Acid-base equilibria in gamma-butyrolactone studied by use of pH-ISFETs

pH-ISFETs were used in the study of acid-base equilibria in gamma-butyrolactone (GEL). After the spectrophotometric determination of the pK(a) value of 3,5-dichloropicric acid, the pK(a) values and homo-conjugation constants of various acids (including the conjugate acids of bases) were determined potentiometrically using a Ta2O5-type pH-ISFET. The values of pK(a) in GEL were in a linear relation with those in propylene carbonate (PC) and 1.0 units smaller on average. The difference in pK(a) between GEL and PC was mainly attributable to the difference in proton solvation. The autoprotolysis constant of GEL, roughly estimated by a rapid titration with a Si3N4-ISFET, was about 30 on the pK(SH) scale. A comparative study was made of the response speeds of the Ta2O5- and Si3N4-type pH-ISFETs and a conventional pH-glass electrode. The result was Si3N4-ISFET > Ta2O5-ISFET > glass electrode. Because GEL is not stable against acids and bases, the use of pH-ISFETs was much more convenient than the use of the conventional glass electrode.