The pH Scale

The pH of a solution indicates its acidity or alkalinity. Hydrogen ions are generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions.

An acid is an inorganic substance that increases the concentration of hydrogen ions (H+) in a solution, usually by having one of its hydrogen atoms dissociate.

A base is an inorganic subsatnce that provides either hydroxide ions (OH - ) or other negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water molecules.

The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids. Conversely, strong bases are those substances that readily donate OH- or take up hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly alkaline and give up OH - rapidly when placed in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are able to thrive in it.

The pH scale is an inverse logarithm and ranges from 0 to 14 ( See pH scale below). Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH of 1 to 2. So how do the cells of the stomach survive in such an acidic environment? How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of the human stomach is completely replaced every seven to ten days.  

The pH of most living cells is close to 7. Even a minor change in pH can be catastrophic for a cell because the chemical processes within the cell are very sensitive to hydrogen ion concentration [H+] and hydroxide ion concentration [OH-] fluctuations. To prevent pH changes in blood, a chemical system is in place to help maintain a stable pH. For instance, if a liter of pure water has 0.01 moles of strong acid added to it, the pH will go from 7 to 2. However, if that same amount of acid is added to blood, the pH only goes from 7.4 to 7.3. This resistance to pH change is due to substances called buffers in the blood. Buffers are substances that accept hydrogen ions [H+] when they are in excess, and donate hydroxide ions [OH-] when they are depleted to prevent changes in blood, which has a narrow pH range of 7.35-7.45. This process minimizes the effect of adding an acid or base. Buffers normally contain a weak acid and its corresponding base, which can combine reversibly with [H+].

The following video discusses pH:

Video 3. View the Introduction to pH video on YouTube (opens in new window).


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