An optical pH sensor based on the fluorescence of a new lipophilic rhodamine dye immobilized in a thin poly(vinyl chloride) membrane was developed. The response to hydrogen ions was a result of reversible changes in the molecular structure of rhodamine. Excited at 528 nm, the membrane fluoresced at 547 nm. The sensor system showed a usable sensitivity over the 9.5-11.9 pH range and very good reproducibility
