128 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS Table 3.2. pK a Values for p-sulfonatocalix[4]arene derivatives C OMPOUND 7 8 9 10 pK a1 8.9 3.08 ± 0.05 2.18 ± 0.05 -2 to 0 pK a2 12.02 ± 0.02 8.45 ± 0.01 1.28 ± 0.09 pK a3 13 (estimated) 11.19 ± 0.15 4.60 ± 0.11 pK a4 >13 (estimated) 11.62 ± 0.12 4.42 ± 0.13 Source: Data from Matsumiya et al. 2002. al. 2002). It has been determined that the phenolic OH of p-sulfonato-tetrathiacalix[4]arene is more acidic than that of p-sulfonatocalix[4]arene, and a large difference has been observed in the case of pK a2 of the second-generation compounds. These differences may be ascribed to the electronic effect of the sulfur moiety as well as the change in the hydrogen bond strength due to enlargement of the ring. It has also been observed that the oxidation of the bridge sulfur to corresponding sulfone further increases the acidity, as indicated by the pK a values in Table 3.2. 7. SPECTRAL PROPERTIES AND CHARACTERIZATION