-22 (1961). ~s~ F. J. METZ~ER, J. Amer. Chem. Soc., 31,523 (1909). 337
338
D. Gz~rr
Effect of glass surface It was found that the occurrence of the cerium(IV)--~ cerium(III) reaction was influenced by the area of glass surface in contact with the solution. Thus the addition of glass surface (in the form of glass helicles) produced a marked increase in the extent of the reaction. The extent is not, however, related linearly to the area of glass exposed to the solution, Fig. 1, although extrapolation to zero surface suggests that
'//"
I
I
,
I
I
I
I
I
l
I I0
0
Wt. pyrex helicles,
I
I
I
G
FIG. l.--Observed increase in the Ce(IV)-H,O reaction as a function of the glass
surface in contact with the solution at reflux temperature Curve A B C
Ce(IV))
(HsSOj)
Time reflux (min)
Total Voi. of soln. (ml).
4.1 4"1 3"1
30 40 30
50 20 10
0.073 0"073 0"145
0"5 g helicles corresponded to approx. 100 cm* of glass area. no reaction would be observed in absence of the glass surface which appears to act as a catalyst.
Possible effects of organic impurities PEa'ZOLDc4) suggested that the thermal decomposition of ceric sulphate in sulphuric acid noted by Wvass and SVaGER,cb~ was due to the presence of small amounts
~,~w. I~TZOLD,Die CerimetrieVerlag Chem. Weinheim/Bergstr. 1955, p. 50. ~5~ L. WEISS,H. SEIOER,Z. Analyt. Chem. 113, 314 (1938).
Thermal instability of cerium(IV) sulphuric acid solutions
339
of organic impurities in the solution. The kinetics of the decomposition process found in the present work shows that an initial zero order reaction ceased after a period of time as would be expected for the oxidation of organic or other impurity in the solution. However, the present study also showed that the reaction ceased due to deactivation of the wall catalyst during the reaction and further reduction could be easily demonstrated for the same solution by reactivating the vessel surface, this being achieved for reactions at reflux temperature by simply cooling to room temperature. Also the reaction was observed as before when especially pure reagents were employed, the cerium(IV) being obtained from a spectroscopically pure sample of ceria. The observed rate was the same as that of solutions obtained from normal
.71 :31
1
I
',
~
Time reflux,
I
1
1
I JO0
rain
Flo. 2.--Comparison of the Ce(IV)-H~O reaction rate as a function of the source of the reagents • points--the ceric sulphate soln. was prepared from commercialgrade re. agents © points--the ceric sulphate soln. was prepared by dissolvingspectroscopically pure in ceria analytical grade sulphuric acid. In both cases (Ce(IV)) was 0.0112 N, (H~SO~) was 2.8 N and the volume studied was 10 ml. The ACe(IV) values found are given as ml of the solution used. commercial reagents, Fig. 2. It is therefore concluded that the reaction observed is not dependent on traces of organic or other impurities which may be present.
Effects of added ions Small amounts of certain added ions have however been shown to have a measureable effect on the reaction. Thus Cu(II), Fig. 3 below 0.004 M retards the reaction while Ag(I), Fig. 4 and also Hg(II) were found to increase the reaction.
Effect of surface other than glass A pronounced increase in the extent of the reaction was observed in cases where cerium containing suspensions appeared during the heating process. This is observed at low sulphuric acid concentrations, (below 2 N). It was possible to redissolve the precipitates by adding sulphuric acid prior to the titration of the cerium(IV), the increased loss of cerium(IV) being clearly due to increased autoreduction caused by the further effect of additional active surfaces. Similar effects were observed when ammonium sulphate was present in the cerium(IV) solution prior to heating. Carborundum chips (commonly used as a boiling agent) were without catalytic effect.
340
D. GR~,rr
2J % "3 1"5
0
<3
I'C
0"~
0
t
I
*
I
Cu(]I)conc.,
Mxl()~
Flo. 3.--Effect of Cu(II) on the Ce(IV)-H20 reaction (Ce(IV)) was 0.058 N, (HsSO,) was 3 N and 25 ml samples were refluxed for 56 rain.
3
% :¢
3
o
2 O
o
I
i Ag(Dconc.,
Mxl(J ~
FIe;. 4.--Effect of Ag(I) on the Ce(IV)-HzO reaction [Ce(IV)] was 0.008 N, (HzSO,) was 9 N and 25 ml samples were refluxed for 33 rain.
Thermal instability of ceriumtlV) sulphuric acid solutions
341
Effect of sulphuric acid concentration A l t h o u g h low acid c o n c e n t r a t i o n produces precipitates o n heating which have a catalytic effect o n the reaction, increasing s u l p h u r i c acid c o n c e n t r a t i o n in the region where n o precipitation occurred also g r a d u a l l y increased the reaction observed. A 35 per cent increase is observed between 2 a n d 18 N s u l p h u r i c acid c o n c e n t r a t i o n both with n o n r e f l u x i n g solutions at c o n s t a n t t e m p e r a t u r e a n d with refluxing solutions where the t e m p e r a t u r e varies with the s u l p h u r i c acid concentration. K I N E T I C S OF THE R E A C T I O N The course of the reaction is the same at all temperatures studied. A n initial zero order reaction stops after a period of time related to the ease of deactivation of the TABLE 1A.--OBSERVED REACTION KINETICS BELOW REFLUX TEMPERATURE
cf. Vol. of soln. Fig. 5 (ml) a e
c d b f
25 25 25 25 25 20 20 20
(Ce(IV)) (N)
(H ~SO,) (N)
Temp. (°C)
Sequence no.
Rate* of initial reaction
0.1065 0.1065 0.1065 0.1065 0.1065 0.1452 0.1452 0.0113
4.7 4"7 4.7 4.7 4"7 3"1 3.1 2.4
53 53 43 77 77 45 85 56
1 3 6 8 10 11 13 --
1.34 2.16 5"5 3.02 1-23 3.42 4.03 0.50
TABLE lB.--DEACTIVATION OF VESSEL WALLS BELOW REFLUX TEMPERATURE
a b c d
Sequence no.
Rate of* initial reaction
Time of initial reaction (rain)
1 2 3 --
1"54 3.15 2-68 13-8
60 15 16 6
Solutions studied: 20mlof0 1450NCe(IV),H2SO, conc., 3"0 N were studied at 64°. a-c shows deactivation by continued use of the same glass surfaces. d shows the effect of reactivation by treating with sulphuric acid. *The rate is expressed in terms of ACe(IV), 10-6 equivs./min. This is considered to be the preferable way of representing the results rather than giving the proportion of the cerium(Iv) present in a given sample which reacts with water, since the most important factor is the glass surface. The sequence numbers shown refer to the use of the same glass surfaces in a series of experiences.
Time of initial reaction (min). 60 25 10 37 45 20 18 35
342
D. Gn.Am"
catalytic surfaces. The reaction is not detected below 40° and between 40 ° and reflux temperature the rate is variable, depending critically on surface conditions. However, at reflux temperature a marked increase in the rate is found and it now shows a less critical dependence on the surface conditions.
Below reflux temperature No change in the rate as a function of temperature or concentration factors has been observed between 40° and reflux temperature. Some results are presented in Table 1A and B and Fig. 5. Table 1B shows the effect of continued use of the same
o
IOO Time,
rain
FIG. 5.~Some reaction curves below reflux temperature for the Ce(IV)-HtO reaction Temp
Curve a b c d e f
(°C) 53 85 77 45 53 56
(Ce(IV))
(HISO.)
(N)
(N)
0"0165 0.1452 O.1065 0-1452 0-1065 0-0113
4"7 3.1 4.7 3.1 4-7 2.4
glass surfaces. The surface becomes catalytically "deactivated if between runs the surface is washed with water only, Table I(B) a-c; if however the walls are treated with boiling 6 N sulphuric acid the activity increases greatly, d. Thus the rate is seen to be controlled by the activity of the glass surface. It is also apparent from Table 1 that the faster rates recorded show a shorter reaction time. This is expected to be the case if the reaction stops by formation of sufficient reaction product at the surface to deactivate it, and if there is an equilibrium of the reaction products between the surface and solution phases.
At reflux temperature The rate was not now as critically dependent on the glass surface and it was possible to obtain data on the effect of concentration factors on the rate of the reaction.
Thermal instability of cerium(IV) sulphuric acid solutions
343
The effect of cerium(IV) concentration. The rate was found to show no dependence on the cerium(IV) concentration from 0.03 to 0.12 N, the rate is however lower at 0.015 N; some results are shown on Fig. 6 and Table 2.
b
r
Time, FIG.
rain
6.--Some reaction curves at reflux temperatures for the C¢(IV)-HsO reaction Curve
(Ce(IV)) (N)
(HsSO,) (N)
Vol. of. soln. (ml)
a b c d e f
0-1191 0.0495 0.0414 0.0305 0.0762 0.0414
5.0 5.0 4.5 5.0 5.0 5-0
25 25 35 35 35 35
The effect of cerium(Ill) concentration. Although no clear cut dependence was observed between the rate (or extent) of the reaction on the cerium(III) concentration in solution the general conclusion was drawn that the solutions obtained from ceria or ceric sulphate which contained variable amounts of cerium(Ill) displayed no critical dependence on the cerium(Ill) concentration. Difficulties were experienced due to precipitation increasing the reaction by catalysis when the solutions were obtained from mixtures of pure cerium(IV) obtained by oxidation of residual cerium(Ill) and then adding controlled amounts of cerium(Ill) sulphate solutions. The effect ofsulphuric acid concentration. The rate increases as the sulphuric acid concentration increases (cf. Table 2). It is not certain what amount of this increase is due to a simple temperature effect since the reflux temperature also increases, e.g. from 100.9 ° to 133.4 ° between 1 and 9 N sulphuric acid concentration; however since little temperature effect was observed on comparing the effect of acid on the observed amount of reaction at constant temperature or at reflux temperature it is likely that sulphuric acid exhibits a per se effect on the rate. The effect of dissolved oxygen. Bubbling oxygen through the reaction solution either before or during the reaction decreased the extent of the reaction. However, deoxygenation by bubbling nitrogen through the solution showed no measurable effect on the kinetics. These observations are in accord with the expected role of
344
D. GRANT TABLE2.~-OBSERVEDREACTIONKINETICSATREFLUXTEMPERATURE of. Fig. 6
Vol of soln. studied (ml)
Initial Ce(IV) cone. (N)
H~SO, cone. (N)
Rate of initial reaction
a b e f d
25 25 35 35 35 35
0.1191 0.0495 0.0762 0.0414 0.0305 0.0153
5.0 5.0 5.0 5.0 5.0 5.0
13.9 13-9 13"9 13.6 13.9 6.5
10 10 20 25 10 35 35
0.0112 0.0112 0"056 0.0496 0"0835 0.0414 0.053
2.5 2.6 2.8 2"9 3'0 4.5 6.2
0.86 1.02 1.26 3.12 7-7 12.8 17.0
The rate as before is expressed as 10-6 equivs./min oxygen as a surface deactivator. Since the system is a potential source of oxygen it is unlikely that complete deoxygenation of the solution is possible. The effect of adding extra oxygen to the solution was found to be one of surface deactivation only since the extent of the reaction rather than the initial rate was altered. Consideration of the time for which the reaction proceeds at reflux temperature as a function of the ratio of solution bulk to air solution interface showed that where there was a large amount of air solution surface the reaction proceeded for longer times at reflux temperature. It is thus indicated that oxygen, which will be one of the products of the reaction, deactivated the surface, since the oxygen concentration in solution will vary with the amount of air-liquid surface present. However, it is postulated that since the characteristics of surface deactivation below reflux temperature are markedly different from those at reflux temperature the effect of cerium(IIl) is predominant at the lower temperatures. DISCUSSION It is obvious from consideration of the thermodynamic equilibrium between cerium(IV) and (ill) in aqueous sulphuric acid solution at 25°, E0 ---- -- 1.44 V (1 M sulphuric acid) 4Ce(III) ~ 4Ce(IV) + 4e and the thermodynamic equilibrium between oxygen and water 2 H20 ~ O a + 4H + + 4e
E o = --1"229 V (s~
that cerium(IV) sulphuric acid solutions are metastable with regard to the oxidation of water to produce gaseous oxygen. However, the attainment of equilibrium is kinetically controlled, the uneatalysed stability of cerium(IV) sulphuric acid solutions being a function of the activation energy barrier. Thus the reaction is only observed in the presence of a catalyst, in the present ease in the absence of added ions the glass vessel wall acts as the catalyst. However, since the glass vessel walls easily become te~ w. M. LATIMER,OxMation Potentials. Prentice-Hall, New York (1952).
Thermal instability of cerium(IV) sulphuric acid solutions
345
deactivated by the products of the reaction the actual experimentally observed stability of cerium(IV) sulphate-sulphuric acid solutions is high when the cerium(IV) concentration and glass areas are favourable. The observed characteristics of the reaction as presented above are all in accord with a heterogeneous reduction of cerium(IV) catalysed by the glass surface. The ease of deactivation observed at temperatures below reflux is greater than at reflux temperatures which underlines the greater stability of solutions at the lower temperatures. At reflux temperature the reaction displays no dependence on the concentration of cerium(IV) in the range studied apart from the observation that at low cerium(IV) concentrations the reaction rate is lower. The surface will not be in full catalytic use at this point. The only clear dependence between a rate and a concentration factor is found with sulphuric acid concentration. Sulphuric acid has been observed to act as a specific surface activator and this is sufficient to explain the observed effect. Although the over-all picture is one of a reaction controlled by physical rather than by purely chemical effects it is also evident that information on the sort of chemical process occurring can be obtained. Thus the surface cannot be the only place where reduction of cerium(IV) occurs since the effect of surface area on the reaction is not a linear one, also ions present in the solution capable of electron exchange with solvent entities affect the reaction, Cu(II) decreasing it while AgO) andHg (II) increase it. It is likely that a smooth increase in the amount of reaction product with time will be occurring at the walls, however, the reaction is observed to stop in an abrupt manner, and the termination of the reaction has been established to be a surface effect. These observations are in line with the occurrence of a chain reaction initiated at the catalyst surface, propagated in the solution phase and terminated both at the walls and in the solution. The precise effect of surface is likely to be a source of reaction initiators which can escape from the walls and lead to formation of gaseous oxygen from cerium(IV) and water; when no further initiators penetrate the solution phase, the overall reaction stops. There is a considerable body of data in the literature indicating the occurrence of radical reactions in cerium(IV) solutions, e.g. exchange of labelled cerium must occur by exchange of electrons between cerium ions and the water molecules; tT~ thus there is considerable likelihood that cerium(IV) reaction of the type discussed above are radical chain reactions. EXPERIMENTAL Standard laboratory techniques were employed, volumetric apparatus being carefully standardised before use. Cerium(IV) was estimated volumetrically by titration with ferrous ammonium sulphate solution using ferroin as indicator. Standardization was done using dichromate solution, values being checked against the direct reduction of cerium(IV) by arsenious oxide. Reagents Various preparations of cerium(IV) sulphate, in sulphuric acid were studied, all showing the same properties. One sample was especially pure being obtained from 'Specpure' curia from Johnson Mathey and Co. Ltd. Acknowledgements--The author thanks Albright and Wilson Ltd. for a grant and Dr. D. S. Payne for much useful discussion and criticism.
~ G. E. CHALLENGER,B. J. MAS'~RS,J. Amer. Chem. Soc., 77, 1063 (1955).
THERMAL
INSTABILITY OF CERIUM(IV) ACID SOLUTIONS
SULPHURIC
D. GRANT* Glasgow University, Scotland (Received 22 March 1963; in revtsed form 25 June 1963) ~et--Autoreduction of cerium(IV) sulphate in aqueous sulphuric acid occurs above 40 ° in the presence of a glass surface which acts as a catalyst. The reaction, which is approximately zero order, stops when the products of the reaction deactivate the catalyst surface. Cerium(IV) containing solids and added Ag(I) and Hg(II) ions also catalyse the reaction. No demonstrable dependence on the bulk cerium(IV) or cerium(III) concentrations could he found. Although no clear cut dependence on temperature was observed from 40 ° to 100 ° the reaction proceeds notably faster at reflux temperature.
WILLARDand YOUNG introduced the use of ceric sulphate as a quantitative reagent in analytical chemistry, tl~ Since these workers found little or no autoreduction of cerium(IV) in sulphuric acid solutions on heating, subsequent workers have largely ignored the possibility of such a reaction. It has now been found that a small amount of reduction of cerium(IV) occurs on heating cerium(IV) sulphuric acid solutions c.f. reference (2), and it was of interest to elucidate the factors influencing this reaction particularly in view of the extensive use of cerium(IV) sulphuric acid solutions in analytical chemistry.
Reaction occurring The most probable process leading to loss of titrable cerium(IV) on heating cerium(IV) sulphuric acid solution is the oxidation of water by the cerium(IV) species. 4Ce(IV) q- 2H20---* 4Ce(III) q- 4H + + O~t. It was verified that in the present case cerium(IV) was reduced to cerium(Ill). This was accomplished by comparing heated and non-heated samples when the final total cerium present in solution was oxidised to the cerium(IV) state by means of sodium bismuthate, is) The titrable cerium(IV) in both cases was now the same. CATALYSIS
OF THE REACTION
A number of experiments were conducted to find the effects of possible factors in the cerium(IV) ---*cerium(III) reaction as a function of the amount of reduction of cerium(IV) under the same conditions, at reflux temperature. * Present address: Monsanto Chemicals Ltd., Ruabon, N. Wales. t~ H. H. WZLLARO,J. P. YOUNG,3". Amer. Chem. Soc. 50, 1322 (1928); series through J. Amer. Chem. Soc. 55, 3260 (1933). ~ D. GRANT, D. S. PAYNE, Analyt. chim. Acta 25, 4-22 (1961). ~s~ F. J. METZ~ER, J. Amer. Chem. Soc., 31,523 (1909). 337
338
D. Gz~rr
Effect of glass surface It was found that the occurrence of the cerium(IV)--~ cerium(III) reaction was influenced by the area of glass surface in contact with the solution. Thus the addition of glass surface (in the form of glass helicles) produced a marked increase in the extent of the reaction. The extent is not, however, related linearly to the area of glass exposed to the solution, Fig. 1, although extrapolation to zero surface suggests that
'//"
I
I
,
I
I
I
I
I
l
I I0
0
Wt. pyrex helicles,
I
I
I
G
FIG. l.--Observed increase in the Ce(IV)-H,O reaction as a function of the glass
surface in contact with the solution at reflux temperature Curve A B C
Ce(IV))
(HsSOj)
Time reflux (min)
Total Voi. of soln. (ml).
4.1 4"1 3"1
30 40 30
50 20 10
0.073 0"073 0"145
0"5 g helicles corresponded to approx. 100 cm* of glass area. no reaction would be observed in absence of the glass surface which appears to act as a catalyst.
Possible effects of organic impurities PEa'ZOLDc4) suggested that the thermal decomposition of ceric sulphate in sulphuric acid noted by Wvass and SVaGER,cb~ was due to the presence of small amounts
~,~w. I~TZOLD,Die CerimetrieVerlag Chem. Weinheim/Bergstr. 1955, p. 50. ~5~ L. WEISS,H. SEIOER,Z. Analyt. Chem. 113, 314 (1938).
Thermal instability of cerium(IV) sulphuric acid solutions
339
of organic impurities in the solution. The kinetics of the decomposition process found in the present work shows that an initial zero order reaction ceased after a period of time as would be expected for the oxidation of organic or other impurity in the solution. However, the present study also showed that the reaction ceased due to deactivation of the wall catalyst during the reaction and further reduction could be easily demonstrated for the same solution by reactivating the vessel surface, this being achieved for reactions at reflux temperature by simply cooling to room temperature. Also the reaction was observed as before when especially pure reagents were employed, the cerium(IV) being obtained from a spectroscopically pure sample of ceria. The observed rate was the same as that of solutions obtained from normal
.71 :31
1
I
',
~
Time reflux,
I
1
1
I JO0
rain
Flo. 2.--Comparison of the Ce(IV)-H~O reaction rate as a function of the source of the reagents • points--the ceric sulphate soln. was prepared from commercialgrade re. agents © points--the ceric sulphate soln. was prepared by dissolvingspectroscopically pure in ceria analytical grade sulphuric acid. In both cases (Ce(IV)) was 0.0112 N, (H~SO~) was 2.8 N and the volume studied was 10 ml. The ACe(IV) values found are given as ml of the solution used. commercial reagents, Fig. 2. It is therefore concluded that the reaction observed is not dependent on traces of organic or other impurities which may be present.
Effects of added ions Small amounts of certain added ions have however been shown to have a measureable effect on the reaction. Thus Cu(II), Fig. 3 below 0.004 M retards the reaction while Ag(I), Fig. 4 and also Hg(II) were found to increase the reaction.
Effect of surface other than glass A pronounced increase in the extent of the reaction was observed in cases where cerium containing suspensions appeared during the heating process. This is observed at low sulphuric acid concentrations, (below 2 N). It was possible to redissolve the precipitates by adding sulphuric acid prior to the titration of the cerium(IV), the increased loss of cerium(IV) being clearly due to increased autoreduction caused by the further effect of additional active surfaces. Similar effects were observed when ammonium sulphate was present in the cerium(IV) solution prior to heating. Carborundum chips (commonly used as a boiling agent) were without catalytic effect.
340
D. GR~,rr
2J % "3 1"5
0
<3
I'C
0"~
0
t
I
*
I
Cu(]I)conc.,
Mxl()~
Flo. 3.--Effect of Cu(II) on the Ce(IV)-H20 reaction (Ce(IV)) was 0.058 N, (HsSO,) was 3 N and 25 ml samples were refluxed for 56 rain.
3
% :¢
3
o
2 O
o
I
i Ag(Dconc.,
Mxl(J ~
FIe;. 4.--Effect of Ag(I) on the Ce(IV)-HzO reaction [Ce(IV)] was 0.008 N, (HzSO,) was 9 N and 25 ml samples were refluxed for 33 rain.
Thermal instability of ceriumtlV) sulphuric acid solutions
341
Effect of sulphuric acid concentration A l t h o u g h low acid c o n c e n t r a t i o n produces precipitates o n heating which have a catalytic effect o n the reaction, increasing s u l p h u r i c acid c o n c e n t r a t i o n in the region where n o precipitation occurred also g r a d u a l l y increased the reaction observed. A 35 per cent increase is observed between 2 a n d 18 N s u l p h u r i c acid c o n c e n t r a t i o n both with n o n r e f l u x i n g solutions at c o n s t a n t t e m p e r a t u r e a n d with refluxing solutions where the t e m p e r a t u r e varies with the s u l p h u r i c acid concentration. K I N E T I C S OF THE R E A C T I O N The course of the reaction is the same at all temperatures studied. A n initial zero order reaction stops after a period of time related to the ease of deactivation of the TABLE 1A.--OBSERVED REACTION KINETICS BELOW REFLUX TEMPERATURE
cf. Vol. of soln. Fig. 5 (ml) a e
c d b f
25 25 25 25 25 20 20 20
(Ce(IV)) (N)
(H ~SO,) (N)
Temp. (°C)
Sequence no.
Rate* of initial reaction
0.1065 0.1065 0.1065 0.1065 0.1065 0.1452 0.1452 0.0113
4.7 4"7 4.7 4.7 4"7 3"1 3.1 2.4
53 53 43 77 77 45 85 56
1 3 6 8 10 11 13 --
1.34 2.16 5"5 3.02 1-23 3.42 4.03 0.50
TABLE lB.--DEACTIVATION OF VESSEL WALLS BELOW REFLUX TEMPERATURE
a b c d
Sequence no.
Rate of* initial reaction
Time of initial reaction (rain)
1 2 3 --
1"54 3.15 2-68 13-8
60 15 16 6
Solutions studied: 20mlof0 1450NCe(IV),H2SO, conc., 3"0 N were studied at 64°. a-c shows deactivation by continued use of the same glass surfaces. d shows the effect of reactivation by treating with sulphuric acid. *The rate is expressed in terms of ACe(IV), 10-6 equivs./min. This is considered to be the preferable way of representing the results rather than giving the proportion of the cerium(Iv) present in a given sample which reacts with water, since the most important factor is the glass surface. The sequence numbers shown refer to the use of the same glass surfaces in a series of experiences.
Time of initial reaction (min). 60 25 10 37 45 20 18 35
342
D. Gn.Am"
catalytic surfaces. The reaction is not detected below 40° and between 40 ° and reflux temperature the rate is variable, depending critically on surface conditions. However, at reflux temperature a marked increase in the rate is found and it now shows a less critical dependence on the surface conditions.
Below reflux temperature No change in the rate as a function of temperature or concentration factors has been observed between 40° and reflux temperature. Some results are presented in Table 1A and B and Fig. 5. Table 1B shows the effect of continued use of the same
o
IOO Time,
rain
FIG. 5.~Some reaction curves below reflux temperature for the Ce(IV)-HtO reaction Temp
Curve a b c d e f
(°C) 53 85 77 45 53 56
(Ce(IV))
(HISO.)
(N)
(N)
0"0165 0.1452 O.1065 0-1452 0-1065 0-0113
4"7 3.1 4.7 3.1 4-7 2.4
glass surfaces. The surface becomes catalytically "deactivated if between runs the surface is washed with water only, Table I(B) a-c; if however the walls are treated with boiling 6 N sulphuric acid the activity increases greatly, d. Thus the rate is seen to be controlled by the activity of the glass surface. It is also apparent from Table 1 that the faster rates recorded show a shorter reaction time. This is expected to be the case if the reaction stops by formation of sufficient reaction product at the surface to deactivate it, and if there is an equilibrium of the reaction products between the surface and solution phases.
At reflux temperature The rate was not now as critically dependent on the glass surface and it was possible to obtain data on the effect of concentration factors on the rate of the reaction.
Thermal instability of cerium(IV) sulphuric acid solutions
343
The effect of cerium(IV) concentration. The rate was found to show no dependence on the cerium(IV) concentration from 0.03 to 0.12 N, the rate is however lower at 0.015 N; some results are shown on Fig. 6 and Table 2.
b
r
Time, FIG.
rain
6.--Some reaction curves at reflux temperatures for the C¢(IV)-HsO reaction Curve
(Ce(IV)) (N)
(HsSO,) (N)
Vol. of. soln. (ml)
a b c d e f
0-1191 0.0495 0.0414 0.0305 0.0762 0.0414
5.0 5.0 4.5 5.0 5.0 5-0
25 25 35 35 35 35
The effect of cerium(Ill) concentration. Although no clear cut dependence was observed between the rate (or extent) of the reaction on the cerium(III) concentration in solution the general conclusion was drawn that the solutions obtained from ceria or ceric sulphate which contained variable amounts of cerium(Ill) displayed no critical dependence on the cerium(Ill) concentration. Difficulties were experienced due to precipitation increasing the reaction by catalysis when the solutions were obtained from mixtures of pure cerium(IV) obtained by oxidation of residual cerium(Ill) and then adding controlled amounts of cerium(Ill) sulphate solutions. The effect ofsulphuric acid concentration. The rate increases as the sulphuric acid concentration increases (cf. Table 2). It is not certain what amount of this increase is due to a simple temperature effect since the reflux temperature also increases, e.g. from 100.9 ° to 133.4 ° between 1 and 9 N sulphuric acid concentration; however since little temperature effect was observed on comparing the effect of acid on the observed amount of reaction at constant temperature or at reflux temperature it is likely that sulphuric acid exhibits a per se effect on the rate. The effect of dissolved oxygen. Bubbling oxygen through the reaction solution either before or during the reaction decreased the extent of the reaction. However, deoxygenation by bubbling nitrogen through the solution showed no measurable effect on the kinetics. These observations are in accord with the expected role of
344
D. GRANT TABLE2.~-OBSERVEDREACTIONKINETICSATREFLUXTEMPERATURE of. Fig. 6
Vol of soln. studied (ml)
Initial Ce(IV) cone. (N)
H~SO, cone. (N)
Rate of initial reaction
a b e f d
25 25 35 35 35 35
0.1191 0.0495 0.0762 0.0414 0.0305 0.0153
5.0 5.0 5.0 5.0 5.0 5.0
13.9 13-9 13"9 13.6 13.9 6.5
10 10 20 25 10 35 35
0.0112 0.0112 0"056 0.0496 0"0835 0.0414 0.053
2.5 2.6 2.8 2"9 3'0 4.5 6.2
0.86 1.02 1.26 3.12 7-7 12.8 17.0
The rate as before is expressed as 10-6 equivs./min oxygen as a surface deactivator. Since the system is a potential source of oxygen it is unlikely that complete deoxygenation of the solution is possible. The effect of adding extra oxygen to the solution was found to be one of surface deactivation only since the extent of the reaction rather than the initial rate was altered. Consideration of the time for which the reaction proceeds at reflux temperature as a function of the ratio of solution bulk to air solution interface showed that where there was a large amount of air solution surface the reaction proceeded for longer times at reflux temperature. It is thus indicated that oxygen, which will be one of the products of the reaction, deactivated the surface, since the oxygen concentration in solution will vary with the amount of air-liquid surface present. However, it is postulated that since the characteristics of surface deactivation below reflux temperature are markedly different from those at reflux temperature the effect of cerium(IIl) is predominant at the lower temperatures. DISCUSSION It is obvious from consideration of the thermodynamic equilibrium between cerium(IV) and (ill) in aqueous sulphuric acid solution at 25°, E0 ---- -- 1.44 V (1 M sulphuric acid) 4Ce(III) ~ 4Ce(IV) + 4e and the thermodynamic equilibrium between oxygen and water 2 H20 ~ O a + 4H + + 4e
E o = --1"229 V (s~
that cerium(IV) sulphuric acid solutions are metastable with regard to the oxidation of water to produce gaseous oxygen. However, the attainment of equilibrium is kinetically controlled, the uneatalysed stability of cerium(IV) sulphuric acid solutions being a function of the activation energy barrier. Thus the reaction is only observed in the presence of a catalyst, in the present ease in the absence of added ions the glass vessel wall acts as the catalyst. However, since the glass vessel walls easily become te~ w. M. LATIMER,OxMation Potentials. Prentice-Hall, New York (1952).
Thermal instability of cerium(IV) sulphuric acid solutions
345
deactivated by the products of the reaction the actual experimentally observed stability of cerium(IV) sulphate-sulphuric acid solutions is high when the cerium(IV) concentration and glass areas are favourable. The observed characteristics of the reaction as presented above are all in accord with a heterogeneous reduction of cerium(IV) catalysed by the glass surface. The ease of deactivation observed at temperatures below reflux is greater than at reflux temperatures which underlines the greater stability of solutions at the lower temperatures. At reflux temperature the reaction displays no dependence on the concentration of cerium(IV) in the range studied apart from the observation that at low cerium(IV) concentrations the reaction rate is lower. The surface will not be in full catalytic use at this point. The only clear dependence between a rate and a concentration factor is found with sulphuric acid concentration. Sulphuric acid has been observed to act as a specific surface activator and this is sufficient to explain the observed effect. Although the over-all picture is one of a reaction controlled by physical rather than by purely chemical effects it is also evident that information on the sort of chemical process occurring can be obtained. Thus the surface cannot be the only place where reduction of cerium(IV) occurs since the effect of surface area on the reaction is not a linear one, also ions present in the solution capable of electron exchange with solvent entities affect the reaction, Cu(II) decreasing it while AgO) andHg (II) increase it. It is likely that a smooth increase in the amount of reaction product with time will be occurring at the walls, however, the reaction is observed to stop in an abrupt manner, and the termination of the reaction has been established to be a surface effect. These observations are in line with the occurrence of a chain reaction initiated at the catalyst surface, propagated in the solution phase and terminated both at the walls and in the solution. The precise effect of surface is likely to be a source of reaction initiators which can escape from the walls and lead to formation of gaseous oxygen from cerium(IV) and water; when no further initiators penetrate the solution phase, the overall reaction stops. There is a considerable body of data in the literature indicating the occurrence of radical reactions in cerium(IV) solutions, e.g. exchange of labelled cerium must occur by exchange of electrons between cerium ions and the water molecules; tT~ thus there is considerable likelihood that cerium(IV) reaction of the type discussed above are radical chain reactions. EXPERIMENTAL Standard laboratory techniques were employed, volumetric apparatus being carefully standardised before use. Cerium(IV) was estimated volumetrically by titration with ferrous ammonium sulphate solution using ferroin as indicator. Standardization was done using dichromate solution, values being checked against the direct reduction of cerium(IV) by arsenious oxide. Reagents Various preparations of cerium(IV) sulphate, in sulphuric acid were studied, all showing the same properties. One sample was especially pure being obtained from 'Specpure' curia from Johnson Mathey and Co. Ltd. Acknowledgements--The author thanks Albright and Wilson Ltd. for a grant and Dr. D. S. Payne for much useful discussion and criticism.
~ G. E. CHALLENGER,B. J. MAS'~RS,J. Amer. Chem. Soc., 77, 1063 (1955).