Quote Originally Posted by labdi01 View Post
Does sequestered iron (which if I understood Chem Geek correctly from another post, are iron ions) show up on a pool-store iron test?

I came home after this visit and shocked w/ 2 lbs of HTH Super Shock and Swim, with the intent of knocking any sequestered ions out in the open to be filtered, and my pool remains clear (it's only been about 10 mins after shock).

Could either or both of you shed some light on the iron result?
I had that same question and asked Taylor Technologies about it a while back and basically the iron test that they use and probably ones that others use test all iron that is present including that which is sequestered since the sequestering agents bind the metal ions somewhat loosely in an equilibrium so that as the free metal ions get used up in the test more are released from the sequestering agent. This is similar to how the FC tests all chlorine including that bound by CYA.

So that would explain why you still measure iron in your pool, yet it is clear. It also explains why you are not able to precipitate it because the concentration that is unbound is fairly low so would take a heck of a high pH to be able to form precipitate (i.e. the metal sequestrant is doing its job well).

Unfortunately, that means there is no easy way for you to tell your "free" metal ion level other than seeing that the water is clear and that you aren't getting any staining. The 5 ppm of iron the pool store is measuring is mostly bound to your metal sequestrant -- you must have used quite a bit of it to sequester that much iron.

The better sequestering agents are slower to get oxidized by chlorine so it isn't easy to break them down to release the iron, but that's the point of a good metal sequestrant anyway. You don't want it to break down easily because if it did then you'd have to add more much more frequently and it would also increase your daily chlorine demand.

Richard