Abstract: To address the limitations of existing methods for assessing the quality of sodium alginate gels, a convenient and stable method for determining the molar ratio (M/G) of β-D-mannuronic acid (M) to α-L-guluronic acid (G) was developed based on the difference in acid dissociation constants between these two monomers. The method, which utilized potentiometric titration, also allowed for the quantification of sodium alginate content as well as the proportions of its alternating MG segments, MM homopolymeric blocks, and GG homopolymeric blocks. Under optimized pretreatment conditions and automatic potentiometric titration parameters, the method demonstrated spiked recovery rates ranging from 102.88% to 103.96% for M/G ratio and alginate content. The relative standard deviation (RSD) for reproducibility was less than 5%, and the precision RSD ranged from 0.22% to 4.77%. The M/G ratio values determined by this method and those obtained by 13C nuclear magnetic resonance (NMR) spectroscopy were fitted to the equation y = 1.32x + 0.076, with a Pearson’s correlation coefficient of 0.999, indicating high accuracy and reliability. This study provides a novel approach for the quality control of sodium alginate.

Key words: sodium alginate; molar ratio of β-D-mannuronic acid to α-L-guluronic acid; uronic acid; automatic potentiometric titration; 13C nuclear magnetic resonance spectroscopy