Q. What pH electrode do I use for a specific application?
A. Follow the general rules below for selecting the right pH electrode:
- Glass bodied pH electrodes may be used in most sample types.
- Epoxy bodied pH electrodes are designed for rugged environments, multiple-user situations, and field or plant applications. Epoxy bodied pH electrodes should not be used in organic solvents.
- For situations containing proteins, sulfide, and TRIS, use ----- electrodes.
- For viscous or dirty samples, use Van London Co. (aka Van London pHoenix Co.) Free-Flow pH electrodes for best results and easy cleaning.
- For custom or difficult applications, please contact one our knowledgeable sales associates for assistance.
Q. How do I store my pH electrode?
A. Proper electrode storage maximizes electrode performance and extends electrode life. There are two ways to store a pH electrode:
- Use Van London Co.'s (aka Van London pHoenix Co.) DEDICATED PH STORAGE SOLUTION CAT NO. R001000
- As a temporary substitute use 200 mL of pH 7 buffer to which 1 gram of potassium chloride (KCl) has been added.
For long-term storage information refer to the appropriate Van London Co. (aka Van London pHoenix Co.) pH electrode instruction manual.
Q. What filling solution do I use?
A. The recommended filling solution depends on the type of electrode. Some Van London Co. (aka Van London pHoenix Co.) electrodes have sealed references and do not require filling solution. For refillable pH electrodes, use the guide below to choose the correct filling solution:
- For single junction type electrodes, use filling solution R001011.
- For double junction type electrodes, use filling solution R001013.
- For Ion Selective electrodes and non-aqueous type electrodes please refer to the electrode instruction manual.
Q. How do I clean my pH electrode?
A. The solution used to clean a pH electrode depends on the possible contaminants. Use the guide below to choose the appropriate solution:
- For general cleaning soak the electrode in 0.1 M HCl or 0.1 M HNO3 for 30 minutes.
- For removing stubborn deposits and bacterial growth, soak the electrode in a 1:10 dilution of household laundry bleach for 15 minutes.
- For removal of protein deposits soak the electrode in 1% pepsin in 0.1 M HCl for 15 minutes.
- For removal of inorganic deposits soak the electrode in 0.1 M tetra sodium EDTA for 15 minutes.
- For removal of oil and grease rinse the electrode with mild detergent or methanol.
After any of the cleaning procedures, thoroughly rinse the electrode with deionized water, drain and refill the reference chamber, and soak the electrode in storage solution for at least 1 hour.
Q. Do pH buffers and filling solutions have a shelf-life?
A. The typical shelf-life for pH buffers and filling solutions is 2 years unopened and 6 months open. For best results, the pH buffer bottles should be sealed promptly to avoid carbon dioxide absorption.
Q. What is a good pH electrode slope range?
A. The acceptable slope range is 92% to 102%. Slopes below 92% indicate that the electrode may require cleaning or if cleaning does not help, the electrode should be replaced. Slopes above 102% indicate that the pH buffers are contaminated.
Q. Do I need an Automatic Temperature Compensation (ATC) probe?
A. The most common cause of error in pH measurements is temperature. The slope of a pH electrode is highly dependent of temperature, and pH buffer values and sample values change with temperature. For the most accurate results an ATC probe is always recommended.
There are three advantages for using an ATC probe. The meter recognizes a particular pH buffer and autocalibrates with the correct pH value at the current temperature. The meter calculates and stores the correct slope value. The meter automatically adjusts the stored slope in memory to display the temperature adjusted pH value of the sample.
Q. Why will my pH system no longer autocalibrate?
A. When the pH system will not autocalibrate, the meter, pH electrode and pH buffers should be checked systematically.
If your meter has a millivolt mode, measure the electrode millivolts in pH buffers. In order for ORION meters to recognize pH buffers for autocalibration, millivolt (mV) limits have been placed on each buffer:
- The electrode millivolts in a pH 7 buffer should be 0+/-30 mV.
- The electrode millivolts in a pH 4 buffer should be 140 to 210 mV.
- The electrode millivolts in a pH 10 buffer should be -140 to -210 mV.
- If the millivolt values are outside of the above ranges, clean the pH electrode. If cleaning does not return the mV to an acceptable range, either manually calibrate or replace the electrode.
Note: As long as the pH electrode has a slope between 92% and 102%, the electrode should be working properly.
The pH buffers should be replaced if the measured millivolts are outside of the acceptable ranges. Contaminated buffers may slightly contribute to shifted millivolt values.
Q. My pH electrode is drifting. What should I do?
A. There are three possible causes for electrode drift:
- If the pH electrode is "out-of-box" and drifting, the electrode may not be properly conditioned. Refer to the appropriate electrode instruction manual for details.
- If the pH electrode is stable in buffers but not in the sample, the electrode may be incompatible with the sample or application. For application assistance contact Van London Co. (aka Van London pHoenix Co.).
- If the pH electrode is drifting in buffers and samples, the electrode may require cleaning. Refer to Question 4.