Water quality analyzing tools such as spectrophotometer are generally expensive and require expertise to handle and can’t be used for in-field sensing. we have demonstrated a smartphone based low cost and field portable photometric sensor for detection of chemical contaminants in drinking water. we have utilized the ambient light sensor (als) of the smartphone as light intensity detector. an android application has been developed which can detect intensity of the light signal propagating through a cuvette containing contaminating water and determine the amount of chemical contaminant present from an internal calibration curve.