The Association Constants of H+ and Ca2 + with 2-Keto-D-Gluconate in Aqueous Solutions

Abstract

2-Keto-D-gluconate (kG) is naturally produced in soils, sediments and rock faces through the microbial oxidation of glucose. Studies have qualitatively shown kG to enhance the dissolution of soil minerals. However, quantitative information, such as the log K values for the formation of metal–kG complexes, are not available. This paper presents the results of potentiometric titration studies that employ H+ and Ca2+ ion selective electrodes (ISEs) to determine the conditional ion association constants (log Q values) for the protonation and deprotonation of kG and the formation of Ca–kG complexes. The experimentally-determined log Q values were then converted to the corresponding ion association constants (the zero ionic strength condition; log K values) by employing a modified Davies equation for charged species and the Setchenow equation for neutral species. The log K values were determined by potentiometric titrations at constant kG concentration, varied ionic strengths, 25 or 22 ∘C, and in the absence of CO2. The computer model GEOCHEM-PC was used to determine the aqueous speciation of ions other than kG and the computer model FITEQL was used to estimate conditional log Q values for reactions in the various chemical models. Based on our evaluations, equilibrium constants for the following reactions were determined: H++ kG– ⇌ HkG0, log Ka1 = (3.00 ± 0.06), kG–⇌ H–1kG2–+ H+, log Ka–1 = –(11.97 ± 0.41), and Ca2++ kG–⇌ CakG+, log K101 = (1.74 ± 0.04).