Methods

pH Overview
       The pH of a solution is a measure of its hydrogen ion [H⁺] activity and is equal to the negative logarithm of the concentration of hydrogen ions, or:

       It is important to remember that a change of one pH unit represents a tenfold change in hydrogen ion concentration; for example, a solution with a pH of 6 has 10 times the hydrogen ions as one of pH 7, and pH 5 has 100 times the hydrogen ions of pH 7.

       pH is measured on a scale that goes from 0 to 14. As a pH reading gets closer to 0, the hydrogen ion concentration [H⁺] gets higher, the hydroxide ion [OH^-] concentration goes down and the solution becomes more acidic. As a pH reading gets closer to 14, the hydrogen ion concentration [H⁺] goes down while the hydroxide ion [OH^-] concentration goes up and causes the solution to become more basic (alkaline). A pH reading of 7 means the [H⁺] concentration and the [OH^-] concentration are equal and the solution is considered to be neutral, being neither acidic nor basic.

       Most living organisms can tolerate only slight pH fluctuations near the neutral region of the pH scale. Under open ocean conditions an effective pH buffering system limits seawater pH values to a narrow range between 7.5 and 8.4. However, dissolved gases such as CO₂, H₂S, and NH₃ can also have a significant effect. The marine biologist is interested in pH and its changes, since they may reflect biological activity and changes in natural chemistry of waters, as well as pollution.

       Variations of water pH can easily be studied in the field using a very accurate, portable pH meter. The instrument is convenient to use and rapidly provides accurate results. The meter may be subject to drift and may be slow in coming to a steady reading. Readings should not be taken before equilibrium has been established between the electrode and the water system. Temperature exerts a significant effect on pH measurements because ionization varies with temperature (higher at higher temperatures). Therefore, the temperature should always be reported along with pH. For instructions on how to use the portable pH meters used for the Bayouside Classroom program, click here.