There are two issues here:
1) Solutions that are difficult to measure
2) Solutions that will damage the sensor

Solutions that are Difficult to Measure
The pH Sensor (GDX-PH, GDX-FPH, PH-BTA or FPH-BTA) will have difficulty measuring the pH of the following liquids: distilled water, water with a low ion concentration, food and wine, ocean water, brackish water, and wastewater. In these liquids, the pH readings will be slow to stabilize, potentially taking several minutes.

Solutions that can Damage the Sensor
The pH Sensor can be damaged if you try to measure the pH of an aqueous solution containing a protein or a Tris buffer. Long-term exposure to these species can cause irreparable damage to the sensor. Vernier sells the Go Direct^® Tris-Compatible Flat pH (GDX-FPH) and Tris-Compatible Flat pH Sensor (FPH-BTA) which, because of its double-junction construction, can be used in these types of solutions.

In addition, do not place the sensor in hydrofluoric acid or an acid or base solution with a concentration higher than 1 M.

Keep in mind that all our pH sensors (GDX-PH, GDX-FPH, PH-BTA or FPH-BTA) are designed to be used in aqueous solutions. The polycarbonate body of the sensor can be damaged by many organic solvents, including but not restricted to: chloroform, acetone, methanol, toluene, xylene, and methyl ethyl ketone.

Options for Specific Solvents
We do offer Glass Body pH probes and electrodes (Go Direct^® Glass-Body pH Sensor (GDX-GPH) and Glass-Body pH Electrode BNC (GPH-BNC)) that will work under all of these conditions. Please consider this electrode if you have specific solvent needs.