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FIGURE 2-3: Structure of a micelle, composed of amphipathic molecules, with their hydrophilic heads pointing out.

Hydrogen Ion Concentration: Acids and Bases

      In aqueous solutions — especially in biological systems — the concentration of hydrogen ions is very important. Biological systems often take great pains to make sure that their hydrogen ion concentration — represented as or by the measurement of pH (the measure of acidity in a solution) — doesn’t change.

      Even minor changes in hydrogen ion concentration can have dire consequences to a living organism. For example, the human body can function properly only if its blood’s hydrogen ion concentration falls within a very small range. Hydrogen ion concentrations higher or lower than this range can cause death.

      Because living organisms are so dependent on pH, we review the concepts of acids, bases, and pH in the following sections.

      Achieving equilibrium

      When the concentrations of hydrogen ions and hydroxide ions are the same, a solution is neutral. If the hydrogen ion concentration exceeds the hydroxide ion concentration, the solution is acidic. If the hydroxide ion concentration is greater, the solution is basic. These ions are related through a chemical equilibrium.

The equilibrium of hydrogen ions is present in all aqueous solutions. Water may or may not be the major hydrogen ion source (usually, it isn’t). Water is a contributor to the hydrogen ion concentration because it undergoes autoionization, as shown by the following equation:

      

You often see represented as .

      The double arrow indicates that this reaction (represented by the equation) is an equilibrium; as such, there must be an associated equilibrium constant (K). The equilibrium constant in the preceding equation is K_w. The value of K_w is the product of the concentrations of the hydrogen ion and the hydroxide ion:

      The value of the constant K_w, like all Ks, is only constant if the temperature is constant. In the human body, where , .

      In pure water, at 25°C, . The hydroxide ion concentration is the same as the hydrogen ion concentration because they’re formed in equal amounts during the autoionization reaction. Keep in mind that only in pure water.

      

M is a concentration term, the molarity. Molarity is the number of moles of solute per liter of solution.

      Understanding the pH scale

      Expressing hydrogen ion concentrations in an exponential form, such as , isn’t always convenient. Thankfully, you have a way of simplifying the representation of the hydrogen ion concentration: the pH. You can calculate the pH for any solution by using the following equation:

      For instance, in the case of a solution with a hydrogen ion concentration of , the pH calculation would be

Table 2-1 gives similar calculations for some hydrogen ion concentrations.

TABLE 2-1 The pH Scale and the Associated Hydrogen Ion Concentration

	pH	Solution Property
	0	Acidic
	1	Acidic
	2	Acidic
	3	Acidic
	4	Acidic
	5	Acidic
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