Resumo

PEREZ-CABALLERO, Guadalupe et al. Potentiometric Study of Acid-base Properties of Thiamine Hydrochloride and Thiamine Mononitrate in Aqueous Medium. J. Mex. Chem. Soc [online]. 2011, vol.55, n.2, pp.126-131. ISSN 1870-249X.

A comparative study of thiamine mononitrate and thiamine hydrochloride potentiometric titrations with a strong base in water was performed. The analysis of resulting curves allowed associating particular reactions to each observed pH break point. The corresponding pKa values were calculated by using the program SUPERQUAD and they were subsequently used to simulate titration curves. The pK_a values obtained in the conditions of the present work are: pK_a1 = 4.80 ± 0.05, pK_a2, > 10.40 ± 0.04, pK_a3 < 8.10 ± 0.04. The system behavior mainly depends on parameters such as the time for measuring pH, NaOH concentration and particularly on the stability of one of the two ampholytes. Comparison of calculated and experimental curves revealed the existence of a dismutation reaction which showed that under steady state conditions and the waiting time between two successive pH measurements, the intermediate "pseudobase (B)", called like this by several researchers, practically does not exist. The results led to find the optimal conditions to propose quantitative alkalimetric titrations. The end-points for the assay of thiamine hydrochloride and thiamine nitrate in raw material were detected by potentiometry. This involves the use of reagents and solvents that are environment-friendly.

Palavras-chave : Potentiometry; thiamine; dismutation; ampholyte; acid-base titrations.

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