Tech Note 630 Detail

pH Sensor Calibration

Affected Products:

PS-2102 PASPORT pH Sensor
PS-2169 PASPORT Water Quality Sensor
PS-2170 PASPORT Chemistry Sensor

Problem/Symptom:
How do I calibrate the pH sensor?

PASCO Solution:

Materials

  • At least two distinct pH buffered solutions.
    • The SC-2321 pH Buffer Set contains buffers of pH 4.00, 7.00 and 10.00 +/- 0.02 .
    • pH buffer solutions can also be made from the following recipies:
      (All values assume a calibration temperature of 25 Celsius.)
      • pH 4.00 buffer solution: 100 mL of 0.1 M Potassium Hydrogen Phthalate + 0.2 mL of 0.1 M HCl
      • pH 7.00 buffer solution: 100 mL of 0.1 M KH2PO4 + 58.2 mL of 0.1 M NaOH
      • pH 10.00 buffer Solution: 100 mL of 0.025 M Na2B4O7.10H2O (Borax) + 36.6 mL of 0.1 M NaOH

Procedure in DataStudio

  1. Connect a PASPort interface to the computer via a USB cable or Bluetooth radio (in the case of the AirLinks).
  2. Connect a PASPort sensor with a well-maintained pH electrode to the PASPort interface.
  3. Click the Setup button in DataStudio.
  4. In the Experiment Setup window, click the Calibrate Sensors... button.
  5. Enter the first known pH buffer value in the Calibration Point 1 Standard Value text box.
  6. Rinse and dry the pH probe.
  7. Place the pH probe in the corresponding buffer solution.
  8. Stir the pH sensor until the value Present Sensor Measurement value stabilizes.
  9. Click the Read from Sensor button.
  10. Place the pH sensor in the second pH buffer solution.
  11. Enter the second known pH buffer value in the Calibration Point 2 Standard Value text box.
  12. Rinse and dry off the pH sensor.
  13. Place the pH Sensor in the second buffer solution.
  14. Stir the pH sensor until the value Present Sensor Measurement value stabilizes.
  15. Click the Read from Sensor button.
  16. Click OK.

Procedure Using the XPlorer GLX

  1. Connect a PASPort sensor with a well-maintained pH electrode to any sensor port of the Xplorer GLX.
  2. Go to Home/Sensors/Sensors/Calibrate (3).
  3. Enter the first known pH buffer value in the Pt 1 (pH) text box.
  4. Rinse and dry the pH probe.
  5. Place the pH probe in the corresponding buffer solution.
  6. Stir the pH sensor until the pH value displayed at the bottom of the screen stabilizes.
  7. Click the Read Pt 1 button (F3).
  8. Place the pH sensor in the second pH buffer solution.
  9. Press the down arrow until you see Pt 2 (Units).
  10. Enter the second known pH buffer value in the Pt 2 (pH) text box.
  11. Rinse and dry off the pH sensor.
  12. Place the pH Sensor in the second buffer solution.
  13. Stir the pH sensor until the pH value displayed at the bottom of the screen stabilizes.
  14. Click the Read Pt 2 button (F4).
  15. Click OK.

Procedure Using the SPARK SLS, SPARKvue, or SPARKvue HD

  1. Connect a well-maintained pH probe to one of the following PASPort sensor boxes:
  2. If you are using a SPARK SLS, connect the PASPort sensor box directly to the SPARK SLS.
  3. If you are using SPARKvue, connect the PASPort sensor box to a PASPort interface.
  4. Connect the PASPort interface to the computer or tablet, using Bluetooth or USB as appropriate.
  5. Press the Home button:
  6. Select pH.
  7. Click Show. (You will see a Graph display.)
  8. Press the Experiment Tools Button:
  9. Select CALIBRATE SENSOR.
  10. In the Sensor field, select the sensor that has the pH probe attached.
  11. In the Measurement field select pH.
  12. Calibration Type should remain 2 point (Adjust Slope and Offset).
  13. Click NEXT.
  14. Enter the first known pH buffer value in the Standard Value box in the Calibration Point 1 section.
  15. Rinse and wipe dry the pH probe.
  16. Place the pH probe in the buffer solution corresponding to the value in the Standard Value box.
  17. Stir the pH sensor in the buffer solution for at least two seconds.
  18. Click the Read From Sensor button.
  19. Place the pH sensor in the second pH buffer solution.
  20. Enter the second known pH buffer value in the Standard Value box in the Calibration Point 2 section.
  21. Rinse and wipe dry the pH probe.
  22. Place the pH probe in the buffer solution corresponding to the value in the Standard Value box.
  23. Stir the pH sensor in the buffer solution for at least two seconds.
  24. Click the Read From Sensor button. (The sensor is now calibrated.)
  25. Click OK twice to return to the graph display.

Temperature compensation

The PASCO pH sensors are temperature compensated. If the temperature is measured, the pH can be calculated from the correction:

pHTemperature-Corrected(T) = 7.0 + (pH - 7.0)* T/T0 .

For best results, the temperature correction should be empirically derived because the isopotential point may differ slightly from 7.0. One can calibrate at another temperature if one has two buffer solutions with published corrections for temperature.

Note that the nominal pH printed on a pH buffer usually only corresponds to pH at 25 C. One must use the temperature correction for the pH of the buffer in order to correctly derive the temperature correction for the pH sensor.

For the pH 10 Carbonate - Bicarbonate buffer standard from the SC-2321 Buffer capsule kit, the following corrections apply:

Temperature [C]

pH

pH Uncertainty

15.0

10.117

0.008

25.0

10.015

0.006

35.0

9.926

0.006


For the pH 4 potassium biphthalate buffer standard from the SC-2321 Buffer capsule kit, the pH varies with temperature as follows:

Temperature [C]

pH

10

4.00

15

4.00

20

4.00

25

4.01

30

4.02

35

4.02

40

4.04

45

4.05

50

4.06

55

4.08

60

4.09

Related Technotes:
TechNote 58 pH Electrode Storage
TechNote 167 Using the pH probe with an ISE Amplifier Box
TechNote 302 pH Electrode Maintenance
TechNote 315 pH sensor reads that pure water is acidic.

Creation Date: 02/11/2008
Last Modified: 02/18/2016
Mod Summary: