Capacity, basic buffering

We have seen from Fig. 3.9 that the buffer capacity is at a maximum at a pH equal to the pK of the weak acid used in the formulation of the buffer system and decreases appreciably as the pH extends more than one unit either side of this value. If, instead of a single weak monobasic acid, a suitable mixture of poly-basic and monobasic acids is used, it is possible to produce a buffer which is effective over a wide pH range. Such solutions are referred to as universal buffers. A typical example is a mixture of citric acid (pJC i = 3.06, pK,2 = 4.78, pK,3 = 5.40), Na HPO (pK, of conjugate acid H2PO4 = 7.2), diethylbarbituric acid (pKji = 7.43) and boric acid (pK i = 9.24). Because of the wide range of pK, values involved, each associated with a maximum buffer capacity, this buffer is effective over a correspondingly wide pH range (pH 2.4-12). 

It is also necessary to know the wine s acido-basic buffer capacity. Thus, in the case of wines from northerly regions, initially containing 6 g/1 of malic acid after malolactic fermentation, tartrating may be necessary to correct an impression of flatness on the palate. Great care must be taken in acidifying this type of wine, otherwise it may have... 

Equation (3.43) gives P as the change of concentration of strong base, d[B], which is required for a given pH change. For a basic buffer, the buffer capacity is given by... 

FIG. 14 A model for the uptake of weakly basic compounds into lipid bilayer membrane (inside acidic) in response to the pH difference. For compounds with appropriate pki values, a neutral outside pH results in a mixture of both the protonated form AH (membrane impermeable) and unprotonated form A (membrane permeable) of the compound. The unprotonated form diffuse across the membrane until the inside and outside concentrations are equal. Inside the membrane an acidic interior results in protonation of the neutral unprotonated form, thereby driving continued uptake of the compound. Depending on the quantity of the outside weak base and the buffering capacity of the inside compartment, essentially complete uptake can usually be accomplished. The ratio between inside and outside concentrations of the weakly basic compound at equilibrum should equal the residual pH gradient. 

When calcium carbonate goes into solution, it releases basic carbonate ions (COf ), which react with hydrogen ions to form carbon dioxide (which will normally remain in solution at deep-well-injection pressures) and water. Removal of hydrogen ions raises the pH of the solution. However, aqueous carbon dioxide serves to buffer the solution (i.e., re-forms carbonic acid in reaction with water to add H+ ions to solution). Consequently, the buffering capacity of the solution must be exceeded before complete neutralization will take place. Nitric acid can react with certain alcohols and ketones under increased pressure to increase the pH of the solution, and this reaction was proposed by Goolsby41 to explain the lower-than-expected level of calcium ions in backflowed waste at the Monsanto waste injection facility in Florida. 

Solutions in which the buffering action is due to the solvent rather than any added solute Strongly acidic or basic aqueous solutions will show httle change in pH when additional increments of acid or base are added (recall that the pK value for H3O+ is -1.74, and that for H2O is 15.74) . Because the solvent is in such high concentration, the buffering capacity for pseudo buffers is larger than for conventional buffers. See Buffer Capacity... 

Lakes respond differently to acid deposition, depending on their underlying rocks. Lakes that rest in a hollow of limestone, for example, are protected from acid deposition to some extent by the buffering capacity of the limestone, which is chemically basic ... 

Since proteins contain a lot of acidic and basic side chains acting as weak acids and bases, respectively, proteins are buffering substances, too. If you mix buffer solutions with protein solutions, pH may be altered because the concentration of protein s buffering residues may exceed the capacity of the (chemical) buffer. For instance, bovine serum albumin contains 59 basic (Lys) and 99 acidic (59 Asp plus 40 Glu) residues per mole a solution of 10 mg/ml (1%) BSA contains 9 mM basic and 14.5 mM acidic residues, and phosphate-buffered saline (PBS) contains only 10 mM phosphate. As a consequence of this example (a) the concentration of the chemical buffer should be high enough to act as a buffer, (b) choose a chemical buffer the pK of which is nearby the pH to be stabilized, and (c) adjust the pH after all components are mixed. 

The effect of pH on the HPLC retention time of an ionisable basic drug. Bupivacaine, which has a piSa of 8.1 is analysed by chromatography on ODS silica gel with a mobile phase consisting of acetonitrile/TRIS buffer pH 8.4 (40 60) at a flow rate of 1 ml/min. The t for the column at a mobile phase flow rate of 1 ml/min is 2.3 min. The retention time of bupivacaine at pH 8.4 is 17.32. If jJf app is the apparent capacity factor Of the partially ionised drug, then for a base ... 

By adding basic salts to matrices of the isopropyl ester of PVM-MA, the pH on the polymer surface and drug release from the matrices could be increased in dissolution medium with poor buffering capacity [4]. 

Why does buffer capacity increase as a solution becomes very acidic (pH = 1) or very basic (pH 13) ... 

Formol titration is a method that estimates amino groups by titration with NaOFI and a phenolphthalein indicator (Vakaleris et ah, 1960). Addition of formaldehyde to the neutralized mixture reduces the pFI by making the amino groups less basic. The amount of NaOFI required to retitrate the mixture has been used as an indicator of proteolysis. Another titrimetric method relies on the increase in buffering capacity of the cheese during ripening and has been applied to study proteolysis in Swiss cheese (Lucey et ah, 1993). 

Lin C, Zhong Z, Lok MC et al (2007) Random and block copolymers of bioreducible poly (amido amine)s with high- and low-basicity amino groups study of DNA condensation and buffer capacity on gene transfection. J Control Release 123 67-75... 

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