Research has been undertaken into the deprotonation degree of the phenolic groups on softwood kraft lignin in black liquor. An attempt was made to mathematically model the ion concentration distributions around lignin fragments in black liquor by studying the possibility of using a modified Poisson-Boltzmann cell model and extending the validity of this model by extrapolating the valid ionic concentration range using the Pitzer method. The model has been utilised to predict the pH dependent deprotonation degree under various conditions that are relevant to the LignoBoost process. It has been demonstrated that the pH and temperature strongly affected the deprotonation degree of phenolic groups on kraft lignin. The effect of the molecular weight of lignin and a small increase in ionic strength on the deprotonation degree was very small in black liquor systems of high ionic strength. It was observed that the pH at the surface of lignin macromolecules was approximately the same in black liquor systems of high ionic strength.