Question: Does the Beer Lambert law apply to the opacity of reagents?

Copying in answer from @stoft, here:

It is my understanding that Beer's law is not the relationship between reagent's effect on color, but is the property of light passing through a uniform substance and the attenuation of that light with distance -- which is linear assuming the substance is uniform. So, if you had some chemical reaction, which was not linear with the concentration of reagent, that non-linearity would appear in a plot of the measured attenuation (from the spectral data) as the concentration changed. The Beer's Law linearity you are relying on is that given that the sample containers are all "identical" then the path-length for light is "identical" and therefore (with all else held constant - like the light source, camera exposure, etc) then the measurements of relative intensity will be directly related (linear or non-linear) with the concentration. [Note: Realize that there are many possible components to the issue of measurement stability and that the default PLab stuff is only a first step. You might find some of my PLab notes on spectrometer stability, noise, time-averaging, calibration and uncertainty of interest.] It's all in the numbers to estimate measurement limits vs your requirements.

https://publiclab.org/questions/interestedperson_ha/06-27-2016/question-can-diy-spectrometer-be-used-for-analysis-of-soil#answer-0-comment-14873

Also noting that @ygzstc cited some work supporting the use of the area under the curve in a spectrum data reading instead of intensity at one wavelength, as a possibly better measure of concentration:

Yes the Beer-Lambert Law is based on -log(I/Io) but it's applicable for a single wavelength value. (https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law) And this law has many limitations. Recent research showed that the area under the peak is much more stable indication of concentration variations (http://www.hindawi.com/journals/jspec/2013/919023/). Hope this is helpful...

Related:

How can we detect contaminants in water samples with a DIY spectrometer using reagents?

https://publiclab.org/notes/warren/12-18-2017/can-a-diy-spectrometer-be-used-to-measure-water-turbidity