**3. Experimental section**

#### **3.1 Calibration curve for the concentration analysis**

Calibration curve was established using freshly prepared stock solution of sodium hydroxide of various concentrations such as 40, 80, 400, 800, 2000–4000 mg/L. The conductivity of each stock solution was measured using a conductivity meter. The values of conductance of each sample were plotted against the concentration of

sodium hydroxide, which was used as a reference for all the experiments to find the unknown concentration of experimental samples.
