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Journal of the Mexican Chemical Society

Print version ISSN 1870-249X

J. Mex. Chem. Soc vol.52 n.1 México Jan./Mar. 2008




Equilibrium Constants Determination of the Species Formation in the Al(III)–H2O System by Integration of 27Al–NMR Signals and Fitting with Species Fractions


Alberto Rojas–Hernández1*, Julio César Botello,1,2 Ma de Lourdes Pacheco–Hernández,1 Atilano Gutiérrez,1 José Manuel Domínguez,3 Gabriela Espinosa,3 and María Teresa Ramírez–Silva1


1 Universidad Autónoma Metropolitana–Iztapalapa, Departamento de Química, Área de Química Analítica, Apdo. Postal: 55–534, 09340 México D. F., México. *Responsible author:

2 Facultad de Estudios Superiores Cuautitlán (UNAM), Sección Química Analítica, Campo 1, Av. 1° de Mayo s/n, 54740 Cuautitlán–Izcalli, Estado de México, México.

3 Instituto Mexicano del Petróleo, Programa de Ingeniería Molecular, Eje Central L. Cárdenas 152, 07730 México D. F., México.


Recibido el 11 de septiembre del 2007
Aceptado el 30 de enero del 2008



A quantitative methodology to determine formation constants that takes into consideration the 27Al–NMR signals of Al(III)–H2O systems has been analyzed, where area fractions beneath the peaks recorded were associated with the corresponding species fractions. The values of the formation constants were compared with the model obtained by potentiometric studies. In agreement with previous reports, the –log*β values obtained by NMR and potentiometry, at 0.020 M ≤ [Al(III)]total ≤ 0.500 M are 12.96 ± 0.02 for Al(OH)3, 13.50±0.02 for Al3(OH)45+, and 109.70±0.09 for Al13(OH)327+, by assuming that Al(OH)2+ has a value of 5.29. Additionally the equilibrium constant values obtained allowed fitting of the conductimetry titrations in the 6.00(10–3) M ≤ [Al(III)]total ≤ 0.120 M. The algorithm presented in this work gives a support to establish that the area fractions of NMR signals are proportional to the species concentration fractions.

Key words: 27Al–NMR, hydrolysis of Al3+, formation constants, potentiometric titrations, conductometric titrations.



Una metodología para determinar constantes de formación por medio de señales de 27Al–NMR para sistemas Al(III)–H2O se aplicó, en donde las fracciones de área bajo los picos se asoció con las fracciones de las especies correspondientes. Los valores de constantes de formación obtenidos de esta forma se compararon con los obtenidos con estudios potenciométricos. Los valores obtenidos de –log*β obtenidos por NMR y potenciometría a partir de sistemas con 0.020 M ≤ [Al(III)]total ≤ 0.500 M son 12.96 ± 0.02 para Al(OH)3, 13.50 ± 0.02 para Al3(OH)45+, y 109.70 ± 0.09 para Al13(OH)327+, considerando que Al(OH)2+ tiene un valor de 5.29, y concuerdan con reportes previos. Además, estos valores de constantes de equilibrio permiten ajustar curvas de valoración conductimétricas, en las cuales 6.00(10–3) M ≤ [Al(III)]total ≤ 0.120 M. Este algoritmo da fundamento para establecer que las fracciones de área de las señales de NMR son proporcionales a las fracciones de concentración de las especies.

Palabras clave: 27Al–NMR, hidrólisis de Al3+, constantes de formación, titulaciones potenciométricas, titulaciones conductimétricas.





The authors wish to acknowledge CONACYT for the grants given to JCBP, and MLPH to undertake their postgraduate studies. Thanks are also given to PEMEX and IMP (through the project FIES–96–31–III) for partial financing support. The authors are indebted to Dr I. Puigdomenech and Prof. Josef Havel for the copy of the program MEDUSA, as well as to Dr A. Vacca and Dr P. Gans for the copy of the program SUPERQUAD and, finally, to Dr. Mario A. Romero–Romo and to Prof. William F Reynolds for helpful comments.



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