Phosphoric acid ionization constant

Remember Thc strongest acids dissociate most readily. Of the 9 acids listed in Table 10-5, the strongest is sulfuric (1), with the highest acid ionization constant, and the weakest is phosphoric (3). 

Write the equation for the ionization reaction and the acid ionization constant expression for the third ionization of phosphoric acid in water. 

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 ... 

Several 1 -phosphates of deoxyfluoro sugars were prepared, and their acid-catalyzed hydrolysis was studied. 2-Deoxy-2-fluoro- (580), 3-deoxy-3-fluoro- (582), 4-deoxy-4-fluoro- (583), and 6-deoxy-6-fluoro-a-D-gluco-pyranosyl phosphates (584) were prepared by treatment of the corresponding per-( -acetylated )9-D-glucopyranoses with phosphoric acid [the p anomer (581) of 580 was prepared by a different method]. The first and second ionization constants (pA a, and pA a2) of these compounds were determined potentiometrically, as well as by the F-n.m.r. chemical shifts at a series of pH values, and then the rate constants of hydrolysis for neutral (B) and monoanion (C) were decided. The first-order rate-constants (k) for 580-584 and a-D-glucopyranosyl phosphate (in Af HCIO4,25 °) were 0.068, 0.175, 0.480, 0.270, 1.12, and 4.10 (all as x lOVs), respectively. The rate... 

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

IB (a) Provided [OH ] is not too high, the hydrogen phosphate ion is not expected to ionize in aqueous solution to a significant extent because of the quite small values for the second and third ionization constants of phosphoric acid. 

Phosphoric acid behaves as a triprotic acid, having three ionizable hydrogen atoms. The hydrogen ions are lost sequentially. The chart below provides the ionization constants and their logs. 

E. Blanc calculated for the ionization of constants of phosphorous acid Ki=0 05, and isT2—2 4 XlO-5. I. M. Kolthoff found that the first ionization constant Ky increases from 0 016 in 0 001 Jkf-soln. to 0 062 in O lJf-soln. and the second ionization constant K2, calculated from the H -ion cone, of mixtures of a secondary phosphite with hydrochloric acid is O O07 at 18°. The first stage of phosphorous acid titration may be conducted with methyl-orange as indicator using a comparison soln. with ps—Z 85. A sharp end-point for the second stage is obtained with thymolphthalein as indicator. For the anodic oxidation of the phosphites, vide infra, perphosphates. P. Pascal studied the magnetic properties. 

J. Kendall and co-workers also computed values of a from the cryoscopic data. Phosphoric acid is a comparatively weak acid which ionizes in three stages H3P04 u H -f H2P0 4 2H-fHP04", at moderate dilutions, and even at extreme dilutions it is but partially resolved HgPO -SH -fPO/". The first hydrogen ion has the strongest acidic function, the third the weakest. 6. A. Abbott and W. C. Bray have also measured the ionization constants of orthophosphoric acid and find at 18° ... 

In the previous problem, we expected some pH change, but in this problem we can assume that the pH is not changed by the addition of the very small amount of phosphoric acid mentioned because it is well buffered, meaning that it is extremely resistant to pH change. Then, if [H+] is fixed, the ratio of two of the desired concentrations can be calculated from each of the ionization constant equations. 

It should be noted that the values for the ionization constants of both hypophosphorous and phosphorous acids (each about 10 2) are very close to that for the first ionization constant of phosphoric acid. Using the principle suggested by Pauling (p. 87) one might infer that the number of nonhydrogenated oxygen atoms per molecule is the same for all three acids. If this wrere the case, hypophosphorous acid would be ... 

For polyprotic acids such as H3PO4 or H3As04, there is usually a factor of approximately 105 difference in successive Ka values. Phosphoric acid has dissociation constants that have the values Kal = 7.5 x 10-3, Ka2 = 6.2 x Itr8, and Ka3 = 1.0 x 10-12. This is because the first proton comes from a neutral molecule, the second from a -1 ion, and the third from a -2 ion. As a result of electrostatic attraction, it is energetically less favorable to remove H+ from species that are already negative. When considering the first and second ionization... 

Strong acids and bases react violently and produce heat and, hence, are not very useful in forming CBCs. Therefore, in the entire study of CBPCs, we will be dealing with phosphoric acid, which is a weak acid in that it dissolves in water slowly compared with strong acids such as hydrochloric acid. Acid phosphates such as KH2PO4 and A1(H2P04)3 and weak bases such as oxides of Mg and A1 dissolve slowly in water. To represent how slowly the weak acids or bases dissolve in water and to what extent, the term dissociation (ionization) constant is used. 

Dissociation Constants of Polybasic Acids Conductance Method.— A polybasic acid ionizes in stages, each stage having its own characteristic dissociation constant for example, the ionization of a tribasic acid HsA, such as phosphoric acid, may be represented by ... 

The ratio of successive ionization constants for a polybasic acid is usually about 10 , as in this case. We see that with respect to its first hydrogen phosphoric acid is a moderately strong acid—considerably stronger than acetic acid. With respect to its second hydrogen it is weak, and to its third very weak. 

Thus far we have considered only monoprotic weak acids. Acids that can furnish two or more hydronium ions per molecule are called polyprotic acids. The ionizations of polyprotic acids occur stepwise, that is, one proton at a time. An ionization constant expression can be written for each step, as the following example illustrates. Consider phosphoric acid as a typical polyprotic acid. It contains three acidic hydrogen atoms and ionizes in three steps. 

The results for dinuclear and polynuclear oxoacids are also consistent with this interpretation. Thus for phosphoric acid, H4P2O7, the successive pKa values are 1.5, 2.4, 6.6 and 9.2 the 10-fold decrease between pATi and pK2 (instead of a decrease of 10 ) is related to the fact that ionization occurs from two different PO4 units. The third stage ionization, however, is 10 less than the first stage and the difference between the mean of the first two and the last two ionization constants is 5 x 10 . 

We see that phosphoric acid is a weak polyprotic acid and that its ionization constants decrease markedly for the second and third stages. Thus we can predict that, in a solution containing phosphoric acid, the concentration of the nonionized acid is the highest, and the only other species present in significant concentrations are and H2PO4 ions. 

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 ... 

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. 

Tremaine and co-workers [487,488] have studied the variations of the ionization constants of 2-naphthol, boric acid, phosphoric acid, and acetic acid in H2O and D2O at high temperatures. The pK s of 2-naphthol, phos phoric acid, and boric acid passed through minima and then increased as the temperature increased from 25°C to 300°C. The ApK, values variec... 

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