I'm trying to determine if it's possible to use a low-grade spectrometer to read 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?

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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).


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 (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.


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