ionic equilibrium titration curves

Titration curves are informative. They are able to give us information about the bascity of the acid, the volume of base required for neutralisation. the pH of the solution and the type of indicator which should be used There are a few situations to take note when we sketch a titration curve.

1. General points to take note.

In drawing the titration curve, we will need to take note what solution would be in the conical flask and what will be in the burette. If an acid is in the conical and the base is in burette, the pH curve will show an increasing pH as the solution in the burette is added into the conical flask.

In sketching the titration curves, the following are points to take note.

• The shape of the curves (e.g is it moving from low pH to high? the number of equivalence points?)
• The volumes where neutralisation has occurred (i.e. the verticals of the graph).
• The volumes and the pH of the effective buffer region. This requires you to check which point the number of moles of weak acid is the same as its conjugate base (or vice versa).
• For the final point, do not assume that the effective buffer capacity is before neutralisation.

2. Examples of weak dibasic acid and weak diacidic base.

Organic bases and acids are generally examples of weak base and weak acid respective. For example CH₃NH₂ and CH₃COOH respectively. The former is a base because N has a lone pair of electrons to form a dative bond with H⁺, hence making it a proton acceptor (thus a base). While the latter has a -COOH which enables a H⁺ (thus an acid) to be donated. The above are examples of organic moles which are dibasic acid and diacidic base.

Using dibasic acids as an example, these substances are able to donate two H⁺. However, the protons are not donated at the same time. The acid (malic acid = HOOCCH₂CH(OH)COOH) will dissociation once first to form HOOCCH₂CH(OH)COO^-, then the acid will dissociate a second time (from HOOCCH₂CH(OH)COO^-) to give ^-OOCCH₂CH(OH)COO^- and H⁺.

Each dissociation has its own acid dissociation constant with the first acid dissociation constant generally larger than the second.  The easiest reason is that the second acid dissociation is the removal of a H⁺ from an anion; which will be more difficult than removing a proton from a neutral acid molecule.  

3. Titrating a weak monobasic acid against a strong base

When CH₃COOH placed in a conical flask and NaOH is added to the acid. The pH increases as more NaOH is added and that is due to decreasing amount of the weak acid CH₃COOH and increasing amount of conjugate base CH₃COO^-. At the point where n(CH₃COOH) = n(CH₃COO^-), that is the maximum buffer capacity. At 2A cm³ of NaOH added, all the acid is used up and only the conjugate base is left, hence the pH is greater than 7.

4. Titrating a weak diacidic base against a strong acid.

When titrating a weak diacidic base against a strong monobasic acid, with both having the same concentration. The volume required for the monobasic acid to completely react with the two basic groups in the diacidic base is twice the volume of the base used. In addition, the effective buffer capacity will be 14 - pK_b values.

However, when we titration against a strong dibasic acid, the volume required by the dibasic acid is the same as the volume of diacidic base used. This is when both acid and base have the same concentration. The pH of the effective buffers remain the same.

5. Titrating a weak dibasic acid against a strong base.

When titrating a weak dibasic acid against a strong monoacidic base, with both having the same concentration. The volume required for the monoacidic base to completely react with the two acidic groups in the dibasic acid is twice the volume of acid used. In addition, the effective buffer capacity will be the pK_a values.

However, when we titration against a strong diacidic base, the volume required by the base is the same as the volume of acid used. This is when both acid and base have the same concentration. Note that the pH of the effective buffers remain the same. 

6.  Conclusion

Hence, when sketching the titration curves, it is good advice to think about how the titration is done.  Do not rush through with fixed model, e.g. the point where maximum buffer capacity exists is always between the starting point and the equivalence points.  Always fist ask yourself what is in the solution at any particular significant point on the curve.

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^{Article written by Kwok YL 2010. (edited in 2011)}

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