Phosphoric acid ionization

Five of the acids listed in Table 13-1 are polyprotic acids, i.e., acids with more than one acidic H. For example, phosphoric acid ionizes in three steps ... 

It is usually true with polybasic acids that one hydrogen radical ionizes with greater facility than the remaining ones. Thus phosphoric acid ionizes primarily as a monobasic acid... 

The carbonic and phosphoric acid ionizations occur only slightly in water solutions. They are used here to illustrate the derivation of the formulas and names of the carbonate and phosphate ions, which are quite abundant from sources other than their parent acids. 

Nucleotides are phosphoric acid esters of nucleosides Those derived from adenosine of which adenosine 5 monophosphate (AMP) is but one example are especially promi nent AMP is a weak diprotic acid with s for ionization of 3 8 and 6 2 respectively In aqueous solution at pH 7 both OH groups of the P(0)(0H)2 unit are ionized... 

Acids which have more than one acidic hydrogen ionize in steps, as shown for phosphoric acid H3PO4 = H+ -f H2PO4- pTsTi = 2.148 = 7.11 X IQ- ... 

Properties. Phosphoric acid is a tribasic acid, in which the first hydrogen ion is strongly ionizing, the second moderately weak, and the third very weak. 

Phosphonic Acid and P(III) Derivatives. Phosphonic or phosphorous acid is a white dehquescent crystalline compound having a melting point of 73.6°C. As evidenced by its stmcture,HP(=0)(0H)2, phosphonic acid is dibasic. The first hydrogen is strongly ionized, pR 1.3—1.7, and... 

In Eq. (6-35), A/Z is the molar heat of ionization of the buffer acid at the conditions (temperature, solvent composition) of the kinetic studies. It happens that for many commonly used acidic buffers this quantity is small. Hamed and Owen give A//2 = —0.09 kcal/mol for acetic acid at 25°C, for example. The very important buffer of dihydrogen phosphate-monohydrogen phosphate is controlled by pK2 of phosphoric acid at 25°C its heat of ionization is —0.82 kcal/mol. 

The results for dinuclear and polynuclear oxoacids are also consistent with this interpretation. Thus for phosphoric acid, H4P2O7, the successive p/fa values are 1.5, 2.4, 6.6 and 9.2 the 10-fold decrease between p/f and p/f2 (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 less than the first stage and the differ-... 

The polymerization reaction starts hy protonating the olefin and forming a carhocation. For example, protonating propene gives isopropyl car-hocation. The proton is provided hy the ionization of phosphoric acid ... 

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 ... 

Phosphorous acid, 372 Phosphorus black, 365 boiling point, 374 chemistry of, 368 compounds, 102 heat of vaporization, 374 ionization energy, 268 melting point, 374 occurrence, 373 oxyacids, 371, 372 preparation, 374, 376 properties, 101 structure, 366 white, 120, 365, 366, 369 Photon, 254... 

Of special biochemical importance are the esters of monosaccharides with phosphoric acid. They are generally termed phosphates (e.g. glucose 6-phosphate). In biochemical use, the term phosphate indicates the phosphate residue regardless of the state of ionization or the counter ions. 

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... 

In the phosphoric acid fuel cell as currently practiced, a premium (hydrogen rich) hydrocarbon (e.g. methane) fuel is steam reformed to produce a hydrogen feedstock to the cell stack for direct (electrochemical) conversion to electrical energy. At the fuel electrode, hydrogen ionization is accomplished by use of a catalytic material (e.g. Pt, Pd, or Ru) to form solvated protons. 

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. 

For a number of reactions of cyclic di- and triesters of phosphoric acid, there are exchange data which can be rationalized on the assumption of trigonal bipyrami-dal intermediates which readily interconvert by pseudorotation. This constitutes a strong argument that at least these cyclic esters react by an associative mechanism and is suggestive evidence that simple trialkyl phosphates also react by this mechanism. The pH dependence of exocyclic versus endocyclic cleavage of methyl ethylene phosphate is readily interpreted in terms of the effect of ionization of the intermediate on the pseudorotation of these pentacoordinate intermediates. ... 

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. 

If the acid contains more than one ionizable hydrogen, the acid is known as a polyprotic acid. Specifically, diprotic acids, such as sulfuric acid, H2S04, contain two ionizable hydrogens and triprotics, such as phosphoric acid, H3P04, contain three ionizable hydrogens. 

In order to use the concept of normality, one must know the reaction involved. Phosphoric acid, H3PO4 undergoes three simultaneous ionizations ... 

Before we consider polyfunctional organic compounds, we should consider the inorganic acids sulfuric acid and phosphoric acid. Sulfuric acid is termed a dibasic acid, in that it has two ionizable groups, and phosphoric acid is a tribasic acid with three ionizable hydrogens. Thus, sulfuric acid has two pMa values and phosphoric acid has three. 

Enols and enolization feature prominently in some of the basic biochemical pathways (see Chapter 15). Biochemists will be familiar with the terminology enol as part of the name phosphoenolpyruvate, a metabolite of the glycolytic pathway. We shall here consider it in non-ionized form, i.e. phosphoenolpyruvic acid. As we have already noted (see Section 10.1), in the enolization between pyruvic acid and enolpyruvic acid, the equilibrium is likely to favour the keto form pyruvic acid very much. However, in phosphoenolpyruvic acid the enol hydroxyl is esterified with phosphoric acid (see Section 7.13.2), effectively freezing the enol form and preventing tautomerism back to the keto form. 

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). 

Here are three successive ionizations of phosphoric acid ... 

When sample components having ionizable groups are chromatographed the use of a background electrolyte and control of the eluent pH with an m>propridte buffer are mandatory. It is advisable to maintain a fairly high concentration of buffer in the medium in order to rapidly reestablish protonic equilibria and to thereby avoid peak sjditting or asymmetrical peaks due to slow kinetic processes. Acetic acid, phosphoric acid, and perchloric acid and their salts have been used for the control of pH. 

Thus, out of the three ionizable hydrogens in phosphoric acid, the first H+ is removed more easily than the second, and the second H" dissociates more easily than the third. When phosphoric acid is titrated with sodium hydroxide,it forms both acidic and basic salts ... 

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