Bouguer–Lambert–Beer Law of Absorption: Spectrophotometry in Electrolyte Solutions

It is shown that with a formal linear dependence of absorbance on concentration А = f(C) and coefficient of correlation r ≈ 0.99 or more for weak electrolytes, a strong deviation from the Bouguer–Lambert–Beer law of experimentally determined coefficient of molar absorption ε_obs can be detected, depending on the concentration of electrolyte solution. Experimental and calculated data illustrating the reasons for the variability of coefficient of molar absorption ε_obs ≠ const in the solutions with concentrations of less than 10⁻³ mol/L. It is shown that when making precision spectrophotometric measurements, dependence ε_obs = f(C) is more informative than A = f(C). A theoretical model that reliably describes dependences ε_obs = f(C) for electrolytes of different strengths is proposed for the first time. The model will allow study and analysis equilibria in electrolyte solutions using the new technology of concentration spectrophotometry.