Neutralization titrations acid-base equilibria

The acid-base equilibrium constant for the Me residue can be determined by spectroscopic pH titration. An example for the titration is shown in Fig. 2. The electrostatic potential

difference between the apparent pK on the charged surface (pKobs) and that on an intrinsic neutral surface (pK1) by... 

As the titration begins, mostly HAc is present, plus some H and Ac in amounts that can be calculated (see the Example on page 45). Addition of a solution of NaOH allows hydroxide ions to neutralize any H present. Note that reaction (2) as written is strongly favored its apparent equilibrium constant is greater than lO As H is neutralized, more HAc dissociates to H and Ac. As further NaOH is added, the pH gradually increases as Ac accumulates at the expense of diminishing HAc and the neutralization of H. At the point where half of the HAc has been neutralized, that is, where 0.5 equivalent of OH has been added, the concentrations of HAc and Ac are equal and pH = pV, for HAc. Thus, we have an experimental method for determining the pV, values of weak electrolytes. These p V, values lie at the midpoint of their respective titration curves. After all of the acid has been neutralized (that is, when one equivalent of base has been added), the pH rises exponentially. 

Figure 6.3. Titration of H30 and Cu aq with ammonia (a) and with tetramine (trien) (b). Equilibrium diagrams for the distribution of NH3-NH4 (c) of the amino coppeifll) complexes (d) and of Cu ", Cu-trien (e). The similarity of titrating with a base and titrating a metal ion with a base (Lewis acid-base interaction) is obvious. Both neutralization reactions are used analytically for the determination of acids and metal ions. A pH or pMe indicator electrode (glass electrode for and copper electrode for Cu " ) can be used for the end point indication.

This equilibrium is described by the constant of 3.1 X 1015 for the case of phosphates, and 4.7 X 1013 for acidic arsenates. For metavanadate solutions an estimation of the value of the similar constant failed owing to the levelling of the acidic properties of this Lux acid. The next stage of the titration is the formation of neutral salts as a result of acid-base neutralization of the formed pyro-anions, this stage is described by the following equation ... 

Etingov ED and Kholodova GV, Kul bakh VO and Kamatushkina AI, Acid-base properties of amphotericin B, Antibiotiki, 17, 301-305 (1972). "Titration of 66% aqueous dimethylformamide solutions of Amphotericin B with methanolic HCl and KOH yields pK s near 5.7 and 10.0. Comparison with N-acetyl-Amphotericin B (pK=6.5) and Amphotericin B-methyl ester (pK=8.8) assigns the two pK s to carboxyl and amino groups respectively. Amphotericin B is found to be almost completely zwitterionic in this solution (tautomeric equilibrium constant Kt = 1000 with respect to the neutral molecule)." S R... 

Fig. 5 shows the results of both titration experiments. The experimental results are in good agreement with the predictions based upon the equilibrium expressions for Kb the Ka for each indicator, and the mass and charge balances[13]. The data from the acid titration show a sharp equivalence point at approximately 10 m HCl, which suggests that B(OH)4 is still a strong base at 350°C and 0.622 g/mL and capable of neutralizing HCl. This strong acid base titration curve, as was also observed for HCl and KOH, may be contrasted with the weak acid-base behavior observed for the sulfuric acid-ammonia system at 380 C[41]. 

Analyze We are asked to determine the pH at the equivalence point of the titration of a weak acid with a strong base. Because tire neutralization of a weak acid produces the corresponding conjugate base, we ejqsect the pH to be basic at the equivalence point. Plan We should first determine how many moles of acetic acid there are initially. This win teU us how many moles of acetate ion there will be in solution at the equivalence point. We then must determine the final volume of the resulting solution, and the concentration of acetate ion. From this point this is simply a weak-base equilibrium problem like those in Section 16.7. 

Chapter 10, Acids and Bases and Equilibrium, discusses acids and bases and their strengths, conjugate acid-base pairs, the ionization of acids, weak bases, and water, pH, and buffers. The reactions of acids and bases with metals, carbonates, and bicarbonates are discussed. Acid-base titration uses the neutralization reactions between acids and bases to calculate quantities of acid in a sample. 

Adsorption of anions or neutral molecules on oxides is controlled by the same parameters (pH, concentration, temperature) as those involved in complexatiqn of cations in solution. The pH of the suspension imposes both the level of protonation of the anion and the surface charge of the oxide. As in the case of cation complexation in solution (see Chapter 5), there is an optimum pH range for anion adsorption. Adsorption is characterized by complexation equilibrium constants, determined using an approach similar to the calculation of complexation constants in solution from acid-base titration of suspensions [48-51]. 

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... 

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

Firstly, ion exchange resins when hydrated generally dissociate to yield equivalent amounts of oppositely charged ions. Secondly, as with conventional aqueous acid or alkali solutions, resins in their acid or base forms may be neutralized to give the appropriate salt form. Finally, the degree of dissociation can be expressed in the form of an apparent equilibrium constant (or pK value) which defines the electrolyte strength of the exchanger and is usually derived from a theoretical treatment of pH titration curves. ... 

Note from Example 14-3 that the analytical concentrations of acid and conjugate base are identical when an acid has been half neutralized (after the addition of exactly 25.00 mL of base in this case). Thus, these terms cancel in the equilibrium-constant expression, and the hydronium ion concentration is numerically equal to the dissociation constant. Likewise, in the titration of a weak base, the hydroxide ion concentration is numerically equal to the dissociation constant of the base at the midpoint in the titration curve. In addition, the buffer capacities of each of the solutions are at a maximum at this point. These points are often called the half-titration points. 

We have performed a study of aeidie properties of boron (HI) and vanadium (V) oxo-compounds in molten Nal at 700"C (see Table 10.4.1). A eomparative study of strength of Lux bases OH", C03 , S04 was determined " " by potentiometrie titration using sodium pyrophosphate as acid. Two moles of the first base may be neutralized by 1 mole of P407 , while two other moles react with the acid in ratio 1 1. On the base of e.m.f. drop magnitude at the equivalence point, the bases have been arranged in sequence 0H>C03 >S04 of basicity decrease. The equilibrium constants were not estimated. "" " ... 

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