Comment on "Simulations of ionization equilibria in weak polyelectrolyte solutions and gels" by J. Landsgesell, L. Nová, O. Rud, F. Uhlík, D. Sean, P. Hebbeker, C. Holm and P. Košovan, Soft Matter , 2019, 15 , 1155-1185.

In a recent review Landsgesell et al. , Soft Matter 2019, 15 , 1155 stated that pH - p K a is a "universal parameter" for titrating systems. We show that this is not the case. This broken symmetry has important implications for constant pH (cpH) simulations. In particular, we show that for concentrated suspensions the error resulting from the use of cpH algorithm described by Landsgesell et al. is very significant, even for suspension containing 1 : 1 electrolyte. We show how to modify the cpH algorithm to account for the grand-canonical nature of the cpH simulations and for the charge neutrality requirement.