Buffer index, acid-base

This quantity gives the capacity of an aqueous system to resist pH changes upon addition of an acid or a base. When one plots the amount of acid A (or base B) added per liter of solution (Ca, Cb) vs. pH change, the slope of the resulting curve (or titration curve) is the buffering index or buffer... 

Here we restrict ourselves to pH buffers. Van Slyke (1922) defined pH buffers as substances which by their presence in solution increase the amount of acid or alkali which must be added to cause a unit change in pH. In aqueous solutions pH buffering is especially due to the interaction of weak acids and bases and their salts with water. The quantification of this effect, the buffer capacity or buffer index, is by definition... 

In Fig. 4 is shown the buffer capacity of mixtures of 0.1 N and 0.2 N acetic acid solutions with a strong acid or with alkali. We see that between pH s of 2 and 3.5 the total buffer capacity is obtained by adding the ordinates of the two dotted lines which represent the buffer capacities of the strong and weak acid. Outside this pH-region, we have to deal only with the buffer index of the individual weak acid, strong acid, or base, without having to consider their mutual buffering action. 

The amount of acid or base that can be added without causing a large change in pH is governed by the buffering capacity of the solution. This is determined by the concentrations of HA and A". The higher their concentrations, the more acid or base the solution can tolerate. The buffer capacity (buffer intensity, buffer index) of a solution is defined as... 

The most frequently used detector in FI systems with gas-diffusion separation is the spectrophotometer. Quite often the gas-diffusion process offers sufficient selectivity to allow relatively non-specific chemical reactions in the acceptor stream to detect the analyte. Thus, carbon dioxide, sulfur dioxide, hydrogen sulfide, ammonia may all be determined using suitable acid-base indicators in appropriate buffer solutions used as the acceptor streams. The concentration of the buffer solutions may be adjusted to suit a certain concentration range for the analyte. In order to further enhance the selectivity and/or sensitivity more specific reagents may be introduced in the acceptor streams. In the previously mentioned example on the determination of cyanide [20] a modified pyrazolone-isonicotinic acid reaction was used for such purposes. Interferences due to Schlieren effects seem not to have been reported in gas diffusion spectrophotometric systems. This is understandable, since the matrix composition of acceptor streams is usually quite uniform, and the refractive index is little affected after absorbing the gaseous analytes. 

Treatment of buffer efficiency in general is not limited to examination of various monoprotic and polyprotic acids, but rather to the behavior of mixtures of acids. Further, from plots of buffer index vs. pH, it is easy to see that strong acids and bases are reasonable buffers for the extreme (low and high) Ph ranges. This leads to the definition of a buffer as a solution that has neutralization capacity, rather the more limited but commonly used definition as a mixture of a weak acid and its conjugate base (See Figure 8.5). 

With this brief application of differential calculus, let us return to questions relating to the sharpness of titration endpoints, buffo indices, and similar characteristics of acid-base mixtures. In effect, the slope of the function of Equation 8-3, dF/dpH, is a measure of the buffer capacity and its reciprocal, dpH/dF, evaluated at the equivalence points, measures the sharpness index of the corresponding titration. 

The buffer index is a measure of the amount of acid or base that can be added to a solution before the pH changed by a given amount. If Equations 8-17 and 8-18 are rewritten as... 

The buffer index can be easily expressed as a function of the pH value and of some parameters related to the buffer. From a general standpoint, let s consider a solution containing Ca mol/L of hydrochloric acid, Cb mol/L of sodium hydroxide, and the analytical concentration C mol/L of the weak acid HA. Before writing the relations that must be satisfied, it is important to notice that the simultaneous presence of sodium hydroxide and weak acid HA results in the formation of the weak base... 

Remark The titration curve slope of an acid by a strong base is equal to dpWdC, that is, to the inverse of the buffer index (see Chaps. 9 and 10). 

Mass action relationships for pH can be evaluated by consideration of the pH buffer index, (3, which is a quantitative measure of a system s ability to withstand changes in pH and represents the inverse slope of the titration curve resulting from the addition of strong acid or base to the buffer system (Stumm Morgan 1981 Hutcheon et al. 1993) ... 

Figure 2.7 High-resolution in situ STM images of the amino acid cysteine in different buffers compared with other alkanethiol-based molecules on different low-index Au electrode surfaces. Overview images of (a) cysteamine [38], (b) mercaptopropionic acid (MPA) [154], (c) cysteine [153, 158], and (d) homocysteine [162], all on a Au(lll) electrode surface. Cysteine on (e) a Au(lll) electrode surface [153, 158] and (f) a Au(llO) electrode surface...

Acid or base capacities are therefore measured in mmol 1 . The pH value up to which the titration takes place, is presented as the index of the relevant abbreviation, e.g. ANC4 3, ANC5 75, BNC7 q, etc. The neutralizing capacity is an integral of the buffer value within given pH range. 

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