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.

HOW TO USE PORTABLE pH METERS

This section outlines how to take a pH reading from a water sample using the Waterproof pHTestr2 from Oakton Instruments.

Calibration

Before sampling with the pHTestr2, the user should consider calibrating the probe. This will ensure accurate and consistent results each week.

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.

Pour enough of the 7.0 buffer solution into a clean beaker to cover the electrode portion of the probe. Remove the cap from the probe and press the ‘On/Off’ button once to turn the probe on. After ‘PH’ flashes off of the display, place the electrode portion of the probe into the solution. It will begin to read the solution on the display. Once the reading on the display gets near the 7.0 mark, press the ‘CAL’ button once. It will continue to read the solution, but the numbers in the display will flash once every second. After at least 30 seconds, press the “Hold/Con” button. It will store the calibration information in the probe. It should now be calibrated and read 7.0 on the display when in the 7.0 buffer solution. You can now turn the probe off by pressing the ‘On/Off’ button once. It will store the calibration information even when turned off. In fact, it will store it until either the calibration process is performed again or the batteries are replaced. Remember to never pour the used buffer back into the original bottle. The used buffer can be saved in a separate air-tight container and will work for weeks.

Sampling

After the probe is calibrated it is ready to take pH measurements. Simply turn the probe on by pressing the ‘On/Off’ button once and wait for ‘PH’ to clear from the display. Take the protective cap off of the electrode end of the probe and place into the sample and stir gently for a few seconds. Give the probe time to settle on one reading; this may take a few minutes, but usually occurs quickly. When the sampling procedure is over, rinse the electrode area of the probe with clean water and replace the cap. Make sure to turn off the probe before packing it away for the week. It will drain the batteries if this step is skipped.