Facile and Accurate Quantification of Sodium Content in Lignosulfonate Based on Ash Analysis

Gravimetric measurement of residual ash after complete combustion is widely used to determine the inorganic content in lignocellulosic biomass. However, this method tends to overestimate the results for lignosulfonate (LS), a type of technical lignin with high inorganic content. This overestimation is attributable to the ash consisting of oxidized metals. This paper proposes a facile and accurate quantification method for LS by correcting the ash yield based on its exact composition. Ash from seven types of LS was obtained under optimized combustion conditions, i.e., 900 °C for 1 h, with a mass yield of 15-30%. Fourier-transform infrared spectroscopy of the ash revealed that the dominant components were Na₂CO₃ and Na₂SO₄. Their weight ratios were calculated based on the C/S ratio determined by elemental analysis of the ash, and the weight percent of Na was estimated. This value was multiplied by the ash yield to determine the Na content in each LS sample, ranging from 5% to 11%. These results were consistent with those obtained using inductively coupled plasma mass spectrometry. The results highlighted the potential of the method for accurately measuring the Na content in LS without the need for special equipment or tedious sample mineralization pretreatment.