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Re: Nitrite/nitrate measurements

On Sun, 2 Jan 2000, Stephen Lindsay wrote:
> I notice people testing for nitrites as quite different (it seems to me) from
> testing for nitrates. When I had a nitrite test kit before, the instructions
> mentioned multiplying the resulting nitrite value by a certain number to
> determine the nitrate value. Hence, only the nitrite test kit is needed to
> measure both. This seems to make sense since nitrite changes to nitrate
> anyway.

Nitrate is a remarkably unreactive ion, so colorimetric nitrate analyses
(like hobby test kits) work by reducing nitrate to nitrite, then analysing
the resulting nitrite.  The number that you get from the kit is actually
nitrate+nitrite, not just nitrate.

Generally there is no fixed ratio between nitrite and nitrate, so there is
no way to estimate nitrate concentrations by mulitiplying a nitrite result
by a constant.

I don't know what kind of kit you have, but I can imagine one way that
someone might use one kit to analyse both ions, with a multiplying factor
to get the nitrate.  The kit would have one color scale for nitrite; to
test for nitrate you would add an additional reagent (a reductant) and an
additional step to the analyses (shake and wait), then would get the
nitrate concentration by comparing the color to the nitrite scale and
multiplying by a factor to convert from nitrite to nitrate.

Roger Miller