8.24: Brønsted-Lowry Acids and Bases: Buffers

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Learning Objectives

Define buffer.
Identify the components of a buffer.
Explain whether a solution containing a given combination of chemicals can be classified as a buffer.

The human body is a highly-complex, organized structure that is comprised of various cells, tissues, organs, and systems that that must work together in order to perform the biochemical processes that are necessary for sustaining life. In order to function properly, each of these physiological systems must contain specific quantities of acids and bases. A small change in the relative amount of acid or base that is present in a particular area of the body will generally not have long-lasting negative repercussions. However, most physiological systems cannot tolerate significantly-altered acid/base ratios, and destructive side-effects, such as headaches, nausea, confusion, tremors, numbness, increased heart rate, loss of consciousness, organ failure, and death, can occur, if the appropriate physiological conditions are not maintained.

In order to avoid these severely-adverse reactions, excess acid or base that is ingested or generated must be quickly neutralized by one of the three buffer systems that are present in the human body. A buffer is defined as a solution that resists change in pH, which, as will be discussed in a later section of this chapter, is a quantitative measurement of the relative amount of acid that is present in a system. A buffer generally consists of a weak Brønsted-Lowry acid and its conjugate base or a weak Brønsted-Lowry base and its conjugate acid. The acidic and basic solutes that are contained in a buffer react with and, subsequently, neutralize any basic or acidic chemicals, respectively, that are introduced into the solution. As a result, the added acidic or basic chemicals are immediately eliminated and, therefore, do not impact the relative amount of acid or base that is present in the system. However, as discussed in Chapter 7, all solutes have a corresponding solubility, which corresponds to the maximum amount of that chemical that can dissolve in a given amount of solvent. Therefore, only finite quantities of acid and base can be dissolved to prepare a buffer, and, consequently, the resultant solution can only neutralize limited amounts of added acidic or basic chemicals.

For example, explain whether a solution containing chloride ions and hydrochloric acid can be classified as a buffer.

As stated above, a buffer generally consists of a weak Brønsted-Lowry acid and its conjugate base or a weak Brønsted-Lowry base and its conjugate acid. By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Because a chloride ion, Cl^–1, and hydrochloric acid, HCl, both contain one chlorine, Cl, and have chemical formulas that differ by exactly one proton, H⁺¹, these chemicals are conjugates.

Recall that, since a Brønsted-Lowry acid donates, or loses, a proton, H⁺¹, during an acid/base reaction, the conjugate base that is produced from this transfer must contain one less proton than the acid from which it was generated. Furthermore, because a Brønsted-Lowry base accepts, or gains, a proton, H⁺¹, during an acid/base reaction, the conjugate acid that is formed as a result of this transfer must contains one more proton than the base that was initially present. Therefore, the chemical that contains more hydrogens, H, will correspond to the acid in a conjugate acid/base pair, and the substance that has fewer hydrogens, H, must be classified as the base. Therefore, because no hydrogens, H, are present in a chloride ion, Cl^–1, and hydrochloric acid, HCl, contains one hydrogen, H, hydrochloric acid, HCl, is the acid in the given solution, and the chloride ion, Cl^–1, is its conjugate base. This assignment can be confirmed by recognizing that hydrochloric acid, HCl, can be classified as a hydrohalogenated, or "HX," acid, because it contains one hydrogen, H, and one chloride ion, Cl^–1, which is a halide anion.

Finally, the chemicals that are present in a buffer solution must be weakly acidic or basic. Based on the information that is presented in Section 8.23, hydrochloric acid, HCl, is classified as a strong acid. Therefore, a solution containing chloride ions and hydrochloric acid cannot be classified as a buffer.

Exercise \(\PageIndex{1}\)

Explain whether a solution containing carbonic acid and bicarbonate ions can be classified as a buffer.

Answer: As stated above, a buffer generally consists of a weak Brønsted-Lowry acid and its conjugate base or a weak Brønsted-Lowry base and its conjugate acid. By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Because carbonic acid, H₂CO₃, and a bicarbonate ion, HCO₃^–1, both contain one carbon, C, and three oxygens, O, and have chemical formulas that differ by exactly one proton, H⁺¹, these chemicals are conjugates.

Furthermore, the chemical that contains more hydrogens, H, will correspond to the acid in a conjugate acid/base pair, and the substance that has fewer hydrogens, H, must be classified as the base. Therefore, because carbonic acid, H₂CO₃, contains two hydrogens, H, and only one hydrogen, H, is present in a bicarbonate ion, HCO₃^–1, carbonic acid is the acid in the given solution, and the bicarbonate ion, HCO₃^–1, is its conjugate base. This assignment can be confirmed by recognizing that carbonic acid, H₂CO₃, can be classified as a polyatomic, or "H_NPoly," acid, because it contains two hydrogens, H, and a carbonate ion, CO₃^–2, which is a polyatomic anion.

Finally, the chemicals that are present in a buffer solution must be weakly acidic or basic. Based on the information that is presented in Section 8.23, carbonic acid, H₂CO₃, is not classified as a strong acid and, therefore, is categorized as weak, "by default." Additionally, the bicarbonate ion, HCO₃^–1, is not one of the eight strong bases and, consequently, is classified as weak, "by default." Therefore, a solution containing carbonic acid and bicarbonate ions can be classified as a buffer.

Exercise \(\PageIndex{2}\)

Explain whether a solution containing sulfite ions and sulfurous acid can be classified as a buffer.

Answer: As stated above, a buffer generally consists of a weak Brønsted-Lowry acid and its conjugate base or a weak Brønsted-Lowry base and its conjugate acid. By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. While a sulfite ion, SO₃^–2, and sulfurous acid, H₂SO₃, both contain one sulfur, S, and three oxygens, O, the chemical formulas of these substances differ by two protons, H⁺¹. Therefore, these chemicals are not conjugates, and a solution containing sulfite ions and sulfurous acid cannot be classified as a buffer.

Exercise \(\PageIndex{3}\)

Explain whether a solution containing CH₃NH₂ and CH₃NH₃⁺¹ can be classified as a buffer.

Answer: As stated above, a buffer generally consists of a weak Brønsted-Lowry acid and its conjugate base or a weak Brønsted-Lowry base and its conjugate acid. By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Because CH₃NH₂ and CH₃NH₃⁺¹ both contain one carbon, C, and one nitrogen, N, and have chemical formulas that differ by exactly one proton, H⁺¹, these chemicals are conjugates.

Furthermore, the chemical that contains more hydrogens, H, will correspond to the acid in a conjugate acid/base pair, and the substance that has fewer hydrogens, H, must be classified as the base. Therefore, because five hydrogens, H, are present in CH₃NH₂, and CH₃NH₃⁺¹ contains six hydrogens, H, CH₃NH₃⁺¹ is the acid in the given solution, and CH₃NH₂ is its conjugate base.

Finally, the chemicals that are present in a buffer solution must be weakly acidic or basic. Based on the information that is presented in Section 8.23, CH₃NH₃⁺¹ is not classified as a strong acid and, therefore, is categorized as weak, "by default." Additionally, CH₃NH₂ is not one of the eight strong bases and, consequently, is classified as weak, "by default." Therefore, a solution containing CH₃NH₂ and CH₃NH₃⁺¹ can be classified as a buffer.