MadSci Network: Chemistry

Re: Nitrate Test for Water

Date: Thu Jul 30 09:17:34 1998
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Chemistry
ID: 901681956.Ch

This question was a stumper, and we resorted to e-mail for further 
elucidation of the problem. I can't say we've solved it, but I reproduce 
the two key e-mail messages as a partial solution.
     David Kopaska-Merkel
     Geological Survey of Alabama

First message: further information from submittor

Hi David!

I will give you a little background about what is going on with this
nitrate test we are having trouble with.  The biotechnology based
pharmaceutical company I work for had some renovations done during the
annual shutdown period earlier this month.  This included an upgrade of the
water system.  We are testing nitrate levels in both Nanopure (filtered
water system) and WFI (Water For Injection).

The nitrate test we are running is based on both British Pharmacoepia (BP)
and European Pharmacoepia (EP) methods for nitrate levels in water.  I did
not actually conduct the test myself but three other analysts have been
trying for the last three weeks to collect some kind of data from the test.

I have not read the STM (Standard Test Method) for the assay but know that
it involves making up a 0ppm, 0.2ppm, and 1ppm nitrate standards
(diphenylamine is the nitrate source).  The test is colorimetric and
involves a 15 minute incubation at 50C (122F).  You are supposed to see a
blue colour in the 0.2ppm standard but we get nothing.  The 1ppm standard
is blue but very light in appearance.  The samples are supposed to be
lighter than the 0.2ppm standard.

We have gone so far as to run the test spectrophotometrically with no luck!
 When they tried this approach, with a set of calibration standards, the
0.2ppm standard gave a negative absorbance.

I have performed nitrate tests in another lab I worked in and we used some
kind of kit.  The test was easy and always worked.  I don't know if we are
using EP and/or BP methods because of FDA or HPB (this is the Canadian
regulatory body) requirements but a test kit is definitely out of the
question in this case.

Any suggestions?

Thanks for your help!  This test is driving everyone crazy! :)


Second message: Response from chemist

Hi Julia,

David forwarded me your comments on your analytical problem. As David
mentioned, we don't use the test mentioned; we use ion chromatography
or the cadmium reduction method ---- and unfortunately I doubt if our 
has the BP or EP methods. It does sound to me that if you're analyzing for 
using diphenylamine as a standard that you may not be getting a complete
oxidation of the amine to produce NO3 --- if that's any help!

Robert E. Meintzer
Geological Survey of Alabama

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