Buffer capacity maximum value

Thus, the buffering capacity depends on the composition of the buffer, i.e. on the concentration of the salt a or b. The maximum value found by differentiation of Eq. (1.4.27) with respect to a corresponds, for an acidic buffer, to b = s. 

Minor differences between the three electrolyte solutions are also observed. First, electrolyte number 3 only shows a peak maximum in the current-potential curves at potentials higher than 8 V. However, this is very clear because its pH value is smaller, indicating that this electrolyte solution possesses a higher buffer capacity against consumption of hydrogen ions in the vicinity of the fibre surface, avoiding hydrogen gas formation and Ni(OH)2 precipitation. Secondly, at a potential of 4V, no deposition occurred in electrolyte solution number 3, indicated by the absence of an increase in the measured electrical current and confirmed by XPS data. Additionally in this case, the lower pH plays an important role because of the lower pH value, the applied potential difference does not overlap with the potential window in which the reduction of Ni(II) occurs. Therefore no deposition is observed. 

It follows, therefore, that the buffer capacity is a maximum when the hydrogen ion concentration of the buffer solution is equal to the dissociation constant of the acid. This condition, i.e., pH is equal to pka, arises when the solution contains equivalent amounts of the acid and its snlt such a system, which corresponds to the middle of the neutralization curve of the acid, has the maximum buffer capacity. The actual value of j3 at this point is found by inserting the condition given by (77) into equation (76) the result is... 

Preparation of Buffer Solutions.—The buffer capacity of a given acid-base system is a maximum, according to equation (77), when there are present equivalent amounts of acid and salt the hydrogen ion concentration is then equal to and the pH is equal to pfca. If the ratio of acid to salt is increased or decreased ten-fold, i.e., to 10 1 or 1 10, the hydrogen ion concentration is then lOfca or O.IAto, and the pH is pA a — 1 or pfca + 1, respectively. If these values for cn are inserted in equation (76), it is found that the buffer capacity is then... 

To make a buffer solution of a given pH, it is first necessary to choose an acid with a pfco value as near as possible to the required pH, so as to obtain the maximum buffer capacity. The actual ratio of acid to salt necessary can then be found from the simple Henderson equation... 

Buffer solutions are widely used in pharmacy to adjust the pH of aqueous solutions to that required for maximum stability or that needed for optimum physiological effect. Solutions for application to delicate tissues, particularly the eye, should also be formulated at a pH not too far removed from that of the appropriate tissue fluid, as otherwise irritation may be caused on administration. The pH of tears lies between 7 and 8, with an average value of 7.4. Fortunately, the buffer capacity of tears is high and, provided that the solutions to be administered have a low buffer capacity, a reasonably wide range of pH may be tolerated, although there is a difference in the... 

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). 

In other words, the pH for the maximum buffer capacity of a weak acid is defined by pH = ipK , and the value of is a function only of Q, the total acid-species concentration. The same reasoning applies to the maximum buffer capacity of weak bases. 

This equation is plotted in Fig. 5.11 which shows that the reaction at equilibrium has about 10 times more buffer capacity than calcite. Because this reaction will not often be at equilibrium, however, the buffer capacity is a maximum possible value. Further, the reaction is usually irreversible with kaolinite more often stable than illite in weathering environments. For this reason the reaction resists a pH decrease, but not an increase. 

The dissociation constant Kg for acetic acid is 10 Given that a water has a total acetic acid concentration of W mol/kg, what is the pH at which the acid has its maximum buffer capacity, and what is the value of the buffer capacity at that maximum ... 

During the water photooxidation reaction by chlorophyll involving proton transfer across the interface, a boundary unstirred layer is formed, enriched in products. The dependence of the Volta potential on the pH of the incubation medium was measured. It proved that A

on the incubation time at different buffer concentrations measured at the initial pH 5.9. If during the reaction of Eq. (20) a boundary unstirred proton-enriched layer is formed, then with decreasing buffer capacity, the photopotential value decreases due to pheophytinization of chlorophyll [67]. 

Buffer capadty is a measure of the strength or quahty of a buffer. It is defined as the reciprocal value of the slope of the pH curve (or titration curve) of the buffer [6]. It depends on two factors (1) the concentration of the buffer (2) the distance between the pH and the pK of the buffer. In Fig. 3, we show the buffer capacities of acetate buffers at concentrations of 5 mM, 10 mM, 20 mM, and 40 mM. The maximum of the buffer capacity is always at the plQ, i.e. 4.75 for the acetate buffer. The maximum buffer capacity increases in direct proportion to the buffer concentration. At the same time, the pH range over which a particular buffer capacity is achieved increases with increasing concentration. For example, a 10 mM acetate buffer has a buffer capacity in excess of0.005 in the pH range from... 

Figure 6.3 summarizes these considerations. It is the diagram of buffer index p/pH for an acetic acid solution (C = 10 mol/L). In the region (8 buffer capacity is due to the acetate ion, the only species present in a noticeable quantity. The buffer index is very weak = 3.5 x 10 . At pH = 2.5, we could believe that the value would be about the same as immediately above because the only present form of the buffer couple is the acidic one. Actually, we find = 8.3 x 10 . The value is far higher than anticipated. In fact, at this pH value, the buffer capacity of the couple H3O+/H2O is already noticeable. Finally, Fig. 6.3 shows that the buffer index of the couple is maximum for pH = pKa. 

Fig. 16E shows an inset of the magnitude of the pulse signal as a function of the chosen pH point for sensing (the pH point is the pH value where the analyte redox system exhibits a reversible potential consistent with the applied sensor potential). It can be seen that the signal is higher at pH 9.5 and a decrease occurs toward pH 7 and towards pH 12. This is caused by the phosphate buffer system. At the point of lowest buffer capacity (= pH 9.5) the maximum sensor response is detected. This establishes a secondary filter where the choice of the sensor potential can enhance or eliminate redox responses dependent on the type of buffer medium and the reversible potential of the analyte. 

Buffer capacity of a weak acid reaches its maximum value when pH = pK. ... 

---