When testing a Go Direct Glass-Body pH, it is best to measure a buffer solution because it is easier to determine if the sensor is reading correctly. Do not test your sensor by measuring distilled water. Distilled water can have a pH reading in the range of 5.5–7.0, due to varying amounts of dissolved carbon dioxide. Furthermore, due to a lack of ions, the pH values reported with the sensor in distilled water will be erratic.
If your pH Sensor is reading differently from the pH of a buffer solution (e.g., reads 6.7 in a buffer 7), you may simply need to calibrate the sensor. See the Calibrating the Sensor section for more information.
Examine the glass bulb. If it is broken, pH readings will be incorrect.
If your readings are off by several pH values, the pH readings do not change when moved from one buffer solution to a buffer solution of different pH, the sensor was stored dry for an extended period of time, or the sensor’s response seems slow, the problem may be more serious. A method called “shocking” can be used to revive pH electrodes. To shock your pH Sensor, perform the following:
- Soak the pH Electrode for 4–8 hours in an HCl solution of 0.1 M–0.5 M.
- Rinse off the electrode and soak the tip in freshly prepared long term storage solution (recipe above) for 30–60 minutes.
- Rinse the electrode and test it with buffer solutions of known pH.
Occasionally, mold will grow in the pH 4 buffer/storage solution. Mold will not harm the electrode and can easily be removed using a mild detergent solution. Mold growth in the storage solution can be inhibited by adding a buffer preservative.
The stored pH calibration equation was determined in aqueous solutions. When determining the pH in organic media or non-aqueous solutions (less than 5% water), the conventional pH range of pH 0 to 14 is not valid because it is based on the dissociation behavior of water.
In applications involving non-aqueous solvents it is common to measure relative rather than absolute pH. Therefore, when doing a pH measurement in non-aqueous samples it is important to remember that the measurement will not give an absolute pH value. To measure quantitatively in non-aqueous solvents, prepare a calibration curve for the pH electrode with different samples that have a known composition corresponding to the conditions of the samples to be measured. This makes it possible to differentiate the different sample compositions during the measurement without having to quantify an absolute value during the measurement.
When testing in non-aqueous solutions, the electrode will lose its hydrated gel layer around the pH-sensitive membrane. To ensure that measurements can still be performed, take care to rehydrate the gel layer in an ion-rich aqueous solution between experiments.
For additional troubleshooting and FAQs, see www.vernier.com/til/3965