Acids Titration weak

Anion of a weak acid titrated with a strong acid. The pH at the equivalence point is given by ... 

These features of the weak acid titration curve are summarized in the flow chart shown in Figure 18-5. 

A flow chart summarizes the major species in solution and the pH calculations for the four key regions of a weak acid titration curve. 

Beyond the buffer region, when nearly all of the acetic acid has been consumed, the pH increases sharply with each added drop of hydroxide solution. The titration curve passes through an almost vertical region before leveling off again. Recall from Chapter 4 that the stoichiometric point of an acid titration (also called the equivalence point) is the point at which the number of moles of added base is exactly equal to the number of moles of acid present in the original solution. At the stoichiometric point of a weak acid titration, the conjugate base is a major species in solution, but the weak acid is not. 

Although the exact pH at the stoichiometric point depends on what weak acid is being titrated, the qualitative result of Example is reproduced for every titration of a weak acid with a strong base. At the stoichiometric point of a weak acid titration, the exact value of the pH is determined by for the conjugate base, and it is always greater than 7.0. 

The sequence of figures, from the initial state to Point C, shows a progressive loss of H A with matching increases in A and H2 O, as we would expect for a weak acid titration. The figures make sense. 

FIGURE 5.1 Acid-base titration curves (a) 0.10 M HCI (strong acid) titrated with 0.10 M NaOH (strong base), (b) 0.010 M HCI titrated with 0.010 M NaOH, and (c) 0.10 M acetic acid (weak acid) titrated with 0.10 M NaOH. 

The above is Equation (17-10) solutions in this region of a weak acid titration are buffers. 

There are a few main types of titrations a strong acid titrated with a strong base (or a strong base titrated with a strong acid) a weak acid titrated with a strong base a weak base titrated with a strong acid and a polyprotic acid titrated with a strong base. Each one of these produces characteristic results and will need to be discussed separately. For the solutions of weak acids and bases, the process is complicated by the common-ion effect. 

Of considerable difference with the titrations of weak acids and weak bases are the buffering effects of the conjugate salts. The titration curves (see Figure 14.6) all contain a buffering region near the equivalence point where most of the solution consists of the conjugate base (for a weak acid titration a conjugate acid for a weak base titration). 

The presence of sodium hydroxide as an impurity in a lignosulfonate in concentrations up to 15% was found not to interfere with the weak acid titration since an excess of hydrochloric acid was available to neutralize the alkali. However, sodium sulfate interfered with the titration when present in lignosulfonates in levels exceeding 5% (Pobiner 1983). 

For the weak acid titration the corresponding change in [HsO" "] is very small, i.e. from... 

The shape of the titration curve depends on the strength of the acid in solution. After the point of equivalence the shape is nevertheless identical but before the shape is characteristic for the acid strength. The climb of the titration curve is more step for the weak acid than for the strong acid. The levelling-off of the curve for the weak acid is caused by the buffer effect as earlier emphasised. A buffer system functions optimal when [HA] = [A ] which is exactly half way to the equivalence point. This is way the weak acid titration curve here is more flat than it is the case for the strong acid at the same location. 

Weak-acid titrations require careful selection of the indicator. 

The shape of the pH curve for a strong base-strong acid titration before the equivalence point is quite different from the shape of the pH curve for a strong base-weak acid titration... 

For a strong base-weak acid titration, the pH is greater than 7 at the equivalence point because of the basic properties of A ... 

Sketch the titration curve for a weak acid titrated by a strong base. When performing calculations concerning weak acid-strong base titrations, the general two-step procedure is to solve a stoichiometry problem first, then to solve an equilibrium problem to determine the pH. What reaction takes place in the stoichiometry part of the problem What is assumed about this reaction ... 

In both strong/weak acid titrations with NaOH then, the Gran relationships lead to give a LINEAR titration curve that permits high precision in locating the equivalence points, particularly in problem cases where the acid is very weak or its concentration is very low. Further, determination of the slope and intercepts ofthe linear portions of the curves can be readily accomplished by the linear regression command /TAR /AR /DAR. ... 

The shape of the curves do not resemble common weak acid titration curves this is due to the high number of different acidic groups, mainly carboxylic and phenolic, with varying dissociation constants the modeling of these curves will be treated in Chapter 13. 

Let us examine the problems of very weak acid titration in some detail. It should be stressed that any method must be tested with known standards to prove that it does what is required—here, produce an end point that agrees satisfactorily with the known equivalence point. The large error that can occur when this is omitted will be illustrated. 

In practice, one can multiply through equation (7-7) by any constant for convenient calculation purposes. For example, with pH data between 7 and 10, it may be easier to use antilogs of (— pH -I- 7) equation (7-7) can be multiplied through by 10. An example plot of the calculated functions for an ideal titration of 0.1 M NH4 (pX° = 9.24) by 0.1 M NaOH is shown in Figure 7-2. The weak acid titration function W, and the base function B, have been multiplied by constants to give one figure before the decimal point. 

Weak Acid Titrated with Strong Base... 

Calculating Points on a Titration Curve Weak Acid Titrated with a Strong Base... 

Here are the principal features of the titration curve for a weak acid titrated with a strong base (Fig. 17-10). 

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