Biochemistry For Dummies. John T. Moore

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target="_blank" rel="nofollow" href="#ulink_54337736-8046-5edc-95d0-b3b86d7612a8">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
	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).

      Calculating pOH

      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:

      Now, if you have the pH or pOH, getting the corresponding or is a simple task:

      For example, a solution with a pH of 7.35 has a .

      Applying the Brønsted-Lowry theory

      Because the acidity (pH) of the biological medium is so very important, in the following sections, we take a look at one of the most widely accepted theories concerning acids and bases — the Brønsted-Lowry theory. According to this theory, acids are proton donors, and bases are proton acceptors.

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