Biochemistry For Dummies. John T. Moore

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.

      Swapping hydrogens between acids and bases

      Acids increase the hydrogen ion concentration of a solution (they lower the pH, in other words). Some acids, known as strong acids, are very efficient at changing hydrogen ion concentration; they essentially completely ionize in water. Most acids — particularly biologically important acids — aren’t very efficient at generating hydrogen ions; they only partially ionize in water. These acids are known as weak acids.

Bases accept (react with), rather than donate, hydrogen ions in solutions. Bases decrease the hydrogen ion concentration in solutions because they react with these ions. Strong bases, although they can accept hydrogen ions very well, aren’t too important in biological systems. The majority of biologically important bases are weak bases.

      

The Brønsted-Lowry theory helps to explain the behavior of acids and bases with respect to equilibrium. A Brønsted-Lowry acid is a hydrogen ion donor, and a Brønsted-Lowry base is a hydrogen ion acceptor. Acetic acid, a weak acid found in vinegar, partially ionizes in solution, evidenced by the following equation:

      The double arrow indicates that the acetic acid doesn’t completely ionize. (For a strong acid, complete ionization would occur, indicated by a single arrow.) The equilibrium arrow indicates that all three chemical species are present in the solution: the acetic acid, the acetate ion, and the hydrogen ion, along with the water solvent.

      In the Brønsted-Lowry theory, you consider the acetate ion to be a base because it can accept a hydrogen ion to become acetic acid. According to this theory, two substances differing by only one hydrogen ion — such as acetic acid and the acetate ion — are members of a conjugate acid-base pair. The species with one additional