Buffer capacity, bases

One method that is httle-known, or at least rarely used to avoid this total acidity imbalance, consists of partially or completely eliminating the malic acid by chemical means, using a mixture of calcium tartrate and calcium carbonate. This method precipitates the double calcium salt, tartromalate, (Section 1.4.4, Figure 1.9) and is a very flexible process. When the malic acid is partially eliminated, the wine has a buffer capacity based on those of both tartaric and malic acids, and not just on that of the former. Tartrate buffer capacity is less stable over time, as it decreases due to the precipitation of monopotassium and calcium salts during aging, whereas the malic acid salts are much more soluble. 

A second way to achieve constancy of a reactant is to make use of a buffer system. If the reaction medium is water and B is either the hydronium ion or the hydroxide ion, use of a pH buffer can hold Cb reasonably constant, provided the buffer capacity is high enough to cope with acids or bases generated in the reaction. The constancy of the pH required depends upon the sensitivity of the analytical method, the extent of reaction followed, and the accuracy desired in the rate constant determination. 

FIGURE 2.15 A buffer system consists of a weak acid, HA, and its conjugate base, A. The pH varies only slightly in the region of the titration curve where [HA] = [A ]. The unshaded box denotes this area of greatest buffering capacity. Buffer action when HA and A are both available in sufficient concentration, the solution can absorb input of either H or OH, and pH is maintained essentially constant. 

Buffer capacity Amount of strong acid or base that can be added to a buffer without causing a drastic change in pH, 390 Buret, 7... 

The concentration of the acid is usually of the order 0.05-0.2 mol L" Similar remarks apply to weak bases. It is clear that the greater the concentrations of acid and conjugate base in a buffer solution, the greater will be the buffer capacity. A quantitative measure of buffer capacity is given by the number of moles of strong base required to change the pH of 1 litre of the solution by 1 pH unit. 

Buffer capacity also depends on the relative concentrations of weak acid and base. Broadly speaking, a buffer is found experimentally to have a high capacity for acid when the amount of base present is at least 10% of the amount of acid. Otherwise, the base is used up quickly as strong acid is added. Similarly, a buffer has a high capacity for base when the amount of acid present is at least 10% of the amount of base, because otherwise the acid is used up quickly as strong base is added. 

Buffer capacity is determined by the amounts of weak acid and conjugate base present in the solution. If enough H3 O is added to react completely with the conjugate base, the buffer is destroyed. Likewise, the buffer is destroyed if enough OH is added to consume all of the weak acid. Consequently, buffer capacity depends on the overall concentration as well as the volume of the buffer solution. A buffer solution whose overall concentration is 0.50 M has five times the capacity as an equal volume of a buffer solution whose overall concentration is 0.10 M. Two liters of 0.10 M buffer solution has twice the capacity as one liter of the same buffer solution. Example includes a calculation involving buffer capacity. 

With a given weak acid, a buffer soiution can be prepared at any pH within about one unit of its p vaiue. Suppose, for exampie, that a biochemist needs a buffer system to maintain the pH of a soiution ciose to 5.0. What reagents shouid be used According to the previous anaiysis, the weak acid can have a p Z a between 4.0 and 6.0. As the p deviates from the desired pH, however, the soiution has a reduced buffer capacity. Thus, a buffer has maximum capacity when its acid has its p as ciose as possibie to the target pH. Tabie 18-1 iists some acid-base pairs often used as buffer soiutions. For a pH - 5.0 buffer, acetic acid (p Za — 4.75) and its conjugate base, acetate, wouid be a good choice. 

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. 

If more and more acid is added, however, the pH will eventually change. Once all the ethanoate ions have been converted to ethanoic acid, an excess of hydrogen ions will build up, leading to a more acidic solution. Likewise, if too much base is added, the ethanoic acid will be used up and excess hydroxide ions will raise the pH. The amount of acid or base that can be added to a buffering solution before the pH changes significantly is called the solutions buffer capacity. 

While it is conceivable that an excess of bases in the cell solution might be protective against mild sulfur burn, this possibility has not yet been tested. On the other hand, a small increase in buffer capacity might reduce sulfur burn. An example of this effect may be seen in the buffer curves of the leaf sap in two of the United States Department of Agriculture s muskmelon varieties. No. 5, which is susceptible to sulfur burn, has a buffer curve which lies 0.2 to 0.3 pH unit closer to the acid side than the buffer curve of the sulfur burn-resistant variety, No. 11353 (Figure 1). 

The slope of the tangent to the curve at the inflection point where oc = is thus inversely proportional to the number of electrons n. The E-oc curves are similar to the titration curves of weak acids or bases (pH-or). For neutralization curves, the slope dpH/doc characterizes the buffering capacity of the solution for redox potential curves, the differential dE/da characterizes the redox capacity of the system. If oc — for a buffer, then changes in pH produced by changes in a are the smallest possible. If a = in a redox system, then the potential changes produced by changes in oc are also minimal (the system is well poised ). 

Buffers are used mainly to control the pH and the acid-base equilibrium of the solute in the mobile phase. They can also be used to influence the retention times of ionizable compounds. The buffer capacity should be maximum and should be uniform in the pH range of 2-8 commonly used in HPLC. The buffers should be soluble, stable, and compatible with the detector employed, e.g., citrates are known to react with certain HPLC hardware components. 

Define buffer solution, conjugate acid, conjugate base, conjugate acid-base pair, buffer capacity, and buffer region. 

These equations allow us to calculate the pH or pOH of the buffer solution knowing Kof the weak acid or base and the concentrations of the conjugate weak acid and its conjugate base. Also, if the desired pH is known, along with K, the ratio of base to acid can be calculated. The more concentrated these species are, the more acid or base can be neutralized and the less the change in buffer pH. This is a measure of the buffer capacity, the ability to resist a change in pH. 

The buffer capacity is a quantitative measure of the ability of a buffer to resist a change in pH. The more concentrated the acid-base components of the buffer, the higher its buffer capacity. 

Epidermis Complete removal of the dermis may be achieved by several mechanical, thermal, and chemical techniques. Most commonly, the epidermal-dermal junction is split by heating the skin to 60 C for 30-120 s [83, 84], Pitman et al. [85] could show that such a treatment does not impair the barrier function. The use of ethylene diamine tetraacetic acid, sodium bromide, or ammonia fumes has also been reported [80, 83, 86], It may, however, be suspected that the use of sufficiently strong acids or bases may change the buffer capacity of skin, which would especially influence the penetration behavior of ionizable drugs. 

Figure 9.5 Generation of a pH gradient by ampholytes within a capillary flanked by an acid as anodic solution and base as cathodic solution. Ampholyte solutions are composed of high numbers of low-molecular weight amphoteric electrolytes (from which the name is derived) with slightly different pi values. Because ampholytes possess buffering capacity, they maintain a pH value in the specific area occupied by the different molecular species. The sample, which is also amphoteric, focuses in between ampholytes with higher and lower pi. To achieve resolution, there must be at least one ampholyte with a pi intermediate to the two sample components of interest.

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