The pK Measurement of Monoprotonated Amines by Indicator Method in Acetonitrile

Abstract

The pKa of amines, mainly aniline and pyridine derivatives, were measured by indicator method in acetonitrile. In acetonitrile, the pKa of those amines have never been measured except for several amines by Kolthoff et al. As an indicator (HI), two types of indicator (different charge on HI; 1+ and 0) were used. It was found that the pKa of amines in the range of 9.55-16.1 could be easily measured by the indicator method. However, we failed to find the appropriate indicator for the measurement of pKa of amines belonging to pKa>16.1 and pKa<9.55. The values of pK obtained were almost the sam e independent of indicator used. Furthermore, the linear plot of pKa (in acetonitrile) against pKa (in water) was observed (the linear plot contains the values of Kolthoff et al.) The values of pKa of amines obtained here would be expected to be used for another studies requiring numerical analysis in acetonitrile solution.