Acid-base Equilibria involving Oxygen-containing Molecules in Dilute Aqueous Solution

Abstract

IT has been shown recently¹ that methanol and isopropanol are more basic than water in dilute aqueous solutions containing mineral acids, in contrast to the basicity of alcohols in alcohol-rich solutions^2,3. The concentration of p-nitroaniline determined spectrophotometrically in mineral acids with and without added alcohol, C_R and c, respectively, in dilute aqueous solution, obeys the relationship¹: c₀ is the initial concentration of p-nitroaniline without added acid or alcohol, r is the concentration of added alcohol and w = [H₂O] before addition of alcohol. This expression can be deduced from the three equilibria: where N and NH are unprotonated and protonated p-nitroaniline respectively, K₁F₁ = [NH][H₂O]/[N][H₃⁺O] and all hydroxylated species, protonated and unprotonated, are assumed to be complex hydrates⁴. The foregoing expression was tested for methanol and isopropanol by obtaining linear plots¹ for CC_R/(C_R–c) against C_R/(C₀–C_R) : values for were calculated from K_b′ = c₀w/rw_R × slope. K_b′ was constant for methanol and isopropanol over the concentration range2–10 per cent v/v alcohol.