versão On-line ISSN 1678-4618
Eclet. Quím. v.26 São Paulo 2001
ABSTRACT: Al(C9H6ON)3.2.5H2O was precipitated from the mixture of an aqueous solution of aluminium ion and an acid solution of 8-hydroxyquinoline, by increasing the pH value to 9.5 with ammonia aqueous solution. The TG curves in nitrogen atmosphere present mass losses due to dehydration, partial volatilisation (sublimation plus vaporisation) of the anhydrous compound followed by thermal decomposition with the formation of a mixture of carbonaceous and residues. The relation between sublimation and vaporisation depends on the heating rate used. The non isothermic integral isoconventional methods as linear equations of Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose (KAS) were used to obtain the kinetic parameters from TG and DTA curves, respectively. Despite the fact that both dehydration and volatilisation reactions follow the linearity by using both methods, only for the volatilisation reaction the validity condition, 20£ E/RT£ 50, was verified.
KEY-WORDS: 8-hydroxyquinolinate, aluminium, kinetic parameters, TG, DTA, volatilisation.
1. Aluminium 8-hydroxyquinolinate
It has been found out that the complete precipitation of the aluminium ion by the 8-hydroxyquinolinate anion to form Al(C9H6ON)3, occurs from pH 4.29 to higher pH values. This reagent has been used to separate the aluminium ion from the beryllium, alkali earth metal and magnesium cations as well as from the phosphate anion2,3,9.
Borrel and Paris4 studied the precipitation of several metallic oxinates, among them that of the aluminium ion. Al(C9H6ON)3 was prepared in 0.1 mol L-1 HCl, with 10% excess of 8-hydroxyquinoline, heated up to 70-80°C, washed by hot water decantation, filtered under vacuum and stored at room temperature. TG curves showed dehydration and thermal decomposition with formation of Al2O3.
Charles and Langer8 prepared complexes of aluminium ion in aqueous solutions, which were washed with distilled hot water in order to obtain total elimination of the ligand excess. The compounds were previously dried at room temperature and the drying was completed in vacuum at 50oC. TG curves allowed to verify the volatilisation of these compounds.
The thermal behaviour of aluminium quinolinate and their derivatives were studied by Wendlandt and Horton18 through the differential thermal analysis in an argon atmosphere. The partial volatilisation of the aluminium complex was observed by Charles and Perroto7 through TG and DTA curves also in argon atmosphere.
The thermogravimetric study of aluminium oxinate obtained by three different procedure was done by Keattch13. TG curves showed the dehydration and thermal decompositions to Al2O3 at 150 and 700°C, respectively.
Critical study using 8-hydroxyquinoline as gravimetric reagent for the aluminium ion was done by Chalmers and Mohammed Abdul Basit6, through modification in experimental conditions described by Berg9. They verified the co-precipitation of 8-hydroxyquinoline and formation of polynuclear species, besides the volatilisation when this compound was submitted to heating.
2. Theoretical of non-isothermal integral isoconvertinal methods
The general equation10 for the reaction rate in an isothermal condition has been written as:
in which da/dt is reaction rate, f(a) is reaction model, A is the pre-exponential factor, E is the activation energy, T is the temperature, and R is the gas constant.
Under non-isothermal condition, at a constant heating rate b =dT/dt, the explicit temporal dependence in equation (1) may be rewritten as:
Considering the integral form and E/RT=x:
Considering E/RT0>>1, the integral temperature Pn may be obtained as being:
Assuming n=0, the equation (4) may be simplified to the form p(x), whose Doyles approximation  from 20£ x£ 46 is given as:
Using the Doyles approximation and logarithmic form of the equation (3) the linear equation of Ozawa-Flynn-Wall12,15 can be obtained:
To small intervals and considering the same extension of reaction a in a series of experiments in different heating rate :
The pre-exponential factor, A, may be obtained through the following equation:
and the lifetime (tf) may be estimated from Toop equation16.
where Tf is the temperature in which the system is exposed and Tp is the temperature at which the mass loss is 5%.
The Kissinger-Akahira-Sunose method1,5,14, KAS method, may be originally obtained through derivation of the equation (2), logarithm application and posterior rearrangement as given bellow:
as (peak temperature)
This equation is valid at any given conversion and according to which, 20£ x£ 50:
An aqueous solution of aluminium ion was added, under constant stirring, to an excess of 8-hydroxyquinoline dissolved in acetic acid, and a precipitate was obtained when raising the pH to 9.5. The precipitate was washed with diluted ammonia aqueous solution to eliminate the excess of 8-hydroxiquinoline, dried at 60°C in a mechanical convection oven.
After wet digestion with a mixture of concentrated sulphuric acid and hydrogen peroxide 30% solution in water, the aluminium content was determined by gravimetric analysis3 as Al2O3.
The 8-hydroxyquinolate content was determined by a bromatometric method using standard Na2S2O3 solution and soluble amide as an indicator3, as well as, by elemental analysis (C, N, H).
The water content was determined from the TG curves.
TG/DTA curves were obtained using a SDT 2960 equipment from TA Instruments in a dynamic atmosphere of N2 (100 mL min-1), a-Al2O3 crucible and sample weight of about 2 mg. The data were evaluated to obtain kinetic parameters by KAS method1,14. Ozawa15 and Flynn et Wall12 methods were also used by means of a software of the TA instruments.
Results and Discussion
Through analytical and thermal analytical methods it has been verified that the compound presents the composition Al(C9H6ON)3.2.5H2O, Table 01.
Through the TG curves of the compound in a nitrogen atmosphere (Figure. 01) two steps of mass loss, the first due to dehydration at 210°C and the second attributed to sublimation/vaporisation of the anhydrous complex at 250-480°C, were observed. During the sublimation/vaporisation process and probably in its very ending, which takes place until 500°C, some thermal decomposition is occurring, resulting in a mixture of carbonaceous and oxide residues. The DTG curves (Figure 01) indicate that the process of sublimation/vaporisation occurs through consecutive steps, Table 02.
The DTA curves showed a first broad endothermic peak due to dehydration (Figure 02 and Table 02). Regarding the sublimation/vaporisation and thermal decomposition process (Figure 03 and Table 02) it can be observed that for the 2.5°C min-1 rate only the sublimation occurs while for the 5 and 10°C min-1 rates three different processes can be verified: partial sublimation, fusion of the anhydrous compound at about 414°C and the vaporisation with probable thermal decomposition until 480°C. The relation between sublimated and vaporised complex and the remaining residues quantities depend on the heating rate.
For a equal a constant, the plot log b vs. 1/T, Ozawa/Flynn/Wall method (Figures 4 and 5) show straight lines whose slopes give Ea/R values. From these values a number of activation energies are obtained depending on the extent of the conversion that consequently allows the obtainment of the pre-exponential factor and half-lifetime. In Table 03 are presented values of Ea, log A, T1/2 and t1/2 in 5% of conversion to both first and second stages of reaction.
In the Figures 6 and 7, the plot log b/T2 vs. 1/T, KAS method, show a straight line obtained through linear fit, whose slope gives Ea/R value from which is obtained the activation energy that gives subsides to estimate the pre-exponential factor, Table 03.
The most precise approximation to kinetic parameters, that is, when 20£ x£ 50, was only obtained in the second stage by using both methods, despite the fact the linearity has also been obtained in the first stage which is the dehydration reaction.
Comparing the results of the kinetic parameters from both methods it can be verified differences between values found. The lower Ea value obtained in the second stage through KAS method may be ascribed to the influence of the partial thermal decomposition, generally an exothermic reaction, on the sublimation/volatilisation process which is endothermic. It might not have happened in the Ozawa method because the evaluation was done at the beginning of the process, probably still without any contribution of the thermal decomposition.
Although the Ea values to dehydration reaction eliminate the validity interval required to x, it has been shown by Vyazovkin17 that the linear procedures gives quite satisfactory values of Ea at x>13 to KAS method.
The isoconversional methods allow the Ea to be determined as a function of the extent of conversion and/or temperature. This dependence is determined without making any assumptions about the reaction model.
Both of the advantages that can be attributed to KAS method are that it can locate the peak maximum temperature besides the unnecessary knowledge of the reaction mechanism to calculate the Ea. Nevertheless its dependence on the accuracy of the peak position may be the main disadvantage of the method.
Considering the difficulty in locating the values the a maximum and other parameters associated with the peak maximum temperature by using KAS method, the Ozawa method probably gave better results.
The authors acknowledge the Fapesp (Proc. 90/2932-4 and Proc. 93/0256-0), and CNPq/PADCT II (Proc. 62.0651/94.6) for financial support.
RIBEIRO, C. A. et al. Cinética não isotérmica de desidratação e volatilização do 8-hidroxiquinolinato de alumínio no estado sólido. Ecl. Quím. (São Paulo), v.26, p. , 2001.
RESUMO: Al(C9H6ON)3.2,5H2O foi precipitado a partir de mistura de solução aquosa do íon alumínio e solução ácida de 8-hidroxiquinolina, e o pH ajustado a 9,5 com solução aquosa de amônia. Curvas TG em atmosfera de nitrogênio apresentam perdas de massa devido a desidratação, volatilização parcial ( sublimação e vaporização) do composto anidro seguido por decomposição térmica com a formação de uma mistura de resíduos carbonaceos. A relação entre sublimação e vaporização dependem da razão de aquecimento utilizada. Os métodos isoconvenciais integrais não isotérmicos como as equações lineares de Ozawa-Flynn-Wall e Kissinger-Akahira-Sunose (KAS) foram usados para obter parâmetros cinéticos a partir de curvas TG e DTA, respectivamente. Apesar das reações de desidratação e de volatilização seguirem a linearidade por ambos métodos, somente foi verificado a condição de validade, 20£ E/RT£ 50, para a reação de volatilização.
PALAVRAS-CHAVE: 8-hidróxiquinolinato, ion alumínio, parâmetros cinéticos, termogravimetria, análise térmica diferencial, volatilização.
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Recebido em 9.3.2001.
Aceito em 6.4.2001.
* Departamento de Química Analítica, Instituto de Química, UNESP, C.P. 355, 14801-970, Araraquara, São Paulo, Brazil