15 mL of a 25 wt% aqueous KOH solution containing 5-phenyl tetrazole (50 mL) and N(CH3)4Cl (25 mM, as internal standard) was prepared by dissolving in MilliQ water in a 50 mL poly(tetrafluoroethylene) (PTFE) test tube. A 0.5 mL sample was taken out for 1 H NMR analysis, and the tube was then sealed and kept at 80 °C for nearly 4000 h. Sampling was carried out regularly by taking out 0.5 mL of the liquid from the tubes after it had been cooled to room temperature. The concentration of 5-phenyl tetrazole was calculated from the sample to internal standard peak integral ratio, assuming that the N(CH3)4Cl concentration remained constant.