A summarized titration procedure reported by Kolthoff and
Stenger (1947) is as follows: (1) dilute the sample solution to about
30 mL; (2) add some sodium bicarbonate in the solution to avoid
oxygen-oxidation of iodide; (3) add 4 g of potassium iodide, swirl
to dissolve the iodide, and then allow standing for 15 min; (4)
add several drops of starch indicator; (5) acidify the solution with
1–2 mL of acetic acid; (6) titrate it with standard thiosulfate (e.g.,
0.01 N) to the starch end-point. The sodium persulfate concentration
can be then calculated as follows