In situ Measurement of the pH-dependent Transmission Infrared Spectra of Aqueous Lactic Acid Solutions.

The transmission infrared absorption spectra of an aqueous lactic acid solution was measured over a pH range of 2.66 - 1.59 using a cell with an optical path length of 12 µm. Absorption peaks were then identified by density functional theory (DFT) calculations. The observed pH-dependent changes in the absorbance spectra were consistent with the properties of the assigned functional groups. We therefore believe that the present method can be applied to the contactless and non-invasive quantitative evaluation of pH values in aqueous systems, such as bioprocesses and bioimaging.