Phosphoric acid dissociable protons

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

A solution containing 0.20 M H3POj, phosphorous acid, is tested with indicators and the H +(aq) concentration is found to be 5.0 X 10 2 M. Calculate the dissociation constant of H3PO3, assuming that a second proton cannot be removed. 

Protons are mainly derived from two sources—free acids in the diet and sulfur-containing amino acids. Acids taken up with food— e.g., citric acid, ascorbic acid, and phosphoric acid—already release protons in the alkaline pH of the intestinal tract. More important for proton balance, however, are the amino acids methionine and cysteine, which arise from protein degradation in the cells. Their S atoms are oxidized in the liver to form sulfuric acid, which supplies protons by dissociation into sulfate. 

Numerous acids have several functional groups capable of releasing protons (Table 3.1). Taking phosphoric acid (H3P04) as an example, we see that three protons can be released. Each proton has its own typical pK, as shown in the titration curve in Figure 3.2. Other titration curves are also shown for comparison purposes. The dissociation of phosphoric acid can be represented as... 

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... 

Phosphate Esters. An ester is formed by elimination of H20 and formation of a linkage between an acid and an alcohol (or phenol) (Fig. III-22). Phosphomonoesters, especially of monosaccharides, are very common (Fig. ffl-23). Because phosphoric acid is a tribasic acid, it can also form di- and triesters (Fig. III-24). Phosphotriesters are rarely found in nature, but diesters are extremely important, particularly as the fundamental linkage of the nucleic acid polymers, which are sequences of ri-bose (or deoxyribose) units linked by 3 —> 5 phos-phodiester bonds (see Fig. III-25). Like phosphoric acid, which has three dissociable protons (Fig. III-26), phosphomono- and phosphodiesters are acidic and typically ionize as shown in Fig. HI-27. Note the similarities between the pvalues for... 

For example, consider phosphoric acid, which when mixed in water, loses hydrogen and forms protons (H ) and anions (H2PO4, HPO , and PO ) of phosphates. The number of protons lost during the dissolution depends on the pH in which this dissociation takes place. These dissociation reactions are represented by the following equations ... 

Other common species that have an effect on the pH and buffering capacity of natural systems include phosphates, borates, amino acids, and some organic compounds (generally weak acids). Phosphoric acid is a polyprotic acid that liberates one proton in each of its three dissociation steps, leaving a weaker acid... 

Some important acids, such as sulfuric acid (H2S04) and phosphoric acid (H3P04), can furnish more than one proton per molecule and are called polyprotic acids. A polyprotic acid always dissociates in a stepwise mar.aer, one proton at a time. 

Phosphoric acid is a triprotic acid (three protons) that dissociates in the following steps ... 

Step 1 Water molecules (HOH) are partially dissociated into hydroxyl groups (HO") and protons (H"). The first step in enzyme-catalyzed hydrolysis may involve the attack of a proton on the target bond. Molecules of phosphoric acid H3PO4) dissociate to give phosphate anions (FO, ) and protons. The first step in enzyme-catalyzed phosphorolysis may involve the attack of a proton. 

The situation becomes more complex for acids or bases that contain two or more reactive hydrogen or hydroxide ions with different tendencies to dissociate. With certain indicators, for example, only the first of the three protons in phosphoric acid is titrated ... 

As expected, the confinement of phosphoric acid in the PBI matrix does not give rise to any relevant electroosmotic drag. Of course, the main reason is the fact that proton conductivity is dominated by structure diffusion, that is the transport of protonic charge carriers and phosphoric acid are effectively decoupled. The other reason is that protonic charge carriers are produced by self-dissociation of the proton solvent (phoshoric acid), that is the number of positively and negatively... 

Acids with more than one ionizable proton are polyprotic acids. In a solution of a polyprotic acid, one proton at a time dissociates from the acid molecule, and each dissociation step has a different K. For example, phosphoric acid is a tripro-tic acid (three ionizable protons), so it has three values ... 

This resonance would be acid-strengthening, as the dissociating proton is nearer to the positive pole of the imposed dipole than to its negative pole. The third dissociation constant of phosphorous acid... 

The acids are produced from normal fuel metabolism. The major acid is carbonic acid, which is formed from water and CO2 produced in the TCA cycle and other oxidative pathways. The oxidation of sulfur-containing amino acids (methionine and cysteine) ultimately produces sulfuric acid (H2SO4), which dissociates into 2H+ + S04 , and the protons and sulfate are excreted. The hydrolysis of phosphate esters produces the equivalent of phosphoric acid. What other acids produced during metabolism appear in the blood ... 

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