Phosphoric acid equilibrium constant

This reaction shows that the hydrated oxide SiCV-xHjO is acidic, since it reacts with a base-As we mentioned earlier, phosphorus can be found in four different oxidation states. The hydroxides of the +1, +3, and +5 states of phosphorus are hypophosphorous acid, H3P02, phosphorous acid, H3P03, and phosphoric acid, H3P04. Their structures are shown in Figure 20-4. As suggested by their names, these compounds are distinctly acidic, and are of moderate strength. The equilibrium constant for the first ionization of each acid is approximately 10-2 hypophosphorous acid ... 

TABLE 1.7. Equilibrium constants for addition or elimination from phosphoric acid esters. ... 

By methods analogous to those used for the tetrahedral intermediates related to carboxylic acid derivatives, Guthrie proceeded from the heat of formation of pentaeth-oxyphosphorane to free energies of the P(OEt) (OH)5 species. °° This allowed the calculation of the equilibrium constants for addition of water or hydroxide to simple alkyl esters of phosphoric acid see Table 1.7. 

The fact that the three equilibrium constants are well separated in phosphoric acid is not relevant. The spreadsheet can deal automatically with any 3-protic acid. Figure 3-20 is the result of replacing the logarithms of the protonation constants in the cells B4 D4 with the values for citric acid (6.4, 4.8, 3.1) ... 

Equilibrium constants for ionization reactions are usually called ionization or dissociation constants, often designated Ka. The dissociation constants of some acids are given in Figure 2-16. Stronger acids, such as phosphoric and carbonic acids, have larger dissociation constants weaker acids, such as monohydrogen phosphate (Ill Of ), have smaller dissociation constants. 

H3PO4 (phosphoric acid)). The dissociation reactions for each pair are shown where they occur along a pH gradient. The equilibrium or dissociation constant (fCa) and its negative logarithm, the p/Ca, are shown for each reaction. 

The equilibrium constant, K , is called hnc stria dissociation constant. Similarly for a polyprotic acid (i.e. phosphoric acid), the equilibrium reactions are f ... 

Nickel and cobalt often occur with copper, and must be separated in pure form from hydrometallurgical leach liquors. Organic acid extractants can quite readily separate copper from cobalt and nickel, but the separation of cobalt from nickel is rather difficult. In one Ni/Co separation process, di-2-ethyl hexyl phosphoric acid (D2EHPA) is used as extractant, with strict control of the pH of the aqueous phase to take full advantage of the slightly different equilibrium constants for the Co and Ni reactions. Pulsed column contactors are used rather than mixer-settlers, and nickel impurity is removed from the loaded organic phase by scrubbing it with a cobalt-rich phase. 

Inorganic phosphate ligands are important with respect to the behavior of actinides in the environment and as potential waste forms. There have been a number of experimental studies to determine the equilibrium constants in the actinide-phosphoric acid system, but they have been complicated by the formation of relatively insoluble solid phases and the formation of ternary actinide complexes in solution. 

The less protonated species are relatively insignificant. Crutchfield" " usedan expression which did not include the first term in (3) and consequently achieved best fit with different values of the rate coefficients to those reported by Goh et al. . The equilibrium constants, K, K2, for peroxodiphosphoric acid have been estimated as 2.0 and 0.3 by consideration of the corresponding values for hypo-phosphoric and pyrophosphoric acids. The values of the rate coefficients and the mechanistic interpretation of the kinetics are thus somewhat uncertain. Some data in terms of (3) has been published (Table 20). The activation energies for the overall process vary" from 18.3 (pH 0) to 28 kcal.mole (pH 3). 

The pH of polyfunctional systems, such as phosphoric acid or sodium carbonate, can be computed rigorously through use of the systematic approach to multiequilibrium problems described in Chapter 11. Solution of the several simultaneous equations that are involved is difficult and time consuming, however. Fortunately, simplifying assumptions can be invoked when the successive equilibrium constants for the acid (or base) differ by a factor of about 10 (or more). With one exception, these assumptions make it possible to compute pH data for titration curves by the techniques we have discussed in earlier chapters. 

The application of standard electrode potential data to many systems of interest in analytical chemistry is further complicated by association, dissociation, complex formation, and solvolysis equilibria involving the species that appear in the Nemst equation. These phenomena can be taken into account only if their existence is known and appropriate equilibrium constants are available. More often than not, neither of these requirements is met and significant discrepancies arise as a consequence. For example, the presence of 1 M hydrochloric acid in the iron(Il)/iron(llI) mixture we have just discussed leads to a measured potential of + 0.70 V in 1 M sulfuric acid, a potential of -I- 0.68 V is observed and in 2 M phosphoric acid, the potential is + 0.46 V. In each of these cases, the iron(II)/iron(III) activity ratio is larger because the complexes of iron(III) with chloride, sulfate, and phosphate ions are more stable than those of iron(II) thus, the ratio of the species concentrations, [Fe ]/[Fe ], in the Nemst equation is greater than unity and the measured potential is less than the standard potential. If fomnation constants for these complexes were available, it would be possible to make appropriate corrections. Unfortunately, such data are often not available, or, if they are, they are not very reliable. 

In a very early study Patat (1945) investigated the hydrolysis of aniline to phenol in a water-based acidic solution in near-critical and supercritical water (Tc = 374.2°C, Pc = 220.5 bar). Phosphoric acid and its salts are used as the catalyst for this reaction. The reaction proceeds extremely slowly under normal conditions and reaches equilibrium at low conversion levels. For these reasons, Patat chooses to study the reaction in supercritical water to temperatures of 450°C and to pressures of 700 bar in a flow reactor. He finds that the reaction follows known, regular kinetics in the entire temperature and pressure space studied and the activation energy of the hydrolysis (approximately 40 kcal/mol) is the same in the supercritical as well as in the subcritical water. He suggests that the reaction is catalyzed by hydrogen ions formed from dissolution of phosphoric acid in supercritical steam. Very small amounts of phosphoric acid and the salts of the phosphoric acid are dissolved in the supercritical steam and are split into ions. Patat lists several dissolution constants for primary ammonium phosphates in supercritical steam. In this instance, the reaction performance is improved when the reaction is operated homogeneously in the mixture critical region and, thus, in intimate contact between the reactants and the catalyst. 

Many acids or bases are polyfunctional, that is, have more than one ionizable proton or hydroxide ion. These substances ionize stepwise, and an equilibrium constant can be written for each step. Consider, for example, the ionization of phosphoric acid ... 

Another way of determining whether the observed plateau does really correspond to [M]e is to perform polymerization at various ratios of [M to [I]0. The properly determined value of [M]e should be independent of this ratio, provided that [M]o/[I]o is not too small. Thus, for non-living systems it is necessary to carry out polymerizations with increasing initial initiator concentration until a constant ultimate monomer conversion is reached. This method of approaching the equilibrium concentration gave reliable thermodynamic parameters for the cationic polymerization of cyclic esters of phosphoric acid, in spite of termination observed in these systems 11 ... 

We now consider Fe hydrolysis. The hexaaquaflFerric cation[Fe(H20)e] is more acid than hexaaquaferrous cation [Fe(H20)g]. The equilibrium constant of hydrolysis is approximately one order lower than that in phosphoric acid, whereas the equilibrium constant of the hydrolysis of Fe " is approximately one order higher than that in boric add. During the hydrolysis the following essentially mononuclear complexes are produced [FeOH] ", [Fe(OH)2]" , [Fe(OH)3(aq)]° and [Fe(OH)4]. By other reactions a series of polynuclear complexes is formed, for example, [Fe2(OH)2], [Fe3(OH)4] , [Fe4(OH)g] , etc. (for simplicity, the coordinated water molecules are omitted). First, colloid hydroxo complexes are formed and finally there is a precipitate of hydrated ferric oxide which is in fact a mixture of different polynuclear complexes. The distribution of polynudear complexes depends not only on pH, but also on the initial concentration of iron. In diluted solutions of ferric salts a precipitate of hydrated Fe203 is separated only at a higher pH. The equilibrium between particular polynuclear complexes is established only very slowly. 

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