TexasSplash posted this quote from chemgeek in another thread. I thought I would start a separate thread to avoid hijacking the OP.
Does this mean that once the iron is oxidized, it is not detectable? If the oxidized iron is not detectable, can a sample be treated with some reducing agent to take it back to ferrous where it can be detected?There's no sense in testing iron in the pool once you've added chlorine. The iron test kits are useful for testing ferrous iron before it is oxidized, such as testing the fill water. Once you've added chlorine it oxidizes nearly all the iron to ferric iron that then forms iron oxides. So now the approach is to physically remove these precipitates either through vacuum-to-waste, through sweeping to get to the filter (or to skimmer socks or batting material) and then you'll be left over with a smaller amount of iron in the water. At that point you can either use a metal sequestrant or physically remove it with something like Metal Free that tries to capture and precipitate more into the filter.
Given that phosphoric acid is a reducing agent, do HEDP sequestrants just reduce the iron back to ferrous so it is soluble? Or do they react with it to "tie it up?"