Abstract: A turbidity measuring device for determining the concentration Kj of a substance Sj in a medium includes measuring arrangements, in which the intensities of scattered light at different angles are registered and convertable into current values of at least a first measured variable M1 and a second measured variable M2, which have different dependences on the concentration Kj of a substance Sj (Mi(Kj)=fij(Kj)). The turbidity measuring device has stored for the measured variables Mi for a number of substances Sj calibration functions gij, with which, in each case, a concentration of a substance Sj is determinable (Kj=gij(Mi)). The turbidity measuring device further includes a computing unit, which is suitable for evaluating the ascertained concentration values gaj(Ma), gbj(Mb), wherein a?b, for different substances Sj as regards their plausibility and so to identify a plausible substance Sj, or to check the plausibility of an earlier identified or predetermined substance Sj.