Abstract: The invention relates to a device for determining the concentration of a substance present in a solution, using ellipsometry. Said device is used for determining the rate of precipitation, which is a measure of the concentration of the substance to be determined The object of the invention is to provide an improved measuring technique based on optical ellipsometry or another technique for measuring the mass bonded to a solid surface (substrate), such as “surface plasmon resonance” (SPR), “quartz crystal microbalance” (QCM) etc, by means of which the concentration of a substance present in a liquid, which liquid is in contact with the substrate, can be measured with a higher degree of accuracy.