How to use a spectrometer to determine NPK

I'm trying to determine if it's possible to use a [low-grade spectrometer](http://www.hamamatsu.com/us/en/product/category/5001/4016/C12666MA/index.html) to read [NPK](https://en.wikipedia.org/wiki/NPK) values of a solution.


I believe I should be able to take readings of the individual compounds (e.g., calcium nitrate) and then use that to "subtract" from a reading of the mixed/final solution and basically deduce how much of each compound is present - is that correct?




Your spectrometer detection range is 340-780 nm. That is to say mostly visible, and a bit of UV. If your compounds are colorless, it's a first indication that you won't see anything on your spectra. These compounds seem to be inorganic, no transition metal or anything exotic, so it's not a good method.


Apart from this, yes, you would need to calibrate with solutions of known concentration (unless you know the absorption coefficient, which would give you the concentration with Beer-Lambert law).


 2

*Autumn*'s answer has already indicated that your spectrometer might not be suitable for the **direct** determination of nitrate, phosphate, and potassium.


In [*Journal of Research of NBS, Section A: Physics and Chemistry*, **1972**, *76A*, 469-482](http://dx.doi.org/10.6028/jres.076A.041) (free PDF) the uv spectrum of potassium nitrate in aqueous solution is given. The spectrum exhibits a maximum at $\lambda = \pu{302 nm}$ with a molar absorption coefficient $\epsilon$ as low as $\pu{7 l\,mol\,cm^{-1}}$.


Even if this wavelength wasn't below your detection range, the low $\epsilon$ wouldn't get you very far anyway.


Admittedly, I didn't bother to check for the other spectroscopical data, but without derivatization or complexation, that would turn the ions in question into species with a strong and specific absorption in the visible range, you're aout of luck. 


 1