3.2.9.6. Beer’s Law or Lambert-Beer Law?
There is a wide consensus that Beer’s Law is applicable only to homogenous solutions where attenuation of light is due absorption by dissolved ions or molecules, and it does not apply to heterogenous solutions because light attenuation is due to scatter on suspended particles. This widely accepted misconception of the Law (Table 2) should be corrected. However, why and how it happened will be difficult to find, given that the initial discovery of this Law was based on observing light attenuation due to scattering by airborne particles. Since the purpose of textbooks is to teach present state of knowledge, textbook summaries (Table 2), do it exactly and correctly. But it is difficult to understand how a sucker lasts 200 years and and continues, is difficult, because already the first step of discovery of this Law was the study of loss of light intensity due to scatter on particles in air.
It was:
• Pierre Bouher, a French astronomist (1729), theorized that each layer of atmosphere absorbs an equal fraction of light that enters it. This, exponential decay of light intensity within heterogenous medium (air, dust, water vapor) is caused by light scattering. [6]
• Johann Lambert (1760) formulated an equation showing that the logarithmic loss of light is proportional to optical density and thickness of medium (Photometria 1760). He developed and verified universal theory of light attenuation by stacking glass plates of uniform composition thus increasing path length. Glass is a frozen liquid, and it was probably heterogenous (particles, microbubbles). It was neutral density filter. The glass also might have been colored. We do not know, but it was scattering and perhaps also absorbing the light. [7]
• August Beer, mathematician and physicist discovered that light absorption is directly proportional to concentration of absorbing substance dissolved in the solution (c) thus completing Lambert equation (Bestimmung der Absorption des rothen Licht in farbingen Flussigkeiten 1852). Following standard laboratory practice, he used filtered solutions of copper sulphate, potassium dichromate and potassium permanganate in distilled water thus linking absorbance to concentration. His solutions selectively filtered light within narrow set of wavelengths. [8]
We can only guess that Beer's fundamental contribution spotlighted measurement of absorbance of homogenous solutions as the only application of what was until recently known as Lambert-Beer’s Law.
TAKEOUT
Recognizing that logarithmic attenuation of light, regardless of whether it is caused by absorption or by scattering, justifies the renaming of the Lambert-Beer Law is beneficial because it will contribute to the understanding of the Law, by focusing on its mathematical and optical foundation and applications, such as the contribution of suspended matter to the limit of detection and reagent or sample blank.
Therefore, our call for renaming is not merely a semantic dispute, but it is a challenge to the general misrepresentation that obscures the bond between turbidimetry and its parent spectrophotometry, and on the interference of suspended matter on spectrophotometric determination of target analytes, and on what is measured by chromogenic reactions.