The AVand POVof the samples were determined
according to the Chinese Official Methods of Analysis (GB/
T5009.37-2003). In brief, the AV was determined by titration
with potassium hydroxide standard solution (0.050 M) and
phenolphthalein indicator, the POV was determined by titration
with potassium iodide saturated solution and sodium thiosulfate
standard solution (0.0020 M).