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Re: Iron test - how it works ?

At 03:48 AM 2/10/01 -0500, Dennis Chua asked:
>When we use any aquatic Fe test kit and got a reading, say, 0.1ppm Fe.
>What does it really measured ?
>(1) Does it breakdown all the available Fe chemical bound (include all
>available FeEDTA from the trace mix dosage.) and indicate the total Fe++
>available ?
>(2) Or it just measured the existing free Fe++ in the water ? while the
>FeEDTA are very much untouched ?
>(3) Or does it indicate all Fe (whether Fe++ or Fe+++?)

I fear you won't like the answer:  "It depends on the test kit".

Most kits contain a dye that complexes with the iron and changes 
color.   The Hach Ferrozine based kits will complex immediately with 
ferrous iron.

Over time, the dye competes with chelators for additional iron they are 
holding.   Since its attraction (the dye's) is generally stronger than the 
chelators, eventually most of the iron will end  up on the dye, and thus 
indicated.   My experience is that this takes a couple of hours to 
occur.  Methods other than Ferrozine may be less susceptible to 
interference by chelators, but also may be less sensitive to low levels of 
iron.   You can also use a digesting agent to break down the chelator 
before running the test.

Many kits have a reducing agent added to convert ferric iron to the ferrous 
form so that total iron is measured.  Read your kit's instructions.

Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
NEW Planted Aquaria Magazine:        http://www.wcf.com/pam