A device for detecting the concentration of urea in solution with water, comprising: a means for emitting radiation; at least one means for receiving radiation, which is configured for detecting a value of intensity of radiation; and a light guide, which is prearranged for optically connecting said emitting means to said receiving means, wherein said light guide has at least one surface that is to come into contact with said solution so as to determine an interface of separation between said light guide and said solution that causes loss of intensity of the radiation that traverses said light guide and is characterized by the refractive index of the light guide and the refractive index of the solution, and, wherein said device comprises control means configured for obtaining a value indicating the concentration of said urea in said solution as a function of the signal coming from said receiving means, said device being characterised in that: - said light guide has a general U-shaped development defining an elbow-shaped portion of said guide that has a given variation of direction so as to concentrate in it the losses of intensity of the radiation that traverses said light guide, said portion defining at least part of said surface that is to come into contact with said solution; - said emitting means is configured for emitting radiation in at least one first range of wavelengths and one second range of wavelengths that are different and said receiving means is configured for detecting intensity of radiation in said first and second ranges; - wherein said control means are configured for obtaining a value indicating the concentration of said urea in said solution as a function of a first signal coming from said receiving means that is indicative of the intensity of the radiation in said first range of wavelengths, and for verifying whether said value indicating concentration and a value of intensity of radiation derived from a second signal coming from said receiving means that is indicative of the intensity of the radiation in said second range of wavelengths, satisfy a given function, characteristic of said solution, that links the loss of intensity of radiation in said second range of wavelengths to the concentration of urea. - said light guide has a general U-shaped development defining an elbow-shaped portion of said guide that has a given variation of direction so as to concentrate in it the losses of intensity of the radiation that traverses said light guide, said portion defining at least part of said surface that is to come into contact with said solution; - said emitting means is configured for emitting radiation in at least one first range of wavelengths and one second range of wavelengths that are different and said receiving means is configured for detecting intensity of radiation in said first and second ranges; - wherein said control means are configured for obtaining a value indicating the concentration of said urea in said solution as a function of a first signal coming from said receiving means that is indicative of the intensity of the radiation in said first range of wavelengths, and for verifying whether said value indicating concentration and a value of intensity of radiation derived from a second signal coming from said receiving means that is indicative of the intensity of the radiation in said second range of wavelengths, satisfy a given function, characteristic of said solution, that links the loss of intensity of radiation in said second range of wavelengths to the concentration of urea.