testing for iron

[bkfamily1]

TexasSplash posted this quote from chemgeek in another thread. I thought I would start a separate thread to avoid hijacking the OP.

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.

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?

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?"

 

[chem geek]

The Taylor K-1716 iron test does use hydrochloric acid to reduce ferric iron to ferrous that then binds to Tripyridyl-s-triazine dye to form a compound with an intense violet color. So the test does presumably measure total iron. My point was that much of the iron in the fill water will form a precipitate and be removed from the water which makes it harder to test that which remains regardless of its form. One can still test it, but in practice you may have iron solids on your pool walls and not in the water so you may see stains yet measure no iron in the bulk water. My comment was more in regard to trying to determine the type of metal staining so testing fill water for iron before adding chlorine is much more definitive to detecting such iron in the water. We hear that a lot on this forum, that there are iron stains but no measurable iron in the pool water.

As noted in this paper the solubility product for iron hydroxide (Fe₂O₃•3H[/sub]2[/sub]O or Fe(OH)₃) is on the order of 10^-37 to 10^-44 which at a pH of 7.5 implies a ferric iron concentration of 1.8x10^-13 to 1.8x10^-20 ppm (ignoring ion pairs) and as noted in this report the amount of iron that theoretically could be present in solution from pH 5 to 8 is less than 0.01 ppm. If you detect iron in a sample of water that has been chlorinated, then it is likely to be suspended solid iron hydroxide that has not yet settled or been filtered.

 

[bkfamily1]

So one can theoretically detect iron in either form since the test theoretically converts it all to ferrous. However, once the water has been chlorinated, very little iron will remain in solution and it will be below detection limits. The only thing you have a chance of detecting once chlorinated is suspended iron. Correct?

So what are we seeing when the water is crystal clear green from iron that has been oxidized?

Is the story the same for copper?

 

[chem geek]

Yes you are correct. However, it is crystal clear yellow/red water seen after chlorine has been added that is suspended iron hydroxide or other iron oxides. If this is dilute so more yellow, then against a blue vinyl background that will look green (yellow+blue/cyan = green).

Copper is different. It is more soluble and is mostly in one oxidation state so chlorine doesn't need to oxidize it. Instead, higher pH will create copper oxides-hydroxides that stain. So for copper, one can usually measure it in the water regardless of whether chlorine has been added or not. Copper looks green and will do so even against a white background (i.e. against white plaster).