Multiprotic acid

The approach that we have worked out for the titration of a monoprotic weak acid with a strong base can be extended to reactions involving multiprotic acids or bases and mixtures of acids or bases. As the complexity of the titration increases, however, the necessary calculations become more time-consuming. Not surprisingly, a variety of algebraic and computer spreadsheet approaches have been described to aid in constructing titration curves. 

The principles outlined above can readily be extended to multiprotic acids. The alkalimetric titration of an acid H2L added as the salt H2L X (e.g., an amino acid, RNH2COOH = HL) is given by the electroneutrality condition... 

In a multiprotic acid-base system various reference levels (/ = 0, 1,2,- ) may be defined for example, in a sulfide-containing solution the acidneutralizing capacity with reference to the equivalence point defined by the pH of a pure H2S solution (/ = 0, g = 2) is... 

Br nsted Acidity and Lewis Acidity In Figure 6.2 alkalimetric titration curv es for the reaction of phosphoric acid and Fe(H20>6, respectively, with a base (OH ion) are compared. Millimolar solutions of H3PO4 and feiric perchlorate have a similar pH value. Both acids (Fe aq and H3PO4) are multiprotic acids that is, they can transfer more than one proton. 

Multiprotic Acid-Conjugate Base Equilibrium Calculations 125... 

MULTIPROTIC ACID-CONJUGATE BASE EQUILIBRIUM CALCULATIONS... 

A multiprotic acid can donate more than one proton. Consider, for example, sulfuric acid, H2SO4, which can donate two protons,... 

For the general case of a multiprotic acid added to water, we can construct a pC-pH diagram in a similar fashion to that used for a monoprotic acid. The procedure is somewhat more complex because we have additional species to consider as well as additional equilibrium statements. 

Using Eqs. 4-69 and 4-70, or alternatively the table of a versus pH in Appendix 1, we can determine ofo ciiid at various pH values and plot them in a distribution diagram as a versus pH (Fig. 4-9). Distribution diagrams for the multiprotic acids, H2S and H3PO4, are shown in Fig. 4-10 such curves can be obtained using either the table or the general equations for a in Appendix 1. 

We can also use a similar derivation of (3 for a multiprotic acid, H A. If the pKa values are separated by more than 2 pH units, the derived... 

The interaction between multiprotic acids and colloidal particles in alcoholic solutions has been extensively studied [1-3]. Addition of multiprotic acids resulted in more negative potentials, and at sufficiently high acid concentration,... 

The difference in adsorption mechanism of multiprotic acids from aqueous solutions on the one hand and from alcoholic solutions on the other is that in aqueous solutimi, pre-existing ions are adsorbed, and the conductance is depressed. In alcoholic solution, adsorption of neutral molecules produces ions in solution, e.g.. 

The equivalent conductance of proton (lyonium) is substantially higher than specific conductance of other ionic species. This property is well-known for aqueous solutions of acids, and it is also observed in alcoholic solutions [8-10]. This is why the protons in solution produced in reaction (1) or in analogous reactions with other mineral oxides and other multiprotic acids, can be easily detected by means of conductometric measurements. 

The interaction between multiprotic acids and mineral oxides has been studied in dispersions in anhydrous or nearly anhydrous n-alcohols. In this study we present analogous results obtained in mixed water-ethanol and water-methanol solvents. The values of p Ta of multiprotic acids and solvent properties relevant to the present study smoothly change firom the properties of water to the properties of alcohol in those solvents. The study of the interactions between multiprotic acids and colloidal particles in mixed solvents makes it possible to verify the above model, and to test the derived relationship between potential and conductance on the one hand and the p Ta of multiprotic acids and solvent properties on the other. 

Aluminum is a hard trivalent ion, and is usually found as an aquo- or hydroxy-complex or hydrous oxide solid (Martell and Motekaitis 1989). Aluminum can act as a complexing agent of other metals, or as a metal center to other inorganic and organic ligands (Buffle 1990). As the hexa-coordinated hydrolyzed cation, aluminum is octahedrally coordinated by six waters (Fig. 6), and as such acts as a multiprotic acid (Stumm and Morgan 1981). 

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