The citrate ion (Cit3–) is a key intermediate occurring in the Krebs cycle in all aerobic organisms, while the uptake of Cit3– from blood undergoes via Na+-regulated membrane transport. Additionally, Cit3– can occur in radioactive wastes as well as Bayer liquors as an organic contaminant. Understanding the behaviour of Cit3– in aqueous medium requires detailed study about its hydration properties and interactions with Na+ ions. In this study, the hydration and ion-pairing of Cit3– were investigated at 25 °C and at neutral pH. The binding of Na+ ions were monitored by potentiometric titrations using Na+-ion selective electrode. In order to observe the direct effect of Na+ ion-pairing, tetramethylammonium chloride, a non-coordinating electrolyte, was applied to adjust the ionc strength. Upon increasing the concentration of Cit3–, the cell potential considerably shifted to lower values attesting the formation of ion-pairs. The analyis of the titration data provided evidence that not only the NaCit2– but the Na3Cit0 is also formed at high ionic strengths. Additionally, the presence of two ion-pairs was confirmed by the analysis of the UV spectra. In conclusion, citrate has high affinity to coordinate Na+ ions in concentrated solutions. The hydration of the anion as well as the simultaneous changes in the solvent structure and dynamics were probed by Dielectric Relaxation Spectroscopy (DRS). For solutions containing only Na3Cit, the spectra could be fitted well by assuming three Debye-type relaxations. The first, centered around 1 GHz, corresponds to the solute as its amplitude increased markedly with increasing concentration of Na3Cit. Simultaneously, the amplitude of bulk water (at ≈ 18 GHz) and thus the concentration of the solvent decreased significantly implying the strong hydration of citrate. (The third relaxation mode belongs to the so-called fast water relaxation.) As a result, the hydration number of Cit3– was calculated to be 16 with respect to infinite dilution. This is in good agreement with other carboxylates studied previously with the same instrumentation. Increasing the concentration of NaCl in samples containing constant amount of TMA3Cit, the amplitude of the solute mode increased markedly. In conclusion, Na+ ions have significant impact on the relaxation properties of citrate providing sound evidence for ion-pairing processes. Furthermore, the possible structures of the NaCit2– ion-pair were analyzed studied via performing DFT calculations.

Hydration and ion-pairing properties of citrate in the presence of sodium ions

2019 Spring National Meeting

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