A researcher is studying the chromatographic behavior of three organic acids—A, B, and C—on a silica gel column. The acids differ only in the number of hydroxyl groups attached to the aromatic ring (one, two, or three). To quantify retention, the researcher measures the distance each analyte front travels down the column relative to the solvent front after 20 mL of mobile phase has passed through. The column length is 25 cm.
The following data are obtained in two separate runs:
Run 1: Mobile phase = 100 % dichloromethane (nonpolar)
• Acid A travels 19 cm
• Acid B travels 13 cm
• Acid C travels 4 cm
Run 2: Mobile phase = dichloromethane containing 12 % by volume methanol (more polar)
• Acid A travels 23 cm
• Acid B travels 20 cm
• Acid C travels 12 cm
(a) For each run, calculate the Rf value of each acid. Rank the acids in order of increasing polarity and justify your ranking using both data sets.
(b) Using only the Run 1 data, estimate the relative partition coefficients K (defined as the ratio of concentration in the stationary phase to concentration in the mobile phase) for Acids A, B, and C. Assume the distance traveled is inversely proportional to K and provide your reasoning.
(c) Analyze quantitatively how the addition of 12 % methanol affects the partition coefficient of Acid C relative to Acid A. Clearly state any assumptions, show your calculations, and interpret the chemical significance of your result.
(d) The researcher considers adding a third, highly polar solvent to create a step-gradient after 20 mL have eluted. Propose criteria the researcher should use to decide the methanol percentage at which to switch solvents, and predict how such a gradient would influence the elution of Acid C in comparison with the isocratic Run 2.