Phosphoric acid dissociation equilibria

Mesmer RE, Baes CF (1974) Phosphoric acid dissociation equilibria in aqueous solutions to 3(X)°C. J Solution Chem 3 307-322... 

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

Step 1 Write the equation for the dissociation equilibrium of phosphoric acid in water. Then set up an ICE table. 

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 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. 

Since, phosphoric acid is a weak polyprotic acid that has three dissociation constants, four species (PO - , HPO, H2PO4, and H PO ) will coexist in equilibrium with one another although the concentration of some may be negligible at a particular pH. Therefore the dominating species at a particular pH will differ at different pHs. Figure 5-24 shows, the % of the ionized forms of phosphoric acid plotted against the pH. 

For example, at pH 2.0 the majority of the species of phosphoric acid exist as two forms H2PO4 (44 %) and H PO (66 %). The contributions from the hydrogen phosphate and phosphate are much less than 0.01 %. Therefore in this case only two species are present, H3PO4 and H2PO4 , and only one equilibrium may be considered. At pH 2.0 the dissociation of H3PO will make the predominant contribution to [H" ]. At higher pHs the problem becomes more complicated and is... 

A polyprotic acid has several acid constants, corresponding to dissociation of successive hydrogen ions. For phosphoric acid, H3PO4, there are three equilibrium expressions ... 

For this equilibrium, the second dissociation constant of phosphoric acid, Xj2, is close to 1 X 10 , or pX j = 7. Figure 2 shows the effect of adding acid or alkali on... 

By using the dissociation constant of protonated 2-phenyl benzimidazole = 5.9x 10 mol/l) and the first dissociation constant of phosphoric acid K = 6.9 H3PO4 X 10 mol/1), they obtained the equilibrium constant AT = 1.17 x 10. ... 

If the dissociation constant of the acid HA is very small, the anion A- will be removed from the solution to form the undissociated acid HA. Consequently more of the salt will pass into solution to replace the anions removed in this way, and this process will continue until equilibrium is established (i.e. until [M + ] x [A-] has become equal to the solubility product of MA) or, if sufficient hydrochloric acid is present, until the sparingly soluble salt has dissolved completely. Similar reasoning may be applied to salts of acids, such as phosphoric(V) acid (K1 = 7.5 x 10-3 mol L-1 K2 = 6.2 x 10-8 mol L-1 K3 = 5 x 10 13 mol L-1), oxalic acid (Kx = 5.9 x 10-2 mol L-K2 = 6.4 x 10-5molL-1), and arsenic)V) acid. Thus the solubility of, say, silver phosphate)V) in dilute nitric acid is due to the removal of the PO ion as... 

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. 

Care must be taken in evaluating the ionic strength contribution of weak electrolytes. For example, if any of the solutions above contained phosphoric or acetic acid also, the ionic strength would be essentially the same, because only the dissociated phosphoric or acetic acids contributes, and this is generally very small. If, on the other hand, H3PO4 is the only solute present, then an approximate equilibrium calculation must be carried out (see Example 3.3), an ionic strength calculated, and the process repeated until values of I remain constant. This may take one or two successive approximations. 

An excess of phenohc hydroxyl drives the equilibrium toward the dissociated state (Rxn. 11). An increase in temperature also drives the equifibrium toward dissociation. Thus, the phenoxide ion formed by the equifibrium may enable the ionic mechanism of Rxns 5 and 6 to propagate the epoxy cm e. The electrons in the outer shell of the phosphorous in TPP are more loosely held than those of the nitrogen in a corresponding tertiaiy amine. The nitrogen has 3 electrons in its 2p outer shell, whereas the phosphorous has 3 electrons in its 3p outer shell. This may draw the TPP into forming ionic complexes with less acidic phenols. Therefore, two conceivable means of creating an ionic species potentially capable of propagating the reaction for TPP-based systems have been demonstrated. 

Experimental data including the acidic species in the vapor phase within the above concentration range are scarce. Only very few publications of VLE data in that range are available [168, 173]. In contrast, numerous vapor pressure curves are accessible in literature. Chemical equilibrium data for the polycondensation and dissociation reaction in that range (>100 wt%) are so far not published [148]. However, a starting point to describe the vapor-Uquid equilibrium at those high concentratirMis is given by an EOS which is based on the fundamentals of the perturbation theory of Barker [212, 213]. Built on this theory, Sadowski et al. [214] have developed the PC-SAFT (Perturbed Chain Statistical Associated Fluid Theory) equation of state. The PC-SAFT EOS and its derivatives offer the ability to be fuUy predictive in combination with quantum mechanically based estimated parameters [215] and can therefore be used for systems without or with very little experimental data. Nevertheless, a model validation should be undertaken. Cameretti et al. [216] adopted the PC-SAFT EOS for electrolyte systems (ePC-SAFT), but the quality for weak electrolytes as phosphoric... 

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