Is there a reason you want to use silicon carbide? That would be my first question, There has been a lot more work done using Gallium Nitride. Gallium nitride is also a wide band gap material, but it has the advantage for sensing that you can relatively easily form a 2D electron gas at an AlGaN/GaN interface. this can be sensitive to surface charge and make a Ph Sensor. It is also very inert.

For Silicon carbide you could try to make FET like sensor for Ph detection, and have the gate modulated by the Ph of the solution.  An you would see some changes in threshold with changes in Ph. 

For high temperature applications, you have additional considerations. The SiC may be a high temperature material, but the materials you use for the dielectric or gate may not be. If you get very hot you may also have etching concerns if the Ph is high or low, etc.

For sensors in general you need to work about calibration and drift, and at higher temperatures you will get a different response.

You can take a look at ISFETs Ion Selective FET and will also find out that you may really want to have a reference electrode. Those characteristics can also change at elevated temperatures. 

High temperature means different things to different people.