Buffer strength, acid-base

The redoxbuffer strength serves the same role for the potential of a solution as the acid-base buffer strength serves for its pH. In both cases it is assumed that the corresponding equilibria are established quickly on the time scale of the experiment. With redox equilibria, which often involve bond breaking, this condition is less often met than with acid-base equilibria, where fast establishment of equilibrium is the norm. 

Equations 16-9 and 16-10 are analogous to the Henderson-Hasselbalch equation of acid-base buffers. Prior to the equivalence point, the redox titration is buffered to a potential near E+ = formal potential for Fc 1 Fe2+ by the presence of Fe 1 and Fe2+. After the equivalence point, the reaction is buffered to a potential near E+ = formal potential for Ce4+ Ce3+. [R. de Levie Redox Buffer Strength, J. Chem. Ed. 1999, 76, 574.]... 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

One can test for general acid-base catalysis by varying [BH+] and [B] at constant pH. An easy test is to dilute the buffer progressively at a constant ratio of [BH+]/[B], making up any ionic strength change so as not to introduce a salt effect. If the rate is invariant with this procedure, then general acid-base catalysis is absent under the circumstances chosen. 

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

Acids and bases are used to increase ionisation by protonation or deprotonation of the analyte, and it is important to choose the buffer and buffer strength very carefully because both have a noticeable effect on sensitivity. In the positive mode, high PA additives such as triethylamine (TEA) will successfully compete with the analyte for available protons and show an intense ion at miz 102. Sensitivity can drop substantially when TEA is buffered with trifluroacetic acid (TFA). This is shown in Figure 6.3. The top diagram shows the mass spectrum of a compound analysed using acetonitrile/H20/TFA as the eluent and shows MH at mIz 511. The mass spectram of the same compound when analysed using... 

Five series of solutions were prepared in artificial seawater. The solutions were hydrochloric acid, an acetate buffer solution (mHAc/ NaAc = 1), and three equimolal buffer solutions (mBHci/ B = l) prepared from the following bases (B) tris, 2-amino-2-methylpropanediol (bis), and the N-bis(hydroxyethyl) derivative of tris, bis-tris. The pKa values of the protonated bases in water at 25°C are, respectively, 8.075 (16), 8.801 (17), and 6.483 (18). When hydrochloric acid or buffer was added to the seawater solvent, the ionic strength and chloride molality were kept constant by reducing the molalities of sodium chloride or sodium perchlorate as necessary. 

It is important to know the dissociation constant of an indicator in order to use it properly in acid-base titrations. Spectrophotometry can be used to measure the concentration of these intensely colored species in acidic versus basic solutions, and from these data the equilibrium between the acidic and basic forms can be calculated. In one such study on the indicator wj-nitrophenol, a 6.36 X 10 M solution was examined by spectrophotometry at 390 nm and 25°C in the following experiments. In highly acidic solution, where essentially all the indicator was in the form HIn, the absorbance was 0.142. In highly basic solution, where essentially all of the indicator was in the form In , the absorbance was 0.943. In a further series of experiments, the pH was adjusted using a buffer solution of ionic strength I, and absorbance was measured at each pH value. The following results were obtained ... 

The buffer strength for the aqueous solution of a single monoprotic acid and its conjugate base follows from (4.7-1) as... 

Fora mixture of monoprotic acids or bases, the buffer strength is... 

Please note that equations (4.8-5) through (4.8-7), (4.8-12) through (4.8-14), (4.8-19) through (4.8-21), and (4.8-26) through (4.8-28) are identical when the concentration fractions are written merely as a2i a1) and a0j where a2 is the concentration fraction of the fully protonated form (H2A for a dipro -tic acid, HA+for a diprotic amino acid, H2B2+for a diprotic base), a0 that of the fully deprotonated form, while is the concentration fraction of the intermediate form. For the buffer strength of the solution of a diprotic acid and/ or its conjugate bases we then have... 

The simultaneous presence of the oxidized and reduced form of a redox couple can stabilize the redox potential of a solution, just as the presence of an acid and its conjugate base can stabilize the pH. The formalism (R. de Levie, /. Chem. Educ. 76 (1999) 574) is quite similar to that of section 4.7, except that there are no terms for the oxidation or reduction of the solvent, because these are typically non-equilibrium processes which, moreover, are insignificant in the usual range of potentials considered. By analogy to (4.7 -1) we write, for the redox buffer strength B of a one-electron redox couple Ox+ e Red, such as Fe3++ e-—Fe2+orCe4++ e Ce3+,... 

Deliberate modification of the microenvironment of the fluorescing species has also been employed as a means of improving analytical sensitivity. A simple approach has involved selection of an appropriate solvent system that will enhance fluorescence. This has often involved the use of acids, bases, salts, or buffers to modify hydrogen ion concentration or ionic strength in aqueous media. A more recent approach involves modification of the immediate environment of the fluorescent species by complexation with an appropriate molecule. The cyclodex-trins, for example, have been reported to enhance the intensity of fluorescence from a number of fluorescers. This enhancement arises from the ability to form inclusion complexes with appropriately sized molecules, shielding the excited singlet-state species from the nonradiative deactivation processes. Micellar systems have also been employed to enhance the intensity of fluorescence from... 

Knowing an acids strength exponent pKa it is also possible to calculate pH in a buffer solution and this is the subject for the following section. We take a starting point in the well-known acid-base reaction ... 

The reaction rate will be at a maximum at a certain pH, owing to complex acid-base equilibria such as acid dissociation between the substrate, the activated complex, and the products. Also, the maximum rate may depend on the ionic strength and on the type of buffer used. For example, the rate of aerobic oxidation of glucose in the presence of the enzyme glucose oxidase is maximum in an acetate buffer at pH 5.1, but in a phosphate buffer of the same pH, it is decreased. 

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