PNNL

Abstract

Measurement of pH is an integral component of chemical studies and process control; however traditional pH probes are difficult to utilize in harsh or complex chemical systems. Optical spectroscopy-based on-line monitoring offers a powerful and novel route for characterizing system parameters, such as pH, and is well adapted to deployment in harsh environments or chemically complex systems. Specifically, Raman spectroscopy combined with chemometric analysis can provide an improved method of on-line pH measurement. Multivariate curve resolution analysis of Raman spectra can be utilized to determine speciation as a function of pH and subsequent chemometric modeling can be used to correlate spectral response to pH. This was demonstrated with phosphoric acid, a chemical system known to challenge traditional pH probes. Raman spectra exhibit clear changes with pH due to changing speciation, and chemometric modeling can be successfully utilized to correlate those fingerprints to pH. Using this approach, true pH of the phosphoric acid system can be measured with greater accuracy than with traditional pH probes.