An apparatus for separating a substance by crystallization from a solution of the substance, comprising a crystallization vessel comprising an aperture for introducing the solution, a temperature-control device (5, 7) for varying the temperature of the solution introduced and/or to be introduced, a temperature sensor (6, 8) for measuring the temperature of the solution introduced and/or to be introduced, a scattered light probe arranged within the crystallization vessel and with which electromagnetic radiation can be radiated into the solution, wherein the electromagnetic radiation radiated into the solution has the form of a beam, the aperture angle of which is greater than 5 degrees, and is in the visible spectral range or is infrared radiation, and the intensity of the electromagnetic radiation scattered by crystals present in the solution is detectable, a control unit, which is data-coupled to the temperature sensor (6, 8), the scattered light probe and the temperature-control device (5, 7), with which the temperature of the solution in the crystallization vessel can be controlled, wherein the control unit is set up in such a way that, in order to introduce a desired amount of seed crystals, the intensity of the electromagnetic radiation scattered by crystals present in the solution is detected, the intensity detected is compared to a desired intensity (l s), the temperature of the solution is controlled as a function of the difference between the intensity detected and the desired intensity (l s) in such a way that the amount of this difference is reduced and, if the amount of the difference between the intensity detected and the desired intensity (l s) is less than a limiting value, the desired amount of seed crystals for the crystallization process is present and a crystallization process can be controlled by means of which crystals of the substance are obtained, and a separator unit to separate off the crystals obtained. a crystallization vessel comprising an aperture for introducing the solution, a temperature-control device (5, 7) for varying the temperature of the solution introduced and/or to be introduced, a temperature sensor (6, 8) for measuring the temperature of the solution introduced and/or to be introduced, a scattered light probe arranged within the crystallization vessel and with which electromagnetic radiation can be radiated into the solution, wherein the electromagnetic radiation radiated into the solution has the form of a beam, the aperture angle of which is greater than 5 degrees, and is in the visible spectral range or is infrared radiation, and the intensity of the electromagnetic radiation scattered by crystals present in the solution is detectable, a control unit, which is data-coupled to the temperature sensor (6, 8), the scattered light probe and the temperature-control device (5, 7), with which the temperature of the solution in the crystallization vessel can be controlled, wherein the control unit is set up in such a way that, in order to introduce a desired amount of seed crystals, the intensity of the electromagnetic radiation scattered by crystals present in the solution is detected, the intensity detected is compared to a desired intensity (l s), the temperature of the solution is controlled as a function of the difference between the intensity detected and the desired intensity (l s) in such a way that the amount of this difference is reduced and, if the amount of the difference between the intensity detected and the desired intensity (l s) is less than a limiting value, the desired amount of seed crystals for the crystallization process is present and a crystallization process can be controlled by means of which crystals of the substance are obtained, and a separator unit to separate off the crystals obtained.