A sensor (100) to measure a liquid (120), such as an urea solution. The sensor (100) includes a light source (132) operably coupled to a lumen (136) disposed in a liquid solution (120), the light source (132) configured to emit light and communicate the light to the lumen (136); a light detector (134) operably coupled to the lumen (136), the light detector (134) configured to receive at least a portion of the light from the lumen (136); and a controller (140) configured to determine a concentration or quality of the liquid solution (120) based on the light emitted by the light source (132) and the portion of light received by the light detector (134).