John T. Moore - Biochemistry For Dummies

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.

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

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

	pH	Solution Property
	0	Acidic
	1	Acidic
	2	Acidic
	3	Acidic
	4	Acidic
	5	Acidic
	6	Acidic
	7	Neutral
	8	Basic
	9	Basic
	10	Basic
	11	Basic
	12	Basic
	13	Basic
	14	Basic

Solutions with a pH less than 7 are acidic. Solutions with a pH greater than 7 are basic. Solutions whose pH is 7 are neutral. The pH of pure water is 7. Be careful, though: Not every solution that has a pH of 7 is pure water! For example, if you add table salt to water, the pH remains at 7, but the resulting solution is certainly not pure water.

The pH scale is an open-ended scale, meaning that a solution can have a pH greater than 14 or less than 0. For example, the pH of a solution of hydrochloric acid is . John loves to ask questions based on this topic to his advanced chemistry students! The scale is a convenient part of the pH scale for most real-world solutions — especially ones found in biochemistry. Most biological systems have a pH near 7, although significant deviations may exist (for example, the pH in your stomach is close to 1).

You can calculate pOH (related to the concentration of the hydroxide ion) in a similar manner to the pH calculation. That is, you can use the equation . You can calculate the hydroxide ion concentration from the hydrogen ion concentration and the K w(equilibrium constant) relationship:

A useful shortcut to get from pH to pOH is the following relationship: for any aqueous solution .

For example, if a solution has a , its pH would be

The calculation for the pOH of that solution is pretty simple: