and thus, critical iodide concentration for certain concentration of silver TNPs could be calculated by the linear regression equation. If an unknown iodide sample was added to a series of tubes containing different concentration silver TNPs, only the tube which contains silver TNPs solution whose corresponding critical iodide concentration value equals to the unknown iodide sample, will show a critical color.We just need to find the one which shows critical color, and then the concentration of iodide sample can be calculated by silver TNPs concentration.