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Phophate test kit


I while back I complained about the results (or lack thereof) from the
only phosphate kit I ever tried.  Soon afterwords I went to the LFS and
bought a different brand to see what different result I might get.  The
new kit was an Aquarium Pharmaceuticals Dry Tab test.

My actual questions are at the end of this letter, following a rather
lengthy description.

I didn't get the result I expected.  I wrote a letter to AP's customer
support using the tedious form on their website.  Now I've waited a week
for a reply.  I suppose I could post another letter and wait another
week, post another letter and wait another week, post another letter and
wait another week... But I won't.  There's probably some committed young
engineer at AP who is so busy trying to find the solution to my problem
that he just can't find time to answer my questions.

The test has you adding two tablets to a small sample of water, crushing
the tablets with a provided plastic rod, shaking till things get
dissolved, waiting, then comparing the sample color to a color chart.

The first problem was that the tablets wouldn't dissolve.  I shook the
vial for as long as the waiting period was supposed to last and it
didn't dissolve. I probably could have shaken that thing till my arm
fell off.  It wasn't going to dissolve.  After shaking and shaking I had
a test vial with a white, fluffy floc, a slightly blue-colored liquid
and a dark, heavy precipitate.  The color was about equivalent to the
lowest concentration on the color chart, which was 0.5 mg/l PO4.

So then I repeated the test on my tap water, and got exactly the same
result except that the color was maybe a tiny bit lighter.  My tap water
contains no phosphate to speak of.  I even talked to my water utility
about it.  They don't add phosphorus and their tests don't detect
phosphorus in any of our 50 or so wells, so they don't report it on
their website.  Unfortunately their detection limit is 0.5 mg/l as P,
which is too high.  So then I checked USGS published analyses and found
(as I recalled from work early in my career) that water from deep wells
in our aquifer here contains essentially no phosphorus (detection limit
0.01 or 0.02 mg/l as P).

My tap water does contain quite a bit of silica.  Technical information
that I've read about phosphate analyses suggests that silica causes
problems for phosphate tests.

Finally I repeated the test again using distilled water.  That time the
tablets refused to dissolve, but there was no color.  I don't remember
whether I got the heavy precipitate or not.

So here's the questions:

Is it normal for the tablets to refuse to dissolve?  Maybe the tablets
aren't supposed to dissolve completely.  What about the dark heavy
precipitate?  Does anyone else get that result?  I asked AP about the
effect of silica on the test.  Is there anyone else on the list that
knows their water has high silica (say more than 20 mg/l) and has tried
the Dry Tab test?  If so, what sort of result did you get?

Roger Miller
On a sunny day in the desert.