reduction by Hydrazine

The whole thing about pH is that it is, by definition, the negative logarithm, base 10, of the hydrogen ion concentration in moles per litre. It appears as a voltage (from memory about 59mV per pH unit) upon the electrode that is contained within a thin glass envelope, relative to a reference electrode. In order to generate that voltage, both electrodes need to be in contact with the solution that is being measured. It is extremely unlikely that any technique involving electromagnetic radiation will produce a signal that can be captured and used, which is why instrument manufacturers haven't produced one that does - such a thing would corner the market!

Most pH electrodes have an acceptable pH range: a scale of 0-14 is not uncommon, and such a span would encompass both acids and bases. The manufacturer's documentation, shipped with the electrode, will give further information.

One of the possibilities may be introduce a flow chamber into the process, into which the electrodes are inserted, that contains cleaning balls that are retained within the chamber. These balls are made of a soft compound that is agitated by the flow passing through the flow chamber, and their action is to wipe the electrode to prevent it coating, in a non-aggressive way. Instrument manufacturers can give more advice on this.

Another question to ask is, "does the pH need to be measured at this location; could it be measured elsewhere, at a point where coating would not occur?" Another one is, "does the pH need to be measured at all, given a high level of confidence in the repeatability of processes upstream?" In essence, what is being done with the measurement and why is it apparently so critical? Is it so critical, or could it be dispensed with? Given the wide span, "acids and bases", in the original post, possibly not.

If neither of these questions can come up with an acceptable answer at a substitute, then the technical specialists at a number of instrument suppliers are available to give further advice over the telephone. It could be that simply substituting a different probe with a different electrode surface might be all that is neccessary.

You probably need to think a little further outside the box. There are other ways of determining pH. Colorimetric indicators measured pH long before Dr. Beckman invented his handy little box. The Analyst, a journal of the Royal Society of Chemistry, deals with solutions to measurement problems like yours. Their website is http://www.rsc.org/publishing/journals/AN/Index.asp.

You probably need to cultivate a good relationship with a knowledgeable analytical chemist. Good luck.