OPTICAL SENSOR FOR GASEOUS AMMONIA WITH TUNABLE SENSITIVITY

The detection of gaseous ammonia using an optical evanescent wave sens or coated with a PVC film containing a chromogenic calixarene (nitroph enylazophenol calix[4]arene) is described. On addition of various conc entrations of ammonia gas, the absorbance maximum shifts to 500 nm, wh ich is characteristic of deprotonation of the chromogenic group, The s ensitivity of the sensor to ammonia varied depending on whether the fr ee ligand or lithium complex was used in the membrane, Intermediate se nsitivities can be generated by varying the mole ratio of lithium to c alixarene. The response time of the lithium complex measured at 520 nm to a 5-50 ppm change in ammonia concentration is fast (several minute s to steady state) but the signal is slow to return to the baseline wh en the gas is replaced by nitrogen.