Monitoring of free phenol content in lignosulfonates by ClO2 titration and UV difference spectroscopy

Lignosulfonates (LS) are water-soluble polyphenolic macromolecules derived from lignins and common industrial additives in plasticizers and dispersants. A key feature for a more efficient utilization of modified LS is the monitoring of their free phenol (OHₚₕₑₙ) content during various reactions as these phenols govern a large part of their chemical reactivity. Against this background, the UV difference spectroscopy (Δeᵢ method) and ClO₂ titration have been revisited. Synthetic LS model compounds with a SO₃H group on their benzylic position and H, G, and S aromatic rings were synthesized and used to assess the suitability of both the ClO₂ titration and the Δeᵢ method on LS. The methodology combining the two analytical methods was then used to determine the molar extinction coefficient of a given LS allowing the calibration of the Δeᵢ method for a convenient and rapid monitoring of OHₚₕₑₙ in the course of LS reactions involving the OHₚₕₑₙ groups.