Speciation of phytate ion in aqueous solution. Alkali metal complex formation in different ionic media

Abstract

The acid-base properties of phytic acid [ myo-inositol 1,2,3,4,5,6-hexakis(dihydrogen phosphate)] (H(12)Phy; Phy(12-)=phytate anion) were studied in aqueous solution by potentiometric measurements ([H+]-glass electrode) in lithium and potassium chloride aqueous media at different ionic strengths (0< I mol L(-1)< or =3) and at t=25 degrees C. The protonation of phytate proved strongly dependent on both ionic medium and ionic strength. The protonation constants obtained in alkali metal chlorides are considerably lower than the corresponding ones obtained in a previous paper in tetraethylammonium iodide (Et(4)NI; e.g., at I=0.5 mol L(-1), log K(3)(H)=11.7, 8.0, 9.1, and 9.1 in Et(4)NI, LiCl, NaCl and KCl, respectively; the protonation constants in Et(4)NI and NaCl were already reported), owing to the strong interactions occurring between the phytate and alkaline cations present in the background salt. We explained this in terms of complex formation between phytate and alkali metal ions. Experimental evidence allows us to consider the formation of 13 mixed proton-metal-ligand complexes, M(j)H(i)Phy((12-i-j)-), (M+ =Li+, Na+, K+), with j< or =7 and i< or =6, in the range 2.5< or =pH< or =10 (some measurements, at low ionic strength, were extended to pH=11). In particular, all the species formed are negatively charged: i+j-12=-5, -6. Very high formation percentages of M+-phytate species are observed in all the pH ranges investigated. The stability of alkali metal complexes follows the trend Li+ > or =Na+K+. Some measurements were also performed at constant ionic strength (I=0.5 mol L(-1)), using different mixtures of Et(4)NI and alkali metal chlorides, in order to confirm the formation of hypothesized and calculated metal-proton-ligand complex species and to obtain conditional protonation constants in these multi-component ionic media.

Potentiometric Determination of Phytic Acid and Investigations of Phytate Interactions with Some Metal Ions

“…The solution of this complex task was found in the interaction between phytate and metallic ions and formation of a relatively strong complexes of the alkaline cations with phytates. 14,[24][25][26] As mentioned above sodium ions significantly increase acidity of phytate due to complexation (Fig. 2, curve a), but at this low concentration of titrant (low ionic strength, I), complete deprotonation to its final 12 th step does not occur.…”

“…This effect is known well and is due to the complexation of Na + ions with phytate as phytic acid obtains an enhanced acidic character in the presence of alkali ions due to exchange of protons with Na + ions. 14,[24][25][26] However, in both cases only two EPs (EP 1 at pH 5.3 and EP 2 at pH 8.5-9.5) can be determined unambiguously. Further from the course of the titration curves is evident that phytic acid contains three categories of protons.…”

Sequestering ability of phytate towards protonated BPEI and other polyammonium cations in aqueous solution

“…In this study, we observed that phytate forms fairly strong mixed (proton-amine) complexes, in particular when the charges of reactants involved in the formation equilibrium are quite high. In this order, here we investigated the interactions of phytate with an high molecular weight polyammonium cation (MW = 750 kDa), such as BPEI; the interactions were studied at low ionic strength values and without addition of supporting electrolyte, owing to the strong interference that phytate anion may undergo with alkali and alkaline earth metal cations [43,44]. Table 3 reports the overall complex formation constants obtained [equation (1)], whilst the equilibrium constants, according to the reaction (2), are reported in Table 4.…”

“…In considering these formation constants and formation percentages, one must take into account that they were obtained in absence of an interacting supporting electrolyte such as NaCl, which is present in all natural and biological fluids. This is particularly important if we take into account that phytate forms stable complexes with alkali metals [28,43], and this interaction may change significantly the speciation of the system. The interaction of a polyammonium cation with phytate is dependent on the M A N U S C R I P T…”

Speciation of Phytate Ion in Aqueous Solution. Trimethyltin(IV) Interactions in Self Medium

“…In fact, in our previous papers we thoroughly described the peculiarities of this molecule from a chemical, biological and environmental point of view, and we stressed several times that, despite the huge number of literature on phytate and its derivatives [see, for example, references in previous contributions [1][2][3][4][5][6][7][8][9] and in reviews [10][11][12][13] ], few quantitative data are presented on the modeling of their thermodynamic behavior in aqueous media. In this light, in our latest works we paid particular attention to the phytate speciation study in presence of heavy metal and organometal cations, such as mercury(II), 7 cadmium(II), 9 and dimethyltin(IV) cations.…”