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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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  • $\begingroup$ First off, welcome to SE.Chemistry! Although including links to descriptions of your equipment and terminology is a good start, you still need to provide additional details for your experiment. Some brief descriptions in additions to the links could be helpful. And most importantly, you need to show some efforts that you have made to solve this problem. For example, provide an example with calculations for how you said you believe it should be done. This may lead to you answering your own question, and it will also motivate others to put fort their efforts to help you. $\endgroup$ – airhuff Feb 2 '17 at 23:20
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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).

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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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