In situ EDXRD Study of the Crystallisation of [Ni(O3P-CH2-C6H4-COOH)2(H2O)4] under
Ultrasonication
L.-H. Schilling, M. Feyand and N. Stock
Christian-Albrechts-Universität zu Kiel, Germany
Sonochemistry utilises ultrasound to enable a chemical reaction. Ultrasonic energy input has two
effects on a reaction mixture: The mechanical oscillation of the solvent creates cavities in which
locally very high energies are reached; also the ultrasound heats up the solvent causing conventional
thermal reactions. Therefore, the relevant question of sonochemistry is often, whether a reaction
requires mechanical work or whether thermal excitation is sufficient to account for the reaction.
We addressed this problem for the formation of [Ni(O3P-CH2-C6H4-COOH)2(H2O)4] (Fig. 1) from
NiCl2·6H2O and H2O3P-CH2-C6H4-COOH in water. The increase of the product signals was measured
during the sonochemical reaction by in situ EDXRD (Fig. 2, left).
Fig. 1. Section of the crystal structure of [Ni(O3P-CH2-C6H4-COOH)2(H2O)4].
Fig. 2. Time dependent increase of a peak in the in situ EDXRD experiment at 20 % of the maximal amplitude (left).
ARRHENIUS plot of the observed data (right).
The reaction was carried out in a glass vial with an ultrasound finger being lowered into the solution
from above. Five experiments were carried out at different amplitudes (20 %, 40 %, 60 %, 80 % and
100 % of the maximum amplitude) of the ultrasonic sound. The reaction progress was observed via
EDXRD and the temperature of the mixture was measured with a digital thermometer.
The analysis of the obtained data showed that thermal excitation alone is insufficient to cause the rapid
increase of the reaction rate with increasing amplitudes; and the mechanical work of the ultrasound is
therefore a necessary factor in the reaction mechanism. The data doesn’t fit the ARRHENIUS law very
well (Fig. 2, right) and the linear regression gives an activation energy of 20.19 kJ mol-1, which is very
low for this kind of reactions.