Hi all,
I know the forum belief is that your average nitrate test kit is highly inacurate so i was just wandering what an accurate method of measuring would be?
Nitrates are quite tricky to measure by traditional methods, even with lab. scale analytical equipment. This is mainly because nearly all nitrate salts are soluble, and most chemical methods are reliant on producing coloured compounds that you can measure by colorimetry/spectrophotometry.
A more accurate method (in approximate order of accuracy) would either be Ion Chromatography, ISE (ion selective electrode), Kjeldahl (after reduction with "Deverda's alloy") or a spectraphotometry/colormetric method using cadmium reduction and absorbance at 543nm (this is the "red" colour).
Water companies etc can measure NO3 fairly accurately using Ion Chromatography (IC), but even this isn't entirely straight forward because of the presence of other anions, and you may need to change columns and your active phase to get repeatable results. The better test kits use semi-quantitative colormetric methods, but you can still get anomalous results.
We got a HPLC and a GC, but we use nitrate ion selective electrodes for most day to day lab. and project work on soil extracts, water etc., but you could easily spend 1/2 a day doing ~10 samples. People want a simple dip test they can use with their tank water and get a meaningful reading, but there really isn't one.
Even though some of the tanks are actually in the teaching labs., I almost never test any of the water. I occasionally dip the conductivity meter in, as it is the only meter or test that you can use like that and get an accurate result (although not a particularly useful one), and we use the tanks (during the photoperiod) to test whether the membranes on the dissolved oxygen meters are damaged.
If any-one wants to have a look at the ISE methodology it is here: <
http://www.coleparmer.co.uk/Assets/manual_pdfs/27502-12Nitratereplaceable.pdf>. Approximate costs are, the meter would be from about £500 (you could also use it for pH measurement with a pH electrode) and the Nitrate Ion Selective electrode from about £200.
cheers Darrel