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

Abstract

Dedicated to the memory of Prof. Dr. Jurij Bren~ià bstract Determination of correct amount (concentration) of phytic acid is of vital importance when dealing with protonation and/or metal complexation equilibria. A novel approach for precise and reliable assay of phytic acid, based on the difference between end points by potentiometric titration, has been presented. Twelve phytic acid protons are classified into three groups of acidity, which enables detection of 2 to 3 distinct equivalent points (EPs) depending on experimental conditions, e.g. counter-ion concentration. Using the differences between individual EPs enables correct phytate determination as well as identification of potential contamination and/or determination of initial protonation degree. Impact of uncertainty of phytate amount on the calculation of protonation constants has been evaluated using computer simulation program (Hyperquad2013). With the analysis of titration curves different binding sites on phytate ligand have been proposed for complexation of Ca 2+ and Fe 3+ ions.

Complex Formation of Phytic Acid With Selected Monovalent and Divalent Metals

“…where j represents the molar ratio between the added titrant and the phytate: j = n (NaOH)/ n (Phy), and can also be expressed as deprotonation step number. As reported before (Marini et al, 1981 ; Evans et al, 1982 ; Veiga et al, 2014 ; Marolt and Pihlar, 2015 ), the titration curve of phytic acid exhibits relatively complicated characteristics due to the high number of (de)protonation steps, and usually only some of them (2–4 deprotonation steps, depending on experimental conditions) are expressed as distinct EPs because of small differences between their corresponding K i H values and consequently smaller pH steps for given EPs. Therefore, the use of titration curve derivatives ∂pH/∂ n (NaOH) is necessary for the determination of EPs.…”

“…During the ion exchange procedure the column flow was set to 0.25 mL/min and the total contact time between phytate and resin was 5 h. An additional extension of the contact time (using a reduced flow) did not increase the protonation level of phytate, which was typically 11.5 ± 0.2. A differential alkalimetric standardization approach introduced by Marolt and Pihlar ( 2015 ) was used to accurately determine the phytic acid concentration (amount). The potassium, sodium, calcium and magnesium concentrations in the eluate were analyzed with an atomic absorption spectrometer (Varian AA240), and were all below 2.0 × 10 −6 M after single ion exchange procedure.…”

“…These important (biological) properties and effects arise mostly from electrostatic interactions between negatively charged phytate species and metal cations and ability for their chelation (Crea et al, 2008 ). They have been studied by many researchers using various analytical techniques, including potentiometry (Torres et al, 2005 ; Šala et al, 2011 ; Marolt and Pihlar, 2015 ), spectrophotometry (Heighton et al, 2008 ), and NMR (Bebot-Brigaud et al, 1999 ) in combination with computational data analysis (Torres et al, 2008 ), which has led to numerous series of papers reporting the thermodynamic protonation and formation stability constants of phytic acid and its corresponding metal complexes (De Stefano et al, 2006 ; Crea et al, 2008 ). However, many useful information can be derived from these data, but it has been recently shown that a particular attention should be given to the precise standardization of phytic acid prior to measurements, since even a relatively small uncertainty in determination of concentration can cause significant deviations of the derived stability constants using computational methods (Marolt and Pihlar, 2015 ).…”

“…They have been studied by many researchers using various analytical techniques, including potentiometry (Torres et al, 2005 ; Šala et al, 2011 ; Marolt and Pihlar, 2015 ), spectrophotometry (Heighton et al, 2008 ), and NMR (Bebot-Brigaud et al, 1999 ) in combination with computational data analysis (Torres et al, 2008 ), which has led to numerous series of papers reporting the thermodynamic protonation and formation stability constants of phytic acid and its corresponding metal complexes (De Stefano et al, 2006 ; Crea et al, 2008 ). However, many useful information can be derived from these data, but it has been recently shown that a particular attention should be given to the precise standardization of phytic acid prior to measurements, since even a relatively small uncertainty in determination of concentration can cause significant deviations of the derived stability constants using computational methods (Marolt and Pihlar, 2015 ). As a supplement to the thermodynamic data, it has been shown that the detailed analysis of the titration curves provides additional information on the formation of metal complexes, particularly when other processes, such as precipitation and metal hydroxide formation, accompany the investigated complexation reactions (Šala et al, 2011 ; Marolt and Pihlar, 2015 ).…”

“…However, many useful information can be derived from these data, but it has been recently shown that a particular attention should be given to the precise standardization of phytic acid prior to measurements, since even a relatively small uncertainty in determination of concentration can cause significant deviations of the derived stability constants using computational methods (Marolt and Pihlar, 2015 ). As a supplement to the thermodynamic data, it has been shown that the detailed analysis of the titration curves provides additional information on the formation of metal complexes, particularly when other processes, such as precipitation and metal hydroxide formation, accompany the investigated complexation reactions (Šala et al, 2011 ; Marolt and Pihlar, 2015 ).…”