STRATEGY TO HELP UNDERGRADUATE STUDENTS TO CALCULATE ACIDITY CONSTANTS BASED ON SPECTROSCOPIC MEASUREMENTS Diego Airado Rodríguez 1, Florentina Cañada Cañada 2, David González Gómez 3, Jin Su Jeong 4 1 Nofima AS, Osloveien, Ås (NORWAY) 2 Department of Science and Mathematics Education, Education School. University of Extremadura (SPAIN) 3 Department of Science and Mathematics Education. Training Teaching School. University of Extremadura (SPAIN) 4 Department of Graphic Expression. Centro Universitario de Mérdia, University of Extremadura, (SPAIN) 7th International Conference of Education, Research and Innovation … Seville, Spain

7th International Conference of Education, Research and Innovation … Seville, Spain Introduction Calculation methods Results and discussion Conclusions * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology

7th International Conference of Education, Research and Innovation … Seville, Spain INTRODUCTION Importance of calculation of pKa: producing distribution plots that show the extent to which e.g. a natural compound with health propertied is ionized at physiological pH. Important properties such as lipophilicity, solubility, and permeability through biological membranes are pH dependent. Understanding the meaning of pK, should enable students to apply their knowledge by making chemical predictions. Acid-base indicator species have been traditionally used as case molecules for pKa calculations in laboratory experiments. The practice we propose goes one step further, considering the study of organic molecules important in different fields, namely, bioactive compounds in wine. Here we propose an easy laboratory experiment and calculation methodology for the determination of acidity constants of organic molecules with ionizable moieties by undergraduate students. * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology

7th International Conference of Education, Research and Innovation … Seville, Spain * Table of Contents * Introduction * Results and discussion * Conclusions FAH: signal corresponding to the non-dissociated form AH; FB: signal for the conjugated base B F: signal at any intermediate pH value, corresponding to the mixture AH / B Taking logarithms: Let’s define this like R: Plotting log R vs pH pKa: cut of the obtained line with the x axis Stenström and Goldsmith method Wilson and Lester method * Calculation methods * Methodology

7th International Conference of Education, Research and Innovation … Seville, Spain * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology Spectral features are provided to the students in advance, in terms of main spectral characteristics of trans-piceid. Taking this information as starting point, the influence of the acidity of the medium on the absorption spectra of trans-piceid is studied, by performing acid base titration of the target molecule. The study of the influence of the acidity of the medium is carried out in aqueous solvent. The ionic strength of the medium is kept constant by the addition of KCl, in a final concentration of 0.10 mol/L in order to avoid migration phenomena in the pH electrode. The titration (variation of the pH) is performed by the addition of small volumes of diluted solutions of HCl or NaOH. Acidulation and alkalinization are performed on independent aliquots of the solution, since students do not have data about the reversibility of the implied equilibria. METHODOLOGY

7th International Conference of Education, Research and Innovation … Seville, Spain * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology RESULTS AND DISCUSSION Three of the more representative registered spectra, namely spectra registered at pH values 9.8, below 7.0 and above 11.0 are plotted in the next figure. As it can be observed in that figure, the decrease of the medium acidity induces a shift of the absorption maximum to longer wavelengths, from 306 to 344 nm. The decrease of the medium acidity induces a shift of the absorption maximum to longer wavelengths, from 306 to 344 nm.

7th International Conference of Education, Research and Innovation … Seville, Spain * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology RESULTS AND DISCUSSION The variation of the photometric signals at 306 and 344 nm, which are the wavelengths of maximum absorption of the acid and conjugated base, respectively, are represented in the next figure. This plot is actually the plot the students need to get to perform calculations. Based on the data from these experiments, pKa values have been calculated by means of Stenström and Goldsmith method and Wilson and Lester method, as detailed above.

7th International Conference of Education, Research and Innovation … Seville, Spain * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology RESULTS AND DISCUSSION pKa values have been calculated by means of Stenström and Goldsmith method and Wilson and Lester method. The average obtained values are reflected in the next table.

7th International Conference of Education, Research and Innovation … Seville, Spain * Table of Contents * Introduction * Results and discussion * Conclusions * Calculation methods * Methodology CONCLUSIONS An easy and straightforward methodology is proposed for pKa calculation. The methodology is based in simple spectrophotometric measurements, in comparison with other methods proposed in the literature, such as methods based on capillary electrophoresis or electrochemical techniques. This laboratory practice is also easy to implement in common labs since only spectrophotometers and hydrochloric acid and sodium hydroxide solutions are needed, apart of potassium chloride. Performing the experiments and applying the proposed calculation methods will help students in the understanding of chemical equilibrium and pKa concept.

Thank you for your attention! 7th International Conference of Education, Research and Innovation … Seville, Spain Authors thank to Extremadura Government, European Social Funds and Ministerio de Economía y Competitividad for the financial support (Grant GR10075 and EDU ).