Calibration and measurement
- For quick and accurate results, the electrolyte plug should be open during measurements (Note: polymer electrolytes do not have an electrolyte plug).
- The electrode should be immersed far enough to cover the diaphragm. The height to which the electrolyte is filled must always be above that of the sample. This prevents the sample solution from entering into the electrode.
- Always wait for the electrode to reach the same temperature as the sample.
- Between measurements, the electrode should be rinsed with deionized water. If necessary, dab it dry with a paper towel. Never rub the electrode dry with a paper towel, as it will become electrostatically charged an slow response will result.
- To prevent problems, calibrate with Duracal buffer (see page 16/17). If you do not use Duracal buffer, never calibrate in the original bottle. Always use fresh buffer solution for calibration. Close the bottle after use.
- Dispose of used buffer responsibly.
- Read the operating instructions of the pH measurement device.
Temperature Influences
Both the pH/redox value of the sample and the characteristics of the electrode are temperature dependent. Usually, the temperature dependency of the sample is unknown. Therefore, it is important to always record the measurement temperature together with the measured value. The automatic temperature compensation of the measurement devices can only compensate for the temperature dependence of the electrode’s characteristics curve (Nernst-Gleichung). For this purpose, temperature sensors (for example, Pt1000 or NTC 30 kohm) are used. In order to obtain the most accurate measurement, the electrode should always be calibrated at the same temperature at which measurements will later take place. For measurements that will serve as a comparison between laboratory and process values, make sure that the laboratory measurement takes place at the same temperature as the process measurement.
Storage
For storage, place the electrode (with closed electrolyte plug) in the reference electrolyte, or better, in the Hamilton storage solution (PN238931). The storage solution helps to clean both the diaphragm and the pH glass. electrodes must never be stored in deionized water.
Cleaning
Contamination of the diaphragm is the most frequent cause of measurement problems. only infrequently do problems with the glass membrane occur. The diaphragm and the pH membrane should therefore be kept clean in order to avoid measurement errors and long response times. Use soap and water to remove oil, fat, and organic substances. In the event of contamination of the electrode by proteins, submerge the electrode in a fresh solution of 0.4% HCl, and 5 g/l pepsin. After every cleaning, the electrode should be conditioned in HAMILToN storage solution for at least 2 hours. After cleaning always perform a new calibration before carrying out measurements.
To simplify cleaning, Hamilton has developed a special cleaning set (PN 238290) which easily remove most types of contamination from electrode diaphragm and pH glass.
Most frequent causes of calibration problems
The following three problems occur most often during calibration:
- Zero calibration error
- Electrode slope too low
- Slow response, for example, longer than 3 minutes
There are a variety of causes for the problems named above. The most frequent are:
- The buffer solutions used are either contaminated or out-of-date. It could also be that one of the buffer solutions used is no longer the value labeled on the bottle – for this reason, never store buffer solutions in unmarked or dirty containers.
- The reference electrolyte and / or the diaphragm are contaminated.
- An old or defective electrode is used.
- An electrode is used that has not been hydrated long enough (after dry storage or after cleaning with strong acid solution).
- The pH membrane of the electrode is mechanically damaged, and has cracks.
- The electrode is electrostatically charged (through rubbing of the electrode shaft with a cloth instead of careful dabbing with soft paper).
- The temperature difference between electrode and buffer solution is more than 10°C.
- The connection between electrode and measurement device can also cause problems: for instance, a break in the cable, or a short-circuit caused by moisture in the cable or electrode plug.