pH sleeve Junction Electrode

Typical Problems When Measuring pH of Non-aqueous (non-water based) Solutions:

• Readings will not stabilize
• Incorrect results
• Plastic electrode components cracking or chemically changing

Contributing Factors:

• Organic liquids have a low ionic strength making the electrode work very hard to find H+
• Contamination from sample to sample
• Organic liquids and polymers can coat the pH sensing bulb and reference junction

• High sample resistance
• Bulb dehydration
• Large, unstable junction potentials.

Possible Solutions:

• The high resistance (low conductivity) of some organic compounds can cause unstable readings and noise. To eliminate this problem, use an electrode with a low pH glass resistance.
• Between measurements, rinse electrodes with a solvent that will remove the all of the previous sample from the glass membrane.
• Soak the electrode in pH 4 buffer to revive the sensing membrane.
• A free-flow, sleeve junction electrode is also great for low ionic strength samples including non-aqueous solutions.  It provides a constant, user-defined flow of the filling solution into the sample. This increases the ionic strength of the sample that alleviates the above problems. However the addition of KCl in the solution may not be desirable as it changes the composition of the sample. Typically this discrepancy is less than the drifting results without the addition.

Other factors to consider when measuring non-aqueous solutions (keep in mind most common applications can use the above guidelines).

• Calibrating using standard (aqueous) buffers but measuring non-aqueous solutions can cause some discrepancy in pH readings.
• Sample type may not be compatible with the electrode due to the difference in diffusion rates of the internal filling solution. Double junction electrodes allow for more compatible filling solutions.   Using a filling solution of a methanol and deionized water saturated with potassium chloride salt (KCl) is recommended.

Thus, a glass lab electrode with a 0-14 pH glass, double junction design, moveable sleeve reference, and fill solution of KCl in methanol is the most logical choice to at least try in this application.