Buffer ionic equilibria

Some of the problems at the end of the sections on Acids and Bases, Hydrolysis, Polyprotic Acids, and Buffers involve multiple equilibria. They may be omitted in a simplest treatment of ionic equilibrium. 

Consider just a few cases of aqueous equilibria. The magnificent formations i n limestone caves and the vast expanses of oceanic coral reefs result from subtle shifts in carbonate solubility equilibria. Carbonates also influence soil pH and prevent acidification of lakes by acid rain. Equilibria involving carbon dioxide and phosphates help organisms maintain cellular pH within narrow limits. Equilibria involving clays in soils control the availability of ionic nutrients for plants. The principles of ionic equilibrium also govern how water is softened, how substances are purified by precipitation of unwanted ions, and even how the weak acids in wine and vinegar influence the delicate taste of a fine French sauce. In this chapter, we explore three aqueous ionic equilibrium systems acid-base buffers, slightly soluble salts, and complex ions. 

Subarats X, Bosch E, Roses M (2007) Retention of ionizable compounds on high-performance liquid chromatography. XVll. Estimation of the pH variation of aqueous buffers with the change of the methanol fraction ofthe mobile phase. J Chromatogr A 1138 203-215 Siicha E, Kotrly S (1972) Solution Equilibria in Analytical Chemistry. Reinhold, London Butler JN, Cogley DR (1998) Ionic Equilibrium. Solubility and pH Calculations. Wiley, New York... 

The maintenance practice of removal and buffering of electrodes is costly and often counterproductive because it reduces measurement accuracy due to damage of the gel surface of the glass electrode, and the upset to the thermal and ionic equilibrium of the reference electrode. It is possible to improve the performance of pH electrodes and safety during pH maintenance and reduce the cost of pH maintenance by an order of magnitude through more realistic expectation, and the use of a better calibration policy that uses process standardization and relaxed tolerances, described in section 4-9. 

Insert the electrodes into a buffer solution that matches the isopotential pH value and adjust the standardization potential to make it match the buffer. Make sure the buffers are fresh. Use high ionic strength or non-aqueous buffers as necessary to match the process composition and allow enough time for the reference to reach ionic equilibrium. Measure and note the time constant of the measurement electrode response. 

Under the conditions of temperature and ionic strength prevailing in mammalian body fluids, the equilibrium for this reaction lies far to the left, such that about 500 CO2 molecules are present in solution for every molecule of H2CO3. Because dissolved CO2 and H2CO3 are in equilibrium, the proper expression for H2CO3 availability is [C02(d)] + [H2CO3], the so-called total carbonic acid pool, consisting primarily of C02(d). The overall equilibrium for the bicarbonate buffer system then is... 

Polyelectrolyte complexes composed of various weight ratios of chitosan and hyaluronic acid were found to swell rapidly, reaching equilibrium within 30 min, and exhibited relatively high swelling ratios of 250-325% at room temperature. The swelling ratio increased when the pH of the buffer was below pH 6, as a result of the dissociation of the ionic bonds, and with increments of temperature. Therefore, the swelling ratios of the films were pH-and temperature-dependent. The amount of free water in the complex films increased with increasing chitosan content up to 64% free water, with an additional bound-water content of over 12% [29]. 

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

Words that can be used as topics in essays 5% rale buffer common ion effect equilibrium expression equivalence point Henderson-Hasselbalch equation heterogeneous equilibria homogeneous equilibria indicator ion product, P Ka Kb Kc Keq KP Ksp Kw law of mass action Le Chatelier s principle limiting reactant method of successive approximation net ionic equation percent dissociation pH P Ka P Kb pOH reaction quotient, Q reciprocal rule rule of multiple equilibria solubility spectator ions strong acid strong base van t Hoff equation weak acid weak base... 

Although the amino acids have a considerable affinity for the resin, the sodium ions are constantly present in a much higher concentration and, as a result, the equilibrium of the above equation is shifted to the right and the amino acids are displaced from the resin. Thus the molarity of the eluting buffer affects elution and when the ionic concentration of the buffer is increased, the amino acids are eluted more rapidly from the column. 

The pKa of the imidazole ring is near 6 (16) so histamine would only exist as an ion in the acidic (pH = 2-3) mobile phase. One would predict no retention on a bonded phase column under this condition however, it does occur. Figure 3 is the simplest way to account for this retention. Here, the mineral acid acts as the counter-ion, as well as the buffer. All of the histamine in the mobile phase is in the ionic form and is in equilibrium with the ion-pair which is only soluble in the stationary phase chemically bonded to silica. Histamine only elutes in the ionic form and is then derivatized for detection. A sharp peak in the chromatogram with good shape and no change in retention time with variation in sample concentration indicates a working system. However, if the paired ion has some solubility in the mobile phase, peak tailing occurs. 

In order to measure the magnitude of the chemical interactions between various ions and buffer gases, approaches that are based on the measurements of either equilibrium or rate constants for ionic processes can be envisioned. An example of a kinetic method is described in the following. The unimolecular kinetic process known as thermal electron detachment (TED) for negative ions (NT -> M + e), should be particularly sensitive to a chemical effect of the buffer gas. This is because the rate of TED will be given by = constant x where the electron... 

Selected entries from Methods in Enzymology [vol, page(s)] Buffer capacity, 63, 4 choice, 63, 19, 20, 285 metal ion chelation effects, 63, 225, 226, 287, 298, 299 dielectric constant effect on pK, 63, 226 dilution, 63, 20 equilibrium constant effects, 63, 18 heavy water, 63, 226, 227 ionic strength effects, 63, 226,... 

A measurement of the ability of a buffer system to limit the change in pH of a solution upon the addition of an increment of strong base. ft is the reciprocal of the slope of the pH-neutralization curve. Consider the simple equilibrium, HA H+ -h A where K = [H+][A ]/ [HA] in which K is a practical dissociation constant determined under conditions of constant ionic strength. In such systems the practical pK is equal to the pH of solution when there are equal concentrations of the two buffer species. Since the total concentration of the two... 

The equilibrium constant of an enzyme-catalyzed reaction can depend greatly on reaction conditions. Because most substrates, products, and effectors are ionic species, the concentration and activity of each species is usually pH-dependent. This is particularly true for nucleotide-dependent enzymes which utilize substrates having pi a values near the pH value of the reaction. For example, both ATP" and HATP may be the nucleotide substrate for a phosphotransferase, albeit with different values. Thus, the equilibrium constant with ATP may be significantly different than that of HATP . In addition, most phosphotransferases do not utilize free nucleotides as the substrate but use the metal ion complexes. Both ATP" and HATP have different stability constants for Mg +. If the buffer (or any other constituent of the reaction mixture) also binds the metal ion, the buffer (or that other constituent) can also alter the observed equilibrium constant . ... 

The use of Haldane relationships to verify the magnitude of the equilibrium constant or, conversely, to determine (or verify) one of the kinetic parameters requires that aU constants be measured under the same experimental conditions (eg., temperature, pH, buffer species, ionic strength, free metal ion concentrations, etc) If not, the Haldane relationship has no meaning. In addition, kinetic data are often limited in precision, unlike equilibrium measurements. For multisubstrate reactions, there are at least two different Haldane relationships. Thus,... 

A more comprehensive analysis of the influences on the ozone solubility was made by Sotelo et al., (1989). The Henry s Law constant H was measured in the presence of several salts, i. e. buffer solutions frequently used in ozonation experiments. Based on an ozone mass balance in a stirred tank reactor and employing the two film theory of gas absorption followed by an irreversible chemical reaction (Charpentier, 1981), equations for the Henry s Law constant as a function of temperature, pH and ionic strength, which agreed with the experimental values within 15 % were developed (Table 3-2). In this study, much care was taken to correctly analyse the ozone decomposition due to changes in the pH as well as to achieve the steady state experimental concentration at every temperature in the range considered (0°C [Pg.86]

Let us consider ionic systems. In non-equilibrium state, the potential drop across the interface differs from the equilibrium value A tpb (eq). If the adjacent phases a and P chemically buffer the interface on their respective sides, as is normally true considering the large number of particles in the bulk relative to the small number of interface particles, the overall potential drop, Atjb, is only due to the electric potential change 8[Pg.84]

Fig. 4. Study of the equilibrium between pyruvic acid and glycine. 5.6 x 10 4 M Sodium pyruvate, 0.005% gelatin, glycine buffer pH 9.2. Concentration of the form of glycine with the free amino group is given in the polarogram. Ionic strength kept constant by addition of sodium chloride. Curves starting at —0.8 V, S.C. E., 200 mV/absc.,

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