Recent advances in the aqueous chemistry of the calcium(II)-gluconate system – Equilibria, structure and composition of the complexes forming in neutral and in alkaline solutions

• Published in: Coordination chemistry reviews 2020-08, Vol.417, p.213337, Article 213337
• Main Authors: Kutus, Bence, Gaona, Xavier, Pallagi, Attila, Pálinkó, István, Altmaier, Marcus, Sipos, Pál
• Format: Article
• Language: English
• Subjects: Acid-base equilibria; Calcium; Complex formation; Equilibrium constants and thermodynamics; Gluconate; Gluconic acid; Hyperalkaline solutions; Lactonization; Solution structure
• ISSN: 0010-8545; 1873-3840
• DOI: 10.1016/j.ccr.2020.213337

[Display omitted] •The acid-base and Ca(II) complexation equilibria of the D-gluconate ion are reviewed.•The Ca(II) coordination of some related derivatives of D-glucose is also discussed.•The stability of these complexes heavily depends on the nature of functional groups.•Both 1:1 and 1:2 chelate complexes are formed with D-gluconate at close-to-neutral pH.•In alkaline solutions, trinuclear species with Ca-L-Ca-L-Ca chains, are predominant. Of the sugar carboxylates, D-gluconate is clearly the most significant representative: the world’s annual production of this organic compound is estimated to be in the order of 105 tonnes. The reason of its mass production is due to its outstandingly broad range of practical (medical, pharmaceutical, industrial, etc.) applications. D-gluconate is a well-known and exceptionally popular complexing agent; accordingly, it has been the subject of a large number of coordination chemical research investigations. Its complexation properties are specially remarkable in alkaline to hyperalkaline pH conditions, where the deprotonation of one or more of its alcoholic OH groups provides a favourable frame for the formation of very stable chelate complexes with a large variety of metal cations. With the aim to show the state of the art of some relevant issues in the aqueous chemistry of the D-gluconate ion, the current paper focusses on the acid-base properties and calcium(II) complexation of the compound encompassing the entire experimentally available pH-range in water. The accessible literature on the deprotonation of carboxylic and alcoholic OH groups is collected and critically evaluated. The lactonization equilibria of D-gluconic acid are also scrutinized. The available data on the calcium complexes forming in neutral and in (hyper)alkaline solutions (both in terms of composition, formation constants and solution structure) are also discussed. Where feasible, some of these properties are compared with those of D-glucose and its derivatives as well as some less common sugar carboxylates, structurally related to D-gluconate, (i.e., D-heptagluconate, L-gulonate and α-D-isosaccharinate). Special emphasis is laid on the relationship between complex stability and the type of metal-binding groups.