The invention discloses a urea concentration sensor. A sensor to measure a liquid, such as an urea solution. The sensor includes a light source operably coupled to a lumen disposed in a liquid solution, the light source configured to emit light and communicate the light to the lumen; a light detector operably coupled to the lumen, the light detector configured to receive at least a portion of the light from the lumen; and a controller configured to determine a concentration or quality of the liquid solution based on the light emitted by the light source and the portion of light received by the light detector.