A optical sensor for measuring tranmissive properties of a solution having: a cuvette body with an enclosed flow passage for the solution, wherein the flow passage further includes a solution inlet and a solution outlet and a cuvette between the inlet and outlet; a light source projecting light of a predetermined wavelength through the cuvette and solution flowing through the cuvette and to a light sensor, wherein the cuvette has inner wall surfaces opaque to the light of said predetermined wavelength, and the inner wall is in contact with the solution.