I would use PdH electrodes. there is an accurate description in Imokawa et al 2006 in Anal Chem. They did not consider differetn temperature and pressures, but in the past the bibliografy refers to them

I would support that opinion PdHx is a reliable reference electrode

How about metal metal oxide pH electrode. They are stable at high temperature. In the following article Odijk et al. 2010 used iridium oxide electrode as a pH detector:

http://www.sciencedirect.com/science/article/pii/S095656631000477X

I am sorry, I never try at temperature different from room temeprature or 37ºC

Below 300C another system that be used is a metal oxide electrode, such as W/WO3 or Ir/IrO2- calibrate with bufeira solution and use.

You can use antimony pH electrode ! very simple use and at high temperature too!

Then it depends from the ph range you need !

I got my PhD long time ago (1974) working at HT (900C) and I do remember that there are many ind electrodes for pH, namely Pd electrodes, metal oxide electrodes, and so on ; go to Scopus and you would find appropriate systems.

Based on the limited information I would look into: PERpH-X – 3300HT/HTVP, 3400HT/HTVP, 3500P/3500VP (Emerson process Management). To really select a sensor you will need to know the minimum acceptable accuracy and at what temperature/pressure.

Mixed metal oxide or Iridium oxide electrodes might be suitable.

W. Vonau, F. Berthold, H. Jahn, U. Guth, : Long-term investigation of glass electrodes at higher temperatures. Chem. Anal. (Warzawa) 54 (2009) 1181-1192.

Today glass electrodes cover pH ranges up to 130 °C and about 8 bar.. Most of them have Polymer reference electrolytes. See e.g. Metrohm, Hamilton, Mettler.

Marco Mascini is right, everything depends in which range of pH u are interested in to and if you are interested in long time stability (ie monitoring pH variation) or u are more interested in small changes in pH in a defined range or in big variations. a good review on the topic has been published by Maj-Zurawska in the 2009. Personaly, maybe becouse I use them every day, I would go for the PdH that is easy to fabricate and cover the entire pH range.

Before measuring samples over 80°C, check whether the probe is designed to stand such high temperatures. Gel-filled pH probes can be used up to 80°C but no higher. Hach pH probes have an Ag/AgCl reference system which is ideal for higher temperatures. We recommend pH probes with saturated KCl filling solution. As KCl crystals are soluble at higher temperatures, it is a good idea to ensure excess crystals so that the solution remains saturated. If at all possible lower the temperature of the sample to the temperature of the buffers at calibration.

Glass electrode can work above 100C

You need to check for the details, e.g., if it a combination type, then you must select silver/ silver chloride. What Hanan Merey has said above is correct but I thought 0.1M KCl would be best because conc will not change with temperature and it has the lowest temp coefficient.

Need extensive callibration of the electrode, It may also show hysteresis.

You may use a solid state chip pH-ISFET (Ion Sensitive Field Effect Transistor) with a solid state Ag/AgCl Ref Electrode . Just specify the Temperature range for the encapsulation material to be adequat. The pH- ISFET Chip includes a Temperature diode which can be used for temperature compensation. Ask us for more details

Ultra portable but protected against high temperature, harsh field conditions...

Check this link,

http://www.hach.com/sension-5052t-portable-combination-ph-electrode-for-difficult-lis-high-temperature-applications/product?id=7640597338

Kind attention to Dr. Jothi: Yes, I had already gone through the site you have quoted. This Hach Electrode is very good one, no doubt in it. But it cannot be used beyond 100 deg.C. I am looking for pH electrode compatible to high temperature and high pressure application so that I can use it for online use. We have developed a pulsating sensor based instrumentation for pH/ ORP measurement. If I get such type of electrode for solution with high temp (> 100 deg. C) and high pressure it will be easy for me to couple it with our instrument for diverse industrial application.

Kind attention to Dr. Grisel: Thank you for your nice suggestion. What I have understood ISFET pH electrode sensor is a miniaturised type and better than glass electrode from handling and operation point of view. But such electrodes are not as reliable as pH glass electrode from performance point of view. They deviate Nernst linearity. If you have certain ISFET electrode which gives excellent precision and resolution in measurement pl. suggest the particular brand and its availability. Will it be used beyond 100 deg.C solution temperature with high pressure? If so it will be suitable for my application which I have mentioned.

If you do not have a glass electrode that is suitable for these temperatures, or you want to avoid it for other reasons, a piece of antimony suitably mounted (e.g. sealed in glass) and contacted, is effective as a potentiometric pH sensor.

pH-ISFET we advice follows a Nernst linearity (that is 55mV/ pH) over a large pH range. It can be used beyond 100°C . Combine with a miniature Ag/AgCl Ref Electrode the system a solid state device and can also be used at high pressure. For more information and datasheets please ask by e-mail : [email protected] and [email protected]

Dr. Grisel: Thank you for your nice suggestions and clarification regarding pH-ISFET.

Metal Oxide may be a good option. Refer to this old reference paper, it maybe useful. Iridium oxide electrodes in particular have been shown to be stable at high temperature. See Fabrication method for hydrated iridium oxide electrode:

Thin-film IrOx pH microelectrode for microfluidic-based microsystems

Igor A. Ges

Also... Sputtered thin-film pH electrodes of platinum, palladium,

ruthenium, and iridium oxides

by Kenneth G. Kreiger

Thesis on Iridium oxide electrodes:

A PH ELECTRODE BASED ON MELT-OXIDIZED IRIDIUM OXIDE

Santi Chrisanti

Kind attention to Dr. Archibong Thank you for your nice suggestion. Will you please send those reference details which will help me to access? If you have soft copies will you please mail me in my e-mail ID ? [email protected] Or [email protected]

It wil be very much useful to me.

Thanks & regards

I realize this question was posed some time ago, but it may still be referenced by others.  Innovative Sensors Inc. has the M-11-HT pH electrode rated up to 110 Celsius.  Honeywell also has a variety of high temperature pH electrodes available.  We have used both at 80 Celsius and they last about 6 months.