THE PROCESS AND PRODUCTS OF INTERACTION IN THE PHOSPHORUS OXIDE–WATER SYSTEM

14 Norwegian Journal of development of the International Science No 166/2025 CHEMICAL SCIENCES THE PROCESS AND PRODUCTS OF INTERACTION IN THE PHOSPHORUS OXIDE–WATER SYSTEM Antraptseva N. Doctor of Chemical Sciences, Full Professor, Professor at the Department of General, Organic and Physical Chemistry National University of Life and Environmental Sciences of Ukraine, Kyiv Bila G. Candidate of Chemical Science, Docent, Ass. Prof. at the Department of Fat Technology and Chemical Technologies of Food Additives and Cosmetics National University of Food Technologies, Kyiv, Ukraine Melnyk N. Candidate of Technical Science, Docent, Ass. Prof. at the Department of Processes and Apparatus of Food Production National University of Food Technologies, Kyiv, Ukraine Tkachuk Yu. Lecturer, Department of Foreign Languages for Professional Purposes National University of Food Technologies, Kyiv, Ukraine https://doi.org/10.5281/zenodo.17352692 Abstract Experimental results have confirmed that, depending on the phosphorus concentration in the P₄O₁₀–H₂O system, condensed phosphoric acids with various anionic structures are formed – either linear (with a degree of polycondensation n = 2–9) or cyclic (n = 4). The duration of the degradation process in the P₄O₁₀–H₂O system has been evaluated. A contemporary scheme for the hydration process of P₄O₁₀ has been proposed. Keywords: interaction, hydration, degradation, anion structure, condensed phosphoric acids, linear, cyclic. 1. Introduction Due to the inherent anomalies of water, the mechanisms of many chemical reactions involving it are complex and remain insufficiently understood. In recent years, the understanding of the stepwise nature and mechanisms of dehydration reactions of numerous inorganic salts has undergone fundamental revision [1, 2]. Equally complex are the processes associated with the hydration reactions of inorganic compounds. The interaction of H₂O with highly hygroscopic substances such as TiCl₄ or P₄O₁₀ is accompanied by intricate chemical transformations. However, the description of the sequential stages of these typically multistep processes remains poorly substantiated. Works devoted to the study of phosphorus(V) oxide hydration [3, 4] describe the assessment of the composition of the condensed phosphoric acids formed and its dependence on the experimental conditions. However, due to experimental difficulties associated with determining the composition of phosphoric acids, they provide contradictory data on the sequence of their formation. The kinetic features of this process have hardly been studied. According to the established literature [3, 4], the initial stage of P₄O₁₀ hydration involves the addition of water to phosphorus(V) oxide, leading to the formation of ultraphosphoric acid with the composition H₂P₄O₁₁. Further hydration results in the formation of two forms of cyclotetraphosphoric acid with identical compositions, H₄P₄O₁₂ – the “anhydrous” and the “normal” forms. Later, the authors concluded that the structure of cyclotetraphosphoric acid, in which the hydroxyl groups are evenly distributed among the phosphorus atoms, is more favorable. According to [4], the hydration process of P₄O₁₀ can be represented by the following scheme: An alternative mechanism for the hydration of phosphorus(V) oxide was proposed in [5]. In this model, ultraphosphoric acid (H₂P₄O₁₁) undergoes further hydration, yielding two reaction products – cyclotetraphosphoric and isocyclotriphosphoric acids – both possessing the same overall chemical composition, H₄P₄O₁₂. Subsequent hydration and degradation of each of these cyclic acids lead to the formation of linear polyphosphoric acids, with the maximum number of phosphorus atoms in the chain not exceeding four (tetraphosphoric acid, H₆P₄O₁₃). However, the above-described hydration schemes of P₄O₁₀ do not take into account the possible formation Norwegian Journal of development of the International Science No 166/2025 15 of condensed phosphoric acids containing more than four phosphorus atoms per chain, despite the well-documented existence of highly polymerized phosphoric acids with chain lengths of at least 9–10 phosphorus atoms [1, 2, 6]. The objective of the present work is to investigate the chemical transformations accompanying the interaction within the P₄O₁₀–H₂O system and to clarify the composition of the resulting phosphoric acids. 2. Experimental Procedure Since the interaction between phosphorus(V) oxide and water is an exothermic process, an aqueous solution of monophosphoric acid (H₃PO₄) of analytical grade (“pure for analysis,” containing 63% phosphorus, calculated as P₂O₅) was used as the hydrating medium instead of pure water. Phosphorus(V) oxide (P₄O₁₀) was gradually added to the acid solution in small portions under constant stirring. The reaction mixture was cooled externally with an ice–water bath. The amount of P₄O₁₀ added was calculated so as to obtain a series of mixtures with different total phosphorus concentrations. Each prepared mixture was then transferred into a desiccator and kept at room temperature with out water vapor of air. To evaluate the kinetics of the hydration process, samples were taken from the reaction mixture at specified time intervals, neutralized with a cooled aqueous ammonia solution, and analyzed. Given the presence of high–molecular–weight acids in the samples, the total phosphorus content was determined gravimetrically using the quinolinium molybdate method (relative error ±0.2%) after their complete conversion to monophosphoric acid. Complete hydrolytic cleavage of the P–O–P bonds, as established in a separate series of experiments, occurred upon heating the aqueous acid solutions at 80–90° C for three hours. The anionic composition of the condensed phosphoric acids was determined by quantitative chromatographic separation, following a procedure analogous to that described in [2]. The method for determining the anionic composition of the acids was refined in a separate series of experiments. Model samples of monoand polyphosphoric acids (including pentaphosphoric acid) of known anionic composition were prepared for this purpose. The concentration of the phosphate anion in the solution applied to the chromatographic origin was maintained within 18–20 µg (as P₂O₅) per 0.1 ml of solution. As the mobile phase for anion separation, an acidic mixture consisting of 58 ml of acetone, 25 ml of a 20% trichloroacetic acid solution, and 17 ml of water was employed. To prevent degradation, all analyses were performed in a refrigerated chamber at 277–278 K. Quantitative determination of phosphorus in each condensed form was carried out gravimetrically using the quinolinium molybdate method after “wet combustion” in perchloric acid (HClO₄). The results of the anionic composition analysis of the phosphoric acids contained in the control samples are presented in Table 1. Table 1. Anionic composition of polyphosphoric acids in model samples Composition of control sample of polyphosphoric acids, P₂O₅, wt% Determined from the analysis results* Determination error Relative, % Mean square error Total content, 13.74 13.31 3.13 3.41 including: mono4.33 4.26 1.62 0.77 di3.14 3.00 4.46 0.93 tri2.47 2.41 2.43 0.51 tetra3.38 2.27 3.78 0.71 penta-. 1.42 1.37 3.52 0.37 * Average of five parallel determinations. The results of the anionic composition analysis of the model monoand polyphosphoric acid samples indicate that the application of paper chromatography with quantitative evaluation of each polymeric anion provides reliable and reproducible data. To further identify the composition of the polyphosphoric acids, the ³¹P NMR spectra were recorded using a Bruker WP-80 spectrometer, following the methodology described in [7]. 3. Results and Discussion The results of evaluating the duration of the hydration process of the initial P₄O₁₀ and the high–molecular–weight phosphoric acids formed as intermediate products of its degradation in an aqueous solution of monophosphoric acid are presented in Table 2. According to the obtained data, an increase in the duration of hydration of the prepared polyphosphoric acid mixture leads to a gradual decrease in the number of distinct acids present in the composition. Specifically, in a mixture of polyphosphoric acids containing 65.0% total P₂O₅, eight different polyphosphoric acids were initially identified. Three hours after preparation, the mixture contained only six phosphoric acids instead of eight: H₈P₆O₁₉, H₇P₅O₁₆, H₆P₄O₁₃, H₅P₃O₁₀, H₄P₂O₇, and H₃PO₄. Nine hours later, the number of polyphosphoric acids in the mixture decreased to four. The final (and sole) product of hydration was identified as monophosphoric acid, H₃PO₄ (Table 2). 16 Norwegian Journal of development of the International Science No 166/2025 Table 2. Dependence of the number of phosphorus atoms in the acids formed in the P₄O₁₀–H₂O system on the duration of degradation Phosphorus concentration (calculated as total P₂O₅), % Duration of P–O–P bond degradation, hours Equilibrium state of the P₄O₁₀–H₂O system 0.17 1.5 3.0 9.0 20.0 36.0 48.0 65.0 8 7 6 4 3 1 1 1 70.6 8 7 6 5 4 3 2 2 75.1 9 8 8 7 5 4 3 3 77.0 9 8 7 6 5 4 4 4 78.5 ≥9 8 8 7 6 6 5 5 80.5 ≥9 ≥8 8 8 7 7 6 7 82.7 ≥9 ≥9 ≥9 ≥9 9 8 9 9 Experimental data indicate that the hydration and degradation processes were completed within two days after the preparation of the mixture (Table 2). The maximum degree of polymerization of the phosphate anion observed in all experiments was found to be proportional to the total phosphorus content of the mixture. According to the results of chromatographic studies, the composition of polyphosphoric acids with linear anion structures, in addition to low–molecular– weight species, includes relatively high–molecular– weight acids of compositions H₈P₆O₁₉, H₉P₇O₂₂, and H₁₀P₈O₂₅. The intense coloration observed in the region of the chromatogram close to the origin (τ = 0.1 and 1.5 h) indicates the presence in the freshly prepared mixture of unresolved condensed phosphoric acids containing more than 9–10 phosphorus atoms in the chain. An increase in the total phosphorus concentration in the prepared mixture leads to a rise not only in the number of linear polyphosphoric acids but also in cyclic ones. For example, in a mixture with a total phosphorus concentration of 82.7% P₂O₅, cyclotri– and cyclotetraphosphoric acids were identified. NMR spectroscopy of phosphoric acids of varying concentration revealed that almost all spectra contained three distinct signals. The signal located in the highest field was attributed to phosphorus atoms situated in the middle of the –P–O–P– chain. The signal recorded in the lowest field was assigned to phosphorus atoms of monophosphoric acid. The third signal was attributed to the phosphorus atoms of the terminal groups of the – P–O–P– polyphosphoric chain. As the concentration of phosphoric acid increased from 72.4% total P₂O₅ to 85.0%, the intensity of the signal corresponding to phosphorus atoms of monophosphoric acid decreased almost to zero. Conversely, the intensity of the signal of phosphorus atoms of the middle groups increased from zero to a maximum value. The intensity of the signals from phosphorus atoms of the terminal groups initially increased, reached a maximum at 79.4% total P₂O₅, and then decreased with further increases in acid concentration. Summarizing the experimental data obtained in the determination of the composition of condensed phosphoric acids formed during the hydration of P₄O₁₀, it can be concluded that the mechanism of this process is significantly more complex than that proposed by the authors [3–5]. The first and second stages of the interaction between P₄O₁₀ and H₂O proceed sequentially with the initial formation of ultraphosphoric acid of composition H₂P₄O₁₁, which is subsequently converted into cyclotetraphosphoric acid, H₄P₄O₁₂. In this process, the original ring structure of phosphorus(V) oxide remains intact. The destruction of the ring occurs at the third stage of the process, when cyclotetraphosphoric acid (H₄P₄O₁₂) is transformed into tetraphosphoric acid, H₆P₄O₁₃, possessing a linear anionic structure. According to our experimental data, the subsequent behaviour of tetraphosphoric acid is dual. On one hand, it undergoes further hydration and degradation of the –P–O–P– bonds, simplifying its anionic composition: Н6Р4О13 + Н2О = Н5Р3О10 + Н3РО4 (1) Н5Р3О10 + Н2О = Н4Р2О7 + Н3РО4 (2) Н4Р2О7 + Н2О = 2Н3РО4 (3) On the other hand, tetraphosphoric acid participates in condensation reactions, leading to the formation of higher–polymer linear phosphoric acids, which can be schematically represented as: 5Н6Р4О13 = 4Н7Р5О16 + Н2О (4) 6Н7Р5О16 = 5Н8Р6О19 + Н2О (5) 7Н8Р6О19 = 6Н9Р7О22 + Н2О (6) or, in general form: (n+1)Нn+2РnО3n+1 = nНn+3Рn+1О3n+4 + Н2О (7) The sequence of the main stages of the interaction process in the phosphorus(V) oxide–water system can be represented by the following general scheme: Norwegian Journal of development of the International Science No 166/2025 17 Thus, the formation of highpolymer linear phosphoric acids observed based on NMR and paper chromatography data in the initial stage of hydration of phosphorus(V) oxide is explained by the polycondensation of tetraphosphoric acid. The existence of cyclic phosphoric acids in the P₄O₁₀–H₂O system has been described in [1] and is experimentally confirmed in the present work. Their formation from polyphosphoric acids with linear anion structures is most likely the result of dehydration of the latter. The cyclization of two terminal – PO₂H₂ groups of a linear acid leads to the formation of cyclic penta-, hexa-, and higher condensed acids according to the following scheme. In other words, dehydration reactions of linear polyphosphoric acids result in the rearrangement of the linear anion into a cyclic structure: Н5Р3О10 = Н3Р3О9 + Н2О (8) Н6Р4О13 = Н4Р4О12 + Н2О (9) Н7Р5О16 = Н5Р5О15 + Н2О (10) Нn+2РnО3n+1 = НnРnО3n + Н2О (11) It should be noted that the likelihood of the formation of acids with a cyclic anion structure from linear polyphosphoric acids according to equations 8–11 depends on their composition. The longer the chain of the linear acid, the less probable the formation of cyclic acids due to steric hindrance in the approach of terminal groups of the linear acid. In this regard, it should be considered that with an increase in the chain length of linear acids, the formation of not cyclic polyphosphoric acids but branched or conjugated isopolyphosphoric acids of the general formula becomes more probable НnRРnОn(5+R)/2, where R = Н2О/Р2О5. Thus, the present study proposes for the first time an experimentally substantiated complete scheme of the real sequence of the main stages of phosphorus(V) oxide hydration. It has been shown that the duration of reaching the equilibrium state in the P₄O₁₀–H₂O system does not exceed 48 hours. 4. Conclusions It has been experimentally confirmed that, depending on the phosphorus components in the P4O10 – H2O system, condensed phosphate acids with different anion structures occur – linear (with a degree of polycondensation n = 2–9) or cyclic (with n = 4). For the first time, it has been substantiated that the direct precursor for the formation of linear polyphosphoric acids is cyclotetraphosphoric acid – an intermediate product of phosphorus(V) oxide hydration. The origin of condensed phosphoric acids with cyclic anion structures is secondary. They are formed as a result of the condensation of the corresponding polyphosphoric acids with linear anion structures. It has been demonstrated that the duration of attaining equilibrium in the P₄O₁₀–H₂O system does not exceed 48 hours. A modern scheme of the P₄O₁₀ hydration process has been proposed. References: 1. Shchegrov L. Phosphates of divalent metals. – K. : Nаuk. dumka, 1987. – 216 р. 2. Antraptseva N.M., Solod N.V. Solid solutions and double phosphates of bivalent metals. –K : Center of polygraphy КОМPRINТ, 2018. – 342 р. 3. Salmon J.E., Теггеу Н.Т. The System Zinc Oxide-Phosphoric Oxide-Water and Manganese Oxide- 18 Norwegian Journal of development of the International Science No 166/2025 Phosphoric Oxide-Water at Temperatures between 25°C and 100°C / J. Chem. Soc. 1990. № 10. P. 2813– 2824. 4. Acton A.Q. Phosphates – advances in research and application / A.Q.Acton. – Atlanta, Georgia: Scholarly Editions, 2013. – 374 p. 5. Konstant Z.A., Dindune A.P. Phosphates of divalent metals. – Riga: Zinаtne, 1997.– 371 p. 6. Antraptseva N., Solod N., Kravchenko O. Influence of crystal hydrate water on the process and products of heat treatment of magnesium-manganese(II) of dihydrogen phosphates / Functional materials. 2020. 27(4). Р. 820–826. 7. Kawabe M., Ohashi O., Yamaguchi J. NMR of phosphorus in polyphosphates and determination of their hydrolysis rate constants / Bull. Chem. Soc. Jap. 2020. 43. N 12. P. 3705-3710.