To demonstrate the capabilities of our modeling approach, we use it to calculate the partitioning of a weak diprotic acid, i.e., a small solute which can undergo charge regulation. The charge states of this acid depend on the pH and are further modulated by intermolecular interactions, in particular electrostatics. Our model of the interpolyelectrolyte complex contains equal amounts of polycations and polyanions, therefore, the Donnan potential between the the complex and the supernatant phase is zero, causing that pH in both phases is the same. However, the ionic strength in the interpolyelectrolyte phase is much higher than in the supernatant, and simultaneously permitivitty in the interpolyelectrolyte phase is slightly lower than in the supernatant, causing that the electrostatic interactions are much stronger in the former phase. According to the Debye-Hückel theory, an increased ionic strength decreases the activity coefficients of charged species, causing that charged states of weak acids are more preferred than the uncharged ones. Consequently, charged states of the solute should be preferred in the interpolyelectrolyte complex, much more than in the supernatant.