Use of a pH Meter

The WPI Department of Chemistry and Biochemistry has available two types of pH meter: small, hand-held meters that operate on small-battery power and have a built-in electrode; and bench-top pH meters that operator on AC voltage and require that the electrode be attached via a connecting wire. The hand-held models are accurate and easy to use, so those will be discussed here. If you should need instructions for the bench-top model, see your instructor.

Hanna Hand-Held pH Meter. A few general statements about the care and storage of the Hanna meter are given below:

• When not in use, the meter should be turned off and the electrode tip should be covered with the plastic cap.
• If the meter has not been used for some time, it should be soaked for several days in distilled water prior to use, with the power off of course. For effective soaking, the protective cap must be removed. The meter should be placed in a beaker of distilled water with the probe (small, cylindrical end) submerged. Distilled water should be deep enough to cover the probe protective sheath to just above the tops of the slots in the sheath, but not so deep that more than 2/3 of the probe sheath is submerged. Check here for a diagram showing the arrangement. UNDER NO CIRCUMSTANCES SHOULD THE METER EVER BE PLACED IN WATER DEEP ENOUGH TO SUBMERGE ANY PART OF THE UPPER BODY OF THE METER. THIS WILL RUIN THE ELECTRONICS.
• After soaking, remove the pH meter from the water, dry the probe tip with a Kimwipe, and cap the probe until you are ready to use the meter.
• The meter may be laid on a paper towel on the bench top when not in use. However, UNDER NO CIRCUMSTANCES SHOULD THE METER BE INVERTED (TURNED PROBE-UP). THIS MAY CAUSE WATER INSIDE THE PROBE TIP TO RUN UP INTO THE METER ELECTRONICS.
• Before using the meter to determine the pH of your analyte, it may be necessary to standardize it--that is, to immerse it in a buffer of known pH and adjust the meter to display the correct pH. Normally this will be done by the instructor. Check to see whether this is necessary or has been done before starting your measurements.

Instructions for using the meter for pH measurements are given below.

• Pour a 1-inch depth of distilled water into a clean 100- or 250-mL beaker.
• Uncap the meter and place probe-down into the distilled water beaker. The meter should be placed in the distilled water between measurements to keep the probe moist.
• Place the analyte solution in a small beaker. A small Erlenmeyer flask will not work because the pH meter probe will not reach to the bottom of the flask.
• When ready to measure the pH of the analyte solution, remove the meter from the DW beaker, rinse the tip with distilled water from a wash bottle, and dry the probe as well as you can with a Kimwipe.
• Turn the pH meter on and insert it into the analyte solution to a depth between 0.5 inches (enough to cover the probe completely) and 1 inch. If necessary, tip the beaker to enhance the liquid depth. Gently stir the solution in a circular motion with the pH meter probe, then hold the meter steady in the solution until a stable pH reading is obtained on the liquid crystal screen. Record the reading.
• Turn off the meter, remove the probe from the analyte solution, rinse it with DW from a squirt bottle, and return it to the DW beaker.
• When finished with your measurements, dry the probe thoroughly, cap the probe, and make sure the power is turned off. Return the meter to the storage box at the front of the lab.