Phosphoric acid dissociation

Phosphoric acid is a tribasic acid. It is not an oxidizing acid. In aqueous solution phosphoric acid dissociates to H2PO4, HP042 and POs ions. The dissociation constants are as follows ... 

Orthophosphates—The ions and salts of the three phosphoric acid dissociation products dihydrogen phosphates containing the H2PO4 ion, hydrogen phosphate containing the HPO ion, and the phosphates containing the P04 ion. 

However, it has proved possible to exclude certain of these formulations on the basis of the number of phosphoric acid dissociations they would possess and the types of hydrolysis products they would yield. By... 

Now, Gulland and coworkers have found, by electrometric titration, that yeast ribosenucleic acid possesses four acid dissociations per tetranucleotide unit, three of which are primary dissociations, and one a secondary dissociation of phosphoric acid. (It was verified that de-aminated yeast ribosenucleic acid is constituted similarly, thus eliminating the possibility of phosphoamide links.) In addition, they found that mild alkaline hydrolysis diminishes the molecular weight of the poly-tetranucleotide, and the titration results for the product suggested that a further secondary dissociation of phosphoric acid is set free that is, that if it were possible to isolate the fundamental tetranucleotide, the penta-basic product should have three primary and two secondary phosphoric acid dissociations. 

Phosphoric acid dissociates in three steps, all forms behaving as weak acids. 

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

Phosphoric acid dissociates to form dihydrogen phosphate and hydronium ion. Phosphoric acid has a of 7.5 X 10 . Write the equation for the reaction and the acid dissociation constant expression for phosphoric acid. 

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. 

Hexafluorophosphoric Acid. Hexafluorophosphoric acid (3) is present under ambient conditions only as an aqueous solution because the anhydrous acid dissociates rapidly to HF and PF at 25°C (56). The commercially available HPF is approximately 60% HPF based on PF analysis with HF, HPO2F2, HPO F, and H PO ia equiUbrium equivalent to about 11% additional HPF. The acid is a colorless Hquid which fumes considerably owiag to formation of an HF aerosol. Frequently, the commercially available acid has a dark honey color which is thought to be reduced phosphate species. This color can be removed by oxidation with a small amount of nitric acid. When the hexafluorophosphoric acid is diluted, it slowly hydrolyzes to the other fluorophosphoric acids and finally phosphoric acid. In concentrated solutions, the hexafluorophosphoric acid estabUshes equiUbrium with its hydrolysis products ia relatively low concentration. Hexafluorophosphoric acid hexahydrate [40209-76-5] 6 P 31.5°C, also forms (66). This... 

The titration curve of phosphoric acid in the presence of sodium hydroxide is shown in Figure 1. Three steps, corresponding to consecutive replacement of the three acidic hydrogens, and two inflection points, near pH = 4.5 and 9.0, are evident. Dissociation constants are = 7.1 x 10 = 6.3 x 10 ... 

The relation between free phosphoric acid content and total phosphate content in a processing bath, whether based on iron, manganese or zinc, is very important this relation is generally referred to as the acid ratio. An excess of free acid will retard the dissociation of the primary and secondary phosphates and hinder the deposition of the tertiary phosphate coating sometimes excessive loss of metal takes place and the coating is loose and powdery. When the free acid content is too low, dissociation of phosphates (equations 15.2, 15.3 and 15.4) takes place in the solution as well as at the metal/solution interface and leads to precipitation of insoluble phosphates as sludge. The free acid content is usually determined by titrating with sodium... 

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. 

Phosphate, PO, is the fully dissociated anion of triprotic phosphoric acid, H3PO4 ... 

Titrations curves for polyprotic acids have an inflection point for each hydrogen in the formula if the dissociation constant (Ka) for each hydrogen is very different from the others and if any dissociation constant is not too small. The titration curves of the polyprotic acids H2S04 and H3P04 are shown in Figures 5.6 and 5.7. Sulfuric acid has essentially one inflection point (like hydrochloric acid—compare with Figure 5.1(a)), while phosphoric acid has two apparent inflection points. Both hydrogens on the... 

Q dD Phosphoric acid, H3P04(aq) is triprotic. It has three hydrogen ions that may he dissociated. 

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

HaO" ] results from the first dissociation of phosphoric acid, so the second dissociation has a negligible effect on the concentration of hydronium ions or the pH of the solution. You would expect this, because the second dissociation is much weaker than the first. 

---