An introduction to multivariate curve resolution-alternating least squares: Spectrophotometric study of the acid-base equilibria of 8-hydroxyquinoline-5-sulfonic acid

The monitoring of the UV-vis spectral evolution of 8-hydroxyquinoline-5-sulfonic acid (8HQS) with pH was proposed for introduction of students to the multivariate curve resolution-alternating least squares algorithm (MCR-ALS) methodology. MCR-ALS is a chemometric tool that may ectract a large quantity of information about a chemcial system without many previous assumptions. In addition, students were introduced to the MatLab code for chemometric purposes. Students under the first session recorded teh spectra while in the second the work was on the chemometric treatment. A constant 0.50M ionic strength (NaCIO4) was kept throughout the experiment. The inner KCI of the electrode was replaced by 3M NaCIO4 solution saturated with AgCl. Solutions placed include: perchloric acid, sodium perchlorate, tris, aminomethane buffer and an 8HQS solution. Recorded spectra were converted into ASCII files and analyzed using MatLab. Results show that the quality of the estimation of the variation of concentration with pH can be measured by calculating the pKa values by any graphical procedure. Comparison of such calculated pKa values with those obtained in the literature is a valid way determining whether the procedure is working as planned.