7.4.2: Making Sense of Vinegar and Other Acids

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Now let us consider another common acid: acetic acid. If wine is left open to the air, it will often begin to taste sour because the ethanol in wine reacts with oxygen in the air and forms acetic acid. Acetic acid belongs to a family of organic compounds known as carboxylic acids. It has one acidic proton attached to the oxygen.

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If we measure the pH of a 0.10-M solution of acetic acid, we find that it is about 2.8. The obvious question is why the pH of a 0.10-M solution of acetic acid is different from the pH of a 0.10-M solution of hydrochloric acid? The explanation lies in the fact that acetic acid (CH₃COOH) does not dissociate completely into CH₃CO₂^– and H₃O⁺ when it is dissolved in water. A pH of 2.8 indicates that the [H₃O⁺] = 10^–2.8. This number can be converted into 1.6 x 10–3 M. About 1.6% of the added acetic acid is ionized (a form known as acetate ion, CH3COO^–). The rest is in the protonated form (acetic acid, CH3COOH). The specific molecules that are ionized changes all the time; protons are constantly transferring from one oxygen to another. You can think of this process in another way: it is the system that has a pH, not individual molecules. If we look at a single molecule of acetic acid in the solution, we find that it is ionized 1.6% of the time. This may seem a weird way to think about the system, but remember, many biological systems (such as bacteria) are quite small, with a volume of only a few cubic microns or micrometers (a cubic micron is a cube 10^–6m on a side) and may contain a rather small number of any one type of molecule. Thus, rather than thinking about the bulk behavior of these molecules, which are relatively few, it can be more useful to think of the behavior of individual molecules averaged over time. Again, in an aqueous solution of acetic acid molecules, most of the molecules (~98.4%) are in the un-ionized form, so any particularly molecule is un-ionized ~98.4% percent of the time.

We can measure the pH of the solutions of many acids of known concentrations, and from these measurements make estimates of the strength of the acid. Strong acids, such as nitric, sulfuric, and hydrochloric are all totally ionized in solution. Weaker acids, such as organic acids, ionize to a much lesser extent. However, given the low naturally occurring concentrations of hydronium and hydroxide ions in pure water, even weak acids can significantly alter the pH of an aqueous solution. The same behavior applies to weak bases.

Conversely, if weak acids or bases are dissolved in solutions of different pH, the amount of ionization of the group may be significantly changed. For example, as we will see in chapters 8 and 9, if we added a weak acid to a solution that was basic (for example at pH 9), we would find that much more of the acid will ionize. Many biological molecules contain parts (called functional groups) that behave as weak acids or weak bases. Therefore, the pH of the solution in which these molecules find themselves influences the extent to which these functional groups are ionized. Whether a part of a large molecule is ionized or not can dramatically influence a biomolecule’s behavior, structure, and interactions with other molecules. Thus, changes in pH can have dramatic effects on a biological system. For example, if the pH of your blood changes by ± 0.3 pH units, you are likely to die. Biological systems spend much of the energy they use maintaining a constant pH (typically around 7.35-7.45).¹⁴⁰ In addition, the pH within your cells is ¹⁴¹tightly regulated and can influence cellular behavior.

Questions to Answer

How would you calculate the molarity of pure water?
What percentage of water molecules are ionized at 25°C?
If the pH of a solution (at 25^oC) is 2.0, what is the [H3O+]? What is the [–OH]?
If the pH of a solution (at 37^oC) is 2.0, what is the [H3O+]? What is the [–OH]?
What would be the pH of a 0.01-M solution of HCl at 25^oC?
If the pH of a 0.1-M solution of NH4Cl is 5.1, what is the [H3O+]?
Draw out a molecular level picture of what you imagine solutions of NaCl and NH4Cl look like.
Why does acetic acid only have one acidic proton (after all, it does have a total of four protons)?
Why is acetic acid more acidic than ethanol? What is it about the structure of acetic acid that makes it more acidic?

Questions for Later

Why do you think we keep specifying the temperature in our discussions of reactions?

Questions to Ponder

Carboxylic acid groups, –COOH, are common in large biomolecules. What would be the effect of raising or lowering the pH on carboxylate side chains?
What effect do you think that might have on the properties of the biomolecule (solubility, interactions with other molecules, etc.)?
Amino groups are also common. What would be the effect of raising or lowering the pH on an amino group?

References

¹⁴⁰http://www.bbc.co.uk/dna/h2g2/A8819652

¹⁴¹ http://www.ncbi.nlm.nih.gov/pmc/arti...00237-0011.pdf