Aqueous Solubility of Sodium and Chloride Salts of Glycine─"Uncommon" Common-Ion Effects of Self-Titrating Solids.

Abstract

Although glycine is the simplest of the amino acids, its solution and solid-state properties are far from straightforward. The aqueous solubility of glycine plays an important role in various applications, including nutrition, food products, biodegradable plastics, and drug development. There is evidence that glycine in subsaturated pH 3-8 solutions forms a dimer, as suggested by several techniques. However, what takes place below pH 3 and above pH 8 in saturated solutions has been sparsely explored and is thought to exhibit complex properties. Although the solubility measurements in the pH 0-13 range have been reported by several groups, the interlaboratory variance between the data below pH 3 and above pH 8 has been high. In a couple of cases, there appears to be no pH dependence on solubility across the wide pH range, even though the reported glycine pK_a values are 2.34 and 9.61. The solubility of the salt forms of glycine is largely uncharacterized. The solubility products of the simplest salts, glycine hydrochloride and sodium glycinate, appear not to have been published. In this study, five series of precision solubility measurements of glycine and its salts were performed at 25 °C, covering the range of pH -0.4 to 12.4, where in each case, just enough glycine was added to reach saturation. We have developed an equilibrium model to rationalize the complicated salt regions. Elemental analysis of isolated solids from saturated solutions supports the speciation model. At least three different salt forms have been indicated in acidic solutions and one salt form in alkaline solutions. Solubility products are reported here. The presence of a water-soluble cationic dimer is also proposed. Data analysis was performed with the aid of the pDISOL-X computer program. Activity corrections based on the Stokes-Robinson hydration theory have been implemented in saturated solutions with ionic strength in some cases exceeding 5 M. Although salt solubility is not a constant, since it depends on two independently controlled reactant concentrations, the salt solubility product is commonly expected to be a constant. However, in the glycine salt region below pH 3, our solubility measurements demonstrate that the solubility products depend on the total amount of added glycine in a saturated solution. We view this as an "uncommon" common-ion effect.