Calculation of the Solubility Diagrams in the System Ca(oh)2-h3po4-koh-hno3-co2-h2o

Equations previously developed and widely applied to the thermodynamic properties of strong electrolytes are extended to solutions involving a dissociation equilibrium. Excellent agreement is obtained with the data for pure phosphoric acid to 6M and for phosphate buffer solutions. The parameters of the strong electrolyte components of the buffer solutions are taken from other work, and the remaining parameters for H+, H2PO
 4
 −
 , and H3PO4 are evaluated, including a pK of 2.146. The present method avoids ambiguities which formerly arose in treating weak acids with as small pK as this.

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Thermodynamics of electrolytes. vi. weak electrolytes including h3po4

Phosphoric acid in water and heavy water has been studied by Raman and infrared spectroscopy over a broad concentration range (0.00873-1.560 mol kg(-1)) at 23 °C. The vibrational modes of the PO(4) skeleton (C(3v) symmetry) of H(3)PO(4)(aq) and D(3)PO(4)(D(2)O) have been assigned. In addition to the P-O stretching modes a deformation mode has been detected, δPO-H(D) at 1250 and 935 cm(-1), respectively. In addition to the modes of the phosphoric acid and heavy phosphoric acid a mode of the dissociation product H(2)PO(4)(-) and D(2)PO(4)(-) has been detected at 1077 cm(-1) and 1084 cm(-1) respectively. H(3)PO(4) and D(3)PO(4) is hydrated in aqueous solution which could be verified by Raman spectroscopy following the νP[double bond, length as m-dash]O and ν(s)P(OH)(3) mode as a function of temperature. These modes show a pronounced temperature dependence inasmuch as νP[double bond, length as m-dash]O shifts to higher wavenumbers with temperature increase and ν(s)P(OH)(3) to lower wavenumbers. In the range between 300-600 cm(-1) the deformation modes have been observed. In very dilute H(3)PO(4) solutions however, the dissociation product is the dominant species. The dissociation degree, α for H(3)PO(4)(aq) and D(3)PO(4)(D(2)O) as a function of dilution has been measured at 23 °C. In these dilute H(3)PO(4)(aq) and D(3)PO(4)(D(2)O) solutions no spectroscopic features for a dimeric species of the formula H(6)P(2)O(8) and D(6)P(2)O(8) could be detected. Quantitative Raman measurements have been carried out to follow the dissociation of H(3)PO(4) and D(3)PO(4) over a very broad concentration range and also as a function of temperature. From the dissociation data, the pK(1) value for H(3)PO(4) has been determined to 2.14(1) and for D(3)PO(4) to 2.42(1) at 23 °C. In the temperature interval from 24.5 to 99.7 °C the pK(1) values for H(3)PO(4)(aq) have been determined and thermodynamic data have been derived.

Raman- and infrared-spectroscopic investigations of dilute aqueous phosphoric acid solutions

Based on Coulomb's Law alone, electrostatic repulsion between two anions is expected to prevent their dimerization. Contrary to that idea, this Tutorial Review will present evidence showing that anion-anion dimers of protic hydroxyanions can form readily, and describe conditions that facilitate their formation. From X-ray crystal structures, we learn that hydroxyanions dimerize and oligomerize by overcoming long-range electrostatic opposition. Common examples are hydroxyanions of phosphate, sulfate, and carbonate, often in partnership with charged and neutral receptors. Short-range hydrogen bonds between anionic donors and acceptors are defined as anti-electrostatic hydrogen bonds (AEHBs) with insight from theoretical studies. While anion dimers are difficult to identify unequivocally in solution, these solution dimers have recently been definitively identified. The development of the supramolecular chemistry of anion-anion dimers has led to applications in hierarchical assemblies, such as supramolecular polymers and hydrogen bonded organic frameworks.

Recognition and applications of anion–anion dimers based on anti-electrostatic hydrogen bonds (AEHBs)

A cyclic pseudooctapeptide 2 is described containing 1,4-disubstituted 1,2,3-triazole moieties. This compound features eight converging hydrogen bond donors along the ring, namely four amide NH and four triazole CH groups, which enable 2 to engage in interactions with anions. While fully deprotonated sulfate anions exhibit only moderate affinity for 2, protonated anions such as dihydrogenpyrophosphate and dihydrogenphosphate anions are strongly bound. Complexation of the phosphate-derived anions involves sandwiching of a dihydrogenpyrophosphate dimer or a dihydrogenphosphate tetramer between two pseudopeptide rings. X-ray crystallography provided structural information, while 1H NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry demonstrated that these complexes are stable in solution (2.5 vol% water/DMSO) and can even be transferred without decomposition into the gas phase. The observed high thermodynamic stabilities are attributed to the mutual reinforcement of the interactions between the individual complex components, namely, hydrogen-bonding between the anions, multiple hydrogen bonding interactions between the anion aggregates and the triazole CH and NH hydrogen bond donors of 2, and potential dispersive interactions between the closely arranged pseudopeptide rings. Pseudopeptide 2 thus represents a promising lead for the construction of phosphate receptors, whose binding selectivity makes use of the unique ability of certain anions to assemble into higher aggregates.

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Efficient stabilisation of a dihydrogenphosphate tetramer and a dihydrogenpyrophosphate dimer by a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties.

The system NaH2PO4−NaClO4 has been studied from 0.15 to 7.0 mole-kg−1 at 25°C by the isopiestic method. The excess free energy of mixing is large and positive. The results confirm the formation of (H2PO4) 2 −2 dimers with a stoichiometric association constantKa=0.25±0.1 kg-mole−1 at 25°C.

Robert F. Platford

Papers

Free energies of aqueous mixtures of NaH2PO4 and NAClO4: Evidence for the species (H2PO4) 2 −2