Weak Base-Strong Acid Titration Curves

Estimating the p/Cg for a weak acid from its titration curve with a strong base. 

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. 

If directed by your instructor, also obtain weak acid-strong base, strong base-strong acid, and weak base-strong acid titration curves. 

The curve for a weak acid-strong base titration The weak acid here is propanoic acid, CH3CH2COOH. 

In textbooks of computational chemistry you will invariably find examples calculating the pH = - lg [H+]/(mol/l)> in weak acid - strong base or strong acid - weak base solutions. Indeed, these examples are important in the study of acids, bases and of complex formation, as well as for calculating titration curves. Following (ref. 24) we consider here the aquous solution that contains a weak tribasic acid H A and its sodium salts NaH, Na HA and Na A in known initial concentrations. The dissociation reactions and equilibrium relations are given as follows. 

FIGURE 11.13 Phenolphthalein can be used to detect the stoichiometric point of a weak acid-strong base titration, but methyl orange would give a very inaccurate indication of the stoichiometric point. The pH curves are superimposed on approximations to the color of the indicators in the neighborhoods of their end points. 

We can calculate pH titration curves using the principles of aqueous solution equilibria. To understand why titration curves have certain characteristic shapes, let s calculate these curves for four important types of titration (1) strong acid-strong base, (2) weak acid-strong base, (3) weak base-strong acid, and (4) polyprotic acid-strong base. For convenience, we ll express amounts of solute in millimoles (mmol) and solution volumes in milliliters (mL). Molar concentration can thus be expressed in mmol/mL, a unit that is equivalent to mol/L ... 

As an example of a weak acid-strong base titration, let s consider the titration of 40.0 mL of 0.100 M acetic acid with 0.100 M NaOH. Calculation of the pH at selected points along the titration curve is straightforward because we ve already met all the equilibrium problems that arise. 

The results of pH calculations for the titration of 0.100 M CH3C02H with 0.100 M NaOH are plotted in Figure 16.7. Comparison of the titration curves for the weak acid-strong base titration and the strong acid-strong base case shows several significant differences ... 

FIGURE 16.8 Various weak acid-strong base pH titration curves. The curves shown are for titration of 40.0 mL of 0.100 M solutions of various weak acids with 0.100 M NaOH. In each case, the equivalence point comes after addition of 40.0 mL of 0.100 M NaOH, but the increase in pH at the equivalence point gets smaller and the equivalence point gets more difficult to detect as the Ka value of the weak acid decreases. 

Beyond the equivalence point, the curves for the weak acid-strong base and strong acid-strong base titrations are identical because the pH in both cases is determined by the concentration of OH- from the excess NaOH. 

The relative precision is inversely related to the slope of the titration curve at the end point, and important conclusions from this relation are for a strong acid-strong base titration the relative precision is approximately VKJC , for a weak acid-strong base titration the relative precision is approximately JkJKjC, and for an intermediate end point in a polybasic acid titration the relative precision is approximately... 

Figure 20-11. Spreadsheet for weak acid-strong base titration curve.

Weak Acid/Strong Base Titration Curves... 

The previous discussions on salts of a strong acid/strong base, weak acid/strong base, weak base/strong acid, weak acid/weak base and buffers form the basis of the interpretation of pH titration curves. Although neutralisation and acid/base behaviour have some features in common, i.e. they both involve acid/base behaviour and both are proton transfer reactions, distinctions must be drawn between them. 

The curve for a weak acid/weak base titration is the composite of the curves for a weak acid/ strong base and weak base/strong acid titrations, as indicated (Figure 6.5). 

Features of the Curve When we compare this weak acid-strong base titration curve with the strong acid-strong base titration curve (dotted curve portion in Figure 19.8 corresponds to bottom half of curve in Figure 19.7), four key regions appear, and the first three differ from the strong acid case ... 

Figure 19.8 Cur e for a weak acid-strong base titration. The curve for the titration of 40.00 mL of 0.1000 M CH3CH2COOH (HPr) with 0.1000 M NaOH is compared with that for the strong acid HCi dotted curve portion). Phenoiphthaiein (photo) is a suitabie indicator here.

Calculating the pH The calculation procedure for the weak acid-strong base titration is different from that for the strong acid-strong base titration because we have to consider the partial dissociation of the weak acid and the reaction of the conjugate base with water. There are four key regions of the titration curve, each of which requires a different type of calculation to find [H30 ] ... 

In the previous case, we titrated a weak acid with a strong base. The opposite process is the titration of a weak base (NH3) with a strong acid (HCl), shown in Figure 19.9. Note that the curve has the same shape as the weak acid—strong base curve (Figure 19.8), but it is inverted. Thus, the regions of the curve have the same features, but the pH decreases throughout the process ... 

Figure 19.9 Curve for a weak base-strong acid titration. Titrating 40.00 mL of 0.1000 M NH3 with a solution of 0.1000 M HCl leads to a curve whose shape is the same as that of the weak acid-strong base curve in Figure 19.8 but inverted. The midpoint of the buffer region occurs when [NH3] = [NH4 ] the pH at this point equals the pKa of NH4. Methyl red (photo) is a suitable Indicator here.

Understand how an acid-base indicator works, how the equivalence point and end point in an acid-base titration differ, and how strong acid-strong base, weak acid-strong base, and strong acid-weak base titration curves differ explain the significance of the... 

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 ... 

At the various points in your titration curve, list the major species present after the strong base (NaOH, for example) reacts to completion with the weak acid, HA. What equilibrium problem would you solve at the various points in your titration curve to calculate the pH Why is pH > 7.0 at the equivalence point of a weak acid-strong base titration Does the pH at the halfway point to equivalence have to be less than 7.0 What does the pH at the halfway point equal Compare and contrast the titration curves for a strong acid-strong base titration and a weak acid-strong base titration. 

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