Recommended Equipment

• Ohaus pH meter
• Ohaus pH electrode and Temp probe
• Magnetic stir plate and stir bar
• 50 mL beakers and 200 mL waste beakers
• pH 4.01 buffer or equivalent buffer
• pH 7.00 buffer or equivalent buffer
• pH 10.01 buffer or equivalent buffer
• Watch glasses or parafilm
• Deionized water or ultrapure water

Frequency of Calibration

At a minimum, the instrument is calibrated prior to use on the day the measurements are to be performed. A post calibration check at the end of the day is performed to determine if the instrument drifted out of calibration.

pH Calibration and Measurement Recommendations

• Choose the appropriate buffered standards that will bracket the expected values at the sampling locations. If the sample’s pH is unknown, then three standards are needed for the calibration: one close to 7, one at least two pH units below 7, and the other at least two pH units above 7. Instruments that will not accept three standards will need to be re-calibrated if the sample’s pH is outside the initial calibration range described by the two standards.
• If the electrode is refillable, uncover the fill hole during calibration and measurement to ensure a uniform flow of filling solution. The filling solution level inside of the electrode must be at least two centimeters above the buffer or sample level.
• Between buffers or samples, rinse with deionized water, and remove excess water. Then with the next buffer or sample. To reduce the chance of errors due to polarization, avoid rubbing or wiping the electrode bulb.
• Never re-use buffer solution and never return it to its original storage container.
• Use a magnetic stir plate and stir bar to stir all buffers and samples at a moderate, uniform rate.

Electrode Preparation

Prepare the pH electrode according to the instructions in the electrode user guide or instruction manual. Prior to calibration, store the electrode in pH electrode storage solution.

Calibration Buffer Preparation

1. Pour about 30 mL of pH 10.01 buffer into a 50 mL beaker and cover the beaker with a watch glass or parafilm prior to calibration.
2. Pour about 30 mL of pH 7.00 buffer into a 50 mL beaker and cover the beaker with a watch glass or parafilm prior to calibration.
3. Pour about 30 mL of pH 4.01 buffer into a 50 mL beaker and cover the beaker with a watch glass or parafilm prior to calibration.
4. Pour about 30 mL each of the pH 10.01, 7.00 and 4.01 buffers into separate 50 mL beakers. Use these three beakers as the rinse beakers during calibration.
5. Allow all of the buffers to reach the same temperature, since pH readings are temperature dependent.

Calibration

1. Allow all of the buffers to equilibrate to the ambient temperature, since pH readings are temperature dependent. If the buffers are not at 25 °C, temperature compensation is recommended. Measure the temperature of the buffers using a NIST traceable thermometer and manually enter the temperature into the meter or use an temperature probe to automatically transmit the temperature of the buffers to the meter.
2. Prepare the pH 10.01 buffer, pH 7.00 buffer and pH 4.01 buffer as described in the Calibration Buffer Preparation section and uncover the calibration beakers.
3. Rinse the pH electrode first with deionized water and then in the pH 10.01 buffer rinse beaker. Make sure to rinse the electrode with deionized water over a waste beaker to prevent contamination of the buffers. The electrode should never be rinsed in the same buffer beaker that will be used for calibration.
4. Place the electrode into the pH 10.01 buffer calibration beaker, so the electrode tip and junction are fully immersed in the buffer, and stir the buffer at a moderate, uniform rate.
5. Start the calibration on the meter.
6. Wait for a stable reading in the pH 10.01 buffer, at least 1 to 2 minutes. If the temperature of the buffer was entered manually or an ATC probe is in use, the meter should automatically recognize the buffer and display its temperature-corrected pH value. If the meter does not automatically recognize the buffer, enter the value of the pH buffer at its measured temperature using Table 1.
7. Once meter automatically recognize the correct buffer value, prepare to proceed to the next calibration point.
8. Rinse the pH electrode first with deionized water and then in the pH 7.00 buffer rinse beaker. Make sure to rinse the electrode with deionized water over a waste beaker to prevent contamination of the buffers. The electrode should never be rinsed in the same buffer beaker that will be used for calibration.
9. Place the electrode into the pH 7.00 buffer calibration beaker, so the electrode tip and junction are fully immersed in the buffer, and stir the buffer at a moderate, uniform rate. 10. Wait for a stable reading in the pH 7.00 buffer, at least 1 to 2 minutes. If the temperature of the buffer was entered manually or an ATC probe is in use, the meter should automatically recognize the buffer and display its temperature-corrected pH value. If the meter does not automatically recognize the buffer, enter the value of the pH buffer at its measured temperature using Table 1.
11. Once meter automatically recognize the correct buffer value, prepare to proceed to the next calibration point.
12. Rinse the pH electrode first with deionized water and then in the pH 4.01 buffer rinse beaker. Make sure to rinse the electrode with deionized water over a waste beaker to prevent contamination of the buffers. The electrode should never be rinsed in the same buffer beaker that will be used for calibration.
13. Place the electrode into the pH 4.01 buffer calibration beaker, so the electrode tip and junction are fully immersed in the buffer, and stir the buffer at a moderate, uniform rate.
14. Wait for a stable reading in the pH 4.01 buffer, at least 1 to 2 minutes. If the temperature of the buffer was entered manually or an ATC probe is in use, the meter should automatically recognize the buffer and display its temperature-corrected pH value. If the meter does not automatically recognize the buffer, enter the value of the pH buffer at its measured temperature using Table 1.
15. Once meter automatically recognize the correct buffer value, prompt the meter to save and end the calibration.
16. Rinse the pH electrode with deionized water and store the electrode.

• NOTE: pH electrode slope daily by performing at least a two buffer calibration. The slope should be 95 to 102%.

 

Calibration Verification

1. Use the same buffers that were used for calibration or prepare fresh buffers as described in the Calibration Buffer Preparation section. Uncover the calibration verification beakers.
2. Rinse the pH electrode first with deionized water and then in the pH 10.01 buffer rinse beaker. Make sure to rinse the electrode with deionized water over a waste beaker to prevent contamination of the buffers. The electrode should never be rinsed in the same buffer beaker that will be used for calibration verification.
3. Place the electrode into the pH 10.01 buffer calibration verification beaker, so the electrode tip and junction are fully immersed in the buffer, and stir the buffer at a moderate, uniform rate.
4. Start the measurement on the meter.
5. Wait for a stable reading, at least 1 to 2 minutes, and then record the pH and temperature of the buffer.
6. Repeat steps 2 through 5 for the pH 7.00 buffer and then the pH 4.01 buffer.
7. Compare the recorded pH and temperature values of the buffers to those listed in Table 1.
8. Rinse the pH electrode with deionized water and store the electrode in pH electrode storage solution until the samples are ready for measurement.

Table 1 – pH Buffer Values at Various Temperatures

0°C	5°C	10°C	15°C	20°C	25°C	30°C	35°C	40°C	50°C
0.92	0.95	0.98	1.01	1.01	1	1.02	1.03	1.02	1.03
1.67	1.67	1.67	1.67	1.67	1.68	1.68	1.68	1.69	1.71
4.01	3.99	4	4	4	4	4.01	4.02	4.03	4.06
6.98	6.95	6.93	6.89	6.88	6.86	6.85	6.84	6.83	6.83
7.13	7.1	7.07	7.05	7.02	7	6.99	6.98	6.97	6.83
9.46	9.4	9.33	9.27	9.23	9.18	9.14	9.1	9.07	9.01
10.31	10.23	10.17	10.11	10.05	10	9.95	9.91	9.87	9.81
12.79	12.73	12.67	12.59	12.52	12.45	12.36	12.25	12.17	11.96

Products for pH Sensor Calibration

Order number	Description
30100425	Buffer pH4.01 250ml
30100427	Buffer pH7.00 250ml
30100429	Buffer pH10.01 250ml
30100428	Buffer pH9.18 250ml
30100424	Buffer pH1.68 250ml
30100426	Buffer pH6.86 250ml
30100440	Buffer pH12.45 250ml