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Re: Seachem phosphate kit accuracy

John T. Fitch wrote"
> I've been using a Seachem Phosphate Test Kit for about a year now.
> Before I
> started dosing KH2PO4, the phosphate level was so low (~ 0.1 mg/l)
> that I
> didn't think much about it.  But now, on Tom Barr's advice, I'm
> trying to
> keep the level at 0.5 mg/l.  And, lately, I've begun to doubt the
> results of
> the test.  This morning, I tested the reference solution that comes
> with the
> kit and, instead of getting the 1.0 mg/l I should have, I got about
> half
> that.
> So, my questions are: Do the reagents "expire" in as little as a
> year, or is
> this test kit inherently inaccurate? 

I'm not an expert on the kit but I believe there is room for error as
in most home kits.  Slight differences in each test can give different
results.  To check for this, you can try testing all six "dishes" from
the same tank, making, for example, very slight progressive differences
in how much water you drop in each "dish."  The readings will probably
vary by about .5 plus or minus with small differences in water amounts.
 Also, I have found that the longer the test sits, the more the color
changes -- so reading after 3 seconds seems to give milder results than
after, say 10 seconds, up to about 30 or more seconds.  

I think these sorts of inaccuracies occur in many hobbyist kits for pH,
nitrites, nitrates, etc.  But it is more problematic with chemical like
phosphate where the amounts being measured are so small to begin with.

Perhaps Dr. Morin will advise.

Scott H.

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