An apparatus and a method for the determination and regulation of the concentration of at least one dissolved substance in a fluid circuit, in which the fluid circuit includes at least two partial circuits separated by a filter in a semi-permeable manner, includes a detection unit for determining a concentration difference of at least one first dissolved substance, a determination unit for determining the influence of at least one second dissolved substance which is not identical to the at least one first dissolved substance, and a regulation unit for regulating the concentration of the at least one dissolved substance in the fluid circuit.
