Quality of Life (Banja Luka) 5(3-4): 98-106

NON-ISOTHERMAL DEGRADATION OF VITAMIN C BY SIMULTANEOUS
THERMOGRAVIMETRIC AND DIFFERENTIAL THERMAL ANALYSIS

DIJANA JELIĆ¹, ALEKSANDRA ĐUKIĆ-DRVAR^2
1University of Banjaluka , Medical faculty, Pharmacy department, Boulevard Vojvode Petra Bojovica 1a, 78000 Banja Luka, Bosnia and Herzegovina, e-mail: dijana_jelic@hotmail.com
²Krka, d.d., Novo mesto, Slovenija, Bosniа аnd Herzegovina Departmant, Džemala Bijedića 125A, 71000 Sarajevo,
Bosnia and Herzegovina

Abstract: The non-isothermal degradation process of Vitamin C (Galenika, Serbia) was investigated by simultaneous thermogravimetric and differential thermal analysis (TG-DTA), in the temperature range from an ambient one up to 650°C. The scope of presented work is toward stability of active pharmaceutical substance – ascorbic acid and calculation of all relevants kinetics parameters: activation energy (Ea), pre-exponential factor (A) and half-time (t_1/2) for stability assesment in explored formulation. It was established that the degradation proceeds through five degradation stages (designated as I, II, III, IV and V), which involve: the dehydration, the melting of ascorbic acid, as well as the decomposition of excipients present: microcrystalline cellulose, D-glucose, carboxymethylcellulose, magnesium stearate , lactose and corn starch. During thermal degradation, all excipients increase their thermal stability and some kind of solid-solid and/ or solid-gas interaction occur. The kinetic parameters and reaction mechanism was determined for the II stage, where decomposition of ascorbic acid happened using powerfull kinetic software KINETICS05. Two approaches were used: isoconversional and modell fitting method. It was concluded that degradation of ascorbic acid is best described with N-th order model with Ea= 139.550kJ/mol, A= 1.9689∙10¹³s^-1 and n=1.2132.

Keywords: vitamin C, kinetics, non-isothermal, thermogravimetry