Weak acids buffering capacity

In general, we may state that the buffering capacity is maintained for mixtures within the range 1 acid 10 salt and 10 acid l salt and the approximate pH range of a weak acid buffer is ... 

In choosing a buffer, seek one whose pKa is as close as possible to the desired pH. The useful pH range of a buffer is usually considered to be pKa 1 pH unit. Outside this range, there is not enough of either the weak acid or the weak base to react with added base or acid. Buffer capacity can be increased by increasing the concentration of the buffer. 

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. 

The pH then remains relatively constant. The components of a buffer system are chosen such that the of the weak acid is close to the pH of interest. It is at the that the buffer system shows its greatest buffering capacity. At pH values more than one pH unit from the buffer systems become ineffec-... 

The reasons why some anions exhibit strong inhibitive properties while others exhibit strong aggressive properties are not entirely clear. The principal distinction seems to be that inhibitive anions are generally anions of weak acids whereas aggressive anions are anions of strong acids. Due to hydrolysis, solutions of inhibitive anions have rather alkaline pH values and buffer capacities to resist pH displacement to more acid values. As discussed... 

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. 

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 ). 

When the metal is immersed in a solution of a salt of a weak acid (e.g., boric or tartaric), the latter exhibits a buffering capacity and thus provides one mechanism for the removal of hydrogen ions from the interface [cf. Section III(3(iv))]. 

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. 

Pancreatic secretions. In the acinar cells, the pancreas forms a secretion that is alkaline due to its HCOa content, the buffer capacity of which is suf cient to neutralize the stomach s hydrochloric acid. The pancreatic secretion also contains many enzymes that catalyze the hydrolysis of high-molecular-weight food components. All of these enzymes are hydrolases with pH optimums in the neutral or weakly alkaline range. Many of them are formed and secreted as proenzymes and are only activated in the bowel lumen (see p. 270). 

The buffering capacity of a buffer system depends on its concentration and its pKg value. The strongest effect is achieved if the pH value corresponds to the buffer system s pKa value (see p. 30). For this reason, weak acids with pKa values of around 7 are best suited for buffering purposes in the blood. 

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. 

An important characteristic of milk is its buffering capacity, i.e. resistance to changes in pH on addition of acid or base. A pH buffer resists changes in the [H+] (ApH) in the solution and normally consists of a weak acid (HA) and its corresponding anion (A-, usually present as a fully dissociat-able salt). An equilibrium thus exists ... 

At what point in the titration of a weak base with a strong acid is the maximum buffer capacity reached This is the point at which a given small addition of acid causes the least pH change. 

In addition to changing the pH of the water, the uptake and release of CO2 alter the buffer capacity of the water. The effect upon buffer capacity is the result of two factors (1) the dependence of buffer capacity on the hydrogen ion concentration, and (2) the dependence of buffer capacity on the total concentration of weak acid and conjugate base in solution (67, 68). The precipitation of CaCO in natural waters reduces the buffer capacity to a value lower than that predicted on the basis of pH change and respiratory or photosynthetic changes in COL content of the water. 

A solution of a weak acid and its conjugate base is called a buffer solution because it resists drastic changes in pH. The ability of a buffer solution to absorb small amounts of added H30+ or OH- without a significant change in pH (buffer capacity) increases with increasing amounts of weak acid and conjugate base. The pH of a buffer solution has a value close to the pKa (— log Ka) of the weak acid and can be calculated from the Henderson-Hasselbalch equation ... 

Buffer Capacity of a Buffer Solution Containing a Polyprotic Weak Acid or Weak Base and Its Conjugate Base or Acid... 

The maximum buffer capacity of the buffer in Example 2-9 is equal to 0.576(0.172 + 0.043) = 0.124. Therefore, it is recommended that a weak acid whose pKa is close to the required pH should be chosen for a buffer solution. 

If there is a weak acid and a weak base in a solution, the buffer capacity of the solution can be determined by the same approach as in the case of a weak acid... 

In the same manner as described in Section 2.2.5, mathematical expressions for the buffer capacity of polyprotic weak acid/base systems can be developed. When one adds a strong base such as NaOH to an aqueous buffer solution containing a polyprotic weak acid (HnA), the electroneutrality would be ... 

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