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Unit 10: Acids and Bases—The Voyage of the Proton

Section 5: Neutralization and Acid-Base Titrations

We know that in a regular glass of pure neutral water there are small amounts of hydronium (H3O+) and hydroxide (OH-) ions, but they are equal in number and balance each other and we say that the solution is neutral or has a pH of 7. However, what happens when large amounts of hydronium ions and hydroxide ions are combined together by mixing an acidic solution with a basic solution?

Let's consider hydrochloric acid (HCl) which is the major component of stomach acid, and sodium hydroxide (NaOH) which is often referred to as "lye," and is used in soap-making. When dissolving a strong acid like hydrochloric acid in water along with a strong base like sodium hydroxide, at first it produces hydronium ions (H3O+) and chloride (Cl-) ions from the dissociation of HCl, along with hydroxide (OH-) and sodium ions (Na+) from the dissociation of NaOH. But nearly all of the hydronium and hydroxide combine to make water molecules, leaving behind only spectator chloride and sodium ions. If the amount of hydrochloric acid is exactly the same as the amount of sodium hydroxide, in moles, then each hydronium should have a hydroxide to react with leaving behind a solution that is now neutral. In theory, it should be a glass of just pure water now, with a pH of 7 and normal table salt (NaCl) dissolved in it. We start with two extremely caustic substances and end up with benign products.

In general, when acids and bases react together, they neutralize each other and the pH of the solution gets closer to 7. This is called a "neutralization reaction." Only when an equal amount of a strong acid and a strong base are added together is the pH going to be exactly 7. But many times, weak acids and weak bases are combining. Still, the pH will get closer to 7 but the solution won't be neutral. In the case of acid rain, the rocks and soil are often basic, especially carbonate rocks found in certain parts of the country. As the weak acids in acid rain fall on the rock, they are neutralized by the bases on the leaves, in the dirt and in the rocks. However, many of these bases, such as those in the rocks, are non-renewable. Once a century of acid rain falling of the rocks has neutralized all the base in the rocks, the acid rain just enters the ecosystem with nothing to neutralize it.

Titration
Phenolphthalein: A Common Laboratory Indicator

Figure 10-9. Phenolphthalein: A Common Laboratory Indicator

© Wikimedia Commons, Public Domain.

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Phenolphthalein: A Common Laboratory Indicator

Figure 10-9. Phenolphthalein: A Common Laboratory Indicator

The molecule phenolphthalein is clear in the presence of acid and pink in the presence of base. The H2In structure is colorless. When it is in a basic solution with a pH higher than 8.2, it has lost two H+ ions and is now the In2- structure, which is pink or fuchsia. It is an excellent indicator for finding the equivalence point in an acid-base titration for simple acids.

In Section 5 of Unit 8, titrations were introduced as a way of combining two solutions together to have a controlled stoichiometric reaction take place. It requires having a solution of a known concentration, usually in a burette, that will react with a solution of an unknown concentration in a beaker below it. Any reaction will work for a titration, but to be able to tell when the reaction is done, there must be a visible endpoint. For acid-base reactions, indicators can provide just this type of information with a color change to indicate the endpoint. Therefore, acid-base neutralization reactions are one of the most common types of reactions used in titration analyses. Thus, a titration can be used to determine the amount of acid or base in an unknown solution.

When titrating strong acids with strong bases, the endpoint is reached when the pH will be exactly 7 because the solution should just be a pure water solution with neutral electrolytes in it. Once the acid is gone, any more base we add to the flask will not be neutralized, and the solution's pH will start to rise. However, even a small fraction of a drop from the burette of excess strong base will move the pH up a unit or two. We use an indicator to detect the point at which this happens. One such indicator, phenolphthalein (C20H14O4), is clear and colorless in acidic solutions but turns deep pink in basic solutions if the pH is higher than 8.2 (Figure 10-9). The fact that phenolphthalein changes color a little above 7.0 usually leads to only very small error in the experiment.

Acid-base Indicators Come In Many Types.

Figure 10-10. Acid-base Indicators Come In Many Types.

© Left: Wikimedia Commons, CC License 3.0. Author: Xato, 02 January 2007. Right: Wikimedia Commons, Public Domain.

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Acid-base Indicators Come In Many Types.

Figure 10-10. Acid-base Indicators Come In Many Types.

In addition to phenolphthalein, other indicators include bromothymol blue and methyl orange. For solutions that are just only slightly basic at their endpoint, bromothymol blue is a good indicator. Just below a pH of seven, the solution has a yellow color that becomes blue just above a pH of 7. When the endpoint of a titration is going to be rather acidic, methyl orange is used. Methyl orange does not have a sharp transition, but undergoes a change from pink-red to yellow-orange around pH 3.5.

However, when titrating a base with an acid, one would want an indicator that changes color just a little bit below 7. And what happens when titrations involve a strong acid or base being added to a weak acid or base? In that case, the pH at the end point might be higher or lower than 7 and one must choose an appropriate indicator that changes color in the right pH range. Two commonly used indicators include bromothymol blue and methyl orange, and there are many others. Usually, bromothymol blue is useful when you expect the endpoint to have a pH almost exactly near 7, and methyl orange is ideal when the pH at the endpoint is weakly acidic. Figure 10-10 shows the two colors that each of these indicators switch between.

Glossary

Neutralization reaction

A reaction in which an equal amount of an acid and a base are mixed together, cancelling each other out, and making the solution neutral with a pH of 7.

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