At this stage, our smart mobile phone is ready for analyzing the concentration of chlorine in water. Once our smart mobile phone finishes the analysis, the display of the smart mobile phone shows the image of the glass bottle filled with water and KI–starch solution. The amount of chlorine found is also highlighted (see bottom image in Fig. 3). By preparing two additional sets of the above chlorine solutions and randomly putting each one of the whole 26 glass bottles in front of our smart mobile phone one at a time, we find that our smart mobile phone can effectively analyze chlorine concentration in water in the 0.3–1.0 ppm range with a measured 0.01 ppm standard deviation, limited by the chemical reaction between KI–starch solution and chlorine in water. The measured average analytical time is 1.9 s and the size of the application program is only 2100 Kb.