When analysing water samples for Iron content there are a large number of forms the iron can take, from conventional dissolved Iron Salts such as Iron Chloride  to particles of Rust and Magnetite.

Does the Hach Iron test kit pick up all of this Iron or just the soluble Iron content?

Used in the standard mode the insoluble Iron particles will not be detected,. However there is an additional digestion step which can be performed to measure all of the iron in the sample.