# Measurement of Nitrate with a Spectrophotometer



## jcspotless (16 Feb 2019)

Hi,

I wondered if anyone here had tried to use some of the homemade spectrophotometers and https://spectralworkbench.org to measure nitrate or indeed other colourmetric assay rather than a test kit. I am thinking about it but its quite a big project so before I start I wondered what I might expect.

https://publiclab.org/wiki/desktop-spectrometry-kit-3-0

I understand the theory and use much more expensive versions in my day job. I just wondered if these back to basic ones actually work. From what I can seen I can get an emission spectrum from these spectrometers. Plotting intensity (or more correct absorbance vs concentration) will give me a calibration curve for measurement of a sample of tank water which has been taken through whatever test kit you are measuring for.


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## ian_m (16 Feb 2019)

You need to ask why you need to measure nitrate ? What will you do with any results?

Nitrate, in inorganic form is only an issue in 1000's ppm, a factor of 10-20 times the value seen in a planted tank.

So my nitrate is below 1000ppm, brilliant that is ok, no need to measure anything.


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## dw1305 (16 Feb 2019)

Hi all,


jcspotless said:


> understand the theory and use much more expensive versions in my day job.


It would work for orthophosphate (PO4---), but <"spectrophotometry"> is more difficult with nitrate (NO3-).

Because nitrate compounds are soluble, so you need to reduce them to nitrite (NO2-) before you can get a coloured solute ("_Nitrate is reduced to nitrite by a reducing agent. In the presence of an acidic buffer, the nitrite is converted to nitrous acid which diazotizes an aromatic amine, this coupled with N-(1-naphthyl)-ethylenediamine to form a red-violet azo dye_") that you can measure. The alternative to the azo dye method is to use salicylic acid _("Cadmium metal reduces nitrates in the sample to nitrite. The nitrite ion reacts in an acidic medium with sulfanilic acid to form an intermediate diazonium salt. The salt couples with gentisic (salicic) acid to form an amber colored solution"_). Traditionally cadmium (Cd) was used as the reducing agent, but you can use vanadium (V).

If you have the reagents (for either method) you can make up standards from KNO3 etc. I'll copy in @alto, they are more up to date with spectrophotometric techniques for nitrogen testing, but I think cadmium reduction is the preferred method?.

We use <"Ion Selective Electrodes for nitrate in aqueous solution now">, they have the advantage of not needing to prepare the samples and are much more "plug and play".  I don't regularly <"test the aquarium water"> (some of the tanks are in one of the labs.) for nitrate, I just use the <"Duckweed Index"> as a proxy for <"NO3 content"> (like the LCC chart below).






This is some very healthy Amazon Frogbit (@Timon Vogelaar's), my "Duckweed" of choice.





cheers Darrel


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## jcspotless (16 Feb 2019)

Thanks for the information. I was thinking of replicating the API test kit method which I believe is based around the chemistry you have described above http://stuff.iorodeo.com/docs/nitrate/api.html. I was more interested in the principle of using "cheap" everyday materials to build a working spectrophotometer so nitrate was just one water ion. Equally might be fun to look at cations too, residual iron, calcium or magnesium using some "old fashion" chemistry rather than ICP-MS or Ion chromatography


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## Zeus. (16 Feb 2019)

If you donate to UKAPS one of us will do a ppm estimate of you tank without even having a water sample and the result will probably be as accurate as a hobbist level test kit


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## dw1305 (16 Feb 2019)

Hi all, 





jcspotless said:


> I was thinking of replicating the API test kit method which I believe is based around the chemistry you have described above http://stuff.iorodeo.com/docs/nitrate/api.html. I was more interested in the principle of using "cheap" everyday materials to build a working spectrophotometer so nitrate was just one water ion. Equally might be fun to look at cations too, residual iron, calcium or magnesium using some "old fashion" chemistry rather than ICP-MS or Ion chromatography


Go for it, I've got a colleague who recently built a <"functioning spectrophotometer using a smart phone and a shoe box">.

There are "wet chemistry" methods for all analytical techniques, but NO3- is a tricky one, even with an ISE.

cheers Darrel


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## Oldguy (17 Feb 2019)

jcspotless said:


> spectrophotometers





dw1305 said:


> "wet chemistry"



Spectrophotometers are easier than spectrometers especially for quantitative analysis. Takes me back to BDH Lovibond Comparators and Nesslerisers with permanent coloured glass as comparison standards. Unfortunately not every thing readily forms coloured solutions, especially when cooking in the home laboratory. 

Wet chemistry, is there any other? May be more more doable than spectometric methods. Burettes and pipettes and a few items of glassware, all very versatile. Bottom line 50ml beakers make good shot glasses and 250ml beakers make the beer go further, while you chill watching your tank.

Colour blindness can be a pain with some shades, had a colleague who was a dab hand at titrating to shades of grey.  Pale blue and pale lilac in old car wiring looms are my bane.

The project sounds very interesting, but if your plants a growing and your fish are healthy then all is well. However we all need projects, its their madness that keeps us sane.


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