An apparatus (1) for the measurement of a concentration of a charged species in a sample (10) is disclosed. The sample (10) comprises a plurality of types of charged species and at least one insoluble component. The apparatus (1) comprises a first circuit with a voltage control device (54) connectable to two first electrodes (30, 30′) arranged along a channel (12) holding the sample (10) and a second circuit with a conductivity detection device (55) connectable to two second electrodes (5, 5′) arranged in the channel (12). The first circuit and the second circuit are dc and ac electrically isolated from each other.