Estimating the Acid-Base Properties and Electrical Charge of Organic Matter Using Spectrophotometry.

Spectrophotometric acid-base titration is a simple and powerful technique to evaluate the properties of proton binding sites of natural organic matter (NOM) at environmentally relevant concentrations. However, it is challenging to quantify the chemical charges ( Q ) carried by NOM at these concentrations. Based on a previous work, which relates the variation of Q with the specific UV-vis differential absorbance (Δ A λ,pH ) at a given wavelength (λ) and pH of a dissolved NOM sample, the present work proposes a method to investigate any NOM sample. It determines specific features in the absorbance spectra attributed to proton-inert chromophores ( A 0,λ ) and to the deprotonation processes of carboxylic ( A 1,λ ) and phenolic groups ( A 2,λ ). It enables to select sample-specific wavelength (λ mid ), where both these functional groups significantly contribute to the variation of absorbance with pH. The linear regression analysis of A λmid,pH vs Q for various NOM reference samples evidenced that the sample-specific slope ( S NOM ) and intercept ( I NOM ) were related to the intrinsic spectroscopic properties of the sample ( A 0,λmid , A 1,λmid , and A 2,λmid ). This approach can thus be used to approximate the Q values of the NOM samples at environmentally relevant concentrations: a pre-requisite for predicting the fate and behavior of metal ions in natural systems.