4.2. Calibration and test
For chlorine solutions with different 13 concentrations in Fig. 2, the relationship between the calculated CR and the chlorine concentration is shown in Fig. 4. In this case, we find that a fifth-order polynomial of 86.008x5 + 359.04x4 + 556.14x3 − 402.96x2 + 135.15x − 15.804 is mathematically fitted very well (i.e., R2 = 99.76%) with our experimental data in the 0–1.0 ppm chlorine concentration range. This fifth-order polynomial function is then stored in our smart mobile phone for real time analysis.