Buffer capacity Henderson equation

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 ... 

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... 

Plot the pH-buffer capacity curve for mixtures of acetic acid and sodium acetate of total concentration 0.2 n. Points should be obtained for mixtures containing 10, 20, 30, 40, 50, 60, 70, 80 and 90 per cent of sodium acetate, the pH s being estimated by the approximate form of the Henderson equation. Plot the buffer capacity curve for water at pH s 1, 2, 3 and 4, and superimpose the result on the curve for acetic acid. 

This relationship is known as the Henderson-Hasselbalch equation and it shows that the pH will be equal to the when the ratio of conjugate base to acid is unity, since the final term will be zero. Consequently, the pAia of a buffer solution is an important factor in determining the buffer capacity at a particular pH. In practical terms, this means that a buffer solution will work most effectively at pH values about one unit either side of the pATa-... 

An expression for instantaneous buffer capacity, jS, can be derived using calculus. Essentially, /S is the reciprocal of the slope of the titration curve at any point. Starting with the Henderson-Hasselbalch equation ... 

A solution of a weak acid and its salt (conjugate base) or a weak base and its conjugate acid acts as a buffer solution. The quantities of buffer components required to prepare buffers solutions of known pH can be calculated from the Henderson-Hasselbalch equation. The buffering capacity of a buffer solution is maximum at the pK of the weak acid component of the buffer. Universal buffers are mixtures of polybasic and monobasic acids that are effective over a wide range of pH. 

The buffer capacity is defined as the number of moles per litre of strong monobasic acid or base required to produce an increase or decrease of one pH unit in the solution. When the concentrations of salt and acid are equal, the log term in the Henderson-Hasselbalch equation becomes the logarithm of 1, which equals 0. To move the pH of the buffer solution by one unit of pH will require the Henderson-Hasselbalch equation to become... 

If additional acid (or base) is added to a solution that contains a buffer at its p/Q value (a 1 1 mixture of HA and A ), the pH of the solution changes, but it changes less than it would if the buffer had not been present. This is because protons released by the added acid are taken up by the ionized form of the buffer (A ) likewise, hydroxyl ions generated by the addition of base are neutralized by protons released by the undissociated buffer (HA). The capacity of a substance to release hydrogen ions or take them up depends partly on the extent to which the substance has already taken up or released protons, which in turn depends on the pH of the solution. The ability of a buffer to minimize changes in pH, its buffering capacity, depends on the relationship between its pKa value and the pH, which is expressed by the Henderson-Hasselbalch equation. 

The Common-Ion Effect The Henderson-Hasselbalch Equation Buffer Capacity and Range Preparing a Buffer... 

Mathematically and geometrically, buffer capacity, p, is deduced from the Henderson-Hasselbach equation [equation (1.2), (Section 1.4.2)]. Buffer capacity is defined by equation (1.3). 

Figure 17.3 Outline of the procedure used to calculate the pH of a buffer after the addition of strong acid or strong base. As long as the amount of added acid or base does not exceed the buffer capacity, the Henderson-Hasselbalch equation. Equation 17.9, can be used for the equilibrium calculation. 

Explain how a common ion suppresses a reaction that forms it describe buffer capacity and buffer range, and und stand why the concentrations of buffer components must be high relative to the amount of added H3O+ or OH" derive and understand the usefulness of the Henderson-Hasselbalch equation explain how to prepare a buffo- ( 19.1) (SPs 19.1-19.3) (EPs 19.1-19.25)... 

What a Buffer Is and How It Works The Common-Ion Effect 618 The Henderson-Hasselbalch Equation 622 Buffer Capacity and Buffer Range 623 Preparing a Buffer 625... 

At pH = pKa + 1 the buffer capacity falls to 33% of the maximum value. This is the approximate range within which buffering by a weak acid is effective. Note at pH = pK - 1, The Henderson-Hasselbach equation shows that the ratio [HA] [A ] is... 

But why does a buffer resist changes in pH It does so because the strong acid or base is consumed by B or BH+. If you add HCI to tris, B is converted into BH+. If you add NaOH, BHf is converted into B. As long as you don t use up the B or BH+ by adding too much HCI or NaOH, the log term of the Henderson-Hasselbalch equation does not change very much and the pH does not change very much. Demonstration 9-2 illustrates what happens when the buffer does gel used up. The buffer has its maximum capacity to resist changes of pH when pH = pKu. We will return to this point later. 

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