Three batches of dried kelp (Laminaria japonica Aresch, pur-chased from local market) were used to validate the reliability of the method. The dried kelps (0.300 g) were firstly burned to ash for 45 min, in which the iodine is present as iodide. Then, the iodide was dissolved out of the residue by boiling water. The filtered iodide solutions (250.00 mL) were prepared as samples and diluted 10-fold for colorimetric detection by standard additions method. The results shown in Table S2 indicate that the detection of extracted iodide solutions had a recovery of 94.0–113.4% with RSD of 3.42–5.78%. The average content of iodide in the dried kelp is 0.516 mg g−1.