pH
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.
Before sampling with any pH meter, the user should consider calibrating the probe. This will ensure accurate and consistent results each week.
Calibration
Calibration of the pH probes are essential to making sure we have the most accurate and reliable measurements. Over time, the gels in the electrodes located in the tips of the meters age and change over time. These changes can effect the electrode’s output which determines our pH measurements. By calibrating our meters we give the pH meter a comparison point or points of a known pH so that it can measure an unknown with greater accuracy.
In order to calibrate the probe, we will use a one-point calibration with the 7.0 buffer solution. From our observations, this is just as acceptable as using a three-point calibration using three buffer solutions for most locations. Within more complex brackish water location a three-point calibration will ensure more accurate measurements. We suggest a calibration at least every 3 months unless your probes are being used more often.