An optical pH sensor basically comprises two essential parts: A pH sensitive sensor layer and a read-out device (pH meter).

The pH meter consist of a LED and photodiode which excites the compounds in the pH sensor and detects its luminescence emission. The excitation and emission light is transmitted via an optical fiber between the sensor indicator and the pH meter. 

The optical pH sensor is based on the successful PyroScience REDFLASH technology. The sensor consists of a pH insensitive reference indicator and a pH sensitive luminescent indicator. Both are excited with red-light (more precisely: orange-red at a wavelength of 610-630 nm) and show bright luminescence in the near infrared (NIR, 760-790 nm). If the pH indicator is de-pronated at high pH, its fluorescence is quenched and only the NIR emission of the reference indicator is measured. If the pH gets more acidic, the pH indicator is protonated and emits bright NIR luminescence and the emission of both indicators is detected. 

The REDFLASH technology impresses by its high accuracy, high reliability and low power consumption. The red-light excitation of the REDFLASH indicators significantly reduces interferences caused by autofluorescence and reduces background fluorescence from biological samples / media. The measuring principle is based on a sinusoidally modulated red excitation light. This results in a phase-shifted sinusoidally modulated emission in the NIR. The PyroScience read-out device (FireSting pro) measures this phase shift, which is then calculated into pH.

Different pH ranges

Optical pH sensors are sensitive to a pH range of 2-3 pH units. Sensors are offered for different ranges:

-PK5 sensors for pH measurements between pH 4.0 - pH 6.0
-PK6 for pH 5.0 - 7.0
-PK7 for pH 6.0 - 8-0
-PK8 for pH 7.0 - 9.0
-PK8T for pH 7.0 - 9.0 on pH total scale (factory calibrated with the spectrophotometric method)

 Depending on the application it is important to choose the right version.

Calibration of all versions is performed at low pH values (completely protonated indicator) and at high pH values
(completely de-protonated indicator). For this reason, it is possible to use the same pH buffer capsules (PHCAL2 and PHCAL11) for all pH sensor versions. 

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