Rosemount Analytical Model 372 pH Sensor

The Rosemount Analytical Model 372 is an easy to install combination sensor that enables pH measurement of solutions containing HF. Although sensor lifetime will differ based on the temperature, pH, and HF exposure, it will last much longer than conventional glass electrodes.

pH measurement has historically been conducted using thin glass membrane electrodes that slowly dissolve in water, creating a pH sensitive surface layer. Some chemicals and/or conditions accelerate this dissolution process, causing the electrodes to become very fragile and prone to breakage. Examples include high temperatures, caustic exposure, steam exposure, and exposure to hydrofluoric acid. Hydrofluoric acid (HF) is commonly used to etch glass and chemically attacks the silicon-oxygen bonds that hold the glass matrix together. Rosemount Analytical's new Model 372 sensor uses a new glass formulation that slows down this attack considerably, making measurements in up to 1% (10,000 ppm) HF cost effective and economical.

The presence of hydrofluoric acid in an application is affected by the pH. When the pH is high, nearly all the fluoride ion will be present in the relatively benign free Fform. Lower pH values will convert the free fluoride to HF and present a much harsher solution to measure.

Sensor features include:

• Cross linked polymer reference system is resistant to heat, solvents, acids, and most other chemicals
• Reference holds an excess of KCl, assuring saturation at all temperatures and extending in situ sensor life
• The Fluoride and Acid Resistant pH Glass formulation reduces the buildup of fluoride precipitates on the surface, minimizing cleaning and calibration requirements
• Glass Composition reduces HF attack by more than a factor of ten

Recommended applications for the 372 pH Sensor include:

• Waste water containing fluoride or HF
• Etching solutions
• Water quality devices requiring long service life and/or operation at remote locations with no supervision