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Test Kit Blues

I've just completed side by side comparisons of the LaMotte and Hach Fe
test kits with very disappointing results.  The Hach kit appears to be the
hands down winner.  The Hach kit is the IR-21 ($57) which uses TPTZ iron
reagent (recommended by Craig Bingham for measuring chelated iron in
planted tanks).  The LaMotte kit is model P-62, Code 7787.  It uses a
sulfuric acid reagent followed by a sodium metabasulfite, bipyridyl and
sodium slufite reagent in a two-step process.

Both kits measured distilled and RO/DI water as zero Fe.  San Francisco's
tap water also measured zero Fe.  Next I tested my aquarium water (which
gets its iron from Plantex's Chelated Secondary Mix).  The LaMotte kit
measured less than .05 mg./l as it has many times before.  I've been using
the LaMotte kit to try to establish .1 mg/l Fe using the PPMD principles.

The Hach kit measured .3 mg/l for the same water (small wonder I have some
unexplained hair algae).  I repeated the tests twice with the same results
and then diluted the aquarium water sample with 50% RO/DI water and
obtained .15 mg/l with Hach and less than .05 mg/l with LaMotte (no visible
difference from the earlier readings).

Any of you guys who understand this stuff want to speculate as to why the
LaMotte and Hach Fe test kits would show a 600% difference in low range Fe

Regards, Steve Dixon ("Oh Mama, can this really be the end to be stuck
inside of Mobile with the [test kit] blues again."  Bob Dylan)

p.s.  Many thanks for the good write up on PMDD.