The iron test strip measures Fe2+ (ferrous) or Fe3+ (ferric) in water from 0 to 100ppm in just seconds. This test strip has steps of 0, 2, 10, 25, 50, and 100 ppm on the color chart. The iron test strips only work on iron ions that are free in solution. They will not detect Iron that is complexed with a chelating agent, such as EDTA. Iron chelates are often used in products such as fertilizer to aid in improving the availability of the iron as a plant nutrient.