Fluid Phase Equilibria 355 (2013) 104–113 Contents lists available at ScienceDirect Fluid Phase Equilibria j our na l ho me pa ge: www.elsevier.com/locate/fluid Thermodynamic properties of melamine (2,4,6-triamino-1,3,5-triazine) in aqueous solution. Effect of ionic medium, ionic strength and temperature on the solubility and acid–base properties Clemente Bretti, Concetta De Stefano, Gabriele Lando, Silvio Sammartano ∗ Dipartimento di Scienze Chimiche, Università degli Studi di Messina, Viale Ferdinando Stagno d’Alcontres, 31, I-98166 Messina (Vill. S. Agata), Italy a r t i c l e i n f o Article history: Received 28 February 2013 Received in revised form 24 June 2013 Accepted 28 June 2013 Available online 13 July 2013 Keywords: Solubility Protonation constants Weak complexes Ionic strength Activity coefficients a b s t r a c t In this paper new solubility and potentiometric measurements are reported to model the behaviour of melamine in aqueous NaCl and (CH 3 ) 4 NCl ionic media at different ionic strengths (0.1 ≤ I/mol L -1 ≤ 3.8) and temperatures (283.15 ≤ T/K ≤ 318.15). For this purpose, some literature data were used together with experimental data. In NaCl solutions, the solubility of melamine decreases with increasing ionic strength and increases with increasing temperature (H = 30.5 kJ mol -1 at I = 0 mol L -1 ), whereas in (CH 3 ) 4 NCl solution increases with increasing both ionic strength and temperature. In NaCl, at T = 298.15 K, the melamine solubility is 26.1 and 16.3 mmol L -1 at I = 0.104 and 2.304 mol L -1 , respectively; whereas in (CH 3 ) 4 NCl it is 29.9 and 33.6 at I = 0.111 and 1.058 mol L -1 , respectively, at T = 298.15 K. Values at infinite dilution are provided together with solubility values of neutral species calculated at different tem- peratures and ionic strengths. As an example, the solubility of melamine is 0.0271 mmol L -1 in pure water. From solubility data, the Setschenow and the activity coefficients were also determined. The protonation constant are reported in condition similar to the solubility measurements, and their depend- ence on temperature shows that the proton binding is exothermic (H = -26.6 kJ mol -1 at I = 0 mol L -1 ). The entropic contribution is low (TS = 2.4 kJ mol -1 at I = 0 mol L -1 ) and increases with increasing ionic strength in NaCl, whereas in (CH 3 ) 4 NCl it remains almost constant. The ionic strength dependence was modelled by means of the extended Debye–Hückel and the Specific ion Interaction Theory (SIT) and data at infinite dilution are calculated. Finally, comparing the protonation constants in different ionic media, the formation of two weak complexes was noticed between the protonated melamine species, AH + and Cl - and between the tetramethyammonium cation, (CH 3 ) 4 N + and the deprotonated melamine species (A). At infinite diluition (T = 298.15 K) it was found K(AH + + Cl - ) = 0.45 ± 0.05 L mol -1 and K(A + (CH 3 ) 4 N + ) = 0.63 ± 0.03 L mol -1 , in agreement with previous findings for amine ligands. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Melamine (2,4,6-triamino-1,3,5-triazine) (see Fig. 1) is a chem- ical intermediate that in its natural state is a white crystalline powder. It was commercially produced in the 1930s from the common substance urea, which is distilled to produce melamine. Melamine is used for a wide variety of applications, including plas- tics, adhesives, laminates, paints, permanent-press fabrics, flame retardants, textile finishes. In factories, it is mixed in large vats to produce a resin; which is commonly used in the manufacturing of particleboard [1]. Melamine is a monomer in the manufacturing of plastic materials (melamine–formaldehyde plastics) used to make ∗ Corresponding author. Tel.: +39 090 6765747; fax: +39 090 392827. E-mail address: ssammartano@unime.it (S. Sammartano). tableware products, suitable for food contact applications because of its hardness, heat resistance and general stability. These supe- rior characteristics enable the use of melamine-based tableware for this purpose; however, repeated use can increase the possibility of melamine migration into food [2]. Melamine is also added to plas- tic foams to increase density and durability. Melamine and other triazine compounds are used as a nitrogen source in slow release urea-based fertilizer mixtures. Their accumulation and persistence in the environment are well known [3]. Considering that melamine is almost tasteless and that it is very rich in nitrogen (67% by mass), it is added to foods to inflate the apparent protein levels; this adul- teration caused the death of hundreds of pets and, more recently, in China the adulteration of milk for infant, caused renal failure because the formation of insoluble melamine–cyanurate crystals in the kidney [4,5]. According to the Environmental Protection Agency Toxic Chemical Release Inventory, until 1987 [6], 82 000 kg 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.06.056