A device for dispensing disinfectants into washing liquors by means of a dispenser is to dispense the correct quantity of disinfectant in function of the dirt to be removed so that adequate but not excessive quantities of disinfectant are always released. To this end, an analytical cell determines, in particular continuously, the content of unused disinfectant in the washing liquor. A comparator connected to the analytical cell and to the dispenser compares the actual value obtained from the measurement system with a predetermined target value and actuates the dispenser when the the actual value falls below the target value. A sample preparation part which comprises a solids-retaining device with a dialysis membrane is arranged upstream of the analytical cell.