During the pickling of steel the setting of the wanted concentrations between free acid and metal salt in the pickling bath is very important for the pickling result, because the surface of the material is determined by the pickling process in a very decisive way. A continuous in-line procedure for a continuous supervision of the pickling process is therefore demanded. For pickling installations with connected acid regeneration installations, a pickling bath conduction of process data on the basis of comprehensive and continuously captured process data creates an economic advantage.