Strong acids titration

U 5 Calculate the pH at any point in a strong base-strong acid titration (Toolbox 11.1 and Example 11.4). 

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. 

It is interesting to compare the case of a strong acid titrated with a strong base (in the last section) with the titration of a weak acid, such as acetic acid, with a strong base, such as NaOH. The difference between the weak acid-strong base case and the strong acid-strong base case just discussed is that for the same... 

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 base-strong acid titration... 

Weak acid/strong base and weak base/strong acid titrations... 

FIGURE 11.14 Methyl orange can be used for a weak base-strong acid titration. Phenolphthalein would be wholly unreliable because its color change occurs well away from the stoichiometric point. The pH curves are superimposed on approximations to the color of the indicators in the neighborhoods of their end points. 

A strong base-strong acid titration curve. The curve shown is for titration of 40.0 mL of 0.100 M NaOH with 0.100 M HC1. The pH at the equivalence point is 7.00. 

Weak Base-Strong. Acid Titration activity... 

FIGURE 16.9 A weak base-strong acid titration curve. The curve shown is for titration of 40.0 mL of 0.100 M NH3 with 0.100 M HC1. The pH is 11.12 at the start of the titration, 9.25 (the pKa value for NH4+) in the buffer region halfway to the equivalence point, and 5.28 at the equivalence point. Note that methyl red is a good indicator for this titration, but phe-nolphthalein is unacceptable. 

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. 

With a weak acid, strong base titration like this one, the situation is more critical for two reasons. First, the pH changes a bit more gradually than with strong acid titrations. As a result, an indicator like methyl red will change color well before the equivalence point is reached. 

Strong Acid Titrated with Strong Base... 

We turn now to the titration of a weak acid with a strong base (the titration of a weak base with a strong acid is analogous). The equivalence point has the same meaning as for a strong acid titration. At the equivalence point, the number of moles of base added (in volume V ) is equal to the number of moles of acid originally present (in volume Vq). So, once again... 

Note The choice of indicator is critical in weak acid-strong base and weak base-strong acid titrations because the slope of the transition is not so steep and the pH of the equivalence point can be very different from 7. pK (indicator) pH at the equivalence point.)... 

Titrating the water with a strong base such as NaOH, the first inflection point around pH 4.5 is taken as the endpoint of the mineral acidity or strong-acid titration. It corresponds to completion of the reaction... 

Figure 5.6 TTie carbonate distribution diagram of a solution with constant Cf = 2.5 x 10" M showing (a) the strong acid titration curve for the same solution from pH 12 to 3 and strong base titration curve between pH 3 and 12. Dashed straight lines in (b) indicate concentrations of H+ and OH", which are independent of Cj. Modified after V. L. Snoeyink and D. Jenkins, Water Chemistry. Copyright 1980 by John Wiley Sons, Inc. Used by permission of John Wiley Sons, Inc.

Strong acid titrated f 13 1 11 - 9 - J rong basetitrated with strong acid ... 

How does the endpoint pH of a strong base-strong acid titration compare with that of a weak base-strong acid titration ... 

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). 

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