The HI93721-03 are high quality reagents that are pre-measured, allowing for users to achieve fast and accurate colorimetric measurements. These reagents follow an adaptation of the USEPA method 315B (phenanthroline method) and Standard Method 3500-Fe B. This method requires a reagent that contains both a reducing and a complexing agent. The reagent converts all but very most resistant forms of iron present in the sample to the ferrous (Fe²⁺) or soluble state. The reagent when added to a sample containing iron will turn the sample an orange hue; the greater the concentration, the deeper the color. The intensity of color is determined by a compatible photometer and the concentration and based upon the meter will be displayed in mg/L (ppm) as total iron. These reagents are designed to be used with samples that have an expected range of 0.00 to 5.00 ppm iron.