Conclusion

OxP, Bz2OxP and Bz4OxP serve as colorimetric sensors for acids in nonpolar media in wide range of acid volume fractions. The halochromic response of OxP ( \(\varphi (\text {DFA}) \in [ 10^{-6},10^{-3} ]\) ) is very sensitive to small amounts of acid (detects ca. 1 ppm of DFA), however, it also saturates at relatively low acid contents. The color response is shifted to higher acid contents in Bz2OxP ( \(\varphi (\text {DFA}) \in [ 10^{-4},1 ]\) ) and Bz4OxP ( \(\varphi (\text {DFA}) \in [0.01,0.6 ]\) ) due to N-alkylation (Fig.  6.6 ).