Thermodynamic Properties of Dopamine in Aqueous Solution. Acid–Base Properties, Distribution, and Activity Coefficients in NaCl Aqueous Solutions at Different Ionic Strengths and Temperatures

The acid–base properties of dopamine were studied in different experimental conditions by potentiometry, UV-spectrophotometry, and spectrofluorimetry. The distribution measurements between 2-methyl-1-propanol/pure water (or NaCl aqueous solutions up to <i>I</i> = 1.02 mol kg^–1) were carried out at <i>T</i> = 298.15 K and 310.15 K. The potentiometric measurements were used to calculate the total concentration of the ligand in each phase, and from simple mass balance equations, using the free hydrogen concentration and the protonation constants, it was possible to calculate the ligand neutral species concentration and the distribution coefficient. The salting parameter and the activity coefficient of the neutral species were calculated by means of the Setschenow equation. Independently of the experimental conditions, a good agreement between the protonation constants obtained from the different techniques was obtained. The dependence of the protonation constants on ionic strength was modeled by means of the Debye–Hückel type and Specific ion Interaction Theory (SIT) approaches, and the specific interaction parameters of the ionic species were determined. For the protonation constants, the following thermodynamic values were obtained at <i>T</i> = 298.15 K, log <i>K</i>₁^H0 =10.886 ± 0.042; log <i>K</i>₂^H0 = 9.107 ± 0.032; at <i>T</i> = 310.15 K, log <i>K</i>₁^H0 =10.260 ± 0.024; log <i>K</i>₂^H0 = 8.418 ± 0.018. From the distribution measurements the following distribution coefficients at infinite dilution were obtained at <i>T</i> = 298.15 K, log <i>K</i>_D₀⁰ = 0.705 ± 0.013; at <i>T</i> = 310.15 K, log <i>K</i>_D₀⁰ = 0.983 ± 0.014.