A bit off topic but interesting - taken from Progrow's forum:
Calibration
Since glass pH electrodes measure H+ (hydrogen ion) concentration relative to their reference half-cells, they must be calibrated periodically to ensure accurate, repeatable measurements.
Although calibration against one pH reference buffer (one-point calibration) typically ensures accurate pH measurement, frequent two-point or even three-point calibrations ensure the most reliable results. Make sure your pH system includes calibration buffers for a range of pH values.
Handling
Rinse electrodes with distilled water before and after measuring a sample. Blot the end of the electrode with lint-free paper to remove excess water. NOTE: Never wipe the electrode to remove excess water - wiping can create static charges that interfere with correct pH measurement.
Conditioning
pH electrodes are shipped with the electrodes moist. Remove the protective cap or rubber boot from the bottom of the sensor and rinse the electrode with distilled or deionised water.
Place the electrode in a clean container containing one of the liquids i.e. 4.0 M K.C.l. or pH 4.0 buffer or pH 7.0 buffer. Soak electrode for 20 minutes. NOTE: Never condition the electrode in distilled water or deionised water - long term exposure to pure water will damage the special glass membrane.
After conditioning the sensor, rinse the electrode with distilled or deionised water. The electrode is ready for calibration and measurement.
Storing
Always keep pH electrode moist. Proper pH electrode storage maximizes electrode performance and extends electrode life. It is best to store electrodes in clean containers filled with pH storage solution. Do not store an electrode in distilled or deionised water - this will cause ions to leach out of the glass bulb and render your electrode useless.
Cleaning
The solution used to clean pH electrode depends on the presence of possible contaminants. Use the guide below to choose the appropriate solution:
For general cleaning soak the pH electrode in 0.1 M HCl or 0.1 M HNO3 for 20 minutes
For removing stubborn deposits and bacteria soak the pH electrode in a 1:10 dilution of household laundry bleach for 10 minutes
For removal of oil and grease rinse the pH electrode with mild detergent or methanol
For removal of protein deposits soak the pH electrode in 1% pepsin in 0.1M HCl for 5 minutes
After any of the cleaning procedures, thoroughly rinse the pH electrode with deionised water, drain and refill the reference chamber, and soak the electrode in storage solution for at least an hour.
