Subsections

• pH Calibration
• pH Measurement

Using the pH Meter

This meter is used to measure the acidity of the water by comparing readings from a reference electrode and a sample electrode. To determine pH the output of these electrodes must be temperature-compensated, so most pH meters also measure temperature. On the Hach EC10 meter (Fig. 4.8), pH is displayed in the center right of the LCD screen, temperature ( ${\rm ^\circ C }$) is displayed in the lower center. pH meters generally require frequent calibration in the field, and calibration is part of this lab.

Figure 4.8: ph Meter display and controls.

pH Calibration

The procedure for pH meter calibration is:

1. Be sure some electrolyte is present at the tip of the probe (ask a TA to check, or blow off any liquid from the probe tip and depress dispenser button). If needed, dispense some electrolyte by depressing the dispenser button on the top of the electrode until a click is heard. Natural water pH measurements outside the range of 7.0-8.9 probably mean electrolyte wasn't present, and are invalid readings.
2. Turn on device (press I/O) key
3. Be sure meter is in pH mode (triangle at base of display is above the ``pH'' label''). If not, press MODE key until mode is correct
4. Press ``CAL'' button to enter calibrate mode, display will flash
5. Rinse electrode and place into the 4.0 buffer.
6. Press CAL key. ``CALIBRATE'' will be displayed above the main field. After the calibration slope is displayed, P1 will be displayed in the lower field. P1 indicates that the meter is ready to accept the first buffer point.
7. When the electrode is stable, the meter will beep and READY will be displayed along with the temperature corrected value for the buffer (flashing). Press YES key to accept this point.
8. The display will remain fixed momentarily, then P2 will be displayed in the lower field. The meter is now ready for the second buffer (7.0).
9. Rinse the electrode and place it in the second buffer (7.0). Wait for a stable pH reading (the meter will beep) and for READY to be displayed. Press the YES key to accept the second point.
10. The meter will automatically enter measure mode and MEASURE appears above the main display.
11. The meter is now ready to take pH measurements. The EC10 retains this calibration until a new calibration is entered or until power is disconnected from the instrument.

pH Measurement

The procedure for making a pH-temperature reading is:

1. rinse the electrode tip in deionized water
2. depress the dispenser button on the top of the electrode until a click is heard (releases reference electrolyte at tip of electrode)
3. wait until the readings become steady (``READY'' indicator shows, and meter beeps once)
4. record results (including temperature)
5. if readings become erratic, dispense more electrolyte