Optical pH sensors

by Steven
(Washington)

One technique to measure pH is to use a fiber optic sensor. Maybe an uncommon pH technique, but I think it's important to know it exists. The development of optical sensors is of great interest because of their possible application in biotechnology, ecology and medicine.


Principle of operation

The main principle of an optical sensor is to measure the color changes of a pH indicator which are trapped in a thin porous glass membrane.

• An optical light source sends a pulse of light via fiber to the pH sensitive membrane. The membrane changes its color (absorbance) in dependence on pH of the measured sample.

• A reflected light pulse is measured by a photodetector. The reflected pulse depends on the absorbance (color) of the membrane. An increase of the absorbance of the membrane (change of pH) will result in a smaller reflected light pulse.


Fiber optic chemical sensors can offer several advantages over traditional sensors as:

• light weight
• small size
• strong immunity to electromagnetic interference
• free from the handling problems associated with pH glass electrodes