In solution, an acid has a greater concentration of hydrogen ions (H+) (see ION) rather than hydroxide ions (OH-); a BASE has the reverse. The pH scale is used to measure the degree to which a solution is an acid or a base: the pH of any solution is defined as:
That is, pH is the negative log of the concentration of hydrogen ions in a solution. In a neutral solution the concentration of hydrogen ions is 10−7 M (M stands for molar) yielding a pH of 7. Values less than 7 indicate increasing acidity, more than 7 decreasing acidity (and hence increasing alkalinity—the term ALKALI is a synonym for base). The largest value is 14 (the most basic), the lowest 0. Each point on the scale represent a change of a factor of 10: a change in pH of one points represents a 10× change in acidity, a change of two points a 100× change, three points a 1000×change and so on. The degree of acidity was once measured using a litmus test, though this is rarely used now. Most laboratories will possess a pH meter that will have a special pH electrode which, when placed in a solution, will determine the pH, registering this on a meter.
Most physiological fluids have a pH between about 6 and 8, though there are exceptions. Some examples of solutions and their pH are: pure water has a pH of 7; human BLOOD and CEREBROSPINAL FLUID have a pH of a little over 7. Ammonia and bleach are strong bases with pH values over 11. Urine has a slightly acid pH of about 6; black coffee has a pH of around 5. GASTRIC ACID has a pH of about 2.
The technicalities of pH are of less concern to biological psychologists than are the practicalities. Strong acids and bases burn and damage tissue (unless there are specialized protective arrangements, as is the case for highly acidic gastric acid). It is therefore of critical importance when preparing solutions for delivery to subjects—human or animal—to know what the pH is. Extremes of acidity or alkalinity can be avoided by preparing solutions in a BUFFER.
Reference
Campbell N.A., Reece J.B. & Mitchell L.G. (1999) Biology, 5th edn, Addison-Wesley: Menlo Park CA.
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