Acetic Acid titration curve

Figure 14-8 Plots of relative amounts of acetic acid and acetate ion during a titration. The straight lines show the change in relative amounts of HOAc (oo) and OAc faj) during the titration of 50.00 mL of 0.1000 M acetic acid. The curved line is the titration curve for the system.

The titration curve for 100 mL of 0.1 M acetic acid titrated with 0.1 M sodium hydroxide is shown in Figure 8.5. The neutralization reaction is... 

Sketch the titration curve for the titration of 50.0 mb of 0.100 M acetic acid with 0.100 M NaOH. This is the same titration for which we previously calculated the titration curve (Table 9.3 and Figure 9.6). 

FIGURE 2.12 The titration curve for acetic acid. Note that the titration curve is relatively flat at pH values near the pK in other words, the pH changes relatively little as OH is added in this region of the titration curve. 

Titration is the analytical method used to determine the amount of acid in a solution. A measured volume of the acid solution is titrated by slowly adding a solution of base, typically NaOH, of known concentration. As incremental amounts of NaOH are added, the pH of the solution is determined and a plot of the pH of the solution versus the amount of OH added yields a titration curve. The titration curve for acetic acid is shown in Figure 2.12. In considering the progress of this titration, keep in mind two important equilibria ... 

The shapes of the titration curves of weak electrolytes are identical, as Figure 2.13 reveals. Note, however, that the midpoints of the different curves vary in a way that characterizes the particular electrolytes. The pV, for acetic acid is 4.76, the pV, for imidazole is 6.99, and that for ammonium is 9.25. These pV, values are directly related to the dissociation constants of these substances, or, viewed the other way, to the relative affinities of the conjugate bases for protons. NH3 has a high affinity for protons compared to Ac NH4 is a poor acid compared to HAc. 

FIGURE 2.13 The titration curves of several weak electrolytes acetic acid, Imidazole, and ammonlnm. Note that the shape of these different curves Is Identical. Only their position along the pH scale Is displaced. In accordance with their respective affinities for ions, as reflected In their differing values. 

A weak acid-strong base titration. The curve represents the titration of 50.00 mL of 1.000 M acetic acid, HC2H3O2. with 1.000 /W NaOH. The solution at the equivalence point is basic (pH = 9.22). Phenolphthalein is a suitable indicator. Methyl red would change color much too early, when only about 33 mL of NaOH had been added. Bromthymol blue would change color slightly too quickly. 

Weak acid with a strong base. In the titration of a weak acid with a strong base, the shape of the curve will depend upon the concentration and the dissociation constant Ka of the acid. Thus in the neutralisation of acetic acid (Ka— 1.8 x 10-5) with sodium hydroxide solution, the salt (sodium acetate) which is formed during the first part of the titration tends to repress the ionisation of the acetic acid still present so that its conductance decreases. The rising salt concentration will, however, tend to produce an increase in conductance. In consequence of these opposing influences the titration curves may have minima, the position of which will depend upon the concentration and upon the strength of the weak acid. As the titration proceeds, a somewhat indefinite break will occur at the end point, and the graph will become linear after all the acid has been neutralised. Some curves for acetic acid-sodium hydroxide titrations are shown in Fig. 13.2(h) clearly it is not possible to fix an accurate end point. 

Weak acids with weak bases. The titration of a weak acid and a weak base can be readily carried out, and frequently it is preferable to employ this procedure rather than use a strong base. Curve (c) in Fig. 13.2 is the titration curve of 0.003 M acetic acid with 0.0973 M aqueous ammonia solution. The neutralisation curve up to the equivalence point is similar to that obtained with sodium hydroxide solution, since both sodium and ammonium acetates are strong electrolytes after the equivalence point an excess of aqueous ammonia solution has little effect upon the conductance, as its dissociation is depressed by the ammonium salt present in the solution. The advantages over the use of strong alkali are that the end point is easier to detect, and in dilute solution the influence of carbon dioxide may be neglected. 

Mixture of a strong add and a weak add with a strong base. Upon adding a strong base to a mixture of a strong acid and a weak acid (e.g. hydrochloric and acetic acids), the conductance falls until the strong acid is neutralised, then rises as the weak acid is converted into its salt, and finally rises more steeply as excess alkali is introduced. Such a titration curve is shown as S in Fig. 13.2(d). 

The method may be used to titrate a mixture of acids which differ greatly in their strengths, e.g. acetic (ethanoic) and hydrochloric acids the first break in the titration curve occurs when the stronger of the two acids is neutralised, and the second when neutralisation is complete. For this method to be successful, the two acids or bases should differ in strength by at least 10s to 1. 

Determination of iron(III) in the presence of aluminium. Iron(III) (concentration ca 50 mg per 100 mL) can be determined in the presence of up to twice the amount of aluminium by photometric titration with EDTA in the presence of 5-sulphosalicylic acid (2 per cent aqueous solution) as indicator at pH 1.0 at a wavelength of 510 nm. The pH of a strongly acidic solution may be adjusted to the desired value with a concentrated solution of sodium acetate about 8-10 drops of the indicator solution are required. The spectrophotometric titration curve is of the form shown in Fig. 17.23. 

Schematic profile of the titration curve for a weak acid H A titrated with hydroxide ions. The titration can be divided into four regions that differ in the major species present in solution. The pH values are those for titration of 0.500 M acetic acid.

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. 

Beyond the stoichiometric point, in the final region of the titration curve, the concentration of acetic acid is very close to zero. There are no acid molecules to react with any further hydroxide ions, so excess hydroxide ions are... 

Attention is secondly focused on Figure 6.5 (B) which represents the titration curve of a weak acid against a strong base. The poor dissociation of the weak acid is reflected in the initial conductivity being low. The addition of alkali results in the formation of highly ionized sodium acetate and the conductance of the solution begins to increase. 

At first we determined, by means of the DVP method, ifTMAX of 2,4-dinitro-phenolate, 2,5-dinitrophenol picrate, acetate and benzoate, which lay between 10 3 and 10 5. Next, separate potentiometric titrations of 2,5-dinitrophenol and picric acid were carried out on the basis of the previously known (see above) ptfax = 6.5 and P hx2- = 100 for 2,5-dinitrophenol and p.fiTHX = 3.0 for picric acid, we calculated titration curves for estimated values of 0 and obtained, for the best fit between the experimental and calculated curves, K o = 10 21 for both 2,5-dinitrophenol and picric acid. In both instances changing fTMA0H for 1 to 10 6 did not alter the calculated titration curve. Finally, for potentiometric titrations of other acids with TMAOH while using / TMAX values from DVP results, in addition to Kn 0 = 10 21, we obtained the best fit between the experimental and calculated curves again when pifbenzoic acid = 1 (see Fig. 4.12)... 

This can be seen from the titration curve for phosphoric acid [6] shown in Figure 10.2. In practice the mono- and di-sodium salt system is used most extensively, since this covers the pH range over which precise control is most often needed. These phosphate buffers are more resistant than the acetate systems to temperature-induced changes. 

L (a) The two curves cross the point at which half of the total acetate is present as acetic acid and half is present as acetate ion. This is the half equivalence point in a titration, where pH = pK3 = 4.74. 

To select an indicator for an acid-base titration it is necessary to know the pH of the end point before using equation (5.5) or standard indicator tables. The end point pH may be calculated using equations (3.27), (3.29) or (3.30). Alternatively, an experimentally determined titration curve may be used (see next section). As an example, consider the titration of acetic acid (0.1 mol dm 3), a weak acid, with sodium hydroxide (0.1 mol dm-3), a strong base. At the end point, a solution of sodium acetate (0.05 mol dm 3) is obtained. Equation (3.28) then yields... 

The curve shown is for the titration of sodium acetate with hydrochloric acid. Before the equivalence point, there is a slight increase in conductance as Cl- replaces CKCOO which is converted to undissociated acetic acid. After the equivalence point, the conductance increases linearly with the addition of H+ and Cl-. 

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. 

FIGURE 5.2 A family of acid-base titration curves for a 0.10 M strong acid (HC1) and three weak acids, as indicated (0.10 M each), titrated with 0.10 M NaOH (strong base). HAc is a representation of acetic acid. 

In these equations, HA symbolizes a weak acid and A symbolizes the anion of the weak acid. The calculations are beyond our scope. However, we can correlate the value of the equilibrium constant for a weak acid ionization, Ka, with the position of the titration curve. The weaker the acid, the smaller the IQ and the higher the level of the initial steady increase. Figure 5.2 shows a family of curves representing several acids at a concentration of 0.10 M titrated with a strong base. The curves for HC1 and acetic acid (represented as HAc) are shown, as well as two curves for two acids even weaker than acetic acid. (The IQ s are indicated.)... 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

Compare the titration curves for 0.10 M hydrochloric acid and 0.10 M acetic acid each titrated with 0.10 M sodium hydroxide. What parts of the titrations curves are the same and what parts are different Why Compare the inflections points for the two curves and tell what impact the differences have on indicator selection. 

Comparison of an alkalimetric titration curve of an equimolar (1 O 4 M) solution of acetic acid (pK = 4.8) and phenol (pK = 10) with a humic acid that contains 10 4 M carboxylic groups. 

Figure 4.6 A titration curve. Acetic acid (10 ml of a 0.1 mol l-1 solution) was titrated with a sodium hydroxide solution (0.2 mol l-1) and the pH of the resulting solution plotted against the amount of alkali added.

In Investigation 8-A, you performed a titration and graphed the changes in the pH of acetic acid solution as sodium hydroxide solution was added. A graph of the pH of an acid (or base) against the volume of an added base (or acid) is called an acid-base titration curve. 

Figure 2.5. The titration curve Ng (dashed line), and the BI, plotted as 1 - 0 (full line) as a function of pH = -logjQ[fl] for acetic acid with pK =14, pk j = 4.754. (Note that higher pH corresponds to lower concentration [fl].)...

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