3.2.9.6. Beer’s Law Does Apply to Heterogenous Solutions

There is a wide consensus that Beer’s Law is applicable only to homogenous solutions where attenuation of light is due to absorption by dissolved ions or molecules, and it does not apply to heterogenous solutions because there light attenuation is due to scatter (Encyclopedia Britannica, Wikipedia Validity, Google Gemini ask: Does Beer’s apply to heterogenous solutions?).

Yet it was the scattering of light by Earth's atmosphere that Pierre Bouguer, astronomist, led to conclude that the decay of light intensity due to passage through layers of air is exponential and it was Johann Lambert (1760) who formulated the equation showing that the logarithmic loss of light intensity is proportional to optical density and number of equally thick glass plates. The fundamental change happened 100 years later when August Beer, mathematician and physicist, demonstrated that the concentration of homogenous solutions of colored compounds is directly proportional to logarithmic loss of light intensity, thus linking absorbance (A) to concentration (c). Since then, a disconnect between scattering and absorption of light took place. being also obscured by terminology (turbidimetry).

In summary, Beer’s Law applies to total attenuation of light, regardless of whether it is caused by absorption or scattering, or a combination of both, as long as:

1) None of the photons involved in the interaction with individual attenuators will reach the detector.

 2) Difference in optical properties of individual attenuators will be smoothed out by the large number of interactions.

Not by a priori design, but by coincidence, our experimental setup fulfills these requirements.