Modeling S-carboxymethyl-l-cysteine protonation and activity coefficients in sodium and tetramethylammonium chloride aqueous solutions by SIT and Pitzer equations

Solubility and acid–base properties of S-carboxymethyl-l-cysteine (carbocysteine, ccys) in NaClaq and tetramethylammonium chloride, (CH3)4NClaq, at t = 25 °C and at different ionic strengths were investigated. Solubility was studied at 1.0 ≤ I (mol L−1) ≤ 5.0 for NaClaq and 1.0 ≤ I (mol L−1) ≤ 3.0 for (CH3)4NClaq, while potentiometric measurements (by ISE-H+, glass electrode) were performed at 0.1 ≤ I (mol L−1) ≤ 5.0 for NaClaq and 0.5 ≤ I (mol L−1) ≤ 3.0 for (CH3)4NClaq. Solubility data allowed us to determine Setschenow constants and activity coefficients of neutral carbocysteine (H2ccys). Dependence on ionic strength and ionic medium of protonation constants and activity coefficients was modeled by Specific ion Interaction Theory (SIT) and Pitzer equations. SIT and Pitzer approaches give comparable and satisfactory results. Solubilities and activity coefficients of neutral H2ccys species, protonation constants in NaClaq and (CH3)4NClaq at different ionic strengths, SIT interaction coefficients and Pitzer parameters were calculated, together with protonation constants at infinite dilution: log ⁡ T K 1 H = p T K a 3 = 9.358 ± 0.001 , log ⁡ T K 2 H = p T K a 2 = 3.461 ± 0.001 , log ⁡ T K 3 H = p T K a 1 = 1.798 ± 0.004 (in the molar concentration scale, ±standard deviation).